Carbonate source rock with low total organic carbon content and high maturity as effective source rock in China: A review

Journal of Asian Earth Sciences 176 (2019) 8–26

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Carbonate source rock with low total organic carbon content and high maturity as effective source rock in China: A review

T

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Zhipeng Huoa,b,c, , Xiongqi Pangd, Junqing Chend, Jinchuan Zhanga,b, Mingzheng Songd, Kunzhang Guod, Pei Lia,b, Wei Lid, Yutao Lianga,b a

School of Energy Resources, China University of Geosciences, Beijing 100083, China Key Laboratory of Strategy Evaluation for Shale Gas of Ministry of Land and Resources, China University of Geosciences, Beijing 100083, China c Key Laboratory of Tectonics and Petroleum Resources of Ministry of Education, China University of Geosciences, Wuhan 430074, China d State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing 102249, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Carbonate source rocks with low total organic carbon (CSRLTOC) content Effective source rocks Lower limit of TOC (LLTOC) Tarim Basin

Carbonate rocks with high maturity and low total organic carbon content are widely distributed in China. Whether they can be effective source rocks remains controversial, but they have great significance for the evaluation of hydrocarbon generation potential and exploration prospects of carbonate rocks. Dataset reveals that as the depth or maturity increases, total organic carbon (TOC) content, atomic H/C ratio, and hydrocarbon generation potential of carbonate source rocks with low total organic carbon (CSRLTOC) all decrease because of hydrocarbon expulsion. Moreover, some pyrolysis experiments also indicate that the CSRLTOC are able to generate and expel a quantity of hydrocarbons. The above studies indicate that CSRLTOC can be effective source rocks for oil/gas pools. The unique features of carbonate rocks and CSRLTOC demonstrate that they are easier to expel hydrocarbons and contribute to oil/gas reservoirs than shale, and thus, the lower limit of TOC (LLTOC) of effective carbonate source rocks could be smaller than shale (0.5%). The LLTOCs of effective carbonate source rocks with type I-II1 kerogen (mainly sapropelic group) at the levels of immaturity and low maturity, maturity, high maturity and over maturity are TOC = 0.5%, 0.5–0.3%, 0.3–0.2%, and 0.2–0.1%, respectively. If the carbonate source rocks are at the high-over mature stage, we could take 0.2% as the LLTOC. The present organicpoor source rocks with high maturity are possibly moderate or good source rocks at the low mature level in the geological periods, especially carbonate source rocks if type I-II1 kerogen. Some typical oil and gas fields in China derive partly or mostly from the CSRLTOC, proving that CSRLTOC could be effective source rock in China.

1. Introduction In this paper, carbonate rock refers to the sedimentary rock whose carbonate content is over 50%, mostly limestone, argillaceous limestone, dolomite, and argillaceous dolomite (Palacas, 1984; Feng, 1993; Zhang et al., 2004). Carbonate rocks accounts for only 20% of the surface area of global sedimentary rocks, but their hydrocarbon reserves contribute over 50% of global total reserves and 60% of productions (Bai, 2006; Jiang et al., 2008). A large number of giant carbonate oil and gas fields are constantly discovered (Carmalt and St John, 1986; Halbouty, 1980, 1992, 2003). Three hundred and twentyone such fields have been discovered as of 2012, mainly distributed in the Middle East, North America and other areas (Zhang et al., 2014). 58% of these fields are derived from carbonate source rocks (Liu et al., 2017). The exploration and development of oil and gas in carbonate

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reservoirs is promising (Halbouty, 2003; Jiang et al., 2008). Carbonate rocks can be not only source rocks (mainly argillaceous carbonate rocks), but also reservoirs (mainly carbonate rocks with fewer clay minerals) (Palacas, 1984, 1988; Li et al., 1998b; Zhang et al., 2002a; Qin et al., 2005). Carbonate rocks as both source rocks and reservoirs have been discovered in many basins, such as the Persian Gulf Basin, Zagros Basin, Williston Basin, and Southern Mexico Gulf Basin, among others (Palacas, 1984; Klemme and Ulmishek, 1991; Liu et al., 2017). Marine carbonate rocks are widely distributed in China covering an area of 3.44 × 106 km2, which accounts for 40% of the area of China’s sedimentary rocks. There is a tremendous potential for hydrocarbon exploration potential in these marine carbonate rocks (Ma, 2000; Kang, 2010; Jin, 2012; Zhao et al., 2014). Compared with carbonate rocks of other basins in the world, those in China are older (mostly Paleozoic), have lower TOC (generally < 0.5%), higher maturity (generally high-

Corresponding author at: School of Energy Resources, China University of Geosciences, Beijing 100083, China. E-mail address: [email protected] (Z. Huo).

https://doi.org/10.1016/j.jseaes.2019.01.038 Received 8 June 2018; Received in revised form 25 January 2019; Accepted 27 January 2019 Available online 07 February 2019 1367-9120/ © 2019 Elsevier Ltd. All rights reserved.

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Table 1 LLTOC of effective carbonate source rocks from various units and scholars. TOC (%)

Units or scholars

TOC (%)

Units or scholars

0.03, 0.08 0.05–0.10 0.08–0.10 0.1 0.12

Cheng et al. (1996) Liu et al. (1985) Zhou and Jia (1974) Chen (1985) Chen et al. (1996); USA Geochemistry Company Cheng et al. (1996) Fu and Shi (1977) Fu and Liu (1982)

0.2 0.24 0.2, 0.3 0.3 0.4

Ronov (1958); Liu and Shi (1994); Norway Continental Shelf Research Institution (Cheng et al., 1996) France Petroleum Research Institution (Cheng et al., 1996) Hao and Jia (1989); Hao et al. (1996) Hunt (1967, 1979); Tissot and Welte (1978); Yin et al. (2011) Palacas et al. (1978, 1984); Peng et al. (2008)

0.5

Liang et al. (2000); Xia (2000); Zhang et al. (2002a, 2004)

0.1–0.2

prospects. This paper thus aims to integrate existing documentation and datasets on carbonate source rocks, and extract effective indicative proxies. The goals of this work are: (1) to demonstrate the effectiveness of the CSRLTOC, (2) to systematically determine the LLTOC of effective carbonate source rocks, and (3) to provide some enlightenments on the further study on the carbonate source rocks.

over mature), and deeper burial depth and are more strongly influenced by later multiple tectonic movements (Li et al., 1998a; Qin, 2005). One of the main debates among petroleum geologists focused on whether carbonate source rocks with low total organic carbon (CSRLTOC) content and high-over maturity are effective sources for oil/gas pools. Carbonate source rocks have been studied by many scholars, and their total organic carbon (TOC) contents are generally lower than those of mudstone/shale (Gehman, 1962; Hunt, 1967, 1979; Tissot and Welte, 1978; Palacas, 1978, 1984; Klemme and Ulmishek, 1991; Cordell, 1992; Katz, 1995; Katz et al., 2000; Fowler et al., 2001). As shown in Table 1, several influential scholars determined the lower limit of total organic carbon (LLTOC) content of effective carbonate source rocks to be 0.3% (Hunt, 1967, 1979; Tissot and Welte, 1978) or 0.4% (Palacas, 1978; Palacas et al., 1984a) or 0.5% (Peters and Cassa, 1994). Some Chinese scholars consider that the LLTOC of carbonate source rocks should be lower than that of mudstone, and thus generally prefer the LLTOC in the range of 0.1–0.2% (Zhou and Jia, 1974; Fu and Shi, 1977; Chen, 1985; Liu and Shi, 1994). However, several other scholars have recently addressed that the LLTOC of carbonate source rocks should be 0.5%, the same as mudstone (Qiu et al., 1998; Liang et al., 2000; Xia, 2000; Zhang et al., 2002a, 2004). These researchers argue that if the LLTOC is too low, the effective source rocks will be ubiquitous, which should result in many oil and gas fields in carbonate rocks in China. They should never be a major limiting factor with hydrocarbon exploration. However, hydrocarbon exploration in carbonate rocks is not ideal, and there are few reserves of middle-large oil/gas fields found before 2002 in China. Therefore, these scholars support a relatively higher LLTOC of 0.5%, and the CSRLTOC (TOC < 0.5%) should be excluded when assessing the oil/gas resources (Zhang et al., 2004; China Ministry of Land and Resources, 2009). Major breakthroughs in hydrocarbon exploration of carbonate rocks in China, however, have brought about due to the discovery of plenty of oil and gas reserves in recent years. The Puguang and Longwangmiao giant gas fields were discovered in the Sichuan Basin in 2006 and 2011, respectively. In the Tarim Basin, over 30 billion barrels of oil equivalent (BOE) in place were discovered by the end of 2014 in the Tazhong and Tabei areas, which outnumbers the 26.7 billion BOE resources enumerated in the third resource assessment evaluated by China Ministry of Land and Resources based on high TOC (> 0.5%) of the source rocks (2009). Therefore, traditional viewpoint of nonideal conditions for hydrocarbon exploration in carbonate rock reservoirs in China has been proven wrong. The areas and thicknesses of muddy and carbonate source rocks with high TOC are not sufficient to form such large scale of oil and gas reserves. The amount of resources estimated by the resource assessment compared to the reserves found by practical exploration show enormous disparities, which may indicate that the resource assessment technique based solely on the source rocks with high TOC (> 0.5%) can be problematic. The CSRLTOC can be an important source (at least among secondary source rocks). Therefore, the effectiveness of the CSRLTOC for hydrocarbon accumulation should be reconsidered and examined to provide an explanation for the hydrocarbon generation and expulsion mechanism of carbonate rocks and to re-evaluate their hydrocarbon generation potential and exploration

