Possible new method to discriminate effective source rocks in petroliferous basins: A case study in the Tazhong area,Tarim Basin

Abstract

Sediments with organic matter content (total organic carbon) TOC ≤ 0.5% which can act as effective source rocks are critical and challenging in the field of petroleum geology. A new method is proposed through a case study to identify and evaluate the effective source rocks, which is applied to study the changing characteristic of hydrocarbon-generation potential index with depth. The burial depth corresponding to the beginning of hydrocarbon-generation potential index reduction represents the hydrocarbon expulsion threshold in source rocks. Then, new identification standards are established for discrimination of effective source rocks of Middle–Upper Ordovician in Tarim Basin. The critical value of TOC for effective source rocks change with their burial depth: the TOC > 0.5% with source rock depth > 4000 m, TOC > 0.4% with depth >4500 m, TOC > 0.3% with depth > 5000 m, TOC > 0.2% with depth >5500 m. Based on the new criteria, effective source rocks in the Middle–Upper Ordovician are identified and their total potential hydrocarbon resources is evaluated, reaches 0.68 × 10⁹ t in the Tazhong area, which is 65.4% higher than that of previous studies and consistent with the exploration result. Thus, this new method is of significance to resource evaluation and can be applied in the carbonate source rocks and mudstone source rocks with high degree of exploration.

Keywords

Total organic carbon carbonate and mudstone source rocks hydrocarbon expulsion hydrocarbon resources evaluation Tarim Basin

Introduction

Effective source rock refers to the source rocks which have expelled hydrocarbons and made contribution to the formation of hydrocarbon reservoirs in a petroliferous basin (Hunt, 1979; Tissot et al., 1978). Ronov (1958), the former Soviet scholar, is the pioneer who proposed to identify the effective source rock with the value of total organic carbon (TOC) simply. He found that the TOC of the shale source rocks is larger than 0.4% in the petroliferous basins of Siberia platform, and the average organic matter abundance TOC < 0.5% in areas without oils discovered. He believed that it was the difference in organic matter abundance that determined the presence or absence of hydrocarbon reservoirs. Since TOC = 0.5% is the lower limit of effective source rock, this standard has been cited by more and more scholars (Jin, 2005; Qin et al., 2004).

As oil and gas exploration continues to expand deeper, more and more in situ generation-accumulation reservoirs have been discovered in areas with TOC < 0.5% (Liu et al., 2017). The rationality of using simple TOC as an effective source rock discrimination has been questioned and studied by more and more scholars (Jones, 1984; Patience, 2003; Tissot et al., 1978). Firstly, this paper analyzed the irrationality of using TOC value to directly identify effective source rocks. Then, this paper proposed the new method to identify and evaluate effective source rocks. Finally, this method would be applied in Tazhong area, Tarim Basin, and verify its reliability.

An overview of TOC as a standard of source rock

Current thoughts and problems

The deviations of simply using TOC to discriminate and evaluate effective source rocks has long been recognized by geochemists (Erik et al., 2006; Klemme and Ulmishek, 1991; Patience, 2003). Tissot and Welte (1978) had an important discussion in their work “Petroleum Formation and Occurrence”, they point out that the minimum organic carbon value (TOC = 0.5%), as the standard for identifying source rocks, can no longer be applied to source rocks in high maturity stage. In the high maturity stage of source rocks, TOC can only reflect the residual quantity of organic matter, while the original value of TOC may be twice as much as it was. In other words, the effective source rocks in petroliferous basins with different sedimentary environments or with different organic matter types and maturity should have different identifying criteria values of TOC. Peters and Cassa (1994) believed that TOC was not a clear indicator of oil production potential for source rocks. The effectiveness of source rock must be based on TOC content, kerogen type, and thermal evolution degree of organic matter together. The source rocks with low TOC value in some geological conditions have made contribution to hydrocarbon accumulation. Therefore, it is a very complex issue for identifying the effective source rocks simply by a unified TOC value. This view is supported by many scholars (Fowler et al., 2001; Katz et al., 2000).

