Prediction of Favorable Positions for Hydrocarbon Accumulation Controlled by Oil-Source Faults: A Case of Dazhangtuo Fault in the Qikou Sag, Bohai Bay Basin, East China

Oil-source faults play an important role in controlling hydrocarbon accumulation, and traps near these faults are often the most favorable positions for hydrocarbon accumulation. However, not all nearby traps can accumulate oil and gas. Therefore, it is necessary to predict favorable positions of hydrocarbon accumulation controlled by oil-source faults. Based on the analysis of hydrocarbon accumulation patterns near oil-source faults, a new method coupling hydrocarbon transportation positions of oil-source faults and favorable traps to predict favorable hydrocarbon accumulation positions is proposed. In this method, the hydrocarbon transportation positions are identified by the paleo activity rate during the hydrocarbon accumulation period; the favorable traps are identified by superimposing the position of fault traps, the lateral sealing position of faults, and the distribution of sand bodies with the ability to store oil and gas. Finally, the sites of overlap between the hydrocarbon transportation positions and the favorable traps are regarded as favorable positions for hydrocarbon accumulation under the control of oil-source faults. This method is applied to predict the favorable positions of hydrocarbon accumulation in the Ban 2 oil group of the Shahejie Formation near the Dazhangtuo fault, in the Qikou Sag (Bohai Bay Basin, East China). The results showed that the favorable positions of oil and gas accumulation along the Dazhangtuo fault in the Ban 2 oil group are mainly distributed in the northeast and center of the fault, and match well with the locations of the discovered oil and gas. Therefore, this method is useful to predict favorable hydrocarbon accumulation positions controlled by oil-source faults.


INTRODUCTION
Hydrocarbon exploration practice shows that oil-source faults play an important role in controlling hydrocarbon migration and accumulation in the accumulation model of "lower-generation and upper-storage" (Hooper, 1991;Aydin, 2000;Zeng and Jin, 2003;Luo, 2010;Fossen and Rotevatn, 2016;Zhang and Fu, 2022). On the one hand, oil-source faults are the main pathways for vertical hydrocarbon migration (Hooper, 1991;Losh, 1998;Goldsworthy and Jackson, 2001;Luo, 2010;Jiang et al., 2018;. On the other hand, fault activities not only form fault-associated traps but also break rocks to form fractures that can provide storage space for oil and gas (Schlische, 1995;Jackson et al., 2006;Gong et al., 2019;Zeng et al., 2022). Therefore, traps near oil-source faults are often the most favorable positions for hydrocarbon accumulation. However, influenced by the differences in geological conditions, such as fault transportation capacity (Hooper, 1991;Jiang et al., 2018;, reservoir distribution (Luo et al., 2012;Zhou et al., 2019;Liu et al., 2020;Gong et al., 2021a), and fault lateral sealing ability (Zhang et al., 2010;Lyu et al., 2021), not all traps near the same oil-source fault can accumulate oil and gas, only some traps can form oil and gas reservoirs. Therefore, predicting favorable positions for hydrocarbon accumulation controlled by oil-source faults is necessary and has an important guiding significance for the hydrocarbon exploration of the accumulation model of "lower-generation and upper-storage." Regarding the controlling effect of oil-source faults on hydrocarbon accumulation and distribution, there have been many studies on faults transporting hydrocarbons (Aydin, 2000;Fu and Wang, 2019;Cong et al., 2020;Liang et al., 2022), fault zone structures (Allan, 1989;Bense, et al., 2003;, fault lateral sealing ability (Chen et al., 2010;Fossen and Rotevatn, 2016;Lyu et al., 2016), source-fault configurations , faultsand configurations Zhou et al., 2019;Luo et al., 2020), fault-caprock configurations (Kumar et al., 2019;Shi et al., 2019;Wang et al., 2021), the development degree of sand bodies (Luo et al., 2012;Fu and Yu, 2021), fault trap characteristics (Jackson et al., 2006;Hu and Lyu, 2019;Song et al., 2020), etc., many studies have confirmed that the hydrocarbon accumulation position near an oil-source fault is related to the transportation capacity of the oil source fault (Lampe et al., 2012;Jiang et al., 2017;Liu et al., 2017;Fu and Wang, 2018). Not all parts of an oil-source fault can vertically migrate hydrocarbon and there are dominant migration conduits. Sites with relatively high fault activity rates are conducive to the transportation of hydrocarbons and favorable locations for faultcontrolled hydrocarbon accumulation (Jiang et al., 2018;Cong et al., 2020;. In addition, under the action of regional tectonic activity, a large number of fractures can be formed in sand bodies near a fault, which is conducive to the lateral migration of the vertically moving hydrocarbons to the sand bodies (Luo et al., 2012;Wang et al., 2016;Yang et al., 2020;Gong et al., 2021b). Therefore, sand bodies deposited near an oil-source fault are favorable locations for hydrocarbon accumulation. Some studies on the distribution and effectiveness of traps reveal that traps are conducive to the accumulation of oil and gas only when the faults are sealed laterally (Zhang et al., 2010;Lyu et al., 2016;Hu and Lyu, 2019;Song et al., 2020). Thus, the lateral sealing location, which is favorable for hydrocarbon accumulation and preservation, is regarded as a favorable position for oil and gas accumulation.
The above studies have significantly deepened our understanding of the control of oil-source faults on hydrocarbon accumulation and improved the accuracy of hydrocarbon prediction near oil-source faults. However, existing studies on oil-source faults controlling hydrocarbon accumulation are more focused on the study of hydrocarbon transportation capacity and trap conditions (Luo, 2010;Lampe et al., 2012;Fu and Wang., 2018;Song et al., 2020;. Comprehensive studies of hydrocarbon transportation and traps are relatively scarce (Fu and Yu, 2021). There is still no uniform understanding and relatively effective methods for the prediction of favorable positions for hydrocarbon accumulation.
Therefore, this study aims to propose an applicable method of predicting favorable positions for hydrocarbon accumulation controlled by oil-source faults. To achieve this goal, the hydrocarbon accumulation pattern near oil-source faults is first analyzed. Then, a new method to predict favorable positions for hydrocarbon accumulation near oil-source faults is proposed and is applied to the Dazhangtuo fault (an oil-source fault) in the Qikou Sag, Bohai Bay Basin to verify the feasibility of the method. The results are helpful to guide hydrocarbon exploration near oil-source faults in petroliferous basins.