2. Geological setting The objective of this study is carbonate rocks in China, and the involved areas of this study include the Tarim, Ordos, Qiangtang, and Sichuan Basins, and North China. The most important area of these is the Tarim Basin, which will be described in detailed here. The geological setting of some other basins will be introduced in brief in the following paper, especially as part of the exploration examples. The Tarim Basin is the largest petroliferous sedimentary basin in Xinjiang Province, China, with an area of 5.6 × 105 m2 (Fig. 1a). Paleozoic, Mesozoic and Cenozoic strata are all well developed. The Lower Paleozoic is marine sediments and develops many sets of sourcereservoir-caprock assemblages, in which the Cambrian and Ordovician strata are the most important formations of reservoirs and source rocks (Fig. 1b). At present, the main reservoirs are carbonate rocks, and oil reserves of 8.5 × 109 barrel (bbl) and gas reserves of 1.4 × 105 trillion cubic feet (tcf) in place were discovered by the end of 2011. The Cambrian to Lower Ordovician and Middle-Upper Ordovician formations are main source rocks and consist of two types of rock: mudstone and carbonate rocks (Liang et al., 2000; Zhang et al., 2002b, 2004; Gao et al., 2006). Carbonate source rocks are developed in numerous areas including the Tabei Uplift, the Northern Depression (containing the Awaiti sag in the west and the Manjiaer sag in the east), and the Middle Uplift (containing the Bachu uplift, the Tazhong uplift, and the Tadong low uplift form west to east). The burial depth of these carbonate source rocks is relatively deep with main depth of 3000–7000 m. Carbonate source rocks have TOC of 0.03–4.98%. 80.6% of them have TOC ≤ 0.5%. The Cambrian to Lower Ordovician strata are in high-over mature and gas generation stages, with vitrinite reflectance (VR) of 0.84–3.08% (VR values are gained by the transformation of bitumen reflectance), while the Middle-Upper Ordovician strata are in the mature and oil generation stages with VR of 0.64–1.55%. In fact, the thermal maturity of carbonate source rocks in the center of sags are higher (there are no wells in these places). The source rocks of Cambrian to Ordovician were developed in the marine sedimentary environment and their organic parent material is mainly derived from planktonic alga (sapropelinite) so that they mainly contain type I-II1 kerogen (Gao et al., 2006; Huo et al., 2015). 3. Data sources To obtain a relatively comprehensive dataset of carbonate source rocks, especially CSRLTOC, an exhaustive review of relevant references was carefully conducted. The data were mainly obtained from published papers, dissertations and reports. In addition, many data were 9

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Fig. 1. Geological background of the Tarim Basin: (a) location; (b) source-reservoir-caprock.

collected from oil fields, such as the PetroChina Tarim and Changqing Oilfield Companies. The data include TOC, pyrolysis parameters, vitrinite reflectance (VR), organic elements (H, C, and O contents, H/C and O/C atomic ratios), carbon isotopes, biomarker parameter, the results of the pyrolysis simulation experiment, and so on. This study will only use the data of carbonate source rocks with TOC ≤ 0.5%. First, the data from mudstone and shale and oil-bearing rocks were eliminated, leaving only the data from limestone and dolomite without hydrocarbons. Second, the source rocks were screened to select those at levels of maturity or high maturity with the same strata and similar lithology and sedimentary facies. All figures in this study, particularly the scatter diagrams, were drawn exclusively using the data with TOC ≤ 0.5%.

and theories. 4.1. Geochemical evidence for effective CSRLTOC 4.1.1. Decrease in TOC The organic matter of source rocks can decrease greatly during geological evolution. The main factors that influence the loss of organic matter are diagenesis, oxidation, water leaching, and hydrocarbon generation and expulsion (Gehman, 1962; Tissot and Welte, 1978; Hunt, 1979; Fu and Shi, 1977; Fu and Liu, 1982; Qin, 2005; Huo et al., 2016a), and crystallization in carbonate rocks (Fu and Liu, 1982). Among these factors, water leaching, oxidation and crystallization play less important roles (Fu and Shi, 1977; Tissot and Welte, 1978; Qin, 2005); however, diagenesis can result in severe losses of 15–50% organic matter in mudstone (Bordovsiky, 1965; Hartmann et al., 1973) and 80–90% in carbonate rocks (Gehman, 1962; Bathurst, 1971). Generally, diagenesis corresponds to the biological methane stage (immature stage) of organic matter with VR < 0.5% (Tissot and Welte, 1978), at which point the source rocks have not reached the hydrocarbon generation threshold so that the quantity of hydrocarbons generation is small. When the source rocks reach the hydrocarbon generation threshold of VR = 0.5%, the residual organic matter will contribute to later hydrocarbon accumulation. The organic matter destroyed by diagenesis, oxidation, water washing and crystallization at the immature stage makes minimal contributions. Thus, the loss of organic matter after maturity is mainly caused by hydrocarbon generation and expulsion. Generally, the TOC of a rock when VR = 0.5% is defined as the original TOC (Peters et al., 2005; Jarvie et al., 2007; Huo et al., 2015). Similarly, original H/C atomic ratio, hydrocarbon generation potential (S1 + S2) and hydrogen index (HI) in the following are corresponding to those at VR = 0.5%.

4. Discussion Many scholars have studied the evaluation criteria of the effective carbonate source rocks (Table 1). Because TOC = 0.5% plays an important role in the evaluation as the LLTOC (Qiu et al., 1998; Liang et al., 2000; Xia, 2000; Zhang et al., 2002a, 2004), the CSRLTOC in this study is defined as carbonate source rocks with TOC ≤ 0.5%. Effective source rocks include fine-grained sedimentary rocks that can generate and expel hydrocarbons under natural conditions to form commercial oil and gas accumulations (Hunt, 1979; Peters and Cassa, 1994; Katz, 1995). According to this definition and considering the theory of the hydrocarbon expulsion threshold (Pang, 1995), our determination of effective CSRLTOC (TOC ≤ 0.5%) refers to such rock that the quantity of generated hydrocarbons is larger than those of residual hydrocarbons, and the hydrocarbon begins to expel on a large scale with a free phase. The effectiveness and LLTOC of the CSRLTOC were systematically demonstrated using existing datasets by certain methods 10

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Fig. 1. (continued)

(Tissot and Welte, 1978; Leythaeuser et al., 1984; Hao, 1984; Cooles, 1986; Daly and Edman, 1987; Raiswell and Berner, 1987; Baskin, 1997; Cornford et al., 1998; Lu et al., 2003; Peters et al., 2005; Qin et al., 2005; Jarvie et al., 2007; Jarvie, 2014; Dembicki, 2009; RomeroSarmiento et al., 2013; Lewan et al., 2014; Pang et al., 2014; Huo et al., 2016a; Liu et al., 2017). When hydrocarbon expulsion is completed, the ranges of TOC decrease for type I, II and III kerogen (mainly sapropelic group, sapropelic and humic mixed group, and humic group, respectively) are 80%, 50% and 20%, respectively (Daly and Edman, 1987;

Scholars have used various methods to study the evolutional regularity of the TOC of source rocks. Tissot and Welte (1978) reported in their book “Petroleum Formation and Occurrence”: “Finally, it should be point out that this minimum organic carbon value as a source rock criterion no longer applies to the rocks in a very advanced stage of maturity, i.e., in a metagenetic stage when only dry gas is generated. At this stage, 0.3% or 0.5% organic carbon may merely indicate a residual amount of organic matter that initially may have been more than twice as high.” The discharge of oil and gas in source rocks will diminish TOC 11

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Fig. 2. The principle of natural evolution of a parameter of organic matter abundance of source rocks.

