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Predicting Source Rock Distribution of the Ninth Member of the Upper Triassic Yanchang Formation Based on Well-Logging Parameters and TOC Values in the Longdong Area, Ordos Basin, China
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  • Xiao Hui
    Xiao Hui
    National Engineering Laboratory for Exploration and Development of Low Permeability Oil & Gas Fields, Xi’an 710018, China
    Research Institute of Petroleum Exploration and Development, PetroChina Changqing Oilfield Company, Xi’an 710018, China
    More by Xiao Hui
  • Xuan Ke
    Xuan Ke
    State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China
    College of Earth Science and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100039, China
    More by Xuan Ke
  • Yalin Qi
    Yalin Qi
    National Engineering Laboratory for Exploration and Development of Low Permeability Oil & Gas Fields, Xi’an 710018, China
    Research Institute of Petroleum Exploration and Development, PetroChina Changqing Oilfield Company, Xi’an 710018, China
    More by Yalin Qi
  • Shuyong Shi*
    Shuyong Shi
    State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China
    College of Earth Science and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100039, China
    *Email: [email protected]
    More by Shuyong Shi
    Orcidhttps://orcid.org/0009-0004-0789-2942
  • Jing Zhu
    Jing Zhu
    National Engineering Laboratory for Exploration and Development of Low Permeability Oil & Gas Fields, Xi’an 710018, China
    Research Institute of Petroleum Exploration and Development, PetroChina Changqing Oilfield Company, Xi’an 710018, China
    More by Jing Zhu
  • Yunpeng Wang*
    Yunpeng Wang
    State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China
    College of Earth Science and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100039, China
    *Email: [email protected]
    More by Yunpeng Wang
    Orcidhttps://orcid.org/0000-0003-4164-9677
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ACS Omega

Cite this: ACS Omega 2024, 9, 51, 50377–50384
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https://doi.org/10.1021/acsomega.4c06995
Published December 9, 2024

Copyright © 2024 The Authors. Published by American Chemical Society. This publication is licensed under

CC-BY-NC-ND 4.0 .

Abstract

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In recent years, new oil reservoirs have been discovered and exploited in the ninth member (Chang 9 Member, T3y9) of the Upper Triassic Yanchang Formation (T3y) in the Longdong area, Ordos Basin. Some studies have shown that the crude oils of the Chang 9 Member may originate from the Chang 9 source rock in some areas, which may be related to the distribution of the source rock. However, the distribution of the Chang 9 source rock in the Longdong area is still unclear, which hinders further exploration and development of petroleum. In this study, we established a multiple linear regression model for predicting total organic carbon (TOC) based on the relationship between well-logging parameters and measured TOC values of shale core samples from 30 wells in the study area. The results show that the Chang 9 shale is mainly composed of gray and dark mudstones, which mainly belong to the interdistributary bay and front delta depositional subfacies. The TOC values of the shale core samples from this member vary in a range of 0.11–4.8%, with an average value of 0.96%. Compared with traditional and improved Δlog R models, our model shows a higher accuracy of TOC prediction with R2 = 0.9181, which meets the requirements for predicting the distribution of the Chang 9 source rock. In the map of the Chang 9 source rock predicted by our model, the thickness of the source rock (TOC ≥ 1.0%) varies in the range of 1–12 m, showing a decreasing trend from northeast to southwest in the Longdong area. The crude oil in the northeastern areas enjoys a high ratio of 17α(H)-C30 rearranged hopane and C30 hopane (C30*/C30), and the thickness of the Chang 9 source rock is also greater than in other areas. It is speculated that the Chang 9 Member tight oil in the northeast area is mainly from the Chang 9 source rock, while the oil in other areas is from the Chang 7 source rock. In our study, we presented a method for predicting the source rock distribution, which can be widely used for exploring the tight oil of the Chang 9 Member in the study area.

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1. Introduction

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The Ordos Basin is an important oil-bearing basin in China, in which the Upper Triassic Yanchang Formation (T3y) is the most important oil-bearing layer. (1,2) Oil exploration and development to date have mainly focused on the seventh member (Chang 7 Member, T3y7) and the upper members (Chang 1–6 Members, T3y1–T3y6). (1,3,4) After several decades of continuous breakthrough and development of tight oil, a number of oil reservoirs were also discovered in the ninth member (Chang 9 Member, T3y9) of the Yanchang Formation in the Zhidan, Jiyuan, and Longdong areas of the basin. (3,5,6) However, the origin of the crude oil from the Chang 9 Member remains controversial. (7−11) Some scholars suggested that the crude oil mainly originated from the Chang 7 source rock due to its high total organic carbon (TOC), large thickness, and wide distribution in the basin, (3,5,8,10) while other scientists believed that the crude oil in some areas might originate from the Chang 9 source rock. (1,9,12) In comparison with the Chang 7 shale, part of the Chang 9 shale enjoys high abundance of 17α(H)-C30 rearranged hopane (C30*), with the ratio of C30* and C30 hopane (C30) higher than 1.0. (7,8,11) Therefore, it is considered that the crude oil with low C30*/C30 values is mainly originated from the Chang 7 shale, while the oil with high C30*/C30 values is derived from the Chang 9 shale. (8−10,13,14)
The Longdong area is located in the southwestern part of the Ordos Basin (Figure 1a), which is considered as the next favorable area for tight oil exploration of the Chang 9 Member. (3) Numerous research works have focused on the sedimentary facies, petroleum reservoir evaluation, and petroleum accumulation conditions. (1,3,15,16) The crude oil recovered from the Chang 9 Member could be related to the source rock distribution of this member. Compared with the Chang 7 source rock, the distribution of the Chang 9 source rock in the Longdong area remains unclear.

Figure 1

Figure 1. (a) Structural units of the Ordos Basin; (b) stratigraphic column of the Upper Triassic Yanchang Formation. Parts (a) and (b) were reproduced from ref (23). Copyright 2016 Elsevier.

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Total organic carbon is an important component of mudstone, which is also crucial for the evaluation of the source rock. Based on the well-logging parameters and the corresponding TOC values, various TOC prediction models have been established, with the traditional (or improved) Δlog R, (17) multiple linear regression, (18−20) and neural network models (21,22) being widely used for the TOC prediction. Although there are so many models for the TOC prediction, the choice of model depends on the accuracy of the model and the actual geological properties of the source rock.
In this study, we first observed drill cores of the Yanchang Formation from 30 wells in the Longdong area, Ordos Basin, with respect to their lithology, sedimentary structure, and shale development. The TOC values of the shale core samples were also measured. Subsequently, the well-logging parameters, including the acoustic time difference (AC), resistivity (RT), natural γ response (GR), and density (DEN), corresponding to the TOC value of each sample were collected. Finally, we constructed traditional and improved Δlog R, and multiple linear regression models and compared the results of the three models for the TOC prediction. The aim of this study was to select an appropriate model for predicting the distribution of the Chang 9 source rock in the Longdong area in the Ordos Basin. At the same time, accurate prediction of TOC is also important for further oil exploration of the Chang 9 Member in the Ordos Basin.

