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The Differential Enrichment Law of Tight Sandstone Gas in the Eighth Member of Shihezi Formation in the North and South of Ordos Basin

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Title: The Differential Enrichment Law of Tight Sandstone Gas in the Eighth Member of Shihezi Formation in the North and South of Ordos Basin
Authors: Haoyuan Wang, Jingong Zhang, Zishu Yong, Xiumei Qu
Source: Energies, Vol 17, Iss 23, p 5978 (2024)
Publisher Information: MDPI AG, 2024.
Publication Year: 2024
Collection: LCC:Technology
Subject Terms: Ordos Basin, source rock, shanxi formation, gas enrichment difference, sedimentary facies, Technology
More Details: Unconventional oil and gas resources are important energy sources for alleviating energy crises and maintaining sustainable social development. Therefore, the subsequent exploration and development of the basin summarizes and analyzes the reasons for the difference in natural gas enrichment between the southern and northern parts of the Ordos Basin. The tight sandstone gas of the Upper Paleozoic in the Ordos Basin has become a key field for increasing reserves and production of natural gas in the basin. As a high-quality tight sandstone gas reservoir, the eighth member of the Shihezi Formation only has good gas production in the south of the basin. For this reason, the types of natural gas source rocks in the north and south of the Upper Paleozoic were summarized, and the main types of gas source rocks in the north and south were clarified. The sedimentary facies of the eighth member of the Shihezi Formation and the Shanxi Formation were analyzed. The results show that the main gas-producing source rock in the northern part of the basin is coal rock, and in the southern part, it is dark mudstone. The Permian Shanxi Formation in the northern part of the basin mainly develops the delta front, which provides good conditions for the development of coal and rock. Compared with the northern Shanxi Formation, the Permian Shanxi Formation in the southern Ordos Basin mainly develops marine facies and marine-continental transitional facies, and the coal seam is not created. Therefore, the natural gas produced by the coal of the Shanxi Formation can easily migrate to the good reservoir in the eighth member of the box, which is also the reason for the difference in the enrichment of tight sandstone gas in the north and south of the Ordos Basin.
Document Type: article
File Description: electronic resource
Language: English
ISSN: 1996-1073
Relation: https://www.mdpi.com/1996-1073/17/23/5978; https://doaj.org/toc/1996-1073
DOI: 10.3390/en17235978
Access URL: https://doaj.org/article/6ee51d1fe6b444f896386567376978c6
Accession Number: edsdoj.6ee51d1fe6b444f896386567376978c6
Database: Directory of Open Access Journals
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  Value: <anid>AN0181656808;[b1v5]01dec.24;2024Dec17.06:08;v2.2.500</anid> <title id="AN0181656808-1">The Differential Enrichment Law of Tight Sandstone Gas in the Eighth Member of Shihezi Formation in the North and South of Ordos Basin </title> <p>Unconventional oil and gas resources are important energy sources for alleviating energy crises and maintaining sustainable social development. Therefore, the subsequent exploration and development of the basin summarizes and analyzes the reasons for the difference in natural gas enrichment between the southern and northern parts of the Ordos Basin. The tight sandstone gas of the Upper Paleozoic in the Ordos Basin has become a key field for increasing reserves and production of natural gas in the basin. As a high-quality tight sandstone gas reservoir, the eighth member of the Shihezi Formation only has good gas production in the south of the basin. For this reason, the types of natural gas source rocks in the north and south of the Upper Paleozoic were summarized, and the main types of gas source rocks in the north and south were clarified. The sedimentary facies of the eighth member of the Shihezi Formation and the Shanxi Formation were analyzed. The results show that the main gas-producing source rock in the northern part of the basin is coal rock, and in the southern part, it is dark mudstone. The Permian Shanxi Formation in the northern part of the basin mainly develops the delta front, which provides good conditions for the development of coal and rock. Compared with the northern Shanxi Formation, the Permian Shanxi Formation in the southern Ordos Basin mainly develops marine facies and marine-continental transitional facies, and the coal seam is not created. Therefore, the natural gas produced by the coal of the Shanxi Formation can easily migrate to the good reservoir in the eighth member of the box, which is also the reason for the difference in the enrichment of tight sandstone gas in the north and south of the Ordos Basin.</p> <p>Keywords: Ordos Basin; source rock; shanxi formation; gas enrichment difference; sedimentary facies</p> <hd id="AN0181656808-2">1. Introduction</hd> <p>Unconventional oil and gas resources are important energy sources for alleviating energy crises and maintaining sustainable social development [[<reflink idref="bib1" id="ref1">1</reflink>]] (Li, Q et al., 2024). Over the past 20 years, the Upper Paleozoic strata of the Ordos Basin have provided the largest source of tight sandstone gas production in China [[<reflink idref="bib2" id="ref2">2</reflink>]]. The output of tight sandstone gas is concentrated in the central and northern parts of the Ordos Basin [[<reflink idref="bib3" id="ref3">3</reflink>], [<reflink idref="bib5" id="ref4">5</reflink>]]. In recent years, the successful exploration of tight sandstone gas in the Upper Paleozoic on the edge of the Ordos Basin has provided significant support for the continuous increase in natural gas production in the basin, showing the great potential of natural gas resources in the Upper Paleozoic [[<reflink idref="bib6" id="ref5">6</reflink>], [<reflink idref="bib8" id="ref6">8</reflink>], [<reflink idref="bib10" id="ref7">10</reflink>]]. The Upper Paleozoic tight gas resources in the Ordos Basin are 13.32 × 10<sups>12</sups> m<sups>3</sups>, accounting for 61% of China's tight gas resources. The sand bodies of the eighth member of the box are superimposed on each other in the longitudinal direction, and compositely connected in the transverse direction. They are distributed in a large area like a network carpet. The exploration drilling rate is more than 90%, and the distribution of the whole basin is stable [[<reflink idref="bib7" id="ref8">7</reflink>]]. The gas reservoir of the eighth member of the Shihezi Formation takes the Carboniferous Permian and Benxi Shanxi marine-terrigenous coal measures as the gas source rock, the mudstone of the seventh member of the Shihezi Formation as the direct cap rock, and the mud rock of the shore-shallow lake facies from the fourth member of the Shihezi Formation to the first member of the Shihezi Formation as the regional cap rock. Among the main gas-bearing intervals of tight gas in the upper Paleozoic Benxi Formation, the Taiyuan Formation, the second member of the Shanxi Formation (Shan-2 section), the first member of the Shanxi Formation (Shan-1 section), and the eighth member of Shihezi Formation (He-8 section), the gas reservoir of He-8 member is widely distributed in the basin and has the largest scale. However, the gas reservoirs discovered so far are mainly distributed in the northern part of the basin, and the gas reservoirs in the southern part of the basin are distributed primarily in the Shanxi Formation, Taiyuan Formation, and Benxi Formation. Previous studies on gas reservoirs in the eighth member of the Shihezi Formation mainly focused on gas–water distribution characteristics, reservoir characteristics, diagenesis, etc. They lacked research on the distribution differences in the gas reservoirs in the eighth member of the Shihezi Formation. Based on previous studies [[<reflink idref="bib12" id="ref9">12</reflink>], [<reflink idref="bib14" id="ref10">14</reflink>], [<reflink idref="bib16" id="ref11">16</reflink>], [<reflink idref="bib18" id="ref12">18</reflink>]], this paper summarizes and analyzes the law of differential enrichment of natural gas in the north and south of the eighth member of the Ordos Basin.