It is difficult to identify and evaluate the effective source rocks just based on simple TOC, as oil and gas exploration continues to expand into deeper areas. In oil and gas exploration practices, the TOC value cannot guide the oil company to find more reservoirs. In recent years, major breakthroughs in hydrocarbon exploration in China have occurred in the areas with relatively low TOC value. Our study area, the Tazhong area in Tarim Basin, shows that plenty of marine oils have been discovered (Figure 1(a)); they are proved to come from the Middle–Upper Ordovician source rocks with low TOC and Cambrian–Lower Ordovician source rocks with high TOC (Pang et al., 2016; Zhang et al., 2012, 2000). In Tarim Basin, over 30 billion barrels of oil equivalent in place were discovered by the end of 2014 in the Tazhong area. However, the discovered oil and gas reserves in this area were greater than the third resource assessment evaluated by China Ministry of Land and Resources based on high TOC (>0.5%) of the source rocks (2009), indicating that source rocks with TOC < 0.5% could also have made some contributions to hydrocarbon accumulation and be of great importance. In the previous points, the source rocks of Middle–Upper Ordovician with TOC ≤ 0.5% (Figure 1(b)) should not be treated as effective source rocks. And this has caused widespread attention about the possibility of source rocks with low TOC as effective source rocks.

Figure 1.

The relationship between the distribution of discovered marine crude oil and the TOC distribution of the Middle–Upper Ordovician source rock, Tarim Basin. (a) The limited distribution of the effective source rocks with high TOC (>0.5%) is difficult to explain the widely distributed marine oil and gas (the red area is the study area in this paper). (b) Effective source rocks of the Middle–Upper Ordovician with TOC > 0.5% are only found in limited individual wells (modified after Pang et al., 2016).

It is the empirical results from different researchers rather than rational research conclusions to simply use TOC > 0.5% to identify and evaluate effective source rock (Hao et al., 1996). The identification of effective source rocks was based on the statistical results of the TOC distribution in the area with discovered and undiscovered reservoirs in Siberia petroliferous basins by Ronov (1958). The process did not involve the factors such as organic matter type and thermal evolution degree of source rocks which have very important effects on the identification of effective source rocks (Gehman, 1962; Palacas et al., 1986; Tissot et al., 1978). In addition, with the continuous improvement of the exploration, substantial commercial oil and gas accumulations have been discovered in the area with TOC ≤ 0.5% of source rock (Zhao et al., 2014). If taking all these conditions into account, there exist problems in simply using TOC to discriminate and evaluate the effectiveness of source rocks.

Many scholars have made efforts to solve the problems in identifying and evaluating effective source rocks with the TOC standard (Li et al., 2016). They have conducted substantial physical and numerical simulations of hydrocarbon generation and expulsion of source rocks. Then they recognized that the hydrocarbons’ expulsion thresholds are affected by many geological factors. These factors include abundance of TOC in the source rock, type of parent organic materials, thermal maturity (Ro), content, and type of internal clay minerals. Owing to these different factors, different scholars finally get different TOC value criteria (Zhang et al., 2004). Figure 2 lists these research results about the identification criterion for effective source rocks. They indicate that there is no uniform standard value of simple TOC in discrimination and evaluation of effective source rocks. If the standard really exists, it should change with these geological conditions.

Figure 2.

Comparison of identification criterion of TOC for effective source rocks proposed by different scholars, indicating different scholars have different research results for different carbonate source rock; the TOC value varies from 0.05% to 0.4%.

Possible reasons

The basic feature of the effective source rocks is that they can provide hydrocarbons to form commercial reservoirs, so there are many reasons for why simple TOC cannot be used to identify and evaluate effective source rocks, and the most important one is that the hydrocarbon generation amount and expulsion amount from source rocks are not only controlled by the value of TOC.