This means that the present poor source rocks with high or over maturity are possibly moderate or good source rocks at the low mature stage (or original conditions) in geological periods.

Baskin, 1997; Jarvie et al., 2007; Jarvie, 2012, 2014). The higher the hydrocarbon expulsion efficiency is, the larger the degree of TOC decrease is (Baskin, 1997; Zhong et al., 2004; Huo et al., 2016a). Carbonate rocks mainly contain type I kerogen and have higher hydrocarbon conversion rate and hydrocarbon expulsion efficiency than muddy source rocks (Gehman, 1962; Hunt, 1967, 1979; Tissot and Welte, 1978; Qin, 2005; Liu et al., 2010), hence carbonate rocks lose more TOC than muddy source rocks. One method showing hydrocarbon expulsion is natural evolution of organic matter abundance parameter. In fact, the modern parameter values of a layer of similar source rocks at different planimetric positions and depths could be regarded as the evolution of this layer of rocks at the same planimetric position in the geological period (Zhou and Pang, 2002; Pang et al., 2014). Thus, the modern TOC of a set of similar source rocks decreases as the depth or maturity increases, which means they would reduce in the geological period (or with the increase of maturity or hydrocarbon expulsion). This principle also applies to H/ C ratio, S1 + S2 and HI below (Fig. 2). The TOC Natural evolution of the CSRLLTOC of the Lianglitage Formation of the Middle-Upper Ordovician in the Tazhong area in the Tarim Basin shows an obvious decrease with the increase of Tmax (Tmax is the biggest pyrolysis peak temperature, and it is also an index of maturity of organic matter) (the data are from wells: TaC1, TZ10, TZ101, TZ12, TZ162, TZ30, TZ43, TZ52, TZ6) (Fig. 3a). Furthermore, pyrolysis simulation experiments (Hao, 1984; Qin et al., 2005) and numerical calculation (Chen, 1985; Daly and Edman, 1987; Lu et al., 2003; Zhong et al., 2004; Pang et al., 2014; Huo et al., 2016a) have also demonstrated that the TOC of CSRLTOC decreases as the maturity or the hydrocarbon expulsion continue (Fig. 3b and c). The extents of TOC decrease determined by different scholars, however, shows discrepancies due to the use of different methods, dissimilar samples, and other factors. Generally, the higher the TOC, the higher the maturity, the larger their hydrocarbon expulsion efficiency, and the larger the decrease degree in TOC. The extent of TOC decrease is different for various type of organic matter (Type I > II1 > II2 > III) (Daly and Edman, 1987; Qin et al., 2005; Pang et al., 2014; Huo et al., 2016a). For carbonate source rocks with high maturity, low TOC and type I-II1 kerogen in China, a decrease in TOC implies that such rocks should have expelled large amounts of hydrocarbons and are effective source rocks. These carbonate source rocks have higher TOC at the low mature stage, so the original TOC must be restored when we evaluate their quality (Qin, 2005; Huo et al., 2016a).

4.1.2. Decrease in H/C atomic ratio Organic matter mainly consists of five elements, C, H, O, N and S, which account for more than 99.5% of total organic matter (Tissot and Welte, 1978; Hunt, 1979; Pang and Zhou, 1995). The H/C atomic ratio of kerogen is a good indicator of the maturity of organic matter. It would decrease with increasing maturity for a rock or a layer of similar rocks (Huizinga et al., 1988; Baskin, 1997; Zhang et al., 1999; Zhong et al., 2004; Huo et al., 2015). In fact, the increasing maturity of organic matter means it has generated and expelled hydrocarbons. Thus, the reduction of H/C ratio is due to hydrocarbon generation and expulsion, which is a process of deoxidation, dehydrogenation, carbon-enrichment, and condensation of aromatic nuclei. When the H/C ratio decreases to its minimum value, hydrocarbons cannot be generated from the organic matter. Baskin (1997) showed that the quantity of expelled hydrocarbons is proportional to the initial H/C atomic ratio value. Using the relationship between the quantity of oil and gas expulsion of hydrous pyrolysis experiments and initial and spent H/C ratio of kerogen, amounts of hydrocarbon expulsion can be estimated. In general, both hydrous and sealed-tube pyrolysis experiments suggest that the fully-matured (H/C = 0.50) source rocks with type I and II kerogen (their TOCs are 10.6% and 4.52–12.7%, respectively) can generate and expel 100 and 60–65 bbl oil/acre-ft/TOC, respectively. Just as the principle of natural evolution of a parameter mentioned above, Fig. 4 reflects that the decrease in the H/C atomic ratio is corresponding to the change in VR of the CSRLTOC of the Cambrian and Ordovician in the Tarim Basin, China. These data are from collection from the PetroChina Tarim Oilfield Companies, including 9 wells: TaC1, TD2, TZ12, H4, K2, TC1 LN46, LN48, LN54 (Fig. 1a). When VR value increases from 0.5% to 2.5%, H/C ratio decreases from 1.5 to 0.45 with a reduction proportion of more than 70%. Based on the above principle (Baskin, 1997; Pang, 2014), CSRLTOC can expel a large amount of hydrocarbons, and can thus be effective source rocks for hydrocarbon accumulation. 4.1.3. Variation in hydrocarbon generation potential The hydrocarbon generation potential index method is effective for calculating the quantity of hydrocarbons generation and expulsion, and can also determine the hydrocarbon expulsion threshold (Zhou and 12

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Fig. 3. Decrease in TOC of CSRLOMA with hydrocarbon generation and expulsion by various. methods: (a) Natural development of Mid-Upper Ordovician marlite in the Tazhong area, Tarim Basin, China; (b) Pyrolysis simulation of Jurassic marlite in the Qiangtang Basin, China (Qin et al., 2005); (c) Numerical calculation (Huo et al., 2016).

or VR increases (Zhou and Pang, 2002; Jiang et al., 2015). (S1 + S2)/ TOC initially increases because the organic matter undergoes a loss of oxygen during the diagenesis stage, during which CO2 is generated and the amount of total organic carbon decreases (Tissot et al., 1974; Wu, 1986). (S1 + S2)/TOC decreases later as hydrocarbons are expelled from the source rocks. Although the shapes of the evolution curves of various effective source rocks may differ, the trend of an increase in (S1 + S2)/TOC followed by a decrease is the same. The point corresponding to the depth or VR at which (S1 + S2)/TOC begins to decrease is defined as the hydrocarbon expulsion threshold (Zhou and Pang, 2002). A severe decrease in (S1 + S2)/TOC demonstrates that a great many hydrocarbons have been generated and expelled. Fig. 5 reflects the variation in (S1 + S2)/TOC of the Middle-Upper Ordovician CSRLTOC in various areas in the Tarim Basin. Although the shapes of these curves are different, they all tend to increase in the beginning and then decrease. In addition, S1 + S2 and the hydrogen index (HI) can also reflect hydrocarbon generation potential. After source rocks become mature, the decreased values of these two parameters are related to hydrocarbon expulsion (Baskin, 1997). Fig. 6 shows the decrease in S1 + S2 and HI of the Middle-Upper Ordovician CSRLTOC in the Tazhong area in the Tarim Basin (from wells: TaC1, TZ10, TZ101, TZ12, TZ162, TZ30, TZ43, TZ52, TZ6). The lithology of these source rocks is mainly marlstone with 0.90–1.08% of VR. As VR reaches 0.9%, S1 + S2 and HI decrease constantly with the depth, which demonstrates that the source rocks are generating and expelling hydrocarbons and so are effective.