</p> <hd id="AN0181656808-3">2. Regional Geological Survey</hd> <p>The Ordos Basin is located on the western margin of the North China Craton. It is a large multi-cycle craton basin and the most stable block [[<reflink idref="bib19" id="ref13">19</reflink>]], covering up to 25 × 10<sups>4</sups> km<sups>2</sups> [[<reflink idref="bib4" id="ref14">4</reflink>]]. In the regional tectonic unit, the western part of the basin is bounded by the Helan fault fold belt and the Liupanshan arc thrust belt, adjacent to the Alxa block. The north borders on the Guyinshan fold orogenic belt of Yimeng uplift, the east is bounded by the Yishan block, and the southwestern margin is bounded by the Qilian-Qinling orogenic system [[<reflink idref="bib3" id="ref15">3</reflink>], [<reflink idref="bib19" id="ref16">19</reflink>]]. Given the current tectonic characteristics, evolution history, tectonic development, and tectonic characteristics of the Ordos Basin, it is divided into six structural units (Figure 1) [[<reflink idref="bib21" id="ref17">21</reflink>], [<reflink idref="bib23" id="ref18">23</reflink>]].</p> <p>We selected the Sulige area and the Yimeng uplift area in the north of the Ordos Basin and the Longdong and surrounding areas in the south of the basin for research. The exploration area of the Sulige gas field is 4 × 10<sups>4</sups> km<sups>3</sups>. By the end of 2014, the annual production of natural gas has reached 238 × 10<sups>8</sups> m<sups>3</sups>, and the cumulative proven and proven natural gas reserves have reached 4.46 × 10<sups>12</sups> m<sups>3</sups>. It is the first large gas field in China [[<reflink idref="bib24" id="ref19">24</reflink>]]. The main strata in the study area include the Shiqianfeng Formation, Shihezi Formation, Shanxi Formation, Taiyuan Formation, and Benxi Formation from the top to the bottom. The main gas-bearing strata are the Upper Paleozoic Shihezi Formation and Shanxi Formation. The Longdong area is located in the southern part of the Ordos Basin. The main target layers are Shihezi Formation and Shanxi Formation. The sand body thickness is 5~15 m and the burial depth is 3800~4600 m. By the end of 2014, the favorable gas-bearing area was preliminarily determined to be 3600 km<sups>2</sups>, forming a new 100 billion cubic meter scale reserve replacement area. The coal-measure source rocks of the second member of the Shanxi Formation (Shan 2 member) are mainly developed in the Longdong area. The source rocks of the Taiyuan Formation are only produced in some areas, and the Benxi Formation is missing in the whole area. In the Sulige area, the three sets of source rocks of Shan 2 members, Taiyuan Formation and Benxi Formation are distributed across the entire area. Still, the development scale is different in different areas [[<reflink idref="bib25" id="ref20">25</reflink>]].</p> <hd id="AN0181656808-4">3. Analysis Methods</hd> <p>We collected geochemical data from more than 1200 wells to analyze the source rocks and selected source rock thickness, organic carbon content, and vitrinite reflectance to characterize the hydrocarbon generating capacity of these source rocks. By collecting imaging logging data from 162 wells, we observed the development of fractures within the Ordos Basin. The disparity in hydrocarbon generation capacity between source rocks located in the northern and southern regions of the Ordos Basin is expected to influence natural gas distribution across these areas. The distribution of source rocks in both regions is governed by sedimentary facies; therefore, we also gathered data from over 300 wells to investigate these facies further. Core samples and thin section specimens from 43 wells were examined to ascertain rock lithology, while observations regarding rock color and sedimentary structure features were compiled. Additionally, logging data from 78 wells were collected for analysis and classification based on their phase curve characteristics. Building upon previous research findings and considering regional sedimentary patterns, sequence stratigraphy, and sedimentation processes, we identified the primary types of sedimentary facies through integration of core facies analysis and logging-derived sedimentary facies classifications.</p> <hd id="AN0181656808-5">4. Results and Discussion</hd> <p></p> <hd id="AN0181656808-6">4.1. Spatial Distribution Characteristics and Hydrocarbon Generation Intensity of Source Rock...</hd> <p>The Upper Paleozoic strata in the Ordos Basin are predominantly composed of continental clastic rocks and coal measures. The primary source rocks for natural gas in this period are mainly represented by coal seams and dark mudstones from the Upper-Lower Shihezi Formation as well as the Shanxi Benxi Formations, with a minor presence of limestone interbedded within. Coal deposits within the basin exhibit widespread distribution, ranging in thickness from 2 to 30 m (Figure 2a), displaying a trend of increasing thickness towards the north and decreasing towards the south, while also thinning progressively from the periphery toward the center. Approximately 70% of this area features coal layers that are between 3 and 10 m thick; notably thicker deposits measuring between 15 and 30 m can be found in the Lingtao region along the western margin, whereas those in the Majiatan fold belt range from 10 to 20 m in thickness. In contrast, the northern sections of Tianhuan show thicknesses between 5 and 10 m, while both Yimeng slope and Yulin-Suide areas present similar ranges of thickness of approximately 10 to 20 m. Dark mudstones are also extensively distributed (Figure 2b), characterized primarily by a west-to-east thinning trend. The Yinchuan graben exhibits maximum thickness reaching up to approximately 800 m near Zhongning but thins sharply eastward; conversely, vast regions westward outside of the basin do not exceed a thickness of 100 m and gradually thin out toward both northern and southern directions relative to the central basin locations [[<reflink idref="bib26" id="ref21">26</reflink>]]. The total organic carbon (TOC) content in the northern Ordos Basin is higher than that found in the southern Ordos Basin. However, the maturity of the organic matter in the southern Ordos Basin exceeds that of the northern Ordos Basin. Previous investigations have indicated that kerogen types predominantly consist of type III sourced from coal and dark mudstone formations, with only a minor fraction classified as type II within certain mudstone samples (Figure 2c). However, this proportion remains relatively insignificant overall. From an integrated perspective on gas source rock distribution regarding abundance and typology across Ordos Basin as a whole (Figure 2e), it is evident that these gas source rocks are widely dispersed with considerable thicknesses coupled with high abundance levels (Figure 2d), thereby providing an excellent material foundation conducive for forming large- to medium-sized gas fields [[<reflink idref="bib27" id="ref22">27</reflink>]].</p> <hd id="AN0181656808-7">4.1.1. Spatial Distribution Characteristics and Hydrocarbon Generation Intensity of Source Ro...</hd> <p>Two sets of Upper Paleozoic source rocks are mainly developed in the Yimeng uplift, northern Ordos Basin, which are coal seams and dark mudstones of the Taiyuan Formation–Shanxi Formation, respectively. Previous studies suggest that the difference in sedimentary surfaces makes the thickness of the source rocks in the north show the characteristics of a thick southeast and thin northwest. In the braided channel area of the alluvial fan, the thickness of the source rock is thinner, while in the swamp and floodplain area, the thickness of the source rock is thicker. The research data show that the hydrocarbon generation potential of the coal seam is significantly higher than that of mudstone, the S1 + S2 value of the coal sample is 172.52 mg/g, while that of mudstone is only 2.72 mg/g. Although the area of the coal seam and mudstone is equivalent, and the thickness of coal seam is about half that of mudstone, the organic matter abundance of the coal seam is more than ten times to dozens of times that of mudstone. This huge advantage in organic matter abundance allows coal seams to make up for differences in volume, so it can be confirmed that the coal seams in the Yimeng uplift in the northern part of the basin are the main source rocks for natural gas accumulation [[<reflink idref="bib28" id="ref23">28</reflink>]].</p> <p>The main gas-bearing intervals in the northwestern Yishan slope of the Ordos Basin are the eighth member of the Shihezi Formation and the first member of the Shanxi Formation, which are tight sandstone gas reservoirs. The natural gas is derived from the Carboniferous–Permian coal-bearing source rocks [[<reflink idref="bib13" id="ref24">13</reflink>], [<reflink idref="bib29" id="ref25">29</reflink>]]. Previous studies have shown that the natural gas of the Lower Shihezi Formation is derived from the upward migration of natural gas generated by the source rocks of the Shanxi Formation, while the source rocks of the Taiyuan Formation do not have a significant contribution. The dark mudstone in the northwest of the Yishan slope has a certain contribution to hydrocarbon generation, which belongs to a medium to good source rock. As a source rock, the quality of coal rock is good, and it should belong to a medium to good source rock. From the comparison of source rock types, the contribution of the coal rock to hydrocarbon generation is significantly greater than that of dark mudstone [[<reflink idref="bib16" id="ref26">16</reflink>]].