2. Geological Settings and Stratigraphy

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2.1. Geological Settings

The Ordos Basin, covering an area of 3.7 × 105 km2, is tectonically located on the western margin of the North China Craton (Figure 1a). (24) The basin can be divided into six structural units: the Weibei Uplift in the south, the Yimeng Uplift in the north, the Jinxi Fold Belt in the east, the Western Edge Thrust Belt and the Tianhuan Depression in the west, and the Yishan Slope in the central basin. (23) The study area (Longdong area), which crosses the Tianhuan Depression and the Yishan Slope, is located in the southwestern part of the basin (Figure 1a).
The Upper Triassic Yanchang Formation is an oil-producing strata that can be divided into ten members (Chang 1–10 Members, T3y1–T3y10) from top to bottom (Figure 1b). The thickness of this formation varies between 700 and 1200 m in the basin. All members are mainly composed of sandstone, siltstone, and mudstone, in which the Chang 7 Member is mainly composed of mudstone with high TOC. (25−27) Previous studies have shown that the Chang 7 Member is the dominant source rock in the Mesozoic and formed during the transgression of the lakes. (25,27,28) It is widely distributed in the Ordos Basin, with thicknesses varying between 20 and 60 m. (29,30) In addition, the shale from the Chang 9 Member is considered as the secondary source rock, mainly composed of deltaic plain shale and deltaic frontal shale, with an average TOC of less than 2.0%. (31) Previous studies showed that the Chang 9 source rock is only distributed in the Zhidan and Ganquan areas, and the thickness is generally less than 25 m, (30,32,33) but the distribution in the Longdong area is not clear.

2.2. Observation of the Drill Cores and the Sedimentary Facies

The lithologies of the Chang 9 Member are mainly composed of sandstone, fine sandstone, siltstone, and mudstone (Figure 2a–c) in the Longdong area, Ordos Basin. The Chang 9 shale is composed of gray and dark mudstone, indicating a low content of organic matter (Figure 2d–h). It is also worth noting that the shale in the form of mudstone interbeds in the Longdong area is thin and not well-developed. The interdistributary bay and delta font are the main types of mudstones in most areas (Figure 2), and the mudstone formed in the semideep lacustrine is only common in the northeastern part of the study area (Figure 3). Plant fossils were also found in some core samples (Figure 2d,e), indicating a shallow deltaic sedimentary environment.

Figure 2

Figure 2. Pictures of core samples of the Chang 9 Member in the Longdong Area, Ordos Basin. (a) Yuan 427, 2432.5 m, mud graveled sandstone, delta font; (b) Yuan 457, 2327–2331 m, sandstone interbedded with 1 m dark mudstone, delta font; (c) Le 46, 1413.5–1415.5 m, fine sandstone interbedded with dark mudstone, delta font; (d) Mu 23, 2516 m, gray mudstone, stem fossil, delta font; (e) Bai 442, 2338.4 m, gray mudstone, interdistributary bay; (f) Yuan 541, 2294.5 m, plant fossil, delta font; (g) Chang 23, 1726.6 m, gray mudstone, delta font; and (h) Bai 445, 2350 m, mud graveled sandstone and dark mudstone, interdistributary bay.

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Figure 3

Figure 3. Sedimentary facies of the Chang 9 Member in the Longdong area, Ordos Basin. This figure was reproduced from ref (3). Copyright 2018 Elsevier.

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The distribution of the Chang 9 source rock is mainly controlled by the sedimentary facies. During the depositional period of this member, three major sedimentary systems were developed in the northwest, northeast, and southwest of the basin (Figure 3). Previous studies have shown that the Chang 9 Member was mainly formed in shallow delta fronts and plain sedimentary environments. (16,32,34) However, the Chang 9 Member in the northeastern areas was formed in a semideep lacustrine environment, resulting in the source rock being thicker than in other areas in the Longdong area, Ordos Basin. (2,30)

3. Experiments and Methods

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3.1. Total Organic Carbon Analysis

All shale core samples, including the Chang 6, Chang 7, Chang 8, and Chang 9 Members, were collected from 30 wells in the Longdong area. The TOC value was measured by the LECO CS-800 analyzer, and the carbonates were removed with hydrochloric acid (HCl) before measurement.

3.2. Well-Logging Surveys

The well-logging surveys were conducted by the Research Institute of Petroleum Exploration and Development, PetroChina Changqing Oilfield Company. Some common well-logging parameters such as GR, RT, DEN, and AC, which are sensitive to the TOC value, were recorded and processed with the corresponding measured TOC values.

3.3. Traditional Δlog R Model

The Δlog R model, the most common model for the TOC prediction, was proposed by Passy based on the well-logging data of RT and AC. (17) The Δlog R can be expressed by the following equation:
Δlog⁡R=a×lg(RT/RTbase)+b×(AC−ACbase)
(1)
The predicted TOC can be expressed as
TOC=c×Δlog⁡R+d
(2)
After the combination of eqs 1 and 2, the predicted TOC can also be expressed as
TOC=e×lg(RT/RTbase)+f×(AC−ACbase)
(3)
where a, b, c, d, e, and f are the fitting coefficients. RT and AC are the resistivity (Ω·m) and acoustic time difference (μm/s), respectively. RTbase and ACbase are the baseline values of resistivity and acoustic time difference.
Then, the data of RT and AC corresponding to the measured TOC values were brought into eq 3 using Origin software. The TOC prediction model is obtained.
TOC=7.7977×lg(RT/10)+0.0559×lg(AC−200)
(4)
The predicted TOC calculated by eq 4 shows a positive correlation with measured TOC with a R2 value of 0.5802 (Figure 4a).

Figure 4

Figure 4. Relationship between measured TOC and predicted TOC by (a) traditional and (b) improved Δlog R models.

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3.4. Improved Δlog R Model

The improved Δlog R model also considered the DEN and GR response to the TOC value based on a traditional model. (19,21) The improved Δlog R model can be expressed as
TOC=[a×log(GR)+b×DEN+d]×Δlog⁡R+d
(5)
After the combination of eqs 1 and 5, the predicted TOC can be also expressed as
TOC=e×log(GR)+f×DEN+g×Δlog⁡R+h
(6)
where a, b, c, d, e, f, g, and h are the fitting coefficients. GR and DEN are the natural γ response (API) and density (g/cm3), respectively.
Then, the data of GR, DEN, and Δlog R corresponding to the measured TOC were brought into eq 6 by using Origin software. The TOC prediction model is obtained.
TOC=19.235×log(GR)−12.6861×DEN−0.0252×Δlog⁡R−3.7856
(7)
As shown in Figure 4b, the predicted TOC calculated by eq 7 shows a positive correlation with the measured TOC with a R2 value of 0.7711.

