The Depth Limit for the Formation and Occurrence of Fossil Fuel Resources

. Fossil resources are valuable wealth given to human beings by nature. Many mysteries related to them have been revealed such as the origin time and distribution area, but their vertical distribution depth has not been confirmed for people’s different understanding of their origin and the big depth variations from basin to basin. Geological and geochemical 15 data of 13,634 source rock samples from 1,286 exploration wells in six representative petroliferous basins are examined to study their Active Source Rock Depth Limits (ASDL), defined in this study as the maximum burial depth of active source rocks beyond which the source rocks no longer generate or expel hydrocarbons and become inactive, to identify the maximum depth for fossil fuel resources distribution. Theoretically, the maximum depth for the ASDLs of fossil fuel resources ranges from 3,000 m to 16,000 m, while their thermal maturities (Ro) are almost the same with Ro≈3.5±0.5%. A 20 higher heat flow and more oil-prone kerogen are associated with a shallower ASDL. Active source rocks and the discovered 21.6 billion tons of reserves in six representative basins in China and 52,926 oil and gas reservoirs in the 1,186 basins over the world are found to be distributed above the ASDL, illustrating the universality of such kind of depth limit. The data are deposited in the repository of the PANGAEA database: https://doi.pangaea.de/10.1594/PANGAEA.900865 (Pang et al., 2019). 25 according to international standard. The proved coal reserves of anthracite C, B and A in China and world are projected according to their variation tendency. This study also analysed the drilling results for 116,489 samples of target layers from 4,978 exploration wells of the six basins in China (Fig. 7). Exploration practices show that the reservoirs in the six basins are undoubtedly distributed above the ASDL, reflecting the control of ASDL on the formation and distribution of hydrocarbon reservoirs. The probability of drilling oil and gas reservoirs decreases with increasing burial depth, whereas the probability of drilling dry layers increases. 30 At some depth, the probability of drilling oil or gas reservoirs decreases to zero, this depth is regarded as the Hydrocarbon Reservoir Depth Limit (HRDL). The HRDLs six basins are Fig. 7 by a dashed line. The ASDLs deduced (“S 1 S 2 (Table 2) Fig. 7