</p> <p>According to previous statistics, the average TOC value of dark mudstone in the northern section of the Jinxi flexural fold belt is 2.4%, the average TOC value of carbonaceous mudstone is 15.2%, and the average TOC value of coal rock is 64.1%. The coal rock is mainly distributed in the Benxi Formation (average of 9.1 m) and Taiyuan Formation (average of 7.2 m) with large, buried depth, and the thickness of coal rock in Shanxi Formation is thin (average of 1.9 m). Carbonaceous mudstone is thicker in the Benxi Formation (average of 5.6 m) and Taiyuan Formation (average of 9.2 m), and thinner in the Shanxi Formation (average of 3.2 m). The dark mudstone is relatively thin in the Benxi Formation (average of 23.8 m) and Taiyuan Formation (average of 27.1 m), and the mudstone of the Shanxi Formation is the thickest (average of 43.4 m). The coal of the Benxi-Taiyuan Formation is a high-quality source rock, and some dark mudstones also have good hydrocarbon generation capacity, which is a medium-good source rock [[<reflink idref="bib30" id="ref27">30</reflink>]].</p> <p>The hydrocarbon source rocks in the central part of the Tianhuan Sag are mainly located in the Carboniferous Yanghugou Formation, Permian Taiyuan Formation, and Shanxi Formation [[<reflink idref="bib2" id="ref28">2</reflink>], [<reflink idref="bib31" id="ref29">31</reflink>]]. Coal seams are mainly distributed in the Yanghugou Formation and the Taiyuan Formation, with a thickness of 1.0~5.0 m and an average of 3.1 m. The coal seams are thick in the north and south and thin in the middle. The dark mudstone is mainly distributed in the Taiyuan Formation and Shanxi Formation, with a thickness of 25.0~70.0 m and an average of 44.5 m, which is characterized by thin in the north and thick in the south. The coal seam as a whole belongs to the poor-good source rock, and the dark mudstone belongs to the good-good source rock. Therefore, the hydrocarbon generation contribution rate of the dark mudstone is large in the hydrocarbon generation capacity [[<reflink idref="bib32" id="ref30">32</reflink>]].</p> <hd id="AN0181656808-8">4.1.2. Spatial Distribution Characteristics and Hydrocarbon Generation Intensity of Source Ro...</hd> <p>The Paleozoic source rocks in the southern part of the basin are mainly developed in the Carboniferous–Permian, and the lithology is primarily dark mudstone, carbonaceous mudstone, and coal [[<reflink idref="bib33" id="ref31">33</reflink>]].</p> <p>In the southwestern part of the Ordos Basin, a large area of coastal swamp facies coal-bearing source rocks [[<reflink idref="bib14" id="ref32">14</reflink>]] were developed in the Permian. Due to the lack of an Upper Paleozoic–Carboniferous system, two sets of source rocks are mainly produced in the southwestern part of the basin, namely Taiyuan Formation and Shanxi Formation coal-bearing strata. The main source rocks are the coal seams of Shanxi Formation and Taiyuan Formation, and the dark mudstones developed when they were secondary source rocks. The coal seams are mainly distributed at the top of the Taiyuan Formation (generally 1~2 sets are developed), with a cumulative thickness of 1~4 m and an average thickness of 2.5 m. The dark mudstone is developed in the Shan-2 member, with a thickness of 60~90 m, and an average thickness of 72 m. It is stable in the southwest of the basin [[<reflink idref="bib34" id="ref33">34</reflink>]]. The thickness of the Permian coal rock is 2–13 m, and the TOC content is between 73.5% and 91.33%. The thickness of the dark mudstone and carbonaceous mudstone is 3–95.0 m, and the TOC content is between 0.26% and 8.93%. The kerogen type is mainly the humic–humic type. The vitrinite reflectance (Ro) of source rocks is 1.6~3.0%, generally more than 2.0%, which is in the stage of high mature-over-mature dry gas. The average hydrocarbon generation intensity is 15.5 × 10<sups>8</sups> m<sups>3</sups>/km<sups>2</sups> [[<reflink idref="bib35" id="ref34">35</reflink>]], which has good hydrocarbon generation ability [[<reflink idref="bib17" id="ref35">17</reflink>]].The high thermal evolution degree of coal measure source rocks in the southwest makes up for the disadvantage of thickness thinning, so it can provide a sufficient gas source for natural gas accumulation.</p> <p>The upper Paleozoic in the southeast of the Yishan slope in the Ordos Basin is composed of Carboniferous Benxi Formation, Permian Taiyuan Formation, Shanxi Formation, Lower Shihezi Formation, Upper Shihezi Formation, and Shiqianfeng Formation from bottom to top (Figure 2a). The dark mudstone of coal measures is mainly developed in the Shanxi Formation and Benxi Formation. The cumulative thickness of strata is 130~195 m in the vertical direction, and the thickness of dark mudstone of coal measures is generally 70~120 m. The thickness of the strata drilled in the Shanxi Formation is 100~135 m, and the thickness of the dark mudstone in the coal measures 55~85 m, accounting for about 55% of the total thickness of the strata. The thickness of the dark mudstone in the coal measures of the Shanxi Formation is generally 65~75 m, and it becomes thinner in the southwest, and the thickness is less than 65 m. The thickness of the strata drilled in the Benxi Formation is 30~60 m, and the thickness of the dark mudstone in the coal measures is generally 15~35 m, accounting for about 60% of the total thickness of the strata. The thickness of the dark mudstone in the Benxi Formation coal measures gradually thins to the west and south, less than 15 m in the west and less than 20 m in the south. Coal rock is mainly developed in the Shanxi group and Benxi group.</p> <p>The coal rock of the Shanxi Formation is relatively developed, which is mainly developed in the Shan 2 section in the longitudinal direction. The thickness of coal rock is generally 1~4 m, and the maximum thickness can reach 5 m. The Benxi Formation is generally characterized by fewer coal–rock interlayers and larger single-layer thickness than the Shanxi Formation. It is concentrated in the first section (No. 8 coal) in the longitudinal direction. The thickness is generally 1–5 m, and the maximum thickness can reach 6 m. The dark mudstones of the Benxi Formation and Shan 2 Member are generally medium-good source rocks, and the dark mudstones of Shan 1 Member are generally poor source rocks–non-hydrocarbon source rocks [[<reflink idref="bib36" id="ref36">36</reflink>]].</p> <hd id="AN0181656808-9">4.2. Characteristics of Sedimentary Facies in the Ordos Basin</hd> <p></p> <hd id="AN0181656808-10">4.2.1. Sedimentary Facies Division Marks</hd> <p>The sedimentary markers of the sedimentary system include rock color, composition, rock type combination, structure, sedimentary structure, particle size distribution, profile structure, etc. These characteristics are important indicators reflecting the sedimentary environment.</p> <p></p> <ulist> <item> Lithology marks</item> <p></p> <item> (<reflink idref="bib1" id="ref37">1</reflink>) Color and lithology</item> </ulist> <p>In the north, the color of mudstone in the lower and upper sections of He-8 is gray black and dark gray, and there are occasional variegated mudstone (Figure 3g–i). The color of the sandstone is mainly light gray, gray-white, and gray-green (Figure 3a–f). Gray black and dark black are more common in the lower part of box eight and the middle and upper part of box eight [[<reflink idref="bib37" id="ref38">37</reflink>]].</p> <p>For the lower and upper parts of He-8 in the northern part of the basin, the colors of mudstone are mostly variegated, grayish green, brown, gray, and brown, followed by grayish black and dark gray. Brown and variegated are more common in the lower part of He-8 and the middle and upper part of He-8, and variegated or brown mudstones are often interbedded with gray and gray-green mudstones, reflecting that the water level of the sedimentary environment changes frequently. The water body often changes between the oxidizing environment and the reducing environment, and the sediments may be exposed. Combined with its regional sedimentary background, it is speculated that the early sedimentary environment of the lower part of He-8 and the upper part of He-8 may be a seasonal arid climate environment, and the flow rate of the sedimentary water body changes greatly. The sandstones are mostly gray-white, gray-green, and gray. The mudstones of the Shan 1 member of the Shanxi Formation are dark gray and gray-black, and the sandstones are mostly gray-white and light gray, reflecting that the sedimentary climate was humid and the sedimentary environment was weak reduction–reduction environment [[<reflink idref="bib39" id="ref39">39</reflink>]].