TOC value cannot reflect the hydrocarbon generation amount

For the convenience of discussion, this article compares the amounts of generated oil and gas amount per unit weight of TOC under different geological conditions. The amounts of hydrocarbons generated per unit weight of TOC differ greatly with the change of organic types, maturity, and other geological conditions. If these factors are not taken into account carefully, it might draw imprecise conclusion to evaluate the effective source rocks simply based on the relative values of TOC. Figure 3 shows the optimization simulation results of hydrocarbon generation amount based on material balance method (Pang et al., 1992), indicating the changing characteristics of hydrocarbon generation amount per unit weight of TOC with the variation of parent material types and thermal maturities. The method of optimization material balance simulation was first proposed by Yenenect (1954) and improved by Pang et al. (1992). With the increasing of source rock maturity (Ro), the amounts of hydrocarbons generated by the three kinds of organic matters gradually increase, and the accumulated amounts of oil generated per ton of organic matters (TOC) with types I, II, and III at Ro ≈ 3.5% are 308.6, 133.7, and 33.4 kg, respectively (Figure 3(A1). In the same maturity (Ro), the amount of oil generated from type I organic matter is more than type II, meaning a positive deviation. With the Ro increases from 0.5% to 3.5%, their deviations gradually increase from 21.5 kg to 174.9 kg. In the same maturity (Ro), the generated oil amount by type III is smaller than that of type II, showing a negative deviation. With the Ro increases from 0.5% to 3.5%, their negative deviation gradually increases from 15 kg to 100.3 kg (Figure 3(A2)). Similarly, the result of methane also shows this deviation (Figure 3(B)). The hydrocarbon generation research results show that there will be big difference if simple TOC is taken into account in the identification and evaluation of effective source rock, the deviation will be –75.0% to +130.8% at Ro = 3.5%. Therefore, the simple TOC value of the source rock cannot represent the amount of hydrocarbon generation.

Figure 3.

Comparison of hydrocarbon amounts generated by current per unit weight of parent TOC with different organic types and at different maturities (Ro). (a) Variation of liquid hydrocarbon amounts generated by current per ton of parent TOC with different organic types and at different maturities (Ro). (b) Deviations of liquid hydrocarbon generation amounts from source rocks with type II and type I as well as from source rocks with type II and III at different maturities. (c) Variation of natural gas amounts generated by current per ton of parent TOC with different organic types and at different maturities (Ro). (d) Deviations of nature gas generation amounts from source rocks with type II and type I as well as from source rocks with type II and III at different maturities.

TOC value cannot reflect the hydrocarbon expulsion amount

For the convenience of discussion, this article also compares the amounts of oil and gas expelled per unit weight of TOC under different geological conditions. The amounts of hydrocarbon expelled per unit weight of TOC differ greatly as the types, maturity of parent material, and their geological conditions change. If these factors are not considered, it might make big mistakes for identifying and evaluating the effective source rocks. Research results show that the amounts of hydrocarbons expelled per unit weight of TOC vary with the type and maturity of parent materials, and the deviation reaches –75.0% to +130.8%.

Figure 4 illustrates the amounts of hydrocarbons expelled per unit weight of TOC and changes with different organic types and maturity (Ro). The maximum hydrogen index (kg/t TOC) varies differently with different types of parent material, it will reach 200, 600, 1000 with organic types III, II, I, respectively (Peter and Cassa, 1994). When the maturity (Ro) reaches 1.0%, the source rocks start to expel hydrocarbons, and the expelled hydrocarbon amounts will increase with the increasing of depth, and also increase when the organic material types change from III to II, then to I. The maximum amounts of hydrocarbons expelled from source rocks with type III parent materials accounts for 32.7% than that with type II, and the maximum amount with type I account for 145% than that with type II (Figure 4(a) and (b)). In the case of the same parent material, the hydrogen index decreases with the increasing of the buried depth, indicating that the amount of hydrocarbons expelled increases with the buried depth. Figure 4(c) and (d) illustrates the variation of hydrocarbon-generation potential indexes with the depth for the major source rocks of Tarim Basin; the source rock enters the hydrocarbon expulsion threshold at Ro ≈ 1%, and the expelled hydrocarbon amount accounts for 0% of the total hydrocarbon-generation potential. Subsequently, the expelled hydrocarbon amount increases with the increase of burial depth. At Ro = 1.5%, 2.5%, and 3.5%, the corresponding amounts of hydrocarbons expelled from source rocks account for 26.7%, 64.7%, and 88.9% of the total hydrocarbon generation amounts, respectively.