4.1.4. Pyrolysis simulation experiment Many scholars have performed pyrolysis simulation experiments to study the characteristics of hydrocarbon generation and expulsion and oil/gas production yields of carbonate source rocks including numerous CSRLTOC (Table 2) (Qin, 2005; Hao et al., 1993; Cheng et al., 1995; Fan et al., 1997; Xie et al., 2002a; Hu, 2005; Liu et al., 2010). Although the origin, lithology, TOC, type, and VR of the samples chosen in previous experiments are varied in the experiments, all the source rocks generated and expelled a quantity of hydrocarbons. The maximum oil generation yields are 40.45–482.6 kg/t TOC. On the basis of these values, source rocks with thickness of 100 m and an area of 1 km2 can generate (5.27–51.34) × 104 t oils at most. Using data from the Tarim Basin, the largest capacity of residual oil (S1/TOC) is 66.9 mg HC/g TOC, and 94.1 mg HC/g TOC after the correction of light hydrocarbon

Fig. 4. Reduction in H/C atomic ratios with VR of the CSRLOMA of the Cambrian and Ordovician in the Tarim Basin, China.

Pang, 2002; Guo et al., 2014). Among the Rock-Rval pyrolysis parameters, S1 and S2 represent free and pyrolyzed hydrocarbon, respectively. S1 + S2 (the sum of S1 and S2) reflects the residual hydrocarbon generation potential. (S1 + S2)/TOC is defined as the hydrocarbon generation potential index, reflecting generation potential per unit mass of organic carbon. In theory, (S1 + S2)/TOC of effective source rocks initially increases to a maximum value and then decreases as the depth 13

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Fig. 5. Variation in hydrocarbon generation potential for the CSRLOMA of the Mid-Upper Ordovician.

4.2. LLTOC of the CSRLTOC

compensation (Fig. 7). For source rocks with a thickness of 100 m, an area of 1 km2 and TOC from 0.1% to 0.5%, the largest residual amount is (2.4–12.0) × 104 t oils that is generally less than (5.27–51.34) × 104 t of the amount of oil generation. This means that the CSRLTOC can expel certain amounts of hydrocarbons. In conclusion, the decrease in TOC, H/C atomic ratio, (S1 + S2)/ TOC, S1 + S2 and HI with the increase of VR indicates that the CSRLTOC can expulse mass of hydrocarbons to form oil/gas accumulation, and thus, they can be effective source rocks.

Some scholars believe that the LLTOCs are independent of lithology, and the LLTOCs of effective muddy and carbonate source rocks should be the same and both are 0.5% (Bissada, 1982; Peters and Cassa, 1994; Zhang et al., 2002a). Compared with mudstone and shale, however, carbonate rocks and CSRLTOC are unique and their LLTOC could be lower than that of muddy rocks. 4.2.1. Unique feature of carbonate rocks (1) Materials generating hydrocarbon in carbonate rocks—organic acid

Fig. 6. Variation in hydrocarbon generation potential of the CSRLOMA of Mid-Upper Ordovician in the Tazhong area, Tarim Basin, China: (a) variation in S1 + S2 with Tmax; (b) variation in hydrogen index (HI) with Tmax. 14

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Rock

Rock

Rock

Rock

966.3 I

0.6

Hydrous closed system

40.45

4226 Hydrous closed system 0.59 I

I

0.61

Open system

482.6

269

307.53 2393.58 110.64 Hydrous closed system 0.99 II2

0.49 Limestone

0.47 Marlstone

Marlstone

0.4

84.5 0.39

Cheng et al. (1995)

Liu et al. (2010)

Hu (2005)

Qin (2005) Zhang et al. (2009) Qin (2005)

Hao et al. (1993)

Hao et al. (1993)

Limestone

0.8 II1 49.1 0.31 0.38 Marlstone Limestone

0.57 II1 95.3 0.28

0.7 I-II1 80.92 0.24

80.92 0.24 Hao et al. (1993)

Pingquan in Hebei, China Xiahuayuan in Hebei, China Xiahuayuan in Hebei, China Jurong Basin in Jiangsu, China Cuoqin Basin, China Tarim Basin, China Fan et al. (1997)

Dolomitic micrite Dolomitic micrite Micrite

0.17

0.1

Muddy dolomite Limestone Ordos, China Xie et al. (2002)

Traditionally, materials that can generate hydrocarbons in source rocks include kerogen and dispersed organic matter (adsorption, inclusion, crystal enclosed organic matter) (Palacas, 1984; Fu and Jia, 1984; Guan et al., 1998; Wang and Cheng, 2000; Qin, 2005). Some scholars found that organic acid salt is a third type of material (Carothers and Kharaka, 1978; Vandegrift et al., 1980; Zhou et al., 1997; Wu et al., 2005; Lei, 2007; Lei et al., 2010; Liu et al., 2013; Liu et al., 2012, 2017). Organic acid salt refer to the organic matter that is formed by the chemical reaction between organic acid and metal ions and then is preserved in the strata during geological process. These salts are mainly formed in carbonate-saline sedimentary environments (Carothers and Kharaka, 1978; Zhou et al., 1997; Wu et al., 2005; Liu et al., 2013; Liu et al., 2017). Many scholars have proven that the organic acids, mainly including fatty acids (one carboxylic acid) in source rocks, are able to generate hydrocarbons (Cooper and Bray, 1963; Jurg and Eisma, 1964; Shimoyama and Johns, 1972; Andresen et al., 1994; Zhang et al., 1998, 2000; Dias et al., 2002). The organic acids can exist in both muddy and carbonate source rocks. Unlike shale, however, carbonate rocks contain more Ca2+, Mg2+ and other metal ions (rich salts) so that they can easily combine with the organic acid to generate organic acid salts, such as fatty acid salts. Thus, the organic acid salts are extensively developed in marine carbonate rocks (Carothers and Kharaka, 1978; Zhou et al., 1997; Liu et al., 2013). It is beneficial to the preservation of organic matter in the form of salts (Kawamura and Nissenbaum, 1992; Wu et al., 2005; Lei, 2007; Liu et al., 2017). The organic acid salts affect the hydrocarbon potential of carbonate source rocks in two ways. First, they possess strong potential to generate hydrocarbons, which improves the conversion rate of hydrocarbon generation of carbonate rocks. The organic acid salts are stable at low temperatures and can be pyrolyzed to a large number of hydrocarbons (mainly gas) at high temperatures, i.e. above 300 ˚C in most cases (Fig. 8). Its pyrolysis temperature and conversation rate of hydrocarbon generation are higher than those of organic acids (Zhou et al., 1997; Lei, 2007; Liu et al., 2017). Liu et al. (2017) believed that the existence of organic acid salts is critical to generate hydrocarbons on a large scale for carbonate source rocks at the high-over mature level. Furthermore, the real TOC value is underestimated in the conventional TOC test. The organic acid and organic acid salts are dissolved and removed when carbonate rocks are soaked with hydrochloric acid to get rid of inorganic carbon, which leads to large losses (> 40%) of organic matter for carbonate rocks (Roberts et al., 1973; Byers et al., 1978; Froelich, 1980; Bisutti et al., 2004; Lei et al., 2009; Liu et al., 2017). When the carbonate rocks have similar TOC, the TOC loss in conventional tests will increase with the increase of carbonate content in rocks. In addition, the lower the TOC of a rock, the larger the TOC loss (ratio, not absolute value) (Froelich, 1980; Lei et al., 2009; Liu et al., 2013). Thus, the loss of organic matter accounts for a larger percentage for the CSRLTOC (Liu et al., 2017). Liu et al. (2016) put forward a new method for testing TOC in carbonate rocks: the analysis of montmorillonite thickening elements, to avoid the loss of organic matter in the conventional measurement. The TOC value measured by this new method is more than two times that by measured conventional methods (Liu et al., 2016, 2017), which means that the previous ineffective or low-quality source rocks are likely to actually be effective or high-quality ones. Therefore, it is likely that CSRLTOC at the high-over mature stage have high potential for hydrocarbon generation. It is considered that large amounts of organic acid salts are preserved in the Ordovician limestones in the Tazhong and Lunnan areas in the Tarim Basin, and limestones in the Ordivician Majiagou Formation in the Jingbian gas field in the Ordos Basin (Sun et al., 2013; Liu et al., 2013, 2017). Additionally, these organic acid salts are one of the hydrocarbons generating materials of carbonates rocks at the stage of high maturation which improves the hydrocarbon production rate.

