Large scale gas fields in the world are mainly distributed in marine sedimentary basins,for example, the North-South Pars gas field in the Persian gulf, the Urengoy gas field in the west Siberian basin and the Gorgon gas field in the northwest shelf of Australia with huge natural gas geological reserves^[1,2,3]. The number of lacustrine sedimentary basins is relatively small in foreign countries. The semi-deep to deep lacustrine source rocks abroad are mainly distributed in the passive continental margins on both sides of the South Atlantic Ocean, the Meso-Cenozoic rift in Africa, the Maracaibo basin in Central America and Southeast Asia. These areas have mainly oil fields but few large gas fields discovered. So far, two large gas fields, one in Gabon Basin in West Africa and one in Sumatra Basin in Southeast Asia, have been discovered. However, the gas in Gabon Basin is oil-cracking gas^[4], while the main gas source rocks of Paleogene Lemat Formation in Sumatra Basin mainly consist of shale and coal^[5,6], and the gas in the basin is humic gas or coal-type gas. Another important feature of the distribution of large gas fields in China and abroad is that they occur in tectonically stable areas, or in the unstable areas with thick gypsum caprocks developed. The giant North-South Pars gas field in the Persian Gulf is not only located in the tectonically stable area, but also has several sets of extremely thick gypsum caprocks. The Urengoy gas field in the west Siberian Basin and the Gorgon gas field in the northwest shelf of Australia with huge natural gas reserves are located in structurally stable craton basins. Although the Keshen gas field in Tarim Basin and the Puguang gas field in Sichuan Basin are located in the tectonically active region, regional gypsum caprocks have a maximum thickness of 1000 m and 500 m in these areas, respectively^{[7,8,9,10,11]}.

The gas field with proved gas reserves greater than 300×10⁸ m³ in China is named a large gas field. The gas sources rocks of them are mainly marine or transitional facies source rocks, and the gases produced from them are largely coal-derived gas, oil-cracking gas and biogas. For example, Sulige gas field in Ordos Basin and Kela-2 gas field in Tarim Basin have coal-derived gas generated by Carboniferous-Permian coal source rocks, while the Puguang and Anyue gas fields in Sichuan Basin have oil-cracking gas^[7,8,9]. By 2016, large coal- derived gas fields in China had proved geological reserves of 76900×10⁸ m³, accounting for more than 70% of the proved natural gas geological reserves in China. The large oil-cracking gas fields have about 25.8% of the total proved gas reserves, and are largely in deep marine basins. The biogas reserves accounted for only 2.7%, which mainly exist in Sebei No.1 and Sebei No.2 gas fields in Qaidam Basin. Oil-prone pyrolysis gas from kerogen is small in quantity^[6]. The lacustrine source rocks in China mainly deposited during Mesozoic to Cenozoic and are distributed in offshore areas, eastern rift system, central transition area, and western compression area^[10,11]. The hydrocarbon discoveries in the Bohai Bay Basin of eastern China are dominated by oil and supplemented by natural gas, while few large-scale natural gas fields have been found.

The Bohai Bay Basin is surrounded by Jiaodong Uplift and Liaodong Uplift in the east, Taihang Mountain Uplift in the west, Western Shandong Uplift in the south, and Yanshan Fold Belt in the north. It is a Meso-Cenozoic basin developed on the basis of Paleozoic deposition on Sino-Korean Paraplatform and subsequent tectonic movements during Indosinian to Yanshan period^[12]. Through more than 60 years of exploration and development, under the guidance of “terrestrial oil-generation” theory, a series of large oil fields with largely oil reserves have been discovered; seven major oil regions, including Shengli, Liaohe, Dagang, Huabei, Jidong, Zhongyuan and Bohai oil fields, have been built in the Bohai Bay Basin, making it one of the most important petroliferous basins in China, with oil reserves and production both accounting for more than one third of the total in China. The nature gas has also been found in the Liaohe depression, Liaodong Bay depression, Bozhong depression, Huanghua depression and Linqing depression, of which Banqiao gas field and Qianmiiqiao gas field both have reserves of more than 200×10⁸ m^3[12,13].