</p> <p>The northern study area is mainly composed of three rock types: quartz sandstone, lithic quartz sandstone, and lithic sandstone, but the main reservoir rock properties of the He-8 reservoir in different zones vary. The western area is mainly composed of quartz sandstone and lithic quartz sandstone (58.27% and 35.9%, respectively), and a small amount of lithic sandstone (5.82%). The eastern area is mainly composed of lithic sandstone and lithic quartz sandstone (44.21% and 48.06%, respectively), and a small amount of quartz sandstone (7.73%); the proportion of lithic quartz sandstone in the central area is the largest (52.95%), followed by lithic sandstone and quartz sandstone (24.85% and 22.19%, respectively) (Figure 4).</p> <p>In the southern area of the basin, through the detailed observation and description of the cores of 31 wells, the sandstone in the southern part of the basin is mainly light gray and grayish green. The mudstones in the Shan 1 member are mostly variegated, grayish green, and gray, followed by grayish black and black, showing plant debris, charcoal, thin coal seams, and bioturbation. The light gray, grayish green, and purplish red mudstones are common in the He−8 section, reflecting that the sedimentary water body of this section is relatively shallow. A small amount of carbon mudstone and terrestrial plant fragments can be seen in the coring section, reflecting that the water level of the sedimentary environment has changed, and the sedimentary environment is a weak oxidation–reduction environment (Figure 5a–i).</p> <p>The reservoir rock types of the Shanxi Formation and Shihezi Formation in the southern Ordos Basin are mainly quartz sandstone, feldspar quartz sandstone, and lithic quartz sandstone, with a small amount of feldspar sandstone and lithic sandstone. The sandstone is mainly medium sandstone, coarse-medium sandstone, a small amount of coarse sandstone, and fine sandstone. The content of quartz is usually greater than 62%, and the content of feldspar and rock debris is less than 25%. Therefore, the clastic composition of sandstone is mainly quartz, followed by debris, and relatively few feldspar. The roundness of the debris particles is medium, mainly subangular. The detrital composition of sandstone is mainly quartz, followed by debris, and feldspar is relatively less (Table 1).</p> <p></p> <ulist> <item> (<reflink idref="bib2" id="ref40">2</reflink>) Sedimentary structure marks</item> </ulist> <p>In the north, the rock sedimentary structure types of Shan-1 to He-8 are mainly horizontal bedding, parallel bedding, cross-bedding, graded bedding, and erosion surface (Figure 6a–d).</p> <p>Horizontal bedding is mainly developed in the flood-plain of a braided river and the natural levee, crevasse splay, and floodplain of a meandering river.</p> <p>Parallel bedding is often formed under the hydrodynamic conditions of shallow water and rapid flow and is mainly found in the sedimentary environment of the heart beach or side beach with strong hydrodynamic force.</p> <p>The tabular cross-bedding is mainly found in the sedimentary environment of the heart beach or the side beach with strong hydrodynamic force.</p> <p>The wedge-shaped cross-bed is composed of gray-light gray coarse, medium, and fine sandstone. It is characterized by wedge-shaped contact of each layer on the cross-section, mostly formed in the sedimentary environment of the heart beach and the side beach.</p> <p>In the southern area, the observation and description of the core indicate that parallel bedding and tabular cross-bedding are common in the Shan 1 member; the sedimentary structure of the He-8 member is mainly large trough cross-bedding and wedge bedding (Figure 7a–d).</p> <p></p> <ulist> <item> (<reflink idref="bib1" id="ref41">1</reflink>) Parallel bedding is common in channel sedimentary environments such as delta plain surfaces distributary channels.</item> <p></p> <item> (<reflink idref="bib2" id="ref42">2</reflink>) Plate cross-bedding often appears in the delta plain surfaces distributary channel and delta front surfaces underwater distributary channel, estuary sand dam environment.</item> <p></p> <item> (<reflink idref="bib3" id="ref43">3</reflink>) Trough cross-bedding is mostly formed in meandering river beach sand dam or delta front underwater distributary channels and estuary sand dams deposition.</item> <p></p> <item> (<reflink idref="bib4" id="ref44">4</reflink>) Wedge cross-bedding: The interface of the cross-bedding is a non-parallel plane, and the layer system converges to another section due to the change in thickness and intersects in a wedge shape.</item> </ulist> <p>Graph: Figure 7 Basin in the southern region of Shan 1, He−8 core typical sedimentary structure symbol photo: (a) parallel bedding, Hetan 1 well, 3635.49 m, Shan 1; (b) tabular cross−bedding, Qingtan 4 well, 4376.40 m, Shan 1; (c) trough cross−bedding, Qingtan 2 well, 4747.42 m, He−8; (d) wedge cross−bedding, Zhentan 2 well, 5036.25 m, He 8.</p> <p></p> <ulist> <item> (<reflink idref="bib3" id="ref45">3</reflink>) Log facies marker</item> </ulist> <p>There are several types of logging facies in the basin:</p> <p></p> <ulist> <item> Bell-shaped curve</item> </ulist> <p>The characteristics of the bell curve are the curve of the relationship between the bottom mutation and the top gradient, which reflects the sedimentary characteristics of the coarse-to-fine underwater distributary channel sand body composed of medium-fine sandstone, siltstone, argillaceous siltstone, silty mudstone to mudstone (Figure 8).</p> <p></p> <ulist> <item> b. Box-shaped curve</item> </ulist> <p>The box-shaped curve is characterized by a sudden change in the bottom and top or a slight positive rhythm change to a box shape, reflecting the sedimentary characteristics of the underwater distributary channel composed of fine sandstone and silty fine sandstone with multi-rhythm superposition. The internal structure is not uniform, and there may be multiple mudstone interlayers. Therefore, based on the box-shaped curve, the intermediate curve will show the corresponding toothing, reflecting the intermittent hydrodynamic energy change in the water body (Figure 9).</p> <hd id="AN0181656808-11">4.2.2. Types and Characteristics of Sedimentary Facies</hd> <p>In the north, according to the characteristics of the regional sedimentary patterns, sequence stratigraphic characteristics, and sedimentation, on the basis of many previous research results, the main sedimentary facies types are identified by core facies and logging sedimentary facies markers. The northern part of the basin is divided into three sedimentary systems [[<reflink idref="bib40" id="ref46">40</reflink>]], and the division of each facies belt is shown in Table 2.</p> <p></p> <ulist> <item> Impact plain</item> </ulist> <p>Braided river channel surfaces and floodplain surfaces are developed in the alluvial plain.</p> <p></p> <ulist> <item> (<reflink idref="bib1" id="ref47">1</reflink>) Riverbed retention deposits: The lithology is mainly coarse sandstone, gravel-bearing coarse sandstone, and conglomerate at the bottom. When the hydrodynamic conditions are strong, the coarser gravel is deposited at the bottom of the riverbed.</item> <p></p> <item> (<reflink idref="bib2" id="ref48">2</reflink>) Bottomland: The sediment thickness is similar to the riverbed depth, and its width depends on the size of the river.</item> <p></p> <item> (<reflink idref="bib3" id="ref49">3</reflink>) Floodplain: Floodwater flooding can form a wide and flat floodplain sedimentary area. The sediments are not only muddy but also have a large number of sandy deposits. It is mainly developed on the outside of the river channel. The lithology is the variegated and gray-black mudstone formed by suspended matter, and the horizontal bedding and sand texture are developed.</item> <p></p> <item> (<reflink idref="bib4" id="ref50">4</reflink>) Deposition of natural levees: Because the river water overflows the river bank during the high flood period, the fine-grained sediments carried by the river water level drop are long-banded and ridge-shaped deposits formed outside the river channel. The common sedimentary structures are horizontal bedding and sand bedding, with obvious frequent sand mud interbedding.