gas from organic matter is the process of condensation of the aromatic nuclei that enriches carbon by deoxygenation and dehydrogenation. The process can be experimentally investigated by measuring the decrease in the H/C and O/C ratios (Tissot et al., 1974). In theory, kerogen eventually evolves into graphite with increasing thermal maturity with the H/C and O/C ratios drop to 0. This indicates that the active source rocks no longer produce hydrocarbons and thus reach the ASDL. 25 The Rock-Eval pyrolysis parameters can also be utilized to measure the ASDL such as the hydrocarbon generation potential index ("S1 + S2"/TOC). "S1" is the amounts of hydrocarbons released from a source rock sample when it is heated from room temperature to 300 °C, and "S2" is the amount released from 300 °C to 600 °C in Rock-Eval pyrolysis system.
TOC is the measured total organic carbon in the source rock (Espitalie et al., 1985). The concept of hydrocarbon generation potential index was proposed by Pang et al. (2005) to measure the quantity of hydrocarbons that can be generated from a 30 single unit of organic carbon. The hydrocarbon generation potential index of the source rocks first increases and then decreases with increasing burial depth. The turning point of hydrocarbon generation potential index corresponds to the https://doi.org /10.5194/essd-2019-72 Open hydrocarbon expulsion threshold (HET) which was proposed by Pang et al. (1997). It represents that the source rocks begin expelling hydrocarbons. As the hydrocarbon expulsion of the source rock increases, the hydrocarbon generation potential index of the source rock gradually depletes, and when the hydrocarbon generation potential index approaches zero, the source rock can no longer expel hydrocarbons and reaches the ASDL. During the evolution of hydrocarbon generation potential index, the hydrocarbon expulsion ratio (Qe), hydrocarbon expulsion rate (Ve) and hydrocarbon expulsion 5 efficiency (Ke) of source rocks also change with regularity. Hydrocarbon expulsion ratio represents the amounts of hydrocarbons expelled from a single unit of organic carbon. Hydrocarbon expulsion rate represents the hydrocarbons expelled from a single unit of organic carbon when burial depth increases by 100 m, and the hydrocarbon expulsion efficiency represents the ratio of cumulative amounts of hydrocarbons expelled from source rocks to cumulative amounts of hydrocarbons generated. When the source rocks reach the state of ASDL, Qe and Ke approach the constant values and Ve 10 approaches the value of zero.
In addition, hydrocarbon generation is the transformation of original organic matter (kerogen) to transitional compounds and finally to hydrocarbons (Behar, et al., 2006). Therefore, when the transitional compounds or the residual hydrocarbons ("S1" or "A") tend to disappear indicating that the hydrocarbon generation potential is exhausted. "A" is the amounts of hydrocarbons extracted by chloroform solution from a source rock sample. Because some non-hydrocarbon 15 matter can also be extracted, "A" values are generally larger than "S1" values. The residual hydrocarbon content index ("S1"/TOC or "A"/TOC), representation of quantity of hydrocarbons that are retarded in a single unit of organic carbon, can therefore be used to measure the ASDL. Previous study (Pang et al., 2005) indicates that the source rocks entered hydrocarbon expulsion threshold when the residual hydrocarbon content index reached the maximum value and since then, such index began to decrease. The source rocks finally enter the state of ASDL when the residual hydrocarbon content index 20 decrease to a minimum value. This study analysed trends of these parameters as a function of depth (D) or thermal maturity illustrated by measured vitrinite reflectance (Ro). The ASDL is thus represented as the level of D or Ro where the indexes of H/C, O/C, "S1 + S2"/TOC, "S1"/TOC, "A"/TOC, and Ve approach zero, and Ke approaches a constant value.

ASDLs in the Six Representative Basins 25
In this study, we characterized the ASDLs in the six representative basins of China. The main text takes the Junggar Basin located in western China as an example to illustrate the process of characterization (Fig. 2). The same methods were applied to study all the major source rocks from the six representative basins, and the results are shown in Figs. S1-S5 and  hydrocarbon amounts in source rock samples, represented respectively by "A"/TOC or "S1"/TOC, with burial depth. Initially, both the mean and the variance of the residual amounts increase with depth, because hydrocarbons are generated but not yet expelled out of the source rocks. A maximum is reached at a depth of 3,500 to 4,000 m or at Ro≈1.0%, which is corresponding the hydrocarbon expulsion threshold. With further increase of depth, the amount of residual hydrocarbon starts decreasing, and finally reaches zero at the ASDL of 7,850-7,960 m with corresponding Ro of 3.0%. Figure. 2c shows 5 the change of hydrocarbon generation potential index, ("S1 + S2")/TOC, hydrocarbon expulsion ratio (Qe), hydrocarbon expulsion rate (Ve) and hydrocarbon expulsion efficiency (Ke) of the source rock samples with increasing burial depth.
These results point to an ASDL of 8,200 m with corresponding Ro of 3.0%, in good agreement with the values obtained in Fig. 2a and 2b. Besides, the HET is determined to be D of 3,000 m and Ro of 0.9%, and the expulsion peak occurs at D of 4,500 m and Ro of 1.3%. 10