3.5. Multiple Linear Regression Model

Some common well-logging parameters, such as GR, RT, DEN, and AC, are sensitive to the TOC value: (21,22,35) (1) The organic-rich source rock shows a positive correlation with GR (Figure 5a) due to its strong adsorption for radioactive elements such as uranium (U) and thorium (Th). The higher the radioactivity of the source rock, the more sensitive the GR response. (2) The organic-rich source rocks belong to the noneasily conductive materials, which lead to the RT showing an increasing trend with the TOC (Figure 5b). (3) The density of solid organic matter is about 1 g/cm3, which is close to that of water. The organic-rich source rocks generally have a low density (Figure 5c); hence, the DEN curves can be used for the TOC prediction. (4) Organic-rich source rock has a low sonic velocity. The AC usually shows a positive correlation with the TOC (Figure 5d).

Figure 5

Figure 5. Linear relationships between measured TOC and well-logging parameters of the Chang 6–9 shales in the Longdong area, Ordos Basin. (a) Natural γ response (GR) versus measured TOC; (b) resistivity (RT) versus measured TOC; (c) density (DEN) versus measured TOC; and (d) acoustic time difference (AC) vs measured TOC.

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The TOC prediction based on a single well-logging parameter is not accurate enough. To further improve the accuracy of the TOC prediction, a multiple linear regression model for the TOC prediction is constructed. Due to the weak correlation between measured TOC and AC, the model was created considering RT, GR, and DEN. The predicted TOC value can be calculated by the following equation:
TOC∗=0.049×GR+0.046×RT−4.433×DEN+5.939
(8)
In eq 8, TOC* is the predicted TOC, GR is the natural γ response (API), RT is the resistivity (Ω·m), and DEN is the density (g/cm3) of the shale. As shown in Figure 6, the multiple linear regression model improves the accuracy of the TOC prediction with an R2 value of 0.9181.

Figure 6

Figure 6. Relationship between the measured TOC and the predicted TOC by the multiple linear regression model.

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4. Results and Discussion

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4.1. Total Organic Carbon of the Chang 9 Shale

Total organic carbon is an important indicator for evaluating the hydrocarbon generation capacity of shale. The TOC of the Chang 9 shale varies in the range of 0.11–4.8%, with an average value of 0.96% (Figure 7). This is because the Chang 9 shale was mainly formed in a deltaic sedimentary environment (Figure 3). According to China’s industrial criterion for continental source rock, (36) the shale is considered as the source rock when the TOC value is higher than 0.4% and good to very good source rock with the TOC value in the range of 0.6–1.0%. The Chang 9 shale is mainly composed of good to very good source rock with 66% (N = 21); however, the proportion of shale with TOC higher than 2.0% is 3.1% (N = 1, Figure 7).

Figure 7

Figure 7. Histogram of the frequency distribution of the TOC of the Chang 9 source rocks in the Longdong area, Ordos Basin.

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4.2. Comparisons of Traditional, Improved Δlog R, and Multiple Linear Regression Models

Based on the results of the traditional and improved Δlog R and multiple linear regression models for the TOC prediction, the R2 values of the three models are 0.5802, 0.7711, and 0.9181, respectively (Figures 4 and 6). The traditional Δlog R model considers the influence of TOC on AC and RT, while the improved Δlog R model also considers the influence of TOC on DEN and GR. The improved Δlog R model improves the accuracy of TOC prediction compared to the traditional model; however, the R2 value (R2 = 0.7711) of the model (Figure 4b) is lower than that of the TOC prediction model based on GR values (R2 = 0.7781, Figure 5a).
The determination of the baseline values of RTbase and ACbase is subject to uncertainties and randomness, which affects the accuracy of the TOC prediction. The core observation results also show that the Chang 9 Member is characterized by thin and multilayer mudstone interbeds in the Longdong area (Figure 2). Due to the weak correlation between TOC and AC (Figure 5b) and the uncertainty of baseline selection, the traditional and improved Δlog R models may not be suitable for the TOC prediction of this member. In comparison, the multiple linear regression method can take the response of multiple well-logging parameters to TOC into account and improve the accuracy of the TOC prediction. In recent years, this method has also widely been used for the TOC prediction due to its higher accuracy than traditional and improved Δlog R models. (18−20) Besides, the measured TOC values show a good correlation with RT, GR, and DEN, with R2 values of 0.7781, 0.6431, and 0.4841, respectively, but a poor correlation with AC with R2 values of 0.1926 (Figure 5). The multiple linear regression method (R2 = 0.9181, Figure 6) based on RT, GR, and DEN parameters (Figure 5a–c) is more suitable for the TOC prediction of the Chang 9 Member in the Longdong area, Ordos Basin.

4.3. Application for the Distribution of the Chang 9 Source Rock

The multiple linear regression model has a higher accuracy with an R2 value of 0.9181 (Figure 6), which is more suitable for the TOC prediction of the Chang 9 source rock in the Longdong area of the Ordos Basin. Based on the above considerations, we chose the multiple linear regression model for the TOC prediction.
According to the industrial criterion for continental source rock in China, mudstone with TOC ≥ 0.4% is considered as the source rock. Generally, the excess hydrocarbons are expelled when the generated hydrocarbons meet their own adsorbed and free capacity. (37) Regarding the Yanchang source rock, Yao et al. (2) found that the generated hydrocarbons of the Yanchang Formation are expelled when the TOC value exceeds 1.0%. More importantly, the measured TOC values have a poor correlation with the well-logging parameters when the TOC value is below 1.0% (Figures 4–6). From the above considerations, the Yanchang source rock with a TOC above 1.0% is the effective source rock as the hydrocarbons can expel and form conventional oil reservoirs.
Then, we input the machine language “0.049 × GR + 0.046 × RT – 4.433 × DEN + 5.939 ≥ 1.0%” into the Gxplorer software. The software can automatically calculate the thickness of the Chang 9 source rock for each well. For example, we use the multiple linear regression model to predict the TOC and thickness of the Chang 9 source rock from well Bai 445 in the Baibao area. The results show that the effective source rock (TOC ≥ 1.0%) is multiple mudstone interbeds with a thickness of tens of centimeters, and the total thickness in the Chang 9 Member of well Bai 445 is 11.75 m (Figure 8).

Figure 8

Figure 8. Prediction results of the thickness of the Chang 9 source rock from well Bai 445 in the Baibao area. The thickness of the source rock was calculated by a multiple linear regression model.

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Finally, we use the kriging method to construct the distribution map of the Chang 9 source rock in the Longdong area. In this way, we obtain the final distribution of the Chang 9 source rock in the Longdong area of the Ordos Basin by combining sedimentary facies analysis and the prediction of the thickness of the Chang 9 source rock (Figure 9).

Figure 9

Figure 9. Distribution of the Chang 9 source rock (TOC ≥ 1.0%) in the Longdong area, Ordos Basin.