</item> <p></p> <item> (<reflink idref="bib5" id="ref51">5</reflink>) Crevasse splay deposition: in the high water level of the flood season, the flood overtook the embankment accumulation formed</item> <p></p> <item> 2. Braided river delta</item> </ulist> <p> <sups>®</sups> The braided river delta facies develop delta plain surfaces and delta front surfaces.</p> <p></p> <ulist> <item> Delta front facies</item> <p></p> <item> (<reflink idref="bib1" id="ref52">1</reflink>) Underwater distributary channel: It is composed of gravelly coarse sandstone, medium sandstone, and fine sandstone, with medium sorting and low compositional maturity and structural maturity. Sedimentary structures such as bottom erosion, trough cross-bedding, parallel bedding, and horizontal bedding are developed. The natural potential curve and natural gamma curve show bell-shaped, tooth-shaped bell-shaped, and micro-tooth-shaped box-shaped.</item> <p></p> <item> (<reflink idref="bib2" id="ref53">2</reflink>) Estuary sand dam: The lithology is composed of medium-coarse-grained sandstone, medium-grained sandstone, and coarse-grained sandstone. The sandstone is well-sorted and rounded. The sedimentary structures are mainly tabular cross-bedding, parallel bedding, graded bedding, ripple bedding, etc., and flushing bedding is also seen. The profile structure is characterized by upward coarsened reverse grain order.</item> <p></p> <item> (<reflink idref="bib3" id="ref54">3</reflink>) Interdistributary bay: It is mainly composed of mudstone and silty mudstone, with a small amount of siltstone or fine sandstone, and wormholes are developed. Horizontal bedding development, see lenticular bedding and wave marks; it is a low-lying deposit between underwater distributary channels.</item> <p></p> <item> (<reflink idref="bib4" id="ref55">4</reflink>) Far sand dam: Due to the weak transformation of lake waves, the deposition of the far sand dam in the northern part of the basin is relatively undeveloped.</item> <p></p> <item> Delta plain facies</item> <p></p> <item> (<reflink idref="bib1" id="ref56">1</reflink>) Braided channel: The lithology is coarse, often composed of conglomerate, pebbly sandstone, and sandstone, of which the main lithology is pebbly-sand medium and coarse sandstone; the maximum thickness of a single sand body varies from 0.2 m to 2.5 m.</item> <p></p> <item> (<reflink idref="bib2" id="ref57">2</reflink>) Abandoned channel: The filling deposits are often formed by the sudden change in the water system and the diversion of the channel, which are lens-shaped, and the sand bodies gradually become thinner on both sides. The lithology is not much different from the braided channel, and the overall grain size is finer than the river channel.</item> <p></p> <item> (<reflink idref="bib3" id="ref58">3</reflink>) Cross-bank deposition: It is the fine-grained material deposited in the water-filled depressions on both sides of the river during the flood period, which is mainly composed of thin interbeds of siltstone and mudstone, and develops ripple bedding.</item> <p></p> </ulist> <p>• Lakes</p> <p>The lake sedimentary system is developed in the Shanxi Formation and the Lower Shihezi Formation. It is mainly shallow lake facies, distributed on both sides of the delta front, mainly shallow lake surfaces. Shallow lake sediments are located in the area from the inner side of the lakeside subfacies to the wave base surface. The water body is deeper than the lakeside area, and the sediments are strongly affected by waves and lake currents. The lacustrine facies deposits are characterized by dentate linear-type on the logging curve.</p> <p>In the south, based on the characteristics of regional sedimentary pattern, sequence stratigraphic characteristics, and sedimentation, based on many previous research results, through core facies and logging facies markers, combined with provenance and tectonic background, the upper Paleozoic in the south of the basin is divided into 3 types of facies, 5 types of subfacies, and 13 types of microfacies (Table 3).</p> <p></p> <ulist> <item> 3. Delta sedimentary system</item> </ulist> <p>There are two types of deltas in the southern part of the basin, namely the meandering river delta and the braided river delta (Table 4).</p> <p></p> <ulist> <item> Delta plain surfaces</item> </ulist> <p>The delta plain is the aquatic sedimentary part of the delta. It begins at a large number of bifurcations of the river and ends at the shoreline or lake level.</p> <p></p> <ulist> <item> (<reflink idref="bib1" id="ref59">1</reflink>) Branch Channel (braided channel)</item> </ulist> <p>The branch channel is a part of the framework in the delta plain. It is often in contact with the underlying rock layer by bottom erosion. It has a positive sedimentary sequence with upward thinning. The bottom sandstone contains scattered mud gravel, plant stems, and other residual sediments.</p> <p></p> <ulist> <item> (<reflink idref="bib2" id="ref60">2</reflink>) Inter-River Depressions (marshes)</item> </ulist> <p>The inter-channel depression is a low-energy depression area between the main distributary channels. The lithology is mainly dark gray mudstone and silty mudstone, with a small amount of lenticular siltstone and fine sandstone. The horizontal texture is developed in the rock, the biological disturbance is strong, and the natural gamma curve is mostly a high amplitude finger shape.</p> <p></p> <ulist> <item> b. Delta Front surfaces</item> </ulist> <p>The delta front surface is part of the underwater sedimentary body near the estuary of the delta plain distributary channel into the lake basin, and it is also the main component of the delta sedimentary system.</p> <p></p> <ulist> <item> (<reflink idref="bib1" id="ref61">1</reflink>) Underwater Distributary Channel</item> </ulist> <p>Underwater distributary channels are generally distributed in a network. The lithology of the underwater distributary channel in the south of the basin is mainly gray medium (fine)-grained sandstone. The thickness of the single sand body is generally 1.0~3.6 m, and the mudstone interlayer is mainly dark gray and gray-black. The bottom of the sandstone has a scouring surface, which is often in contact with the underlying rock strata.</p> <p></p> <ulist> <item> (<reflink idref="bib2" id="ref62">2</reflink>) Interdistributary Channel (tributary bay)</item> </ulist> <p>The interdistributary bay is a low-energy sedimentary environment, which is usually dominated by the vertical accretion of fine-grained suspended matter in the overflow channel during the flooding period. Therefore, the interdistributary bay is dominated by clay deposition and contains a small amount of silt.</p> <p></p> <ulist> <item> (<reflink idref="bib3" id="ref63">3</reflink>) Underwater Natural Levee</item> </ulist> <p>The lithology of the underwater natural levee in the south of the basin is mainly dark gray siltstone and black-gray argillaceous siltstone.</p> <hd id="AN0181656808-12">4.2.3. Single Well Sedimentary Facies Analysis</hd> <p>Based on the study of various facies markers, combined with core observation, the vertical evolution of sedimentary facies and the distribution law of the plane are studied. Based on careful observation of the cores in the north and south of the basin, the comprehensive histogram of the single well is drawn.</p> <p>Su 422 Well is located in the northern part of the basin, with a depth range of 3866.2–4015 m. The total thickness of Shan 1 and He−8 formations is about 106.4 m, which is generally gray-white medium-coarse-grained quartz sandstone. The Shanxi Formation is a meandering river delta front deposit, and the He−8 member is a braided river delta front deposit. The microfacies include branch channels and interdistributary bays (Figure 10).</p> <p>The Shanxi Formation section is a meandering river delta front deposit, and the lithology is mainly light gray pebbly coarse sandstone with dark gray mudstone. The sedimentary structures include erosion surface, tabular cross-bedding, and horizontal bedding. The natural gamma curves of the gravel-bearing coarse sandstone, coarse sandstone, and medium sandstone are medium-high-toothed box-shaped. The natural gamma curves of mudstone and silty mudstone are low-amplitude micro dentate mudstone baselines. Among them, pebbly coarse sandstone, coarse sandstone, and medium sandstone are riverbed retention deposits, beach deposits or abandoned channel deposits, mudstone, and silty mudstone are flood lake deposits.</p> <p>He-8 section is a braided river delta front deposition, which developed a set of dark gray mudstone. The sedimentary microfacies are braided channels, abandoned channels, and overbank deposits. The braided channel microfacies are gravelly coarse sandstone and medium-fine sandstone, and its natural gamma logging curve is a high-amplitude toothed box type. The abandoned channel microfacies are gray fine sandstone, and its natural gamma logging curve is medium-amplitude toothed box type.