Figure 4.

(a) Variation characteristics of Hydrogen Index (HI, kg/t) and the comparison of hydrocarbon expulsion amount with organic types I, II, III at different maturities. In the same thermal maturity, the hydrocarbon expulsion amounts decrease from organic type I to organic type II, then to organic type III. (b) Variation characteristics of hydrocarbon-generation potential index (“S₁+S₂”/TOC, kg/t) and the comparison of hydrocarbon expulsion amounts with the same organic type of the Middle–Upper Ordovician source rock in the Tazhong area, Tarim Basin, at different maturities. The expelled hydrocarbon amounts increase with the maturities (Ro, %) from 1.0% to 3.5%.

Above analysis shows that the simple TOC value has different geological significance if their types, maturities, and geological conditions are different, indicating different hydrocarbon generation amounts and different hydrocarbon expulsion amounts. Current measured value of TOC can neither reflect the generated hydrocarbon amount nor the expelled hydrocarbon amount from the source rocks, and it can only represent the abundance of organic matter remained in the source rocks. Therefore, it is unreasonable to use simple TOC to identify the effectiveness of source rocks.

Possible new method to identify effective source rock

Hydrocarbon expulsion threshold and its corresponding critical factors are proposed by this paper to identify the effective source rocks Then, the new identification criteria is applied to identify the effective source rocks and evaluated hydrocarbon resources based on a case study in the Tazhong area, Tarim Basin.

Geological background

The Tarim Basin is a large superimposed basin with the rich hydrocarbon resources and a complicated geology in western China (Zhu et al., 2019). More than 4 billion tons of hydrocarbons have been discovered in the Basin. They are mainly distributed in the Tabei Uplift and Tazhong Uplift (Figure 1(a)). About 1 billion tons of hydrocarbons are distributed among the Ordovician, Silurian, and Carboniferous systems in the Tazhong Uplift. The Tazhong area was selected as the study area for relatively concentrated oil distribution. Tazhong area is located in the central part of the Tarim Basin, with an exploration area of 2.2 × 10⁴ km². It is adjacent to the Manjiaer depression in the north, Tangguzibasi depression in the south, Tadong Uplift in the east, and Bachu Uplift in the west. It is a paleo-uplift that has developed over a long period of time and has good conditions for hydrocarbon accumulation. The Tazhong area develops the strata of Paleozoic, Mesozoic, and Cenozoic from the bottom up. The Lower Paleozoic is marine sediments and develops many sets of source-reservoir-caprock assemblages, in which the Cambrian and Ordovician strata are the most important formations of reservoirs and source rocks. The Cambrian–Lower Ordovician and Middle–Upper Ordovician formations are main source rocks and consist of two types of rock: mudstone and carbonate rocks. Carbonate source rocks have TOC of 0.03–4.98%; 80.6% of them have TOC ≤ 0.5%. The Cambrian–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% (Zhang et al., 2004; Gao et al., 2006; Chen et al., 2018).

Data

In this study, a total of 2368 core samples from 164 wells in Tazhong area, Tarim Basin, are taken from the Middle–Upper Ordovician. The TOC contents of Middle–Upper Ordovician source rocks are generally low. These samples are analyzed by using Rock-Eval pyrolysis equipment for their source rock characteristics, such as types of kerogen, thermal maturity, oil retention, and hydrocarbon-generation potential. And the TOC were collected by the oil field company.