A lot of related research work on nature gas has been carried out in the Bohai Bay Basin^{[12,13,14,15,16,17,18,19]}. The source rocks of continental rift Bohai Bay Basin are mainly middle-deep Paleogene lacustrine mudstone, with organic matter abundance of 1.3%-5.1%, 3.0% on average, and type Ⅱ₂-Ⅱ₁ kerogens which generally generate less gas than the traditional type Ⅲ kerogen^{[14,15,16,17,18,19]}. Formations in the Bohai Bay Basin are largely continental deposits with frequent facies change and abundant faults due to intense tectonic activities. The Cenozoic strike- slip and extension movements, especially, caused severe damage to the caprocks, so gas accumulated earlier might dissipate^{[20,21,22,23,24,25]}. In conclusion, poor preservation conditions make it difficult for the gas to accumulate effectively in most parts of the Bohai Bay Basin, or only accumulate into small and medium-sized gas fields. Therefore, the research on the formation conditions of large-scale gas fields in the Bohai Bay Basin has always been a scientific challenge for the industry.

Although the conditions for the formation of large-scale gas fields in the Bohai Bay Basin are harsh^[13], we think large-scale natural gas fields can be formed in areas with "one core element and two key factors" in the oil-prone Bohai Bay Basin after examining more than 60 sags (Fig. 1), the accumulation dynamics, structure and sedimentation in the Basin. The one core factor is strong sealing of regional "quilt-like" overpressured mudstone. The two key factors are the rapid maturation in the late stage and high-intensity gas generation of source rocks and large-scale reservoirs.

The strong sealing of regional "quilt-like" overpressured mudstone is the core element for the formation of large natural gas accumulation. The gas in large natural gas reservoirs is easy to escape due to the higher gas concentration and temperature than that in overlying strata^{[26,27,28,29]}, which results in reduction of geological reserves of natural gas. For example, the study on Liujiazhuang gas field in Ordos Basin shows that this gas field had natural gas geological reserves of about 450×10⁸ m³ 50 Ma ago, but it has only 1.9×10⁸ m³ at present^[30,31]. Because gas dissipates much faster than oil, it has more demanding requirements on caprock to form accumulation. Therefore, large-scale natural gas fields are mainly located in structurally stable areas with excellent preservation conditions, or structurally active areas with thick gypsum caprock^{[31,32,33,34]} (Table 1).

Although the Bohai Bay Basin has experienced strong tectonic activities and has no regional thick gypsum, it is found that the thick overpressured mudstone of Paleogene Shahejie and Dongying Formation can act as good caprocks for large gas accumulation.The third member of Shahejie Formation and the lower member of Dongying Formation deposit in the semi-deep lake and deep lake sedimentary environment characterized by deep water and weak hydrodynamic conditions favorable for the formation of high quality caprock, so the mudstone layers formed are pure, widely distributed, continuous and thick, with an average thickness of 500 m, and maximum thickness of more than 2500 m. Under the joint effect of undercompaction and late stage hydrocarbon generation, especially in the stage of large-scale natural gas generation from source rock, the mudstone reached overpressure rapidly, with pressure coefficient of 1.2-2.0^[35] and displacement pressure between 4.81-27.91 MPa, 10.24 MPa on average, forming high-quality regional caprock^[36,37] (Fig. 2).

Although the fault activities in the Bohai Bay Basin were strong on the whole, they could be divided into early and late stages. The overpressured mudstones in several sags have not been damaged and can still act as effective sealing caprocks for large-scale natural gas accumulations (Fig. 2). Before the sedimentary period of Dongying Formation, a large number of faults developed in the sags. These faults were reactivated in Neogene but differed widely in active intensity. In some sags, such as Nanpu, Qikou and Huanghekou, faults with strong activity in the late stage can break through the mudstone of Dongying Formation, making the regional overpressured mudstone unable to seal scale gas accumulations anymore. In contrast, in some other sags, such as Bozhong sag with the thickest Cenozoic strata, although faults developed in the late stage, they haven’t cut through the overpressured mudstone (Fig. 2), so the regional mudstone "quilt" has been preserved to become a high-quality seal for natural gas. In the third type of sags such as Liaodong Bay, the faults were hardly active in the late stage, and the regional overpressured mudstone "quilt" has been remained intact to act as effective caprock of natural gas reservoirs. Besides, affected by the sandstone content of Dongying Formation, the different sags differ widely in distribution range of overpressured mudstone. In the area with abundant source supply, the sandstone is widely distributed, so the overpressured mudstone is not developed or only developed locally and can’t make an effective caprock for large-scale natural gas reservoirs.Taking Liaozhong sag as an example, the regional overpressured mudstone caprock controls the distribution of oil and gas significantly. The source rocks in the north and south areas of Liaozhong sag are similar in parent material (Ⅱ₁-Ⅱ² kerogen), burial history, present burial depth and gas generation intensity. But exploration results show the north part has rich natural gas, where Jinzhou 20-2 condensate gas field and Jinzhou 25-1 South Oil and gas field with gas cap have been discovered, while the south part has more oil but no gas discovered. Close examination reveals that the differential distribution of Paleogene regional caprock is the root cause of the difference of hydrocarbon accumulations between the north and south of Liaodongw Bay sag. In the north, the Paleogene is mainly composed of overpressured thick mudstone, with a pressure coefficient of 1.2-1.8, which makes a set of thick "quilt" covering the source rock, stopping the vertical migration of oil and gas, so the oil and gas can only migrate laterally to the high part of the uplift to form natural gas reservoirs. On the contrary, in the south part, the Paleogene is mainly composed of sand and mud interbeds with small pressure coefficient, which can’t seal the natural gas to form large-scale reservoirs.