HYDROCARBON ACCUMULATION PATTERNS NEAR OIL-SOURCE FAULTS
In the accumulation model of "lower-generation and upperstorage," hydrocarbons generated in the underlying source rock first vertically migrate to the upper target reservoir along the dominant conduits of an oil-source fault; then, hydrocarbons in the reservoir migrate to nearby traps under the action of buoyancy. The main reason why hydrocarbons can accumulate near oil-source faults is that hydrocarbons transported by oilsource faults couple (match) with traps near oil-source faults (Fu and Yu, 2021). For a specific layer, there are three types of hydrocarbon accumulation patterns near oil-source faults: 1) There exists an effective trap near the dominant transportation conduit of an oil-source fault; thus, hydrocarbons can accumulate in the trap, such as in case A in Figure 1. 2) The position where the trap is located has no dominant transportation conduit, that is, there is no source of hydrocarbons, so hydrocarbon reservoirs cannot be formed in the trap, as shown in case B in  3) There are no effective traps near the dominant transportation conduit, the hydrocarbons will continue to migrate upwards; thus, hydrocarbon reservoirs cannot be formed in the target layer, as shown in case C in Figure 1. Therefore, only when fault-transported hydrocarbon positions are coupled with traps can hydrocarbons accumulate to form reservoirs. If either the hydrocarbons or the trap is absent; or even if both of them are present, but they are not coupled in space, hydrocarbon reservoirs cannot form. The coupling sites of hydrocarbon transportation positions and favorable traps are favorable positions for hydrocarbon accumulation controlled by oil-source faults.

PREDICTION METHOD
According to the above patterns, modified from the method in the literature of Fu and Yu (2021), a new method to predict favorable positions for hydrocarbon accumulation controlled by oil-source faults is proposed in this study. First, hydrocarbon transportation positions of oil-source faults and favorable traps near oil-source faults are identified. Then, by overlapping the identified hydrocarbon transportation positions and the traps, favorable positions for hydrocarbon accumulation are identified along with the areas of overlap. Therefore, the key contents of this method are the identification of hydrocarbon transportation positions and favorable traps.