Rock-Eval analyses of the source rock samples were completed on a Rock-Eval6 analyzer that can measure free hydrocarbons S₁, pyrolysis hydrocarbon S₂, and temperature of maximum pyrolysis yield T_max. The sample quantity ranges from 30 to 40 mg, and thermal vaporization of the free hydrocarbons at 300°C for 5 min to produce the S₁ peak was then followed by programmed pyrolysis at 25°C/min to 650°C in an atmosphere of nitrogen to generate S₂ peak.

The TOC of the drilled Middle–Upper Ordovician source rocks in Tarim Basin is less than 0.5%. In order to figure out the hydrocarbon expulsion threshold, this study calculates (S₁ + S₂)/TOC on 286 samples with TOC ≤ 0.05%, 509 samples with 0.05% < TOC ≤ 0.1%, 828 samples with 0.1% < TOC ≤ 0.2%, 387 samples with 0.2%<TOC ≤ 0.3%, 208 samples with 0.3% < TOC ≤ 0.4%, 150 samples with 0.4% < TOC ≤ 0.5%. And these results are used to identify and evaluate the effectiveness of source rocks.

Methodology

Plenty of studies have shown that the hydrocarbon-generation potential index (“S₁ + S₂”/TOC) changes with buried depth, which reflects the basic characteristics of hydrocarbon generation and expulsion in source rocks per unit TOC (Erik, et al., 2006; Klemme and Ulmishek, 1991; Patience, 2003; Welte and Leythaeuser, 1983) (Figure 5).

Figure 5.

Variation characteristics of hydrocarbon-generation potential index (pink) for source rocks with depth and the model for hydrocarbon expulsion from effective source rocks. In the model, hydrocarbons begin to be expelled at the hydrocarbon expulsion threshold (HET) from source rocks, and the hydrocarbon expulsion amount (light red) from effective source rocks increases when the depth increases, showing its contribution to hydrocarbon. S₂ (blue) is the parent TOC for hydrocarbon generation and S₁ (gray) is the generated but remaining hydrocarbons in source rocks (modified after Pang et al., 2016).

According to the hydrocarbon-generation potential method, the hydrocarbon-generation potential of the source rock stays constant until the generated hydrocarbon satisfies its own various residues. The only reason to reduce the hydrocarbon-generation potential is that some hydrocarbons have been expelled. Therefore, the burial depth corresponding to the beginning of the hydrocarbon-generation potential reduction represents the hydrocarbon expulsion threshold from the source rock. The maximum value of the hydrocarbon-generation potential index corresponds to the hydrocarbon expulsion threshold. When the depth is deeper than the hydrocarbon expulsion threshold, the value of hydrocarbon-generation potential index decreases rapidly, which indicates a mass of hydrocarbon expulsion. The difference between the total accumulative potential of hydrocarbon generation (Qp) and residual amount for hydrocarbons (Qm) reflects the hydrocarbon expulsion amount (Qe) from source rocks, and the difference between the maximum hydrocarbon-generation potential index of the source rock and the residual hydrocarbon-generation potential index is the hydrocarbon expulsion rate. According to the variation characteristics of the hydrocarbon-generation potential index with burial depth, the potential of hydrocarbon resources can be evaluated. Owing to the hydrocarbon expulsion in the evolution process of source rocks, the value of TOC will decrease. Therefore, using today's measured TOC data as a parameter for hydrocarbon-generation potential will lead to the wrong results. So, the TOC values need to be recovered before we use them as parameters for identifying the effective source rocks (Pang et al., 2014).