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4.4. Distribution of the Chang 9 Source Rock and Potential Triggering Factors

Our core observation results show that the Chang 9 source rock in the Longdong area is not well-developed. The main lithology of the Chang 9 Member consists of sandstone or siltstone interbedded with thinly bedded mudstone (Figure 2). In general, the TOC and thickness of the Chang 9 source rock are significantly lower than those of the Chang 7 source rock (Figure 7), mainly due to the differences in sedimentary environments in the Longdong area, Ordos Basin (Figure 3). As shown in Figure 9, the distribution characteristics of the Chang 9 source rock are mainly controlled by the sedimentary environment, (16) and the thickness shows a decreasing trend from northeast to southwest. The thickness of the Chang 9 source rock in the northeastern areas (e.g., Huachi and Baibao) is higher than in other areas and varies between 8 and 12 m. In the Qingcheng and Qingyang areas, the thickness of the Chang 9 source rock is between 3 and 7 m, compared to 1 to 3 m in other areas.
It is worth noting that the crude oils from the Chang 9 Member in the northeastern parts of the Longdong area have a high C30*/C30 ratio, while the C30*/C30 ratio of crude oils in other parts is lower. (8,10) The C30*/C30 ratio of the source rocks of Chang 7 and 9 Members shows significant differences, which may be related to the sedimentary environments. (7,11,38) Some studies have shown that the Chang 9 source rocks of, which formed in semideep lacustrines, have a high abundance of C30* in the Huachi and Baibao areas. In comparison, the Chang 7 source rock with a low abundance of C30* content was mainly formed in deep lacustrines. (9,10) In addition, our results showed that the Chang 9 source rock with a large thickness was mainly distributed in the Huachi and Baibao areas (Figure 9). From the perspective of C30*/C30 and the distribution of the Chang 9 source rock, it can be suggested that the tight oil of the Chang 9 Member in the northeastern areas is derived from the Chang 9 source rock, while the oil in other areas is mainly derived from the Chang 7 source rock.

5. Conclusions

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(1)

The shale of the Chang 9 Member is mainly composed of gray and dark mudstones, which are generally formed in a deltaic sedimentary environment. Total organic carbon (TOC) varies in the range of 0.11–4.8%, with an average value of 0.96%.

(2)

The determination of baseline values of resistivity (RT) and acoustic time difference (AC) is subject to a certain uncertainty that affects the accuracy of the TOC prediction. The TOC prediction value based on the traditional and improved Δlog R models differs greatly from the measured TOC values, with R2 values of 0.5802 and 0.7711, respectively.

(3)

The TOC value shows good correlations with well-logging parameters such as natural γ (GR), RT, and density (DEN), leading to constructing a multiple linear regression model for the TOC prediction. Compared with traditional and improved Δlog R models, the R2 of our model can reach 0.9181, which is more suitable for the TOC prediction of the Chang 9 source rock in the Longdong area, Ordos Basin.

(4)

The map of predicted source rock TOC shows that the distribution of Chang 9 source rock (TOC ≥ 1.0%) is mainly controlled by the sedimentary environment. The thickness varies in the range of 1–12 m and shows a decreasing trend from northeast to southwest in the Londong area, Ordos Basin.

(5)

From the perspective of the relative abundance of C30 rearranged hopane and the distribution of the Chang 9 source rock, it is speculated that the tight oil of the Chang 9 Member in the northeastern areas derives from the Chang 9 source rock, while the oil in other areas mainly from the Chang 7 source rock.

Author Information

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  • Corresponding Authors
    • Shuyong Shi - State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China;  College of Earth Science and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100039, China;  Orcidhttps://orcid.org/0009-0004-0789-2942;  Email: [email protected]
    • Yunpeng Wang - State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China;  College of Earth Science and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100039, China;  Orcidhttps://orcid.org/0000-0003-4164-9677;  Email: [email protected]
  • Authors
    • Xiao Hui - National Engineering Laboratory for Exploration and Development of Low Permeability Oil & Gas Fields, Xi’an 710018, China;  Research Institute of Petroleum Exploration and Development, PetroChina Changqing Oilfield Company, Xi’an 710018, China
    • Xuan Ke - State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China;  College of Earth Science and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100039, China
    • Yalin Qi - National Engineering Laboratory for Exploration and Development of Low Permeability Oil & Gas Fields, Xi’an 710018, China;  Research Institute of Petroleum Exploration and Development, PetroChina Changqing Oilfield Company, Xi’an 710018, China
    • Jing Zhu - National Engineering Laboratory for Exploration and Development of Low Permeability Oil & Gas Fields, Xi’an 710018, China;  Research Institute of Petroleum Exploration and Development, PetroChina Changqing Oilfield Company, Xi’an 710018, China
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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This research was funded by the National Natural Science Foundation of China (Nos. 42273053 and 42203054), the Science and Technology Project of the PetroChina Changqing Oilfield Company (Ji-2021-39) and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA14010103). The authors thank Nan Xue, Yunchao Hou, Min Liu, and Xiangrui Chen for their help in the field work and data collection. We also thank the editor and reviewers for their critical and constructive comments.