Identification of Hydrocarbon Transportation Positions
First, the distribution of oil-source faults is determined according to the method in the literature (Fu and Wang, 2018), i.e., a fault that connects the source rock and target reservoir and stays active during the hydrocarbon accumulation period is regarded as an oil-source fault. Second, based on seismic data, the paleo fault throws of oilsource faults in the target layer during the hydrocarbon accumulation period are identified by using the maximum fault throw subtraction method (David and Bruce, 2009); then, the paleo fault throw is divided by fault activity time to obtain the paleo activity rate of different parts of the oilsource fault during the hydrocarbon accumulation period. Third, the minimum paleo activity rate (V min ) at discovered oil and gas wells in the study area is determined; this minimum value is the threshold required for oil-source faults to transport hydrocarbons, as shown in Figure 2A.
Only when the fault paleo activity rate is greater than the V min are the associated fractures in the fault zone developed; thereby, oil and gas can be transported along oil-source faults and be found in drilling wells. Otherwise, no oil and gas will be found. Lastly, areas where the paleo activity rate of oil-source faults is greater than the V min are identified to be favorable hydrocarbon transportation positions.

Identification of Favorable Traps
Favorable traps near oil-source faults can be identified based on the identification of the position of fault traps, the lateral sealing position of faults, and the position of sand bodies that can store oil and gas. The area where the above three positions overlap is a favorable trap that can entrap hydrocarbons ( Figure 3). The types of fault traps mainly include faulted anticlines, faultblock traps, and fault-screened traps. In terms of identifying the position of fault traps from the aspect of morphology, faulted anticlines, and fault-block traps can be directly identified from structural maps, while fault-screened traps can only be distinguished according to the configuration between the strata and the faults in seismic profiles.
The lateral sealing position of faults can be quantitatively determined by using the shale gouge ratio (SGR) method (Yielding et al., 1997;Song et al., 2020). First, the SGR of faults in the target layer is calculated using Eq. 1 with information from drilling and seismic data.
where SGR is the shale gouge ratio of fault rocks (%), H i is the thickness of the i-th layer that is disrupted by the fault (m), R i is the shale content of the i-th layer that is disrupted by the fault (%), L is the fault throw of the fault (m), and n is the number of layers disrupted by the fault. Then, the SGR at the tested wells in the study area is counted, and the minimum shale gouge ratio (SGR min ) at the oil and gas wells is taken as the threshold required for faults to laterally seal hydrocarbons, as shown in Figure 2B. This is because only when the shale gouge ratio of a fault is greater than the SGR min can oil and gas be trapped by the fault and accumulate in the trap; thus, oil and gas can be found by drilling wells here. Otherwise, no oil and gas will be found. Lastly, areas where the shale gouge ratio is greater than the SGR min are recognized to be the lateral sealing positions of faults.
To determine the position of sand bodies that can store oil and gas, drilling data should be used to calculate the sand net/gross ratio of the target layer near the oil-source faults. Then, the sand net/gross ratio of test wells in the target layer is counted, and the minimum sand net/gross ratio (S min ) at the oil and gas wells is regarded as the threshold required for sand bodies to store hydrocarbons, as shown in Figure 2C. This is because only when the sand net/gross ratio of a layer is greater than the S min can sand bodies have connectivity and store oil and gas; otherwise, oil and gas can hardly migrate from oil-source faults to sand bodies. Lastly, areas where the sand net/gross ratio of the target layer is greater than the S min are circled and considered to be the locations of sand bodies that can store oil and gas.

EXAMPLE APPLICATION
In this study, the Dazhangtuo fault (an oil-source fault) in the Qikou Sag, Bohai Bay Basin, East China is taken as an example to apply the above method to predict favorable hydrocarbon accumulation positions. Moreover, the prediction results and the discovered oil and gas are compared to verify the feasibility of this method.