Based on the above method, firstly, we recover the measured TOC value. Then, we establish the relationship between the hydrocarbon-generation potential and burial depth with different TOC values. Finally, we can acquire critical parameters corresponding to hydrocarbon expulsion threshold and apply them to identify the effective source rocks. Also, this method can be used to evaluate the hydrocarbon resources by establishing the model in different areas. This article adopts this method to discriminate and evaluate the effectiveness of the Middle–Upper Ordovician source rock in the Tazhong area, Tarim Basin (Figures 4(c), (d) and 6). The result shows that the source rocks of the Middle–Upper Ordovician enter the hydrocarbon expulsion threshold after 4150 m and form effective source rock even with TOC < 0.5%. When the depth is larger than the depth with hydrocarbon expulsion threshold, the amounts of hydrocarbons expelled from the source rock increase with the increase in the burial depth. At the depth of 6000 m, the hydrocarbon-generation potential index is more than 1000 kg/t (Figure 4(c) and (d)), even with low TOC (<0.5%), which have made the contribution to hydrocarbon accumulation. In order to establish a detailed discrimination criteria for effective source rocks, this paper separately counts the hydrocarbon-generation potential index and the hydrocarbon expulsion thresholds for effective source rocks with TOC ≤ 0.05%, 0.05 < TOC ≤ 0.1%, 0.1% < TOC ≤ 0.2%, 0.2% < TOC ≤ 0.3%, 0.3 < TOC ≤ 0.4%, 0.4 < TOC ≤ 0.5. After a comprehensive study, identification criteria values of TOC for effective source rocks of Middle–Upper Ordovician with different depths were established as shown in Figure 6: the source rocks with TOC < 0.2% cannot enter the hydrocarbon expulsion threshold to form effective source rocks (Figure 6(a) to (c)), while the source rock with TOC ≥ 0.2% could enter the hydrocarbon expulsion threshold (Figure 6 (d) to (f)), so they have the ability to expel hydrocarbons to form the effective source rocks. The depth of hydrocarbon expulsion threshold becomes shallower with the increasing of TOC value. The depth of the hydrocarbon expulsion threshold is 5500 m for source rocks with 0.2% < TOC ≤ 0.3%, is 5000 m with 0.3 < TOC ≤ 0.4%, is 4500 m with 0.4 < TOC ≤ 0.5. Their cumulative hydrocarbon expulsion amounts (Qe) are about 100 kg/t TOC, 150 kg/t TOC, and 250 kg/t TOC at 7000 m, respectively (Figure 6 (d) to (f)). The source rock with high TOC (>0.5%) is generally considered as an effective source rock because their depth for hydrocarbon expulsion threshold is shallower, their depth is much lower than 4000 m, and the cumulative hydrocarbon expulsion amount is more than 800 kg (Pang et al., 2003).

Figure 6.

The variation characteristics of hydrocarbon-generation potential index with depth and the identification of effective source rocks with different TOC of the Middle–Upper Ordovician in Tazhong area, Tarim Basin (modified after Pang et al., 2018). Source rocks with low TOC (≤0.2%) cannot enter HET and make no contribution to HC accumulation, while these with TOC > 0.2% can enter HET and make contribution to HC accumulation if their burial depth is deep enough. (a) The variation characteristics of Hydrocarbon Generation Potential Index with depth and the identification of effective source rocks with TOC ≤ 0.05%; (b) The variation characteristics of Hydrocarbon Generation Potential Index with depth and the identification of effective source rocks with .05 < TOC ≤0.1%; (c) The variation characteristics of Hydrocarbon Generation Potential Index with depth and the identification of effective source rocks with 0.1% < TOC ≤ 0.2%; (d) The variation characteristics of Hydrocarbon Generation Potential Index with depth and the identification of effective source rocks with 0.2% < TOC ≤ 0.3%; (e) The variation characteristics of Hydrocarbon Generation Potential Index with depth and the identification of effective source rocks with 0.3 < TOC ≤ 0.4%; (f) The variation characteristics of Hydrocarbon Generation Potential Index with depth and the identification of effective source rocks with 0.4 < TOC ≤ 0.5.