</p> <hd id="AN0181656808-13">4.2.4. Analysis of Sedimentary Connected Well Profile</hd> <p>The profile is a north–south sedimentary connected well profile, which passes through five wells of Su 307-Su 400-Su 422-Su 148-Su 121 in turn.</p> <p>From the observation of this section, it can be seen that the sub-facies combination model of braided river delta plain-braided river delta front was formed in the eighth member of the Shihezi Formation. During the sedimentary period of the first member of the Shanxi Formation, the braided river delta plain sub-facies-braided river delta front was successively developed (Figure 11). During the sedimentary period of the second member of the Shanxi Formation, the littoral and shallow sea sedimentary facies were also developed. The Su 121-Su 148 area develops the braided river delta plain surfaces and then transitions to the braided river delta front surfaces. The boundary of the two subfacies mainly appears near the location of the Su 422 well and Su 148 well. In this section, the braided river delta plain surfaces mainly develop large-scale sandy sediments, and the distributary channel microfacies are developed. There are five-channel sand bodies with small thicknesses and narrow ranges and two-channel sand bodies with large thicknesses and wide ranges. The braided river delta front surfaces are widely distributed in this section, and the channel sand body is greatly improved in thickness, connectivity, and distribution range than the braided channel on the delta plain. However, the channel sand is still surrounded by widely distributed inter-river mud. The water body gradually deepens from south to north, and the thickness of the channel sand body gradually increases from south to north. This shows that although the water body as a whole deepened during this period, there was a sudden change in water level in some areas, and the source supply was sufficient. Typical inter-tributary bay microfacies are developed in the Su 400 well-Su 307 well interval, during which a large number of brown or black carbonaceous mudstones are deposited, while the underwater distributary channel microfacies in the front are mainly concentrated in the well control area where the Su 422 well-Su 400 well is located. Among them, the underwater distributary channel of the delta front is mainly coarse-medium sandstone, and the grain size is smaller than that of the delta plain branch channel sand body, but the grain size sorting is better.</p> <hd id="AN0181656808-14">4.2.5. The Plane Distribution Characteristics of Sedimentary Facies</hd> <p>The shallow water delta sedimentary system is mainly developed in the southern Ordos Basin, and the provenance is affected by the ancient water system from the southwest, south, and north.</p> <p></p> <ulist> <item> Sedimentary facies distribution of Shanxi Formation</item> </ulist> <p>The combination form of the meandering river delta plain-meandering river delta front-shore shallow lake surface was formed in the Shan 1 period. The sand body development scale is relatively small, the thickness is thin, the lithology is mainly medium-fine-grained sandstone with less gravel, the bottom erosion phenomenon is not significant, the distributary channel sand body is distributed in the north–south direction, and the characteristics of local contiguous. The delta front surfaces in the northern part of the delta front belt are advancing to the southwest and still form a lobed delta. The range of shallow lake or lakeside mud is large, mainly distributed in the front of the delta front, showing a curved and continuous distribution (Figure 12a,b).</p> <p></p> <ulist> <item> 2. Sedimentary facies distribution of Shihezi Formation</item> </ulist> <p>The braided river delta facies are mainly developed (Figure 13), while the lacustrine facies are only a small-scale development in the middle of the study area. The braided distributary channel sand bodies are vertically superimposed on each other, and the degree of lateral interweaving is slightly increased. The braided river delta plain surfaces are mainly distributed in the area south of Shatan 1–Chengtan 1.</p> <hd id="AN0181656808-15">4.2.6. Imaging Logging</hd> <p>Electrical imaging logging is to convert the resistivity changes caused by the changes in formation lithology, physical properties, cracks, holes, bedding, and other formation characteristics into different chromaticity, and display the characteristics of the formation through images. In the fractured formation, the resistivity of the fracture is significantly lower than that of the surrounding rock due to the invasion of the mud into the fracture, and it is shown as dark stripes on the imaging image. Therefore, the electrical imaging data can be used to identify the fracture. On the imaging logging map (Figure 14), the near-vertical fractures are black lines with small or even parallel angles to the well axis. The low-angle oblique fracture shows a black sine curve (Zhan et al., 2010). The measured strata include the Shihezi Formation, Shanxi Formation, Taiyuan Formation, and Benxi Formation. Statistics show that sandstone is dominated by the near-vertical fractures. The contact interface of mudstone, coal seam, and sand mudstone is mainly parallel to the bedding plane. The development of fractures in the south and north of the basin is the same, so although fractures are important gas migration channels, affecting the distribution and migration of natural gas in source rocks, they are not used as one of the indicators to judge the hydrocarbon generation capacity of source rocks in this paper.</p> <p>In summary, the source rocks are widely distributed in the Upper Paleozoic of the Ordos Basin. From a macro point of view, reservoir rocks are controlled by sedimentary facies. Through the analysis of the source rocks in the northern and southern parts of the basin, it can be seen that the natural gas source rocks in the northern part of the basin are mainly coal seams of Shanxi Formation, and the natural gas source rocks in the southern part of the basin are mainly dark mudstones of Taiyuan Formation and Benxi Formation. The intensity of hydrocarbon generation in the southern Ordos Basin is greater than that in the northern region; however, the quality of natural gas in the north surpasses that found in the south. This discrepancy can be attributed to the predominant development of delta front systems within the Shanxi Formation of the Permian period in the northern Ordos Basin, which creates favorable conditions for coal and rock formation. The natural gas produced by the coal of the Shanxi Formation can easily migrate to the good reservoir in the eighth member of the box. Therefore, the He-8 member in the northern part of the basin has good gas-bearing properties. Compared with the Shanxi Formation in the north, the Permian Shanxi Formation in the southern Ordos Basin mainly develops marine facies and marine-continental transitional facies, and the coal seams are not developed. The natural gas produced by the dark mudstone of the Taiyuan Formation and Benxi Formation is separated from the good reservoir of the southern He-8 section. The Shanxi Formation makes the displacement of natural gas migration longer and the resistance is greater. Therefore, the natural gas in the southern part of the basin is difficult to migrate to the He-8 reservoir in the southern part of the basin, resulting in the natural enrichment in the northern part of the He-8 section of the Ordos Basin. The distribution pattern in the south is less.</p> <hd id="AN0181656808-16">5. Conclusions</hd> <p></p> <ulist> <item> The Upper Paleozoic in the Ordos Basin is dominated by continental clastic rocks and coal measures. The source rocks of the Upper Paleozoic natural gas are mainly coal seams and dark mudstones of the Upper and Lower Shihezi Formations and Shanxi Formation. The main gas-producing source rock in the northern part of the basin is coal rock, and the main gas-producing source rock in the southern part of the basin is dark mudstone.</item> <p></p> <item> The Permian Shanxi Formation in the northern Ordos Basin mainly develops delta front. Permian Shanxi Formation in southern Ordos Basin mainly developed marine facies and transitional facies without coal seams.</item> </ulist> <hd id="AN0181656808-17">Figures and Tables</hd> <p>Graph: Figure 1 Division of tectonic units in Ordos Basin.</p> <p>MAP: Figure 2 Geochemical characteristics of the Upper Paleozoic source rocks in the Ordos Basin. (a) Isoline of coal seam thickness. (b) Thickness contour map of dark mudstone. (c) Total organic carbon content contour map. (d) Vitrinite reflectance contour map. (e) The current cumulative gas generation intensity contour map.