Geological Setting
The Bohai Bay Basin is the largest petroliferous basin in eastern China, it has an area of approximately 2 × 10 5 km 2 (Zhou et al., 2012). The Bohai Bay Basin is divided into seven depressions by four uplifts, among which the Huanghua Depression is located in the center of the Bohai Bay Basin ( Figure 4B; Chen et al., 2014;Song et al., 2021). The Huanghua Depression is rich in oil and gas resources in the Cenozoic strata and comprises two secondary depressions, i.e., the Qikou Sag and the Cangdong Sag (Zhou et al., 2019). The Qikou Sag, the largest sag in the Huanghua Depression, is an intracontinental rift lake basin (Huang et al., 2012). The sag has undergone tectonic evolution from the Paleogene rift stage to the Neogene depression stage. During the rifting stage, the structural framework of the main sag, five sub-sags, and four bulges were formed ( Figure 4C; Huang et al., 2012;Zhou et al., 2019). The Palaeogene and Neogene stratigraphy in the Qikou Sag include, from bottom to top, the Shahejie Formation (Es), the Dongying Formation (Ed), the Guantao Formation (Ng), and the Minghuazhen Formation (Nm) ( Figure 4D; Huang et al., 2012;Song et al., 2022). The Es, which is subdivided into the third (Es 3 ), second (Es 2 ), and first (Es 1 ) members, is one of the main targets for hydrocarbon exploration in the sag. The Es 3 and Es 2 mostly consist of dark-gray mudstone and sandstone, representing a delta-lake-subaqueous fan sedimentary system. During the period of deposition of the Es 1 , the sag was dominated by a lake-delta-carbonate platform sedimentary system where mudstone, sandstone, and limestone were deposited (Song et al., 2022). Es 3 and Es 1 are regarded as the principal source rocks for crude oil ( Figure 4D; Wang et al., 2014;Zhu et al., 2022). Es 3 , Es 2 , and Es 1 are all favorable reservoirs, and many industrial oil and gas reservoirs have been discovered in the lower sub-member (Es 1 x) of the first member of the Shahejie Formation Shi et al., 2019). Hydrocarbon accumulation in the Qikou Sag mainly occurred from the end of the deposition of the Guantao Formation to the present (Jin et al., 2019). In this study, the Es 1 x is the study interval, and the Ban 2 oil group of the Es 1 x is further considered.
Due to multistage tectonic movements, a large number of master faults are developed in the sag (Zhou et al., 2012). Most of these faults are connected with the source rock of the Es 3 or the Es 1 to form oil-source faults; these faults include the Banqiao fault, the Dazhangtuo fault, the Nandagang fault, and the Gangdong fault. (Figure 4C). It has been demonstrated that the vertical migration of hydrocarbons along faults is the main hydrocarbon migration model of oil and gas in the Cenozoic strata (Zhou et al., 2019;Zhu et al., 2022). The discovered oil and gas are obviously controlled by oil-source faults. For example, under the control of the Dazhangtuo fault, oil and gas in the Es 1 x are differentially enriched near the fault. Therefore, the Dazhangtuo fault can be used as a good example for  Figure 4C). It has an extended length of 21.3 km and a dip angle of 55°-62°. The Dazhangtuo fault is a long-term developed normal fault and was active during the hydrocarbon accumulation period (from the end of deposition of the Guantao Formation to the present). Moreover, as shown in Figure 5A, the Dazhangtuo fault connects the source rock of Es 3 and the reservoir of Es 1 x. Therefore, it is an oil-source fault for the Es 1 x. According to exploration results, a large amount of oil and gas have been discovered in the Shahejie Formation near the Dazhangtuo fault, and the Ban 2 oil group of the Es 1 x is a principal producing layer. As can be seen from Figure 5B, the oil and gas discovered in the Ban 2 oil group near the Dazhangtuo fault are abundantly distributed in the northeast with a small amount in the southwest; this result is predominantly controlled by the Dazhangtuo fault. Therefore, whether it is possible to accurately predict the locations of favorable hydrocarbon accumulation spots controlled by the Dazhangtuo fault in the Ban 2 oil group is very important for guiding hydrocarbon exploration in this area.

Hydrocarbon Transportation Positions Along the Dazhangtuo Fault
First, fault throws from different survey lines of the Dazhangtuo fault in the Ban 2 oil group were calculated using 3D seismic data. Second, the maximum fault throw subtraction method was used to restore the fault throws to the paleo fault throws during the hydrocarbon accumulation period. Then, the paleo fault throw is divided by the fault activity time to obtain the paleo activity rate ( Figure 6). It can be seen from the results that the paleo activity rates of the Dazhangtuo fault are mostly greater than 5 m/Ma, and the areas with high paleo activity rates are distributed in the center of the fault, showing a decreasing trend from the middle to the ends of the northeast and southwest ( Figure 6). The minimum paleo activity rate required for oil-source faults to transport hydrocarbons is about 4 m/Ma in the Qikou Sag ( Figure 7A). Therefore, it is determined that except for the northeast and southwest of the Dazhangtuo fault, the rest of the fault are all locations where hydrocarbons were transported (Figure 6).