According to research results above, the identification criteria for effective source rocks of the Middle–Upper Ordovician in Tarim Basin can be determined by combining the TOC and the buried depth (Z): there is non-effective source rocks if TOC < 0.2% and Z < 6000 m; there is effective source rocks if TOC > 0.2% but it changes with depth: 0.2% < TOC ≤ 0.3% and Z > 5500 m; 0.3% < TOC ≤ 0.4% and Z > 5000 m; 0.4% < TOC ≤ 0.5% and Z > 4500 m; TOC > 0.5% and Z > 4000 m. For the same source rocks, it can expel more oil and gas with the deeper depth, and made greater contribution to hydrocarbon accumulation.

Application

Based on the new method and identifying criterion, the effective source rocks will be discriminated. Firstly, this paper re-identified the thickness of effective source rocks in the vertical direction. Then, this paper re-predicts the distribution range of effective source rocks on the plane. Finally, the map of the thickness for effective source rocks by the new method and identifying criterion was made. Figure 7 illustrates an example for this study. Based on the new method, the TOC value for identifying effective source rocks should be more than 0.2% but less than 0.5% at a depth of about 5200 m, and the research result shows that the thickness ratio of the effective source rocks to total source rocks changes from 7.8% to 46.1% in Tazhong area (Figure 7(a)). In addition, this paper chose 24 wells relatively evenly distributed in the Tazhong area, Tarim Basin, and identifies the effective source rocks by recovered TOC; most of them should be classified as effective source rocks based on the previous identifying TOC criteria with 0.5% (Figure 7(b)). This study re-identified the effective source rocks by the new identifying TOC criteria and also by the traditional identifying TOC criteria to their recovered original TOC with Ro = 0.5% for convenient comparing with previous research results. By using the new criteria, the ratio of wells with effective source rocks to the total wells changes from 33.3% to 58.3%. Figure 8 shows the comparison of effective source rock thickness between the previous predicted results and the current predicted results: Figure 8(a) is the thickness distribution of Middle–Upper Ordovician effective source rocks predicted by the previous identification criterion with TOC = 0.5%, their cumulative largest thickness is 250 m; Figure 8(b) is the thickness distribution of Middle–Upper Ordovician effective source rocks predicted by the new identification criterion with 0.2% < TOC < 0.5 %, changing with depth, and the cumulative largest thickness is 550 m. Thickness ratio of effective source rocks to the total source rock thickness changes from 7.8% to 46.1% in Tazhong area, Tarim Basin.

Figure 7.

Prediction of the effective source rocks of Middle–Upper Ordovician in Tazhong area. (a) Prediction and comparison of effective source rock thickness distribution vertically based on two different criteria of TOC: non-effective source rock is expressed by light gray with TOC <0.2%, effective source rock with 0.2% ≤ TOC ≤ 0.5% by middle gray, good effective source rock with TOC > 0.5% by dark. The thickness ratio of cumulative effective source rocks to total source rocks changes from 7.8% to 46.1% in Tazhong area. (b) Prediction and comparison of the effective source rock thickness distribution based on the measured TOC and the recovered TOC, the ratio of wells with effective source rocks to total wells in Tazhong area changes from 33.3% to 58.3%.

Figure 8.

Comparison of effective source rock thickness between the original predicted results and the current predicted results: (a) the thickness distribution of Middle–Upper Ordovician effective source rocks predicted by previous identification criterion with TOC > 0.5%, the largest thickness is 250 m; (b) the thickness distribution of Middle–Upper Ordovician effective source rocks predicted by the new identification criterion with 0.2% <TOC < 0.5%, changing with depth, the largest thickness is 550 m. The ratio of effective source rock changes from to the total source rocks changes from 7.8% to 46.1% in Tazhong area, Tarim Basin.