Figure 2:
The identification of ASDL in the Junggar Basin using different indicators. a, the variation of H/C ratios with depth and identification of ASDL. b, the variation of residual hydrocarbon amounts (represented by "A"/TOC and "S1"/TOC) with depth. c1, the variation of hydrocarbon generation potential index (represented by "S1 + S2"/TOC) with depth and identification of ASDL; c2, the variation of Qe with depth and identification of ASDL; c3, the variation of Ve with depth 15 and identification of ASDL; c4, the variation of Ke with depth and identification of ASDL.
According to the results of ASDLs in the six representative basins (Table 2), three kinds of features of ASDLs can be drawn from them. First, the ASDLs of the same basin derived from the six indexes are same or very close in values. For example, the derived depths of the ASDL in the Junggar Basin vary from 7,850 m to 8,450 m with an average value of 8,168 20 m and a deviation of 7.6%. Second, ASDLs in different basins can be very different. ASDLs of the six representative basins range between 5,280 m and 9,300 m with an average value of 7,094 m and a deviation of >76%. Third, for all ASDLs of the six representative basins, the corresponding thermal maturities have much smaller variation than the depths. Ro values vary from 3.0% in the Junggar Basin to 4.0% in the Songliao Basin, with an average of 3.5% among the six basins and a deviation of 33.3% which is much smaller than the 76.1% deviation of the depths. This implies that ASDL is mainly controlled by the 25 thermal maturity of source rocks. The average thermal maturity level of 3.5% can be regarded as the identification criterion for ASDL in general geological settings.

Organic Matter Type
The original organic matter in source rocks, named as kerogen, is generally classified into three types based on its 30 origin (Peters, 1994;Tissot et al., 1974). It has different organic element composition and different pyrolytic parameters, and therefore has different hydrocarbon generation potential. The hydrocarbon generation potential index of different type source The trends of hydrocarbon generation potential index of different organic matter types varying with thermal maturity (Ro) are very similar: the index increases first and then decreases with increasing Ro. The intercept of the dashed lines on the vertical axis marks the ASDLs for different organic matter types. It was found that source rocks with type I (oil-prone), type 5 II, and type III (gas-prone) of kerogen reach ASDLs at Ro of about 3.0%, 3.5% and 4.0%, respectively. This indicates that the oil-prone source rocks are more likely to reach the ASDL and stop generating and expelling hydrocarbons at a shallower burial depth under similar geological conditions. the hydrocarbon generation potential index of different types of source rocks were plotted versus Ro. a, the variation of hydrocarbon generation potential index ("S1 + S2"/TOC) with Ro for type I source rocks and identification of corresponding ASDL. b, the variation of hydrocarbon generation potential index ("S1 + S2"/TOC) with Ro for type II source rocks and identification of corresponding ASDL. c, the variation of hydrocarbon generation potential index ("S1 + S2"/TOC) with Ro for type III source rocks and identification of corresponding ASDL. 15

Heat Flow and Geothermal Gradient
ASDL is shallow in petroliferous basins with high heat flow and high geothermal gradient. The ASDLs in the six basins span from 5,400 m to 9,300 m as determined from hydrocarbon generation potential index (Fig. 4). The basins in western China are characterized with low heat flow and low geothermal gradient (1.5-2.8 °C/100 m) and thus have the deepest ASDLs ranging from 8,200 m to 9,300 m. The basins in eastern China are of high heat flow and high geothermal gradient 20 (3.0-4.2 °C/100 m) and then have the shallowest ASDLs, ranging from 5,400 m to 5,900 m. The basins in central China are moderate in terms of heat flow and geothermal gradient, and the depths of ASDLs change from 6,600 m to 7,700 m. In addition, it was found that the burial depths corresponding to the HET vary similarly: lower burial depths relate to higher heat flow and higher geothermal gradient (Fig. 4).
In fact, the ASDL is also influenced by some other factors, such as tectonic uplift and the stratigraphic age of source 25 rocks. As previous stated, the ASDL is mainly characterized by thermal maturity, and Ro= 3.5% is regarded as general threshold for ASDL in common geological settings. However, the corresponding depth of ASDL for different source rock layers is highly variable. Due to the irreversible nature of the vitrinite reflectance (Hayes, 1991;Peters, 2018), the corresponding depth of ASDL for those older source rocks that was historically uplifted after reaching the ASDLs (Ro=3.5%) is relatively shallower compared with those younger source rocks. The Sichuan basin that experienced several stages of 30 tectonic uplift in the geological history epitomizes the influence of these factors on ASDL. For example, the Ro of the upper Triassic source rock is about 1.0% at the depth of ~2000 m in the southern Sichuan basin . At the same burial depth of southern Sichuan basin, however, the Ro of the lower Triassic source rock can reach about 2.0% (Zhu et al.,