1.2.1. Late-stage rapid maturation and high-intensity gas generation of source rock

High-intensity gas generation is the most fundamental factor for gas accumulation. Nature gas is easier to dissolve, diffuse and volatilize than crude oil, so to form large gas accumulation, there must be not only good seal condition and high-intensity gas generation, but also sufficient and continuous gas supply, especially rapid and concentrated supply in late period^{[38,39,40,41]}. Dai et al. ^[33] reveals that forming large and medium gas field requires gas generation intensity greater than 20×10⁸ m³/km², and the higher the gas generation intensity and the later the major gas generation period, the more conducive it is for large gas field to be formed. Compared with gas-prone basins in China and abroad, the Bohai Bay Basin is medium in buried depth, and the main gas source layer of Shahejie Formation has an average organic matter abundance of 3% and mainly types Ⅱ₂—Ⅱ₁ kerogens which are more likely to produce oil than the traditional type III kerogen, so the Bohai Bay Basin is more likely to generate oil, and has overall gas production much less than basins with marine source rock and coal layers in west China. Moreover, the Bohai Bay Basin experienced intense tectonic activity in the late stage, so the natural gas might have dissipated due to damage to the structure.Studies show that due to regional tectonic activities, some sags in the Bohai Bay Basin experienced late-stage rapid subsidence and high speed maturation of source rock, so the source rock might generate at high intensity, to provide material foundation for the formation of large natural gas reservoirs.The Bohai Bay Basin has undergone multi-stage tectonic uplifting and denudation^{[20-23, 42-43]}, source rocks in different sags differ widely in burial depth and thermal evolution degree in different periods. For example, in the deep sags such as Bozhong, Qikou, and Qinnan, the rapid late subsidence rate was more than 200 m/Ma and the corresponding maturation rate of source rock was more than 0.25%/Ma. Especially in Bozhong sag, during the E₂s_1-2 and E₂s₃ periods, the overall stratigraphic thickness was stable; during Dongying period, the active extension of the mantle action and the passive extension caused by the dextral strike slip and pull apart worked together to facilitate the rapid subsidence of the Bozhong sag, so the subsidence rate rose significantly than before, and strata of over 3500 m deposited in the main sub-sag of Bozhong sag. Since Neogene, the area turned into post-rift thermal subsidence stage, and the sedimentary center converged to Bozhong sag^[39,40]. The subsidence rate has been up to 320 m/Ma since 5.1 Ma before present (Fig. 3) and the formation deposited can reach up to 3 000 m. Due to the rapid subsidence during this period, the burial depth of the source rock of the Shahejie Formation increased, making the thermal evolution of the source rock accelerate, with the maturation rate of source rock in Bozhong sag reaching up to 0.41 %/Ma (Fig. 3). Based on the thermal simulation experiments with golden tubes, the amount of gas generated by the source rock before 5.1 Ma accounted for only 16.6% of the total, while since 5.1 Ma to present, the amount of gas generated accounted for 83.4% of the total, which is 5 times as much as the gas generated in the early stage (Fig. 4). This proves that this type of rapid subsidence and sedimentation has accelerated the thermal evolution of source rock and was conducive to large-scale gas generation in the late period. Based on the simulation results of thermal evolution, it is inferred from basin simulation that the gas generation intensity of Bozhong, Qikou, and Qinnan sag exceeded 20×10⁸ m³ since 5.1 Ma. The gas generation intensity of Bozhong sag was as high up to (50- 200)×10⁸ m³/km². The large amount of gas generation during this period well matched with the overpressure forming time of Dongying mudstone and the regional hybrocarbon accumulation time, which made the large amounts of natural gas generated in the late stage likely to preserve and accumulate under the regional overpressure mudstone cap.