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    Org.-rich black shale and dark-gray mudstone are widespread in the Seventh Member of the Yanchang Formation, Ordos Basin. There are obviously differences in geochem. characteristics and hydrocarbon generation potential between black shale and dark-gray mudstone. Based on the detailed org. geochem. examn., this paper reveals the differences in hydrocarbon generation processes and characteristics (activation energy and conversion rate of hydrocarbon generation) between dark-gray mudstone and black shale and the causes for the differences. Black shale is characterized by moderate to very high total org. carbon (TOC) contents (5.08%-39.39%) and its kerogen type belongs to type II1 and type I. Dark-gray mudstone has a characteristic of low to moderate TOC contents (0.64%-7.35%) and its kerogen types are mainly composed of type II2 and type II1. The hydrocarbon generation activation energy of black shale mainly oscillates around 250-310 KJ/mol, and the corresponding hydrocarbon generation threshold is high (the simulated temp. is 400 °C). The process of hydrocarbon generation is characterized by fast conversion rate and short cycle. By contrast, the hydrocarbon generation activation energy of dark-gray mudstone has a wide range (170-360 KJ/mol), and the corresponding hydrocarbon generation threshold is low (the simulated temp. is 230 °C). The process of hydrocarbon generation is characterized by slow conversion rate and long cycle. The difference of hydrocarbon generation process and characteristics between black shale and dark-gray mudstone is mainly affected by macerals compn. and org. matter source. The org. matter of black shale mainly originated from algae, and its macerals was mainly composed of alginite. The org. matter of dark-gray mudstone mainly originated from the mixt. of algae and terrestrial higher plants (conifers), and its macerals was mainly composed of vitrinite, liptodetrinite and inertinite. The complex macerals compn. and org. matter source make the activation energy distribution of dark-gray mudstone more dispersed than that of black shale. In addn., affected by the macerals (e.g sporinite and resinite) with low activation energy from terrestrial higher plants, the activation energy of dark-gray mudstone contg. sporinite and resinite in the macerals is lower than black shale. Therefore, the dark-gray mudstone contg. algae and terrestrial higher plants corresponds to low hydrocarbon generation threshold, slow conversion rate and long hydrocarbon generation cycle. The black shale contg. a large no. of algae corresponds to high hydrocarbon generation threshold, fast conversion rate and short hydrocarbon generation cycle. Moreover, the conversion rate of black shale contg. radioactive elements and more clay minerals is higher than that of dark-gray mudstone.
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    Yang, Weiwei; Liu, Guangdi; Feng, Yuan
    Marine and Petroleum Geology (2016), 71 (), 238-249CODEN: MPEGD8; ISSN:0264-8172. (Elsevier Ltd.)
    Relatively high abundance of 17α(H)-C30 rearranged hopane has been detected in some oils and rock exts. from Yanchang Formation of Longdong area, southwestern Ordos basin. Oil shale, silty mudstone and carbonaceous mudstone exts. from Yanchang Formation all have low abundance of C30 rearranged hopane, while high concns. of C30 rearranged hopane were obsd. in the dark mudstone exts. Anal. on the geochem. parameters of the org. matter origin, thermal maturity, and sedimentary org. facies were carried out. The result showed that the depositional-redox environment and lithol. may be the crit. factors for the rearrangement of hopanes which usually occurred in suboxic condition with clay-catalyzed reactions, as is consistent with the viewpoint of Peters and Moldowan (1993). According to the relative abundance of diahopane, three oil classes can be identified. Class 1 oil with low abundance of diahopanes was originated from oil shale of Chang 7 member, while Class 2 oil and Class 3 oil with middle to high abundance of diahopanes were derived from dark mudstones of Chang 4 + 5 to Chang 9 members of Yanchang Formation. Of the 151 analyzed oil and exts. samples from different stratigraphic intervals, 105 samples belong to Class 1 oil coming from oil shale, while 46 samples belong to Class 2 and Class 3 oil derived from mudstones. So oil shale of Chang 7 member is the main effective source rock of Longdong area, and dark mudstones of Chang 4 + 5 to Chang 9 member are also important source rocks.
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    Marine and Petroleum Geology (2016), 73 (), 554-571CODEN: MPEGD8; ISSN:0264-8172. (Elsevier Ltd.)
    Org. shales deposited in a continental environment are well developed in the Ordos Basin, NW China, which is rich in hydrocarbons. However, previous research concerning shales has predominantly focused on marine shales and barely on continental shales. In this study, geochem. and mineralogical analyses, high-pressure mercury intrusion and low-pressure adsorption were performed on 18 continental shale samples obtained from a currently active shale gas play, the Chang 7 member of Yanchang Formation in the Ordos Basin. A comparison of all these techniques is provided for characterizing the complex pore structure of continental shales. Geochem. anal. reveals total org. carbon (TOC) values ranging from 0.47% to 11.44%, indicating that there is abundant org. matter (OM) in the study area. Kerogen anal. shows vitrinite reflectance (Ro) of 0.68%-1.02%, indicating that kerogen is at a mature oil generation stage. X-ray diffraction mineralogy (XRD) anal. indicates that the dominant mineral constituents of shale samples are clay minerals (which mainly consist of illite, chlorite, kaolinite, and negligible amts. of montmorillonite), quartz and feldspar, followed by low carbonate content. All-scale pore size anal. indicates that the pore size distribution (PSD) of shale pores is mainly from 0.3 to 60 nm. Note that accuracy of all-scale PSD anal. decreases for pores less than 0.3 nm and more than 10 μm. Exptl. anal. indicates that mesopores (2-50 nm) are dominant in continental shales, followed by micropores (<2 nm) and macropores (50 nm-10 μm). Mesopores have the largest contribution to pore vol. (PV) and sp. surface area (SSA). In addn., plate- and sheet-shaped pores are dominant with poor connectivity, followed by hybrid pores. Results of research on factors controlling pore structure development show that it is principally controlled by clay mineral contents and Ro, and this is different from marine systems. This study has important significance in gaining a comprehensive understanding of continental shale pore structure and the shale gas storage-seepage mechanism.
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    International Journal of Coal Geology (2014), 128-129 (), 32-46CODEN: IJCGDE; ISSN:0166-5162. (Elsevier B.V.)
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  • Abstract

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    Figure 1

    Figure 1. (a) Structural units of the Ordos Basin; (b) stratigraphic column of the Upper Triassic Yanchang Formation. Parts (a) and (b) were reproduced from ref (23). Copyright 2016 Elsevier.

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    Figure 2

    Figure 2. Pictures of core samples of the Chang 9 Member in the Longdong Area, Ordos Basin. (a) Yuan 427, 2432.5 m, mud graveled sandstone, delta font; (b) Yuan 457, 2327–2331 m, sandstone interbedded with 1 m dark mudstone, delta font; (c) Le 46, 1413.5–1415.5 m, fine sandstone interbedded with dark mudstone, delta font; (d) Mu 23, 2516 m, gray mudstone, stem fossil, delta font; (e) Bai 442, 2338.4 m, gray mudstone, interdistributary bay; (f) Yuan 541, 2294.5 m, plant fossil, delta font; (g) Chang 23, 1726.6 m, gray mudstone, delta font; and (h) Bai 445, 2350 m, mud graveled sandstone and dark mudstone, interdistributary bay.

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    Figure 3

    Figure 3. Sedimentary facies of the Chang 9 Member in the Longdong area, Ordos Basin. This figure was reproduced from ref (3). Copyright 2018 Elsevier.

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    Figure 4

    Figure 4. Relationship between measured TOC and predicted TOC by (a) traditional and (b) improved Δlog R models.

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    Figure 5

    Figure 5. Linear relationships between measured TOC and well-logging parameters of the Chang 6–9 shales in the Longdong area, Ordos Basin. (a) Natural γ response (GR) versus measured TOC; (b) resistivity (RT) versus measured TOC; (c) density (DEN) versus measured TOC; and (d) acoustic time difference (AC) vs measured TOC.

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    Figure 6

    Figure 6. Relationship between the measured TOC and the predicted TOC by the multiple linear regression model.

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    Figure 7

    Figure 7. Histogram of the frequency distribution of the TOC of the Chang 9 source rocks in the Longdong area, Ordos Basin.

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    Figure 8

    Figure 8. Prediction results of the thickness of the Chang 9 source rock from well Bai 445 in the Baibao area. The thickness of the source rock was calculated by a multiple linear regression model.

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    Figure 9

    Figure 9. Distribution of the Chang 9 source rock (TOC ≥ 1.0%) in the Longdong area, Ordos Basin.