East Lake in Qiangtang Basin, China Williamston Basin, USA Tongkou in North Sichuang, China Ordos, China

Rock Rock 136.36 1071 1921 106.83

Kerogen 304 370 134

Rock 443 524 291

331 411 175 I-II1

0.7

0.84

2.43

Whole rock heating (semi-closed system) Closed system, high pressure Anhydrous closed system Anhydrous closed system Anhydrous closed system Hydrous closed system Hydrous opened system

140

294

115

Maximum gas generation rate (m3/t TOC) Maximum oil generation rate (kg/t TOC) Experiment conditions VR (%) Organic matter type Carbonate content (%) TOC (%) Lithology Sample place Experimenter

Table 2 Oil and gas production rates from pyrolysis simulation experiments of CSRLOMA from various scholars.

Kerogen

Rock

Rock

salts

Total hydrocarbon generation rate (kg/t TOC)

Sample type

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mainly type I and type II1, which have higher hydrocarbon generation potential. Another reason is that carbonate rocks are richer in material generating hydrocarbon—organic acid salts. The capacities of hydrocarbon adsorption are different for various minerals. The capacities of organic matter and carbonate minerals are the highest and the lowest, respectively, with clay minerals in between (Jarvie, 2014). Organic matter and clay mineral content in carbonate rocks are lower, as a result, the adsorption of hydrocarbons is much lower than that of muddy rocks, reducing the residual hydrocarbons in carbonate rocks (Gehman, 1962; Wu, 1986; Qin, 2005). With the higher hydrocarbon conversion yield and less residual hydrocarbons, carbonate source rocks will reach peak residual hydrocarbons quantity sooner and then begin to expel hydrocarbons with higher hydrocarbon expulsion efficiency using the hydrocarbon generation potential index method (Zhou and Pang, 2002; Jiang et al., 2015). The hydrocarbon expulsion efficiency for carbonate rocks and mudstone with the same 1.0% of average TOC value is calculated. When VR = 2.5%, the hydrocarbon expulsion efficiency for carbonate rock can reach to 93.5%, which is higher than the 80.6% of mudstone (Huo et al., 2016, 2018). (3) Expelled hydrocarbons have lower loss during migration and after accumulation to form oil/gas pools more easily. Fig. 7. Capacity of residual hydrocarbon of the CSRLTOC.

Expelled hydrocarbons will undergo various dissipations during migration, including retention in reservoirs, disappearance before caprock formation, water runoff, and diffusion. After being accumulated in traps, hydrocarbons will be subsequently readjusted or destroyed by later multiple tectonic movements. The remaining hydrocarbons after dissipation would be the final oil and gas resources (Pang et al., 2000; Luo et al., 2008; Pang et al., 2012; Huo et al., 2016b). Carbonate rocks can not only be source rocks but also be used as a reservoir (Palacas, 1988; Li et al., 1998b; Zhang et al., 2002a; Qin et al., 2005; Liu et al., 2017), so they can form self-generated and self-accumulated oil/gas reservoirs in some conditions. If so, the hydrocarbon migration distance is shorter and the hydrocarbon loss during migration is likely to be much lower, particularly for the CSRLTOC. Furthermore, carbonate rock strata generally form contact with a layer of gypsum-salt rock, which is a better caprock than mudstone. Thus, there may be a smaller quantity of hydrocarbons that are destroyed and diffused in carbonate rock reservoirs, and the formation of large oil and gas reservoirs is easier (Hunt, 1967; Hunt and McNichol, 1984; Jones, 1984; Huo et al., 2016b). The development of regional caprocks of large-scale gypsum-salt rock brings out huge oil and gas reserves in carbonate reservoirs in many basins, including the Sichuan

(2) It is easier to expel hydrocarbons from carbonate rocks than from muddy rocks Some scholars believe that the rates of hydrocarbon conversion (HC/TOC) of carbonate rocks are much higher than that of shale, which they have examined by testing the quantities of hydrocarbons (chloroform bitumen “A”) and TOC in carbonate rock and shale (Gehman, 1962; Hunt, 1967, 1979; Tissot and Welte, 1978). This view has been argued by other scholars (Jones, 1984; Xia, 2000; Zhang et al., 2002). It has been proven that the carbonate and carbonate-evaporite rocks have higher yields of lipid-like extractable organic matter (immature and low-mature oil) than shales (Palacas et al., 1984b; Tannenbaum and Aizenshtat, 1985; Orr, 1986). Under similar conditions, pyrolysis experiments suggested that the hydrocarbon conversion rates of carbonate rocks are indeed greater than those of mudstone/ shale (Fig. 9) (Hu et al., 2004; Qin et al., 2005, 2006; Tao, 2008; Liu et al., 2010). The main reason is that the kerogen type of carbonate source rocks is better than that of muddy rocks, because carbonate rocks mainly comprised marine low algae organisms (the organic maceral is mainly sapropelitic group). Thus, their kerogen type is

Fig. 8. Hydrocarbon generation of calcium stearate (Liu et al., 2017). 16

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Fig. 9. Gas production rate of source rocks with different lithologies: (a) Jurassic samples from the Qiangtang Basin, China (Qin et al., 2006); (b) Permian samples from the Sichuan Basin, China (Liu et al., 2010).

Basin in China, Persian Gulf Basin in the Middle East, Williston and Michigan Basin in USA, Alberta Basin in Canada, and Karakum Basin in Central Asia (Hunt, 1967; Jones, 1984; Palacas, 1984; Qin et al., 2005).

less to oil and gas pools. If the source rocks with high TOC are the chief source rocks, the CSRLTOC can be the secondary source rocks and should be examined at greater length.

4.2.2. Unique feature of the CSRLTOC Compared with source rocks with high TOC (> 0.5%), the CSRLTOC generate fewer hydrocarbons and expel hydrocarbons later (Huo et al., 2016b). First, the source rocks with high TOC discharge a certain quantity of hydrocarbons before the CSRLTOC begin to expel hydrocarbons. Most of these hydrocarbons will be lost during migration, including retention and dissipation in migration paths. For the carbonate reservoirs, long distance hydrocarbon migration is not required, so that the migration paths may be retained partly or totally by hydrocarbons generated from source rocks with high TOC. Subsequently, the hydrocarbons expelled later for the CSRLTOC will have smaller loss over migration paths and will migrate more easily into the traps to form oil/gas pools. Second, for the CSRLTOC, hydrocarbons that are expelled later are more likely to accumulate at the late stage, and there will be fewer hydrocarbon loss caused by tectonic movements (Pang et al., 2012; Huo et al., 2016b). Forming a reservoir with a certain scale and industrial value will be easier, or reserves will occupy a larger percentage in the final hydrocarbon resources. Finally, the CSRLTOC can make a greater contribution to gas reservoirs because they mainly generate and expel gas at the high-over mature stage (Huo et al., 2016b). Moreover, carbonate source rocks are heterogenous, and source rocks with low and high TOC are often alternately mixed such that source rocks with high TOC are much thinner. If the source rocks with high and low TOC are primary and secondary source rocks, respectively, this means that a set of source rocks contain multiple layers of primary and secondary source rocks. Both sources of oil and gas will migrate along the migration path together. Therefore, although the source rocks with low TOC may contribute less to the commercial oil and gas reservoirs than those with high TOC, they indeed contribute to hydrocarbon reservoirs to some extent. They make no contribution only when the TOC is too low to expel oil and gas. The contribution of each type of source rock depends on the quantity of hydrocarbon expulsion. If a layer of CSRLTOC expels fewer hydrocarbons and its contribution to oil and gas reservoirs is low, multiple layers of CSRLTOC will expulse large amounts of hydrocarbons and may make a greater contribution. Undoubtedly, carbonate source rocks with TOC > 0.5% are effective and of high quality. There are carbonate source rocks with high TOC in most of the world’s known carbonate oil/gas fields (Palacas, 1984; Klemme and Ulmishek, 1991; Bai, 2006). Therefore, some petroleum geologists regard carbonate rocks with high TOC as merely effective and ignore the importance of CSRLTOC. Through the various examinations discussed above (variation in TOC, H/C, O/C and hydrocarbon generation potential, pyrolysis experiments), however, the CSRLTOC can expel quantities of hydrocarbons and contribute more or