1.2.2. Large-scale reservoirs

Large-scale gas accumulations are mostly “under caprock” type with large burial depth^[34]. The overpressure cap of Dongying Formation in the Bohai Bay Basin is generally more than 3000 m deep, and the underlying reservoirs mainly include metamorphic rock, carbonate rock, igneous rock of Pre-Cenozoic buried hill and clastic rock of Cenozoic Shahejie Formation. Statistics show the main reservoirs in large gas fields are mostly granite, igneous rock, carbonate rock, metamorphic rock and clastic rock (Table 2), which means buried hills of different lithologies are primary reservoirs of gas accumulations, this is because the quality of carbonate rock, igneous rock and metamorphic rock etc is less affect by burial depth, moreover, subject to multi-stages of uplift and erosion, and dissolution by meteoric water and deep fluids^[44,45,46], weathered crust and inside of buried hill have large amounts of fractures, which can provide enough storage space for natural gas.

The development degree of buried hill reservoir is controlled by base rock type, fracture-making capacity of tectonic stress and dissolution intensity of subsurface fluids. Among them, tectonic stress is the dominant factor, while the base rock type is the material foundation. The buried hills in the Bohai Bay Basin generally experienced multiphase tectonic movements, which are crucial for weathering and fracture generation. The multiphase tectonic events lifted the formation rock to the surface to suffer weathering on one hand, more importantly, under the multiphase and multidirectional stress, the formations and minerals broke and generated fractures of various trends which provided storage space for later hydrocarbon accumulation. Large faults induced by intense tectonic movements communicate the mantle, allowing various kinds of fluids to enhance the reservoir development of weathered crust and inner buried hill. The reworking processes of various fluids on reservoir mainly include two types, leaching of atmospheric water; and dissolution by deep CO₂ and hydrocarbon-bearing fluids. The basement rocks, long exposed, suffering weathering and corrosion, tend to form thick weathered crust and high quality reservoirs, especially in the higher and gentle position of buried hill. The deep fluids, including mantle-derived CO₂, hydrocarbon fluid and magma, also can improve the quality of buried hill reservoir considerably. They can re-activate and expand the early fractures by dissolution^[44,45].

Archaeozoic metamorphic rock buried hill in the Bohai Bay Basin is characterized by rich felsic components and the rock types mainly include plagiogneiss, amphogneiss, migmatitic granite and granulite^[46,47,48] etc with high brittleness. In response to multiphase tectonic stresses, these metamorphic rocks have many fractures in different directions intersecting with each other, which lays the foundation of fracture network of inner buried hill reservoir. The multiple tectonic movements since Indo-china period controlled the formation of Archaeozoic metamorphic rock fractured reservoir^{[49,50,51,52]}. Affected by the collision of Yangtze plate and North China plate during Indo-china period, a large number of NWW thrust faults and compressional fractures were generated. During Yanshanian stage, as the Pacific plate subducted NNW toward the east Asia continent, the Tancheng-Lujiang fault had sinistral strike-slip compression, inducing a large number of NE compressional fractures. The mantle plume motion in the early Himalayan period caused basin rifting, giving rise to many EW extension faults and fractures. These fractures formed in the 3 stages constitute 3 fracture systems. The upper weathered crust reservoir and thick inner fracture section in metamorphic buried hill constitute huge reservoir space (Fig. 5).

The above analysis shows that in the Bohai Bay Basin, where there is mature source rock of Shahejie Formation, overpressure mudstone caprock and large-scale reservoirs, large scale nature gas accumulation is likely to occur.

The natural gas generated with crude oil can migrate laterally and accumulate in large-scale reservoirs under the overpressured mudstone cap in the oil-prone Bohai Bay Basin. But in some sags where the Dongying Formation is mostly sandstone or the “quilt-like” mudstone cap is damaged by later faults, largely oil fields, even heavy oil fields, or small-scale gas fields will come up. Generally, the natural gas is likely to accumulate in large-scale in the low buried hill, small gas or oil and gas field in the medium height buried hill and large-scale oil fields in structurally high buried hills (Fig. 6). Consequently, the natural gas fields, medium-light oil fields, and heavy oil fields appear in ring shape around the source rock.