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      Zheng, Ruihui; Wang, Yifan; Li, Zhipeng; Zhang, Zhihuan; Wang, Guangli; Zhang, Heng
      International Journal of Coal Geology (2022), 257 (), 104012CODEN: IJCGDE; ISSN:0166-5162. (Elsevier B.V.)
      Org.-rich black shale and dark-gray mudstone are widespread in the Seventh Member of the Yanchang Formation, Ordos Basin. There are obviously differences in geochem. characteristics and hydrocarbon generation potential between black shale and dark-gray mudstone. Based on the detailed org. geochem. examn., this paper reveals the differences in hydrocarbon generation processes and characteristics (activation energy and conversion rate of hydrocarbon generation) between dark-gray mudstone and black shale and the causes for the differences. Black shale is characterized by moderate to very high total org. carbon (TOC) contents (5.08%-39.39%) and its kerogen type belongs to type II1 and type I. Dark-gray mudstone has a characteristic of low to moderate TOC contents (0.64%-7.35%) and its kerogen types are mainly composed of type II2 and type II1. The hydrocarbon generation activation energy of black shale mainly oscillates around 250-310 KJ/mol, and the corresponding hydrocarbon generation threshold is high (the simulated temp. is 400 °C). The process of hydrocarbon generation is characterized by fast conversion rate and short cycle. By contrast, the hydrocarbon generation activation energy of dark-gray mudstone has a wide range (170-360 KJ/mol), and the corresponding hydrocarbon generation threshold is low (the simulated temp. is 230 °C). The process of hydrocarbon generation is characterized by slow conversion rate and long cycle. The difference of hydrocarbon generation process and characteristics between black shale and dark-gray mudstone is mainly affected by macerals compn. and org. matter source. The org. matter of black shale mainly originated from algae, and its macerals was mainly composed of alginite. The org. matter of dark-gray mudstone mainly originated from the mixt. of algae and terrestrial higher plants (conifers), and its macerals was mainly composed of vitrinite, liptodetrinite and inertinite. The complex macerals compn. and org. matter source make the activation energy distribution of dark-gray mudstone more dispersed than that of black shale. In addn., affected by the macerals (e.g sporinite and resinite) with low activation energy from terrestrial higher plants, the activation energy of dark-gray mudstone contg. sporinite and resinite in the macerals is lower than black shale. Therefore, the dark-gray mudstone contg. algae and terrestrial higher plants corresponds to low hydrocarbon generation threshold, slow conversion rate and long hydrocarbon generation cycle. The black shale contg. a large no. of algae corresponds to high hydrocarbon generation threshold, fast conversion rate and short hydrocarbon generation cycle. Moreover, the conversion rate of black shale contg. radioactive elements and more clay minerals is higher than that of dark-gray mudstone.
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    5. 5
      Liu, X.; Deng, X.; Zhao, Y.; Zhang, X.; Han, T. Hydrocarbon Migration Law and Model of Chang 9 Reservoir in Jiyuan Area, Ordos Basin. Lithol. Reserv. 2011, 23 (5), 9– 15
      There is no corresponding record for this reference.
    6. 6
      Shen, M.; Zhu, X.; Li, C.; Zhang, W.; Cheng, Y.; Jiang, M. Diagenesis of the Chang 9 Oil-Bearing Interval of Triassic Yanchang Formation in Zhengning area, Ordos Basin. J. Palaeogeogr. 2020, 22 (3), 539– 554
      There is no corresponding record for this reference.
    7. 7
      Zhang, W.; Yang, H.; Hou, L.; Liu, F. Distribution and Geological Significance of 17α(H)-Diahopanes from Different Hydrocarbon Source Rocks of Yanchang Formation in Ordos Basin. Sci. China, Ser. D: Earth Sci. 2009, 52 (7), 965– 974,  DOI: 10.1007/s11430-009-0076-1
      There is no corresponding record for this reference.
    8. 8
      Liu, G.; Yang, W.; Feng, Y.; Ma, H.; Du, Y. Geochemical Characteristics and Genetic Types of Crude Oil from Yanchang Formation in Longdong Area, Ordos Basin. Earth Sci. Front. 2013, 20 (2), 108– 115
      There is no corresponding record for this reference.
    9. 9
      Yang, H.; Zhang, W.; Peng, P.; Liu, F.; Luo, L. Oil Detailed Classification and Oil-Source Correlation of Mesozoic Lacustrine Oil in Ordos Basin. J. Earth Sci. Environ. 2016, 38 (2), 196– 205
      There is no corresponding record for this reference.
    10. 10
      Yang, W.; Liu, G.; Feng, Y. Geochemical Significance of 17α(H)-Diahopane and Its Application in Oil-Source Correlation of Yanchang Formation in Longdong Area, Ordos Basin, China. Mar. Pet. Geol. 2016, 71, 238– 249,  DOI: 10.1016/j.marpetgeo.2015.10.016
      10
      Geochemical significance of 17α(H)-diahopane and its application in oil-source correlation of Yanchang formation in Longdong area, Ordos basin, China
      Yang, Weiwei; Liu, Guangdi; Feng, Yuan
      Marine and Petroleum Geology (2016), 71 (), 238-249CODEN: MPEGD8; ISSN:0264-8172. (Elsevier Ltd.)
      Relatively high abundance of 17α(H)-C30 rearranged hopane has been detected in some oils and rock exts. from Yanchang Formation of Longdong area, southwestern Ordos basin. Oil shale, silty mudstone and carbonaceous mudstone exts. from Yanchang Formation all have low abundance of C30 rearranged hopane, while high concns. of C30 rearranged hopane were obsd. in the dark mudstone exts. Anal. on the geochem. parameters of the org. matter origin, thermal maturity, and sedimentary org. facies were carried out. The result showed that the depositional-redox environment and lithol. may be the crit. factors for the rearrangement of hopanes which usually occurred in suboxic condition with clay-catalyzed reactions, as is consistent with the viewpoint of Peters and Moldowan (1993). According to the relative abundance of diahopane, three oil classes can be identified. Class 1 oil with low abundance of diahopanes was originated from oil shale of Chang 7 member, while Class 2 oil and Class 3 oil with middle to high abundance of diahopanes were derived from dark mudstones of Chang 4 + 5 to Chang 9 members of Yanchang Formation. Of the 151 analyzed oil and exts. samples from different stratigraphic intervals, 105 samples belong to Class 1 oil coming from oil shale, while 46 samples belong to Class 2 and Class 3 oil derived from mudstones. So oil shale of Chang 7 member is the main effective source rock of Longdong area, and dark mudstones of Chang 4 + 5 to Chang 9 member are also important source rocks.