Quantitative prediction of ASDLs 10
According to the relationship of ASDLs with heat flows and kerogen types discovered with six basins in China, a quantitative model was established by statistical analyses using software Origin 2017 to predict ASDLs in different basins of the world using the available data. We analyzed the maximum depth of ASDLs as a function of heat flow with a linear model. The ASDL for each basin was represented by the average depth obtained from various indicators, and the heat flow used in the model was the average value of each basin (Table 1). We found a significant interaction between ASDLs and 15 heat flows with coefficients larger than 0.9 (Fig. 5a). It was discovered that ASDLs for basins worldwide could range between 3,000 m and 16,000 m. Typically, ASDLs are less than 5,500 m in basins with high heat flow (>70 mW/m 2 ), and are larger than 9,000 m in basins with relatively low heat flow (<40 mW/m 2 ). Thus, the heat flow played an important role in variation of ASDLs, and the equation could be utilized to predict the ASDLs of basins from the documented heat flow.
Given that the depth of ASDL is also influenced by organic matter type according to Fig. 3, we further analyzed the effects 20 of organic matter type on ASDL by adding the hydrogen index, indicator of organic matter type, in the linear model. In our study, to quantize the influence of organic matter types on ASDLs, the type I, I-II, II, II-III and III kerogen corresponded to where, ASDL is the active source rock depth limit, m; HI is the hydrogen index value of the major source rock in a basin, mg HC/g TOC; HF is the average heat flow value of a basin, mW/m 2 .
We predicted the depth value of ASDL by using Eq. (1). The parameters such as heat flow and organic matter types for each basin were shown in Table 1. The result showed a strong correlation between the depth of ASDLs and measured depth 30 of ASDLs, represented by the average values of the results obtained from different methods (Table 2) (Fig. 5b), indicating that the Eq. (1) is reliable. Such kind of understanding with practical implications provides the geological basis and boundary condition for the prediction of fossil fuels distribution and the evaluation of its potential.