Favorable Traps Along the Dazhangtuo Fault
According to the above method, favorable traps along the Dazhangtuo fault are identified from three aspects: the position of fault traps, the lateral sealing position of the fault, and the distribution of sand bodies that can store oil and gas.
In terms of the position of the fault traps, it can be seen from Figure 5B that there are fault anticlines in the northeast and southwest of the hanging wall, and fault-block traps in the northeast of the footwall of the Dazhangtuo fault in the Ban 2 oil group. Although there are no structural traps developed in the   Frontiers in Earth Science | www.frontiersin.org June 2022 | Volume 10 | Article 908812 7 using drilling and seismic data. Then, the SGR was calculated using Eq. 1 ( Figure 8A). According to the relationship between the SGR and the oil and gas distribution in the Qikou Sag, the minimum shale gouge ratio required for faults to laterally seal oil and gas is about 24% ( Figure 7B). Therefore, the lateral sealing positions of the Dazhangtuo fault are mainly distributed in its central and northeastern parts ( Figure 8A).
In terms of the distribution of sand bodies that can store oil and gas, the sand net/gross ratio of the Ban 2 oil group near the Dazhangtuo fault was calculated using drilling data (e8B). According to the relationship between the sand net/gross ratio of the Ban 2 oil group and the oil and gas distribution in the Qikou Sag, the minimum sand net/gross ratio required for sand bodies to store oil and gas is about 15% ( Figure 7C). Based on this, it is  Frontiers in Earth Science | www.frontiersin.org June 2022 | Volume 10 | Article 908812 8 determined that sand bodies with the ability to store oil and gas in the Ban 2 oil group near the Dazhangtuo fault are mainly located in the central and northeastern parts of the hanging wall and the northeastern part of the footwall ( Figure 8B).
By overlapping the position of fault traps, the fault lateral sealing position, and the distribution of sand bodies with the ability to store oil and gas, favorable traps near the Dazhangtuo fault in the Ban 2 oil group were identified, these traps are mainly distributed in the central and northeastern portions of the fault (Figure 9).

Favorable Hydrocarbon Accumulation Positions Along the Dazhangtuo Fault
By superimposing the identified hydrocarbon transportation positions and the favorable traps along the Dazhangtuo fault, it was found that the favorable positions (coupling sites) for hydrocarbon accumulation in the Ban 2 oil group are mainly distributed in the northeast and center of the fault ( Figure 10). Meanwhile, the discovered oil and gas near the Dazhangtuo fault are just located at or near the favorable hydrocarbon accumulation positions identified by the newly proposed method ( Figure 10). However, almost all of the water-bearing wells are located in unfavorable positions for hydrocarbon accumulation. Due to a lack of oil-gas supply or a lack of favorable traps, no oil and gas reservoirs are present in unfavorable positions for hydrocarbon accumulation. These include Well B59 which is not near hydrocarbon transportation position and Wells B801 and B84 which are not near favorable traps ( Figure 10). Therefore, the sites where hydrocarbon transportation positions overlap with favorable traps are the main locations for hydrocarbon accumulation. This is because only in the overlapping positions could hydrocarbons generated in the source rock of the Es 3 migrate to the overlying Ban 2 oil group through conduits of the Dazhangtuo fault and then subsequently laterally migrate to favorable traps to form oil and gas reservoirs.
In addition, due to the inaccurate identification of the fault traps, the area of proven oil and gas is larger than that of the predicted favorable positions for hydrocarbon accumulation. At present, the accurate identification of traps is still a key problem in hydrocarbon exploration, and many factors may lead to the inaccurate identification of traps, such as the accuracy of the structural map, the prediction accuracy of sand bodies, and the recognition accuracy of fault and sand configuration. Therefore, on the basis of adopting this method, the identification of favorable traps should be strengthened, which is also the aspect that the authors want to improve later.

CONCLUSION
1) The sites where hydrocarbon transportation positions overlap with favorable traps are the favorable locations for hydrocarbon accumulation controlled by oil-source faults. 2) A method of coupling hydrocarbon transportation positions and favorable traps to predict favorable hydrocarbon accumulation positions controlled by oil-source faults is established. In the method, the paleo activity rate is used to determine hydrocarbon transportation positions; the morphology of fault traps, fault lateral sealing position, and the distribution of sand bodies storing oil and gas are superimposed to obtain the locations of favorable traps. 3) The favorable hydrocarbon accumulation positions along the Dazhangtuo fault in the Ban 2 oil group in the Qikou Sag are the northeastern and central parts, which is consistent with the discovered oil and gas distribution. 4) The successful application of the proposed method to the Dazhangtuo fault shows that it is suitable for predicting favorable hydrocarbon accumulation positions controlled by oil-source faults in petroliferous basins.

DATA AVAILABILITY STATEMENT
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

AUTHOR CONTRIBUTIONS
FJ and GF presented the idea and design of the research. FJ wrote the manuscript and led the data analysis and interpreted the results with HW and XH. GH, XD, and WZ contributed to review and writing of the revised manuscript. All authors contributed to manuscript revision and approved it for publication.