</p> <p>Graph: Figure 3 Core photos of Shan 1 member and He−8 member in the northern basin: (a) gray−white sandstone, Su 353 well, 3495.3 m, He 8; (b) gray−white coarse sandstone, Shan 269 well, 2943.7 m, Shanxi Formation; (c) light gray fine sandstone, Shan 265 well, 3304.67 m, Shan 1; (d) gray−white medium sandstone, Su 353 well, 3529.55 m, Shanxi Formation; (e) gray−white fine sandstone, Shan 394 well, 2820.9 m, He 8; (f) light gray medium sandstone, Shaan 400 well, 3769.2 m, Shan 1; (g) Su 121 well sand mudstone interbed; (h) Su 76 well sandstone layer; (i) sand−mudstone interface of well Mi11.</p> <p>Graph: Figure 4 Rock types of different blocks in the northern Ordos Basin.</p> <p>Graph: Figure 5 Core photos of Shan 1 and He 8 sections in the south of the basin: (a) gray−white sandstone, well Qingtan 3, 4243.4 m, Shan 1; (b) gray−green sandstone, Hetan 1, 3592.65 m, He−8; (c) purple red mudstone, Zhentan 2 well, 5091.2 m, Shan 1; (d) gray−green mudstone, Xiang 1 well, 3743.08 m, Shan 1; (e) black mudstone containing plant debris, Lian 1 well, 3464.69 m, Shan 1; (f) grey mudstone, Qingtan 2 well, 4709.5 m, He 8; (g) long 28 well Quartz detrital sandstone; (h) long 38 well mudstone; (i) long 58 well feldspathic lithic sandstone.</p> <p>Graph: Figure 6 Basin in the northern region of Shan 1, He−8 core typical sedimentary structure symbol photo: (a) parallel bedding, Shan 307 well, 3672.65 m, He−8; (b) cross−bedding, Shan 293 well, 4265.03 m, He−8; (c) plate cross−bedding, Shaan 331 well, 3323.07 m, He−8; (d) wedge cross-bedding, Shaan 267 well, 2899 m, Shan 1.</p> <p>Graph: Figure 8 Long 38 well clock−type GR curve.</p> <p>Graph: Figure 9 Box−type GR curve of Well Qingtan 4.</p> <p>Graph: Figure 10 Comprehensive histogram of sedimentary facies of Shanxi Formation−He8 in Su422 well in the northern part of the basin.</p> <p>Graph: Figure 11 Su 121 well−Su 307 well Shanxi Formation, He−8 section sedimentary connected well profile.</p> <p>MAP: Figure 12 Sedimentary facies map of the Permian−Shanxi Formation in the Ordos Basin: (a) sedimentary facies map of Shan 1 of the Permian−Shanxi Formation in the Ordos Basi; (b) sedimentary facies map of Shan 2 of the Permian−Shanxi Formation in the Ordos Basin.</p> <p>Graph: Figure 13 Plane distribution of sedimentary facies of the He−8 member in the basin.</p> <p>Graph: Figure 14 Image of the fracture imaging logging in the Ordos Basin: (a) well Su233; (b) Su 359 well; (c) long 82 well; (d) long 89 well.</p> <p>Table 1 Rock types in southern Ordos Basin.</p> <p> <ephtml> <table><thead><tr><th align="center" style="border-top:solid thin;border-bottom:solid thin">Horizon</th><th align="center" style="border-top:solid thin;border-bottom:solid thin">Quartz Sandstone (%)</th><th align="center" style="border-top:solid thin;border-bottom:solid thin">Quartz (Feldspar) Lithic Sandstone (%)</th><th align="center" style="border-top:solid thin;border-bottom:solid thin">Lithic Sandstone (%)</th><th align="center" style="border-top:solid thin;border-bottom:solid thin">Feldspar Lithic Sandstone + Lithic Feldspar Sandstone (%)</th></tr></thead><tbody><tr><td align="center" valign="middle" style="border-bottom:solid thin">Shan1 section</td><td align="center" valign="middle" style="border-bottom:solid thin">35</td><td align="center" valign="middle" style="border-bottom:solid thin">39</td><td align="center" valign="middle" style="border-bottom:solid thin">16</td><td align="center" valign="middle" style="border-bottom:solid thin">10</td></tr><tr><td align="center" valign="middle" style="border-bottom:solid thin">He8 section</td><td align="center" valign="middle" style="border-bottom:solid thin">32</td><td align="center" valign="middle" style="border-bottom:solid thin">33</td><td align="center" valign="middle" style="border-bottom:solid thin">23</td><td align="center" valign="middle" style="border-bottom:solid thin">12</td></tr></tbody></table> </ephtml> </p> <p>Table 2 Upper Paleozoic sedimentary system division table in the northern Ordos Basin.</p> <p> <ephtml> <table><thead><tr><th align="center" style="border-top:solid thin;border-bottom:solid thin">Sedimentary Systems</th><th align="center" style="border-top:solid thin;border-bottom:solid thin">Sedimentary Facies</th><th align="center" style="border-top:solid thin;border-bottom:solid thin">Subphase</th><th align="center" style="border-top:solid thin;border-bottom:solid thin">Microfacies</th><th align="center" style="border-top:solid thin;border-bottom:solid thin">Distribution Location</th></tr></thead><tbody><tr><td rowspan="2" align="center" valign="middle" style="border-bottom:solid thin">Alluvial System</td><td rowspan="2" align="center" valign="middle" style="border-bottom:solid thin">Flood Plain</td><td align="center" valign="middle" style="border-bottom:solid thin">Braided River Channel</td><td rowspan="2" align="center" valign="middle" style="border-bottom:solid thin">River bed detention, channel bar, crevasse splay, natural levee, flood plain</td><td rowspan="2" align="center" valign="middle" style="border-bottom:solid thin">Shihezi Formation</td></tr><tr><td align="center" valign="middle" style="border-bottom:solid thin">Flood Land</td></tr><tr><td align="center" valign="middle" style="border-bottom:solid thin">Lake System</td><td align="center" valign="middle" style="border-bottom:solid thin">Continental lakes</td><td align="center" valign="middle" style="border-bottom:solid thin">Shallow Lake</td><td align="center" valign="middle" style="border-bottom:solid thin">Shallow sea mud</td><td align="center" valign="middle" style="border-bottom:solid thin">Shanxi Formation</td></tr><tr><td rowspan="2" align="center" valign="middle" style="border-bottom:solid thin">River-dominated Delta System</td><td rowspan="2" align="center" valign="middle" style="border-bottom:solid thin">Braided River Delta</td><td align="center" valign="middle" style="border-bottom:solid thin">Delta Plain</td><td align="center" valign="middle" style="border-bottom:solid thin">Braided Channel, Abandoned Channel, Overbank Deposition</td><td rowspan="2" align="center" valign="middle" style="border-bottom:solid thin">Shihezi Formation</td></tr><tr><td align="center" valign="middle" style="border-bottom:solid thin">Delta Front</td><td align="center" valign="middle" style="border-bottom:solid thin">Underwater Distributary Channel, Estuary Sand Dam, Tributary Bay, Distal Sand Dam, Sheet Sand</td></tr></tbody></table> </ephtml> </p> <p>Table 3 Upper Paleozoic sedimentary system division table in the southern Ordos Basin.</p> <p> <ephtml> <table><thead><tr><th align="center" style="border-top:solid thin;border-bottom:solid thin">Sedimentary Facies (System)</th><th align="center" style="border-top:solid thin;border-bottom:solid thin">Subphase</th><th align="center" style="border-top:solid thin;border-bottom:solid thin">Microfacies</th><th align="center" style="border-top:solid thin;border-bottom:solid thin">The Main Distribution Layer</th></tr></thead><tbody><tr><td align="center" valign="middle" style="border-bottom:solid thin">Barrier Coast System</td><td align="center" valign="middle" style="border-bottom:solid thin">Tidal Flat</td><td align="center" valign="middle" style="border-bottom:solid thin">Sand flat, Mixed Flat, Mud Flat</td><td align="center" valign="middle" style="border-bottom:solid thin">Taiyuan Formation, Benxi Formation</td></tr><tr><td rowspan="2" align="center" valign="middle" style="border-bottom:solid thin">Meandering River Delta</td><td align="center" valign="middle" style="border-bottom:solid thin">Delta Plain</td><td align="center" valign="middle" style="border-bottom:solid thin">Distributary Channels, Interdistributary depressions, peat swamps</td><td rowspan="2" align="center" valign="middle" style="border-bottom:solid thin">Shanxi Formation</td></tr><tr><td align="center" valign="middle" style="border-bottom:solid thin">Delta Front</td><td align="center" valign="middle" style="border-bottom:solid thin">Subaqueous Distributary Channel, Interdistributary Bay, Mouth Bar, Natural Levee, Crevasse Splay, Distal Bar and Sheet Sand</td></tr><tr><td rowspan="2" align="center" valign="middle" style="border-bottom:solid thin">Braided River Delta</td><td align="center" valign="middle" style="border-bottom:solid thin">Delta Plain</td><td align="center" valign="middle" style="border-bottom:solid thin">Distributary Channel, Interdistributary Depression</td><td rowspan="2" align="center" valign="middle" style="border-bottom:solid thin">Shihezi Formation</td></tr><tr><td align="center" valign="middle" style="border-bottom:solid thin">Delta Front</td><td align="center" valign="middle" style="border-bottom:solid thin">Underwater Distributary Channel, Underwater Interdistributary Bay</td></tr></tbody></table> </ephtml> </p> <p>Table 4 Comparison of the sedimentary characteristics of Shan 1 meandering river delta and He−8 braided river delta in the southern part of the basin.