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    11. 11
      Zou, X.; Chen, S.; Lu, J.; Zhang, H.; Wang, L.; Zhou, S. Composition and Distribution of 17α(H)-Diahopane in the Yanchang Formation Source Rocks, Ordos Basin. Geochimica 2017, 46, 252– 261,  DOI: 10.19700/j.0379-1726.2017.03.005
      There is no corresponding record for this reference.
    12. 12
      Zhao, Y.; Yao, J.; Duan, Y.; Wu, Y.; Cao, X.; Xu, L.; Chen, S. Oil-Source Analysis for Chang-9 Subsection (Upper Triassic) of Eastern Gansu Province in Ordos Basin. Acta Sedimentol. Sin. 2015, 33 (5), 1023– 1032
      There is no corresponding record for this reference.
    13. 13
      Li, X.; Liu, X.; Zhou, S.; Liu, H.; Chen, Q.; Wang, J.; Liao, J.; Huang, J. Hydrocarbon Origin and Reservoir Forming Model of the Lower Yanchang Formation, Ordos Basin. Pet. Explor. Dev. 2012, 39 (2), 172– 180,  DOI: 10.1016/S1876-3804(12)60031-7
      There is no corresponding record for this reference.
    14. 14
      Luo, L.; Li, J.; Yang, W.; Ma, J.; Li, H.; Wu, K. Characteristics and Hydrocarbon Generation Potential of Chang 9 Source Rocks on Yishaan Slope, Ordos Basin. Xinjiang Pet. Geol. 2022, 43 (3), 278– 284,  DOI: 10.7657/XJPG20220304
      There is no corresponding record for this reference.
    15. 15
      Wu, F.; Li, W.; Li, Y.; XI, S. Delta Sediments and Evolution of the Yanchang Formation of Upper Triassic in Ordos Basin. J. Palaeogeogr. 2004, 6 (3), 307– 314
      There is no corresponding record for this reference.
    16. 16
      Li, C.; Zhang, W.; Lei, Y.; Zhang, H.; Huang, Y.; Zhang, Z.; Sun, M. Characteristics and Controlling Factors of Oil Accumulation in Chang 9 Member in Longdong Area, Ordos Basin. Earth Sci. 2021, 46 (10), 3560– 3574,  DOI: 10.3799/dqkx.2021.007
      There is no corresponding record for this reference.
    17. 17
      Passey, Q. R.; Creaney, S.; Kulla, J. B.; Moretti, F. S. A Practical Model for Organic Richness from Porosity and Resistivity Logs. AAPG Bull. 1990, 74 (12), 1777– 1794
      There is no corresponding record for this reference.
    18. 18
      Jiang, D.; Jiang, Z.; Zhang, H.; Yang, S. Well Logging Prediction Models of TOC Content in Source Rocks: A Case of Wenchang Formation in Lufeng Sag. Lithol. Reserv. 2019, 31 (6), 109– 117
      There is no corresponding record for this reference.
    19. 19
      Miao, H.; Wang, Y.; Ma, Z.; Guo, J.; Zhang, Y. Generalized ΔlogR Model with Spontaneous Potential and Its Application in Predicting Total Organ Carbon Conten. J. Min. Sci. Technol. 2022, 7 (4), 417– 426
      There is no corresponding record for this reference.
    20. 20
      Chen, H.; Zhang, F.; Zhang, B.; Sun, Y.; Chen, Z. Logging Evaluation Method of the Organic Carbon Content of Source Rocks Based on Logging Data: A Case Study of Shanxi Formation in Ganquan Area, Ordos Basin. Complex Hydrocarb. Reserv. 2023, 16 (1), 43– 49
      There is no corresponding record for this reference.
    21. 21
      Hu, S.; Zhang, H.; Zhang, R.; Jin, L.; Liu, Y. Quantitative Interpretation of TOC in Complicated Lithology Based on Well Log Data: A Case of Majiagou Formation in the Eastern Ordos Basin, China. Appl. Sci. 2021, 11 (18), 8724  DOI: 10.3390/app11188724
      There is no corresponding record for this reference.
    22. 22
      Han, X.; Hou, D.; Cheng, X.; Li, Y.; Niu, C.; Chen, S. Prediction of TOC in Lishui–Jiaojiang Sag Using Geochemical Analysis, Well Logs, and Machine Learning. Energies 2022, 15 (24), 9480  DOI: 10.3390/en15249480
      There is no corresponding record for this reference.
    23. 23
      Jiang, F.; Chen, D.; Wang, Z.; Xu, Z.; Chen, J.; Liu, L.; Huyan, Y.; Liu, Y. Pore Characteristic Analysis of a Lacustrine Shale: A Case Study in the Ordos Basin, NW China. Mar. Pet. Geol. 2016, 73, 554– 571,  DOI: 10.1016/j.marpetgeo.2016.03.026
      23
      Pore characteristic analysis of a lacustrine shale: A case study in the Ordos Basin, NW China
      Jiang, Fujie; Chen, Di; Wang, Zhifang; Xu, Ziyang; Chen, Jian; Liu, Li; Huyan, Yuying; Liu, Ying
      Marine and Petroleum Geology (2016), 73 (), 554-571CODEN: MPEGD8; ISSN:0264-8172. (Elsevier Ltd.)
      Org. shales deposited in a continental environment are well developed in the Ordos Basin, NW China, which is rich in hydrocarbons. However, previous research concerning shales has predominantly focused on marine shales and barely on continental shales. In this study, geochem. and mineralogical analyses, high-pressure mercury intrusion and low-pressure adsorption were performed on 18 continental shale samples obtained from a currently active shale gas play, the Chang 7 member of Yanchang Formation in the Ordos Basin. A comparison of all these techniques is provided for characterizing the complex pore structure of continental shales. Geochem. anal. reveals total org. carbon (TOC) values ranging from 0.47% to 11.44%, indicating that there is abundant org. matter (OM) in the study area. Kerogen anal. shows vitrinite reflectance (Ro) of 0.68%-1.02%, indicating that kerogen is at a mature oil generation stage. X-ray diffraction mineralogy (XRD) anal. indicates that the dominant mineral constituents of shale samples are clay minerals (which mainly consist of illite, chlorite, kaolinite, and negligible amts. of montmorillonite), quartz and feldspar, followed by low carbonate content. All-scale pore size anal. indicates that the pore size distribution (PSD) of shale pores is mainly from 0.3 to 60 nm. Note that accuracy of all-scale PSD anal. decreases for pores less than 0.3 nm and more than 10 μm. Exptl. anal. indicates that mesopores (2-50 nm) are dominant in continental shales, followed by micropores (<2 nm) and macropores (50 nm-10 μm). Mesopores have the largest contribution to pore vol. (PV) and sp. surface area (SSA). In addn., plate- and sheet-shaped pores are dominant with poor connectivity, followed by hybrid pores. Results of research on factors controlling pore structure development show that it is principally controlled by clay mineral contents and Ro, and this is different from marine systems. This study has important significance in gaining a comprehensive understanding of continental shale pore structure and the shale gas storage-seepage mechanism.
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    24. 24
      Yang, Y.; Li, W.; Ma, L. Tectonic and Stratigraphic Controls of Hydrocarbon Systems in the Ordos Basin: A Multicycle Cratonic Basin in Central China. AAPG Bull. 2005, 89 (2), 255– 269,  DOI: 10.1306/10070404027
      There is no corresponding record for this reference.
    25. 25