4.2.3. Consideration for determination of LLTOC of CSRLTOC The LLTOCs of muddy source rocks determined by most researchers are relatively consistent and mainly show TOC = 0.5% (Ronov, 1958; Hunt, 1979; Tissot and Welte, 1978; Zhang et al., 1999, 2002a), while fewer scholars suggest LLTOC = 1.0% (Bjørlykke, 1989). The LLTOCs of effective carbonate source rocks are not uniform and range from 0.05% to 0.5% (Table 1), mainly owing to the differences in study areas and methods, in addition to the diversity in lithology, types, geological ages, and maturity of source rocks. Moreover, differences in the definition of LLTOC are also crucial factor. There are three LLTOCs for effective source rocks: the LLTOC of hydrocarbon expulsion, the LLTOC of hydrocarbon accumulation, and the LLTOC of formation of large scale of oil/gas field (Peng et al., 2008). The LLTOC of hydrocarbon expulsion refers to the lowest TOC that a source rock can have to generate a certain quantity of hydrocarbons that is larger than the largest residual hydrocarbons and begins to expel with a free phase. This LLTOC can be obtained from pyrolysis experiments and numerical analysis and is generally smaller than 0.2% (Liu et al., 1985; Chen et al., 1996; Hao et al., 1996; Qin et al., 2005; Xue, 2010). The LLTOC of hydrocarbon accumulation refers to the effective TOC by which expelled hydrocarbons can satisfy the loss of hydrocarbons during migration and tectonic movements and then form a reservoir. This LLTOC is the same as the definition of effective source rocks proposed by Hunt (1979). The LLTOC of formation of large scale of oil/gas fields refers to the TOC required for the source rock to generate hydrocarbons to sufficiently form large and medium oil and gas fields. In fact, TOC = 0.5% is the LLTOC of formation of large scale of oil/gas fields and is therefore larger. According to the uniqueness of carbonate rocks, their expelled hydrocarbons are easier to form oil and gas reservoir. Therefore, the LLTOC of effective carbonate source rocks is smaller than shale. For carbonate source rocks, the LLTOC of hydrocarbon accumulation is larger than that of hydrocarbon expulsion, but the difference in the two values is not great. As a result, we primarily aim at determining the LLTOC of hydrocarbon expulsion, and then slightly increase this value to become the LLTOC of effective source rocks. Furthermore, the LLTOC should be a range of values rather than a sole value. It gradually decreases as the organic matter maturity increases, the type of organic matter improves, and the thickness of source rocks increases (Pang et al., 1993; Xue, 2010). Therefore, it is not scientific to consider TOC = 0.5% as the only LLTOC of effective carbonate source rocks. In fact, 0.5% can be regarded as the LLTOC of immature and low mature effective source rocks. In China, carbonate rocks are generally at levels of high or over maturity. The low TOC may be the result of the expulsion of many hydrocarbons in the process of 17

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(Pang and Zhou, 1995), the liquid hydrocarbon generation per unit organic carbon for carbonate rocks was calculated to determine the oil production rate (Fig. 12a). Fig. 12b shows the variation in the LLTOC of hydrocarbon expulsion with VR in the Tarim Basin. At the immature level (VR < 0.5%), LLTOC ≥ 0.60%; at the low mature level (VR = 0.5–0.7%), LLTOC = 0.6–0.41%; at the mature level (VR = 0.7–1.3%), LLTOC = 0.41–0.22%; at the high mature level (VR = 1.3–2.0%), TOC = 0.22–0.12%; and at the over mature level (VR ≥ 2.0%), LLTOC < 0.12%. These results show that the LLTOC is higher at the lower mature level, whereas the LLTOC decreases rapidly with the increase of maturity (Fig. 12b). Similarly, Xue (2010) used the method of numerical simulation to determine that the LLTOC of carbonate source rocks in the Tarim Basin and Bohai Bay Basin are 0.15% and 0.12%, respectively.

geological evolution. Therefore, a LLTOC smaller than 0.5% is reasonable for carbonate source rocks with high maturity. 4.2.4. Determination of LLTOC based on variation in hydrocarbon generation potential According to the above discussion, the hydrocarbon generation potential of effective source rocks initially increases and then decreases with depth or VR (Zhou and Pang, 2002; Jiang et al., 2015). The decrease in potential occurs because the source rocks begin to expel hydrocarbons. The depth or VR corresponding with the turning point from large to small is the hydrocarbon expulsion threshold. Because noneffective source rocks do not expel a great number of hydrocarbons, however, the hydrocarbon generation potential of such rocks always increases with depth or VR, and the hydrocarbon expulsion threshold does not appear. Theoretically, if the depth or VR is the same, the quantity of generated hydrocarbons will also be gradually augmented with increasing TOC. The generation quantity is initially less than the maximum residual hydrocarbons of a rock and they later equalize. The TOC value or range at which the hydrocarbon expulsion threshold appears is the LLTOC of effective source rocks. To determine the LLTOC of CSRLTOC, TOC from 0% to 0.5% are divided into 10 sections with the same 0.05% interval, and the variations in hydrocarbon generation potential with the TOC of different ranges are then drawn for CSRLTOC in the Tarim Basin, China (Fig. 10). When the TOC is small, the (S1 + S2)/TOC always increases with depth but is initially lowered within the range of 0.11% ≤ TOC ≤ 0.15%, and appears to be the hydrocarbon expulsion threshold. Therefore, 0.13% of TOC (the average value of 0.11% and 0.15%) is determined as the LLTOC of hydrocarbon expulsion. It should be noted that the deepest pyrolysis data for TOC ≤ 0.05% and 0.06% ≤ TOC ≤ 0.10% are obtained at 6400 m, with few data indicating greater depth. According to the observation, the hydrocarbon expulsion threshold may appear at depths greater than 6400 m for the source rocks with TOC ≤ 0.05%, or it may not appear at all. Generally, the hydrocarbon expulsion threshold occurs from deep to shallow depths with an increase in TOC. In addition, because of fewer hydrocarbon generation and later hydrocarbon expulsion, the CSRLTOC may have reached levels of late maturity or high maturity when the hydrocarbon expulsion threshold appears. Therefore, the CSRLTOC reflected by the data in Fig. 10 are mostly mature and high mature rocks in the Tarim Basin. As a result, the determined LLTOC is only for source rocks at levels of maturity and high maturity.

4.2.6. Determination of LLTOC based on TOC evolution This paper has discussed the TOC evolution with the depth or VR from three perspectives: natural development, a pyrolysis simulation experiment and numerical simulation (Fig. 3). The TOC evolution characteristics of type I-II1 source rocks per unit volume with depth or VR is studied by numerical simulation based on material balance in the Tazhong area, as shown in Fig. 3c (Huo et al., 2016a). The main principle is the balance of organic carbon content in source rocks, which means the original TOC is equal to the sum of the residual TOC and expelled TOC. TOC decreased dramatically at VR = 0.5–2.0% with 62% of the TOC reduction degree (Fig. 13). This paper takes the LLTOC = 0.5% put forward by some scholars as the LLTOC of effective carbonate source rocks at the immature and low mature stages. According to the TOC evolution (Fig. 13), the residual TOC value of 0.5% at different stages of maturity can be taken as the LLTOC at the corresponding mature stage. Therefore, in theory, the LLTOC of effective source rocks is continuously varying. To better use the LLTOC, this paper makes the TOC corresponding to VR mid-value at each maturity stage the same as the LLTOC at each stage. The LLTOC of effective carbonate source rocks with type I-II1 kerogen at the immature, mature, high mature and over mature stages are 0.50%, 0.29%, 0.21% and 0.16%, respectively. The abovementioned studies show that the CSRLTOC could be effective in suitable conditions and their LLTOCs gradually decrease with an increase of VR. According to the results from the hydrocarbon generation potential, simulation calculation, TOC evolution and previous research, the LLTOCs for effective carbonate source rocks of type I-II1 kerogen in this study are as follows: immature and low mature level, TOC = 0.5%; mature level, TOC = 0.5–0.3%; high mature level, TOC = 0.3–0.2%; and over mature level, TOC = 0.2–0.1%. If the carbonate source rocks are at the high-over mature stage, we could take 0.2% as the LLTOC for the sake of expedience.