</p> <p> <ephtml> <table><thead><tr><th align="center" style="border-top:solid thin;border-bottom:solid thin">Deltaic Type</th><th align="center" style="border-top:solid thin;border-bottom:solid thin">Meandering River Delta</th><th align="center" style="border-top:solid thin;border-bottom:solid thin">Braided River Delta</th></tr></thead><tbody><tr><td align="center" valign="middle" style="border-bottom:solid thin">Rock Type</td><td align="center" valign="middle" style="border-bottom:solid thin">Medium-Fine Sandstone</td><td align="center" valign="middle" style="border-bottom:solid thin">Gravel Coarse Sandstone, Coarse Sandstone</td></tr><tr><td align="center" valign="middle" style="border-bottom:solid thin">Bedding Type</td><td align="center" valign="middle" style="border-bottom:solid thin">Common Parallel Bedding, Plate Bedding and Small Cross-Bedding</td><td align="center" valign="middle" style="border-bottom:solid thin">Common Large Trough Cross-bedding, Wedge Cross-bedding, Cross-bedding</td></tr><tr><td align="center" valign="middle" style="border-bottom:solid thin">Logging Curve</td><td align="center" valign="middle" style="border-bottom:solid thin">Most Of Them Are Bell-shaped</td><td align="center" valign="middle" style="border-bottom:solid thin">Multi-box Type, Toothed-Box Type</td></tr><tr><td align="center" valign="middle" style="border-bottom:solid thin">Dual Structure</td><td align="center" valign="middle" style="border-bottom:solid thin">Development</td><td align="center" valign="middle" style="border-bottom:solid thin">Undeveloped</td></tr><tr><td align="center" valign="middle" style="border-bottom:solid thin">Sand Ratio</td><td align="center" valign="middle" style="border-bottom:solid thin">In Low</td><td align="center" valign="middle" style="border-bottom:solid thin">Very High</td></tr></tbody></table> </ephtml> </p> <hd id="AN0181656808-18">Author Contributions</hd> <p>Conceptualization, J.Z. and Z.Y.; methodology, Z.Y.; validation, H.W.; formal analysis, J.Z. and Z.Y.; investigation, H.W. and X.Q.; data curation, H.W. and X.Q.; writing—original draft preparation, H.W.; writing—review and editing, H.W.; visualization, H.W.; supervision, J.Z. and Z.Y.; project administration, J.Z. All authors have read and agreed to the published version of the manuscript.</p> <hd id="AN0181656808-19">Conflicts of Interest</hd> <p>Haoyuan Wang, Jingong Zhang and Zishu Yong declare no conflict of interest. Xiumei Qu has employed by petrochina Yantai Sales Branch, Shandong Province, and I hereby declare the following regarding conflicts of interest: I undertake to avoid conflicts of interest (even superficial ones)with the Company, its shareholders and customers. I undertake to ensure that my personal conduct follows the following guidelines and appropriately reports in the event of an actual or potential conflict. These conflicts of interest may arise from my immediate family, other family members or stakeholders. Therefore, the abstention in this commitment also involves my immediate family, other family members or stakeholders. I undertake to write to the President or Vice President of the Company regarding any conflict of interest caused or likely to be caused by my immediate family, other family members or stakeholders.</p> <ref id="AN0181656808-20"> <title> Footnotes </title> <blist> <bibl id="bib1" idref="ref1" type="bt">1</bibl> <bibtext> Disclaimer/Publisher's Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.</bibtext> </blist> </ref> <ref id="AN0181656808-21"> <title> References </title> <blist> <bibtext> Li Q., Li Q., Han Y. A Numerical Investigation on Kick Control with the Displacement Kill Method during a Well Test in a Deep-Water Gas Reservoir: A Case Study. 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  Data: The Differential Enrichment Law of Tight Sandstone Gas in the Eighth Member of Shihezi Formation in the North and South of Ordos Basin
– Name: Author
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  Data: <searchLink fieldCode="AR" term="%22Haoyuan+Wang%22">Haoyuan Wang</searchLink><br /><searchLink fieldCode="AR" term="%22Jingong+Zhang%22">Jingong Zhang</searchLink><br /><searchLink fieldCode="AR" term="%22Zishu+Yong%22">Zishu Yong</searchLink><br /><searchLink fieldCode="AR" term="%22Xiumei+Qu%22">Xiumei Qu</searchLink>
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  Data: Energies, Vol 17, Iss 23, p 5978 (2024)
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  Data: MDPI AG, 2024.
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  Data: 2024
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  Data: <searchLink fieldCode="DE" term="%22Ordos+Basin%22">Ordos Basin</searchLink><br /><searchLink fieldCode="DE" term="%22source+rock%22">source rock</searchLink><br /><searchLink fieldCode="DE" term="%22shanxi+formation%22">shanxi formation</searchLink><br /><searchLink fieldCode="DE" term="%22gas+enrichment+difference%22">gas enrichment difference</searchLink><br /><searchLink fieldCode="DE" term="%22sedimentary+facies%22">sedimentary facies</searchLink><br /><searchLink fieldCode="DE" term="%22Technology%22">Technology</searchLink>
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  Data: Unconventional oil and gas resources are important energy sources for alleviating energy crises and maintaining sustainable social development. Therefore, the subsequent exploration and development of the basin summarizes and analyzes the reasons for the difference in natural gas enrichment between the southern and northern parts of the Ordos Basin. The tight sandstone gas of the Upper Paleozoic in the Ordos Basin has become a key field for increasing reserves and production of natural gas in the basin. As a high-quality tight sandstone gas reservoir, the eighth member of the Shihezi Formation only has good gas production in the south of the basin. For this reason, the types of natural gas source rocks in the north and south of the Upper Paleozoic were summarized, and the main types of gas source rocks in the north and south were clarified. The sedimentary facies of the eighth member of the Shihezi Formation and the Shanxi Formation were analyzed. The results show that the main gas-producing source rock in the northern part of the basin is coal rock, and in the southern part, it is dark mudstone. The Permian Shanxi Formation in the northern part of the basin mainly develops the delta front, which provides good conditions for the development of coal and rock. Compared with the northern Shanxi Formation, the Permian Shanxi Formation in the southern Ordos Basin mainly develops marine facies and marine-continental transitional facies, and the coal seam is not created. Therefore, the natural gas produced by the coal of the Shanxi Formation can easily migrate to the good reservoir in the eighth member of the box, which is also the reason for the difference in the enrichment of tight sandstone gas in the north and south of the Ordos Basin.
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  Data: 1996-1073
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  Data: 10.3390/en17235978
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        Value: 10.3390/en17235978
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      – SubjectFull: Ordos Basin
        Type: general
      – SubjectFull: source rock
        Type: general
      – SubjectFull: shanxi formation
        Type: general
      – SubjectFull: gas enrichment difference
        Type: general
      – SubjectFull: sedimentary facies
        Type: general
      – SubjectFull: Technology
        Type: general
    Titles:
      – TitleFull: The Differential Enrichment Law of Tight Sandstone Gas in the Eighth Member of Shihezi Formation in the North and South of Ordos Basin
        Type: main
  BibRelationships:
    HasContributorRelationships:
      – PersonEntity:
          Name:
            NameFull: Haoyuan Wang
      – PersonEntity:
          Name:
            NameFull: Jingong Zhang
      – PersonEntity:
          Name:
            NameFull: Zishu Yong
      – PersonEntity:
          Name:
            NameFull: Xiumei Qu
    IsPartOfRelationships:
      – BibEntity:
          Dates:
            – D: 01
              M: 11
              Type: published
              Y: 2024
          Identifiers:
            – Type: issn-print
              Value: 19961073
          Numbering:
            – Type: volume
              Value: 17
            – Type: issue
              Value: 23
          Titles:
            – TitleFull: Energies
              Type: main
ResultId 1