Reliability

In order to verify the accuracy of the conclusion, this paper re-evaluated the potential resources of Middle–Upper Ordovician in Tazhong area by the previous method (Jiang and Pang, 2011). After identifying the effective source rocks and predicting their distribution and volume (Vs), considering the weight ratio of organic materials (TOC), the hydrocarbon generation rate of unit weight of the organic materials (Rp) and the effective hydrocarbon expulsion ratio (Ke), finally calculated as the amount of hydrocarbon resources of Middle–Upper Ordovician source rock in Tazhong area, account for 9.8 × 10⁹ t. Taking out the hydrocarbon amount of 8.08 × 10⁹ t, which was the destroyed hydrocarbon amount by tectonic movement (Pang et al., 2010), the resource potential of the Tazhong area is up to 1.72 × 10⁹ t. After deducting the discovered oil and gas reserves of 1.04 × 10⁹ t, the remaining resource potential is predicted to be more than 0.68 × 10⁹ t, showing a good exploration prospect. Compared with the early results of resource evaluation, our predicted result increases by 65.4%. Over the past 10 years, significant progress has been made in oil and gas exploration in the Tazhong area, and a lot of new hydrocarbon reservoirs have been discovered. The distribution area of oil and gas has expanded by 67%, which confirms the reliability of hydrocarbon resource potential evaluation by the new method and new identification criterion of TOC with depth.

Discussion

It should be emphasized that we do not deny the importance of carbonate source rocks with high TOC (>0.5%), which are of high quality for hydrocarbon reservoirs. However, this study presents the evidence for the possibility of source rocks with low TOC as effective source rocks. At a depth deep enough, the source rocks with TOC < 0.5% do generate and expel hydrocarbons and contribute to hydrocarbon accumulations. The source rocks with TOC < 0.5% have the possibility to become the secondary source rocks. Also, some points need to be noted here. First, when we establish the identification standard of effective source rocks, we need lots of data like S₁, S₂, and TOC. So, this method can only be used in the high exploration areas. Without enough data, we cannot discriminate the hydrocarbon expulsion precisely. In addition, the evaporative hydrocarbons of S₁ are easily lost due to the inappropriate preservation measures. Therefore, the hydrocarbon-generation potential index by using S₁ is often lower than it is, which will underestimate the evaluation of hydrocarbon resources. Therefore, the source rocks with low TOC may provide a certain amount of oil and gas resources; we should consider more in-depth studies of low TOC source rocks (TOC < 0.5%) in the future.

Conclusions

Simple TOC index of source rocks in petroliferous basin cannot be used as the identification criterion for effective source rocks; it can neither reflect their generated hydrocarbon amount nor their expelled hydrocarbon amount.

Based on the hydrocarbon-generation potential index, this paper established the discrimination for the effectiveness of source rocks in Tazhong area, Tarim Basin, and the criterion are TOC ≥ 0.2% with depth Z > 5500 m, TOC ≥ 0.3% with depth Z > 5000 m, TOC ≥ 0.4% with depth Z > 4500 m, TOC ≥ 0.5% with depth Z > 4000 m.

The new identification criterion is applied to predict the distribution of the effective source rocks of the Middle–Upper Ordovician in Tazhong area, Tarim Basin, and evaluate the hydrocarbon potential resources. The undiscovered hydrocarbon amount is about 0.68 × 10⁹ t, verified by the exploration and discoveries in the past 10 years.

Highlights

The examples that TOC cannot be used to identify and evaluate effective source rocks are listed and the reasons are clarified in this paper.

The new method for identifying effective source rocks using the hydrocarbon expulsion threshold and the corresponding conditions is proposed.

The practical application of the new method in the Tazhong area, Tarim Basin, demonstrates the practicability and validity of the new method.

Footnotes

Acknowledgements

Our research work was supported by the National Basic Research Program of China (2011CB201100) and National Natural Science Foundation Project(U1262205).

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

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