4.2.5. Determination of LLTOC by numerical simulation According to the material balance principle, numerical simulations can calculate the hydrocarbons generation quantity, residual hydrocarbons critical saturation quantity, and hydrocarbons expulsion quantity of a source rock during burial process. Moreover, it can determine the expulsion threshold when hydrocarbons generation is equal to residual hydrocarbons (Pang, 1995). Before the expulsion threshold, oil expulses in free phase and water solution. The gas residual effect mainly includes three types of phases: oil solution, water solution, and absorption, which are more complex than those of oil. Gas could expel in an oil solution, water solution, by diffusion or in free phase. The amount of hydrocarbons expulsion in the free phase is effective for hydrocarbon accumulation, and can be obtained (Pang, 1995). The effective expulsion quantity (the difference of generated and residual hydrocarbon amounts) is equal to zero at the hydrocarbon expulsion threshold. At this point, the TOC required by the hydrocarbon generation quantity is the LLTOC (Fig. 11). In the Tarim Basin, the marine carbonate rocks of the Cambrian and Ordovician consist mainly of type I-II1 kerogen (Gao et al., 2006; Huo et al., 2015), which chiefly generate and expel liquid hydrocarbons at the mature stage. Therefore, the TOC of the expulsion oil threshold can be considered as the LLTOC of hydrocarbon expulsion. According to the method of numerical optimal simulation based on material balance

4.3. Exploration examples Numerous large and superlarge carbonate oil and gas fields have been found in the Middle East, North America, Europe and Asia-Pacific region (Bai, 2006; Jiang et al., 2008). Carbonate rocks are source rocks in many of these fields (Owen, 1964; Hunt, 1967; Palacas, 1984; Claypool and Mancini, 1989; Klemme and Ulmishek, 1991; Qiu et al., 1998; Bai, 2006; Liu et al., 2017). Although most carbonate source rocks have higher TOC (> 0.5%), oil/gas and source correlation showed that there are oil and gas fields worldwide, including China, that are partly or mostly derived from the CSRLTOC, which indicates that the CSRLTOC can indeed generate and expel quantities of hydrocarbons and are thus effective. This also supports the view of this study. 4.3.1. The marlstone of Middle-Upper Ordovician in the Tabei area In China, one typical example that the CSRLTOC can be effective source rocks is the marlstone of the Middle-Upper Ordovician in the Tabei area of the Tarim Basin. The Tabei area is located in northern 18

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Fig. 10. Variation in hydrocarbon generation potential of the CSRLOMA with TOC of various ranges, Tarim Basin, China. (S1 + S2)/TOC initially reduces within the range of 0.11% ≤ TOC ≤ 0.15%, which means the hydrocarbon expulsion threshold corresponds to TOC = 0.11–0.15%.

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Fig. 11. Principle of determining the LLTOC of hydrocarbon expulsion by numerical simulation based on material balance.

Fig. 12. LLTOC of hydrocarbon expulsion determined by numerical simulation in the Tarim Basin, China. (a) Oil production rate of hydrocarbon generation optimizing the simulation of carbonate rocks; (b) Variation of LLTOC with VR. LLTOC decreases with the increase of VR.

Tarim Basin, with a total area of 3.66 × 104 km2. It is the richest area of oil and gas resources in the Tarim Basin. The Tahe, Lunnan, YingMaiLi and Halahatang oil/gas fields were found there in succession. By the end of 2015, the oil reserves in place were found to be more than 80 × 108 bbl, and the gas reserves in place are over 18 tcf in Ordovician strata. These oil and gas fields are derived from two sets of source rocks of the Cambrian–Lower Ordovician (Є-O1) and Middle-Upper Ordovician (O2+3) strata. The crude oil contains a higher dibenzothiophene series, C29-hopane/C30-hopane, and C35-hopane/C34-hopane, and so their sources are obviously from marine carbonate source rocks rather than mudstone/shale (Li et al., 2015; Liu et al., 2017). According to the oil source correlation, most scholars believe that these oils have mixed origins but are mainly derived from the O2+3 carbonate source rocks with a smaller contribution by the Є-O1 carbonate source rocks (Fig. 14) (Hanson et al., 2000; Zhang et al., 2000, 2002, 2004; Lu et al., 2007; Wang et al., 2004, 2008, 2010; Li et al., 2010, 2015; Yu et al., 2011; Li et al., 2012). Some researchers have argued that oil sources are located in the Tabei and adjacent southern areas (Wang et al., 2004, 2008,

2010; Lu et al., 2007; Li et al., 2010, 2015), but the TOCs of the O2+3 carbonate source rocks in these places are much lower than 0.5% (Jia, 1997; Zhang et al., 2004; Zhao et al., 2008; Huang et al., 2016). Our studies show that the sample number of the O2+3 source rocks with TOC > 0.5% accounts for less than 10% of the total samples (Fig. 15), and the TOC of the sole well of Lungu39 is between 0.20% and 0.85% with an average value 0.59%. Some other scholars believed that these oils originate mainly from the source rocks in the Manjiaer sag (Zhao et al., 2008), which is located in the southeast of the Tabei area. The O2+3 strata is the flysch clastic deposits of super compensation basin facies, so the possible source rocks should be shale, not carbonate rocks, which does not correspond to the viewpoint that the Tabei area oils are derived from marine carbonate source rocks. Furthermore, the TOCs of these shales are relatively low (generally < 0.3%) and that they are ineffective or poor source rocks (Zhang et al., 2004). Some scholars suggested that the effective source rocks for the majority of oils in the Tarim Basin have not yet been drilled (Yu et al., 2012), but this is an assumption that has not been proved. In contrast, a few scholars argue that the Tabei area oils are mainly attributed to the Є-O1 source rocks 20

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4.3.2. The carbonate rocks of the Ordovician Majiagou Formation in the Ordos Basin The carbonate rock of the Ordovician Majiagou Formation in the Ordos Basin is also a example in China. The TOC of carbonate rocks of the Majiagou Formation is below than 0.5%, with 0.2–0.3% of average value (Fig. 16) (Li et al., 1999; Xia, 2000; Dai et al., 2005; Liu et al., 2013). There is a dispute over whether these carbonate rocks can be effective source rocks. The reservoirs of the Jingbian giant gas field are also carbonate rocks of the Majiagou Formation. According to the gassource correlation, the gas primarily derives from coal measure strata of Carboniferous-Permian, and partially from carbonate rocks of the Majiagou Formation (Huang et al., 1996; Liu et al., 2004; Dai et al., 2005; Cai et al., 2005; Chen, 2002; Liu et al., 2009). In recent years, commercial gas reservoirs have been discovered in the fifth member of the Majiagou Formation in the east of the Central Uplift and the west of Jingbian. Some scholars believe that these gases are mainly from the coal-bearing layers of Carboniferous-Permian strata in the Jingbian gas field (Zhao et al., 2015; Liu et al., 2016). However, other scholars hold the view that gypsum-salt rocks developed in the Majiagou Formation in the eastern basin and that there are self-generated and self-accumulated gas reservoirs of carbonate rocks in the lower formation of gypsum-salt-bearing strata. Because these gas reservoirs are far from Carboniferous-Permian coal-measure source rocks and have a certain thickness of gypsiferous rocks (good barrier for gas migration) between reservoirs and source rocks, the migration pathway and accumulation condition for large-scale upper source and lower reservoir are considered to be nonexistent (Yang et al., 2009; Liu et al., 2016; Yao et al., 2016; Liu et al., 2017). As a consequence, the effectiveness of carbonate rocks in the Majiagou Formation has aroused attentions from scholars, and they are believed to be effective source rocks. In addition, the distribution of these source rocks has been studied (Xie et al., 2002b; Chen et al., 2014). The carbonate rock of the Ordovician Majiagou Formation in the Ordos Basin is still a possible case for the CSRLTOC, and its effectiveness must be further investigated.