      Zou, C.; Wang, L.; Li, Y.; Tao, S.; Hou, L. Deep-Lacustrine Transformation of Sandy Debrites into Turbidites, Upper Triassic, Central China. Sediment. Geol. 2012, 265–266, 143– 155,  DOI: 10.1016/j.sedgeo.2012.04.004
      There is no corresponding record for this reference.
    26. 26
      Tang, X.; Zhang, J.; Wang, X.; Yu, B.; Ding, W.; Xiong, J.; Yang, Y.; Wang, L.; Yang, C. Shale Characteristics in the Southeastern Ordos Basin, China: Implications for Hydrocarbon Accumulation Conditions and the Potential of Continental Shales. Int. J. Coal Geol. 2014, 128–129, 32– 46,  DOI: 10.1016/j.coal.2014.03.005
      26
      Shale characteristics in the southeastern Ordos Basin, China: Implications for hydrocarbon accumulation conditions and the potential of continental shales
      Tang, Xuan; Zhang, Jinchuan; Wang, Xiangzeng; Yu, Bingsong; Ding, Wenlong; Xiong, Jinyu; Yang, Yiting; Wang, Long; Yang, Chao
      International Journal of Coal Geology (2014), 128-129 (), 32-46CODEN: IJCGDE; ISSN:0166-5162. (Elsevier B.V.)
      Continental shales in China are generally characterized by low thermal maturity and high clay content, characteristics that are significantly different from those of marine shales documented in the USA and elsewhere. Whether such continental shales have com. hydrocarbon potential as marine shale in the USA is a question for the China petroleum industry. Recently, vertical drills in the lacustrine shales in the Yanchang Formation produced an av. of two to five tons of oil and 1000-3000 m3 of gas per day by reservoir hydraulic fracturing, which demonstrates that the continental shales can also achieve viable hydrocarbon prodn. Shale and gas samples collected from the Chang 7 (C7) and Chang 9 (C9) Members, Yanchang Formation in the southeastern Ordos Basin were examd. for geochem., petrol. and gas content anal. The results show that the C7 and C9 Members developed a huge vol. of org.-rich shales (with 2-4 wt.% TOC) in the deep or semideep lacustrine, characterized primarily by type-II kerogen, with a relatively low thermal maturity and a vitrinite reflectance ranging from 0.5 to 1.5% Ro, which decreases from west to east. The hydrocarbon product varies with the thermal maturity. In the southwestern corner, the gas content measured by canister desorption equals 1.15-3.49 m3/t rock, and the gas-absorption capacity ranges from 3 to 5 m3/t rock, whereas in the eastern part, the gas content is low, and oil prodn. is 1-4 m3/day on av. Nanometer-scale pores and micro-fractures are well developed. All of this indicates that the continental shales in the southeastern Ordos Basin might have huge shale hydrocarbon resources. However, the clay content of the continental shales of C7 and C9, ranging from 40 to 60% of the bulk mineral content, are much higher than for gas produced in marine shales, which might lead to significant challenges for successful development.
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    27. 27
      Yang, W.; Song, Y.; Jiang, Z.; Luo, Q.; Wang, Q.; Yuan, Y.; Zhang, C.; Chen, L. Whole-Aperture Characteristics and Controlling Factors of Pore Structure in the Chang 7th Continental Shale of the Upper Triassic Yanchang Formation in the Southeastern Ordos Basin, China. Interpretation 2018, 6 (1), 175– 190,  DOI: 10.1190/INT-2017-0090.1
      There is no corresponding record for this reference.
    28. 28
      Zhang, W.; Yang, H.; Yang, W.; Wu, K.; Liu, F. Assessment of Geological Characteristics of Lacustrine Shale Oil Reservoir in Chang7Member of Yanchang Formation, Ordos Basin. Geochimica 2015, 44 (5), 505– 515,  DOI: 10.19700/j.0379-1726.2015.05.010
      There is no corresponding record for this reference.
    29. 29
      Fu, J.; Li, S.; Niu, X.; Deng, X.; Zhou, X. Geological Characteristics and Exploration of Shale Oil in Chang 7 Member of Triassic Yanchang Formation, Ordos Basin, NW China. Pet. Explor. Dev. 2020, 47 (5), 870– 882,  DOI: 10.1016/S1876-3804(20)60107-0
      There is no corresponding record for this reference.
    30. 30
      Cui, J.; Zhang, Z.; Liu, J.; Liu, G.; Huang, X.; Qi, Y.; Mao, Z.; Li, Y. Hydrocarbon Generation and Expulsion Quantification and Hydrocarbon Accumulation Contribution of Multiple Source Beds in Yanchang Formation, Ordos Basin. Nat. Gas Geosci. 2021, 32 (10), 1514– 1531,  DOI: 10.11764/j.issn.1672-1926.2021.05.011
      There is no corresponding record for this reference.
    31. 31
      Guo, K. Active Source Rocks of Chang 7 Member and Hydrocarbon Generation and Expulsion Characteristics in Longdong area, Ordos Basin. Pet. Geol. Exp. 2017, 39 (1), 15– 23
      There is no corresponding record for this reference.
    32. 32
      Li, J.; Wu, H.; Lu, S.; Xue, H.; Huang, Z.; Wang, K.; Shi, L.; Wang, X. Development and Hydrocarbon Expulsion Efficiency of Source Rock in 9th Member of Yanchang Formation, Ordos Basin. J. Jilin Univ. Sci. Ed. 2012, 42 (S1), 26– 32,  DOI: 10.13278/j.cnki.jjuese.2012.s1.004
      There is no corresponding record for this reference.
    33. 33
      Zhang, W.-z.; Yang, H.; Li, S. Hydrocarbon accumulation significance of Chang 9 1 high-quality lacustrine source rocks of Yanchang Formation, Ordos Basin. Pet. Explor. Dev. 2008, 35 (5), 557– 562,  DOI: 10.1016/S1876-3804(09)60088-4
      There is no corresponding record for this reference.
    34. 34
      Zhu, J.; Zhu, Y.; Xin, H.; Li, W.; Li, T. The Sedimentation during Chang 9 Oil Formation in Yanchang formation, Ordos Basin. J. Northwest Univ. Nat. Sci. Ed. 2013, 43 (1), 93– 100
      There is no corresponding record for this reference.
    35. 35
      Feng, R.; Liu, W.; Meng, Y.; Jiang, L.; Han, Z.; Liu, L. Optimization and Application of Organic Carbon Logging Prediction Models for Source Rocks: A Case Study of Chang 9 Member of Yanchang Formation in Ansai Area, Ordos Basin. J. Jilin Univ. Earth Sci. Ed. 2024, 54 (2), 688– 700
      There is no corresponding record for this reference.
    36. 36

      Petroleum Industry Standards of PRC. SY/T 5735-1995. Geochemical Evaluation Method for Terrestrial Source Rocks, 1996.

      There is no corresponding record for this reference.
    37. 37
      Abrams, M. A. Understanding Multiple Factors That Impact Unconventional Production: Guidelines to Evaluate Liquid-Rich Unconventional Resource Plays. Interpretation 2023, 11 (4), 1– 10,  DOI: 10.1190/INT-2022-0120.1
      There is no corresponding record for this reference.
    38. 38
      Xiao, L.; Wang, T.; Li, M. Discussion on Biological Origin and Formation Mechanism of Rearranged Hopanes in Sediments and Crude Oils. Earth Sci. 2023, 48 (11), 4190– 4201,  DOI: 10.3799/dqkx.2021.255
      There is no corresponding record for this reference.