ASDL controlling the vertical distribution of fossil fuel resources
Fossil fuel resources are formed from organic matter in the course of millions of years. They are currently the primary energy sources in the world, and can be utilized for various aspects in the industry. Oil and gas are the products during the 10 evolution of organic matter, while coal is the residue of organic matter. On the other side, the ASDL is the critical condition or the dynamical boundary at which oil and gas expulsion ends. It controls the formation and distribution of favourable hydrocarbon reservoirs. Once the burial depth of organic matter exceeds the ASDL, the oil and gas could no longer be produced from the source rocks, and the coal which has been evolved to graphite would also lose their industrial value as fuel. Theoretically, the depth corresponding to ASDL represents the maximum depth of the formation and distribution of 15 fossil fuels. According to Fig. 6, 97.7% of coal resources in China and 97.3% of recoverable coal reserves over the world are distributed above the ASDL corresponding to Ro of 4.0% (CCRR, 1996;CNACG, 2016;Conti et al., 2016). Therefore, ASDL controls the maximum depth of hydrocarbon reservoir distribution, including oil, gas and coal. This study also analysed the drilling results for 116,489 samples of target layers from 4,978 exploration wells of the six basins in China (Fig. 7). Exploration practices show that the reservoirs in the six basins are undoubtedly distributed above the ASDL, reflecting the control of ASDL on the formation and distribution of hydrocarbon reservoirs. The probability of drilling oil and gas reservoirs decreases with increasing burial depth, whereas the probability of drilling dry layers increases. 30 At some depth, the probability of drilling oil or gas reservoirs decreases to zero, this depth is regarded as the Hydrocarbon Reservoir Depth Limit (HRDL). The HRDLs of the six basins are marked in Fig. 7 as yellow dots and connected by a dashed red line. The ASDLs deduced from ("S1 + S2")/TOC (Table 2)  Meanwhile, according to the vertical distribution characteristics of proved hydrocarbon reserves, it is discovered that all proved hydrocarbon reserves in the six representative basins are controlled by HRDL which is above ASDL (Fig. 7; Fig. S6).
This means a basin's HRDL is controlled by its ASDL and should always be above the ASDL. The currently discovered natural gas hydrate over the world are also distributed above the ASDL with corresponding Ro < 4.0% (Dai et al., 2017). We further extended the research to 52,926 reservoirs in 1,186 basins over the world recorded in IHS (2010). HRDL for each 5 basin was derived from the actual reservoir depth data in IHS (2010) using the same way as described in the previous paragraph (Fig. 7) and the results were shown in Fig. 8. ASDL for each basin was assumed to be at Ro of 3.5%, and the corresponding depth can be calculated from the documented heat flow of that basin. We found that HRDL (represented as depth) is universally above ASDL for all the basins.  Hydrocarbon is generally classified in two big categories as natural gas and liquid petroleum, which have distinct 25 physical properties. By definition, ASDL marks the end of generation of any hydrocarbon from source rocks. This concept can be modified to incorporate the types of hydrocarbons, such as ASDL for gas and ASDL for oil. ASDL for oil indicates that the source rocks can no longer generate oil and is named oil supply limit, while ASDL for gas indicates that the source rocks can no longer generate gas and is named gas supply limit. The hydrocarbon components are mainly liquid oil and gaseous hydrocarbons when the thermal maturity is low. The gaseous hydrocarbons become the dominant components with 30 nearly no liquid oil when the thermal maturity is high. Therefore, theoretically speaking, the burial depth and thermal maturity corresponding to ASDL for oil should be lower that of ASDL for gas. To investigate the ASDLs for different fluids, the high temperature (room temperature to 600 °C) and high pressure (50 MPa results, source rocks reached oil supply limit at Ro of about 2.0% (Fig. 9), and the same source rocks reached gas supply limit at Ro of 3.0 to 4.0%, which is also the overall ASDL. rate with Ro and identification of oil supply limit (ASDL for oil). b, the variation of gas production rate with Ro and identification of gas supply limit (ASDL for gas) which varies from Ro of 3.0% to Ro of 4.0%.
Besides, Pang et al. (2005) proposed the concept of hydrocarbon expulsion threshold, which marks the starting point of source rocks expelling hydrocarbons at certain depth. The HET, oil supply limit and gas supply limit divide a basin into three 10 regions along the vertical direction, and they control the types of hydrocarbon reservoirs and their distributions (Fig. 10).
The top area (blue area in Fig. 10) is favourable for hydrocarbons migrating upward to form conventional reservoirs in traps, and the source rocks in this area do not yet expel hydrocarbons. The middle area (pink area in Fig. 10) is favourable for source rocks to generate, expel and retain hydrocarbons to form various kinds of oil reservoirs, and this area also supplies hydrocarbons that migrate into the top area. The bottom area (yellow area in Fig. 10) is favourable for source rocks to 15 generate, expel and retain natural gas to form mainly unconventional resources. Figure.

Conclusions
(1) ASDLs, the maximum burial depth for source rocks to generate and expel oil and gas due to exhausting of hydrocarbon generation potentials, commonly exist in petroliferous basins. The thermal maturity of 3.5% can be regarded as the ASRL in general geological conditions.
(2) The ASRLs of all basins over the world vary from 3,000 m to 16,000 m, and the variation is mainly influenced by 30 different heat flows and different organic matter types. As the heat flow gets lower and the organic matter gets more gasprone, the ASDLs become deeper.