Fig. 13. LLTOC evolution of effective carbonate source rocks with I-II1 type kerogen. The line of TOC = 0.5% represents the LLTOC evolution.

(Sun et al., 2003; Wang and Xiao, 2004; Cai et al., 2009). Current compositions, however, are overwhelmingly dominated by late charges from the O2+3 source rocks (Jia et al., 2010; Tian et al., 2012). This means that the O2+3 carbonate source rocks should make a relatively large contribution to the early oil pools, which are mainly derived from the Є-O1 source rocks. All in all, the thickness and area of the O2+3 carbonate source rocks with TOC > 0.5% in the Tabei area are limited, and the resources evaluated using them are less than the reserves of 8 billion barrels oil equivalent discovered by the end of 2015. Therefore, at least the CSRLTOC should make some contribution to the Tabei oil/ gas fields.

4.3.3. The dolomite pinnacle reefs of Silurian Niagara in the Michigan Basin, USA There are a few exploration examples in which the CSRLTOC form the main oil/gas source worldwide, which further proves the merit of the results of this study. One classic example is the dolomite pinnacle reefs of Silurian Niagara in the Michigan Basin, USA (Gardner and Bray, 1984). An important characteristic of the Niagara reefs development zone is that its upper part is covered and closed by impermeable

Fig. 14. Single molecule carbon isotope of oil in the Tabei area, Tarim Basin, China (Li et al., 2015). Oil from wells TZ62 and TD2 derives from Cambrian source rocks and its single molecule carbon. isotope is lighter. Oil from wells YG2 and YG2-1C derives from Ordovician source rocks and its single molecule carbon isotope is heavier. 21

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Fig. 15. TOC distribution of carbonate source rocks of the Middle-upper Ordovician in the Tabei area, China.

Fig. 16. TOC of carbonate rocks of the Majiagou Formation of the Ordovician in the Ordos Basin, China (Xia, 2000). (a) Total sample; (b) Samples from gypsum saline lake and open sea.

Fig. 17. Silurian reef profile in the Southeast Michigan Basin, USA.

spectrograms of C18+ aromatics, and the stable whole oil infrared spectrogram. Ordovician and Devonian oils are similar to each other yet different from Silurian Niagara oils (Mooer, 1976; Vogler et al., 1981; Gardner and Bray, 1984). In other words, Ordovician, Silurian and Devonian oils suggest three distinct oil chemistries and three sources (Illich and Grizzle, 1983; Pruitt, 1983; Rullktter et al., 1986; Obermajer et al., 1998). But there is a general agreement that Niagara reef-hosted oil has a separate source and is originated from the generating oil of the Silurian layers themselves. The carbonate rock of the Silurian Salina

gypsum-salt-bearing rock (the Silurian Salina Formation A-2). The bottom of the Niagara reefs is also impermeable because the underlying formation is Clinton shale; the intermediate part between A-2 and the Clinton shale is the carbonate rocks of the Silurian Salina Formation A-1 and dolomite of Brown Niagara facie (Fig. 17). Many scholars have conducted oil-source correlations for crude oil of the different layers (Ordovician, Silurian and Devonian) in the Michigan basin by analyzing carbon isotopes (Fig. 18a), gas chromatography (GC), gas chromatography and mass spectrometer (GC-MS) (Fig. 18b), infrared

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Fig. 18. Oil and source correlation of the Silurian in the Michigan Basin, USA: (a) Distribution of δ13C of oil and kerogen (Gardner and Bray, 1984); (b) Biomarker cross-plot of C27 diasteraneC27 regular sterane ratio vs. C30 hopaneC29 sterane ratio (Obermajer et al., 1998).

moderate or good source rocks at the low mature stage, especially for carbonate source rocks with type I-II1 kerogen. We should conduct more in-depth studies of CSRLTOC in the future. The hydrocarbon generation and expulsion of source rocks depend on hydrogen (H) content as well. When different source rocks have same or similar TOC, those with higher H content are able to generate and expel a larger quantity of hydrocarbons. The LLTOC determined above does not reflect the influence of the H content. For the CSRLTOC in which TOC are more than LLTOC, their H contents are different, and the degree of decrease of their TOC are also different. Therefore, CSRLTOC can be divided into four types: (1) source rocks with original TOC > 0.5% and more H, which have generated and expelled a large number of hydrocarbons and their degree of TOC decrease is relatively larger; (2) those with original TOC < 0.5% and moderate H, which have generated and expelled some hydrocarbons and their degree of TOC decrease is moderate; (3) those with original TOC < 0.5% and lower H, which have generated and expelled a few hydrocarbons and TOC decrease is small; (4) those with original TOC < 0.5% (or < 0.2%) and few H, which have not expelled hydrocarbons and TOC decrease is negligible. Methods used to identify these three types of source rocks and to evaluate their contributions to hydrocarbon accumulation deserves further research.

Formation A-1 is the principal source rock (Fowler, 1984; Gardner and Bray, 1984; McMurray, 1985; Rullkotter et al., 1986; Dunham et al., 1988; Goodman, 1991; Obermajer et al., 1998, 2000) with a maximum and average TOC of only 0.6% and 0.28%, respectively. Moreover, the dolomite of the Brown Niagara facies is the secondary source rock with an average TOC of 0.27% (Gardner and Bray, 1984). In recent years, some scholars have found that there are a few organic-rich argillaceous interlayers in the Salina Formation A-1 with TOC ranging from 0.4% to 3.5%, and they have good source rock potential (Obermajer et al., 2000). However, the total thickness of these organic-rich source rocks is relatively thin. The oil and gas reserves of Middle Silurian pinnacle reef are enormous, and the recoverable reserves are estimated at approximately 1.2 billion BOE (Gill, 1994). Therefore, the dolomite with TOC < 0.5% should make some contribution to these enormous reserves. 5. Conclusion CSRLTOC is defined as carbonate source rocks with TOC less or equal to 0.5%. The decrease of TOC, H/C, and hydrocarbon generation potential and results of pyrolysis simulation indicate that the CSRLTOC can expel hydrocarbons to contribute to oil/gas reservoirs and can thus be effective source rocks. The unique feature of the carbonate rocks and CSRLTOC demonstrate that their effective LLTOC could be lower than shale (0.5%). The LLTOCs of effective carbonate source rocks with type I-II1 kerogen at immature and low mature, mature, high mature and over mature stages are 0.5%, 0.5–0.3%, 0.3–0.2%, 0.2–0.1%, respectively. There are some typical oil and gas fields derive partly or mostly from the CSRLTOC in China. It should be emphasized that we do not deny the importance of carbonate source rocks with TOC > 0.5%. They are of high quality and are primary source for oil/gas fields. However, we should not overlook the possibility of the CSRLTOC as effective source rocks. The CSRLTOC do generate and expel lower amounts of hydrocarbons than carbonate source rocks with TOC > 0.5%, but they can indeed expel hydrocarbons and contribute to oil/gas accumulations because of their uniqueness. Therefore, the CSRLTOC may be the secondary source rocks and can provide a certain amount of oil/gas resources. In addition, the organic-poor source rocks with high or over maturity are possibly

Conflict of interest The authors declared that there is no conflict of interest. Acknowledgements We wish to thank the Open Project of Key Laboratory of Tectonics and Petroleum Resources of Ministry of Education (TPR-2016-06), Special grant of China Postdoctoral Science Foundation (2018T110124), National Natural Science Foundation of China (41702152), and National Key Fundamental Research Plan “973” Project (2011CB201102) for funding this study. We are very grateful to PetroChina Tarim Oilfield Company, particularly the Research Institute of Exploration and Development, for their data on TOC and pyrolysis parameters. 23

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