There are more than 60 sags distributed in 7 major onshore and offshore oil regions of the Bohai Bay Basin. According to formation conditions mentioned above, combined with exploration practices, four natural gas enrichment and depletion models have been identified.

(1) The gas enrichment model under the regional overpressure mudstone. In the strongly gas-producing humus-type or high-mature saprolite-type sags and surrounding areas, the hugely thick “quilt-like”overpressure mudstone cap in Dongying Formation (Shahejie Formation) above the source rock, if not damaged by the late-stage faults and with overpressure development remained, would block the gas generated by Paleogene from dissipation, the natural gas would migrate laterally and accumulate in reservoirs, to form large gas field beneath the cap possibly. Especially in the low buried hill in the sags, in places near the sags, the quilt is more developed and the natural gas charge is stronger, so larger gas fields are more likely to be formed (Fig. 7a). For example, the northern area of Liaodong Bay sag and the Bozhong sag.

The giant condensate gas reservoir of BZ19-6 in Bozhong sag of the Bohai Sea is a representative of regional overpressure mudstone enrichment model, which has undergone continuous gas-invasion accumulation process after oil accumulated in the trap. The gas, mainly associated with crude oil and condensate, is originated from the mudstone of Paleogene, Shahejie Formation in Bozhong sag. Yellow-green and blue-white fluorescent oil inclusions were found under microscope, indicating there were two phases of accumulation of lower maturity and higher oils. Brine inclusions coexisting with the oil inclusions have a homogeneous temperature range of 90-160 °C, which indicates the period of crude oil charge was mainly 12.0-5.1 Ma before present according to the recovery of the buried history. And we also observed many gas inclusions. The brine inclusions associated with them have a homogeneous temperature range of 140-180 °C, and the natural gas filling period was mainly 5.1 Ma to the present according to recovery of burial history. Rich oily asphalt is found on the top of the buried hill, which has a vitrinite reflectance range from 1.3% to 1.6%, indicating the origin of gas invasion. All these show that the BZ19-6 condensate gas reservoir had oil and then gas accumulation: in the middle Miocene to early Pleistocene (12.0-5.1 Ma), the source rock in Shahejie Formation generated oil extensively, and oil accumulated from in the trap (Fig. 7a); with the advent of new tectonic movement (5.1 Ma), a part of crude oil in the deep strata migrated along the faults to the shallow Neogene trap, giving rise to medium-sized BZ19-4 oilfield (Fig. 7b); from the Pleistocene (5.1 Ma), the source rock in high to over mature stage has been generating natural gas massively, and the gas charged into the oil reservoirs as gas invasion. Under the cover of the "quilt-like" overpressure mudstone cap, the invasion process has lasted to this day, transforming the oil reservoirs gradually into large-scale condensate gas reservoirs (Fig. 7c).

(2) The gas depletion model under the local overpressure mudstone. Although the mudstone of Dongying Formation (Shahejie Formation) is widespread, but at the slope of the depression, sand-mudstone interlayers were formed due to far source drainage, where the fluid could be discharged timely during sedimentation and compaction. As a result, the strong regional seal does not exist there, and the natural gas would migrate and dissipate in vertical direction at large. This type of mudstone in Dongying Formation cannot seal a large amount of natural gas but can seal crude oil to form large and medium- sized oil fields. The central Liaozhong sag in central part of Liaodong Bay is a representative area of this type (Fig. 8a).

(3) The gas depletion model in sand-rich sags. During the depositional period of the Dongying Formation after the formation of source rocks, sandstone and glutenite widely deposited due to adequate supply of terrigenous source, while the mudstone was limited in distribution and not well-developed, so it could hardly seal natural gas and light groups in crude oil. In a consequence, only heavy oil has left beneath the low- grade seal to form heavy oilfields, which is represented by the southern area of Liaozhong sag in Liaodong Bay (Fig. 8b).

(4) The gas depletion model under the intensive activity of the late-stage fault. In the center and surrounding area of some strong gas-producing sags, although thick mudstone in Dongying Formation (Shahejie Formation) deposited, late-stage faulting was very strong, so faults cut deep into the mudstone and are wide-spread, even cut through the whole mudstone in the sag and slope, making the regional overpressure mudstone cap ineffective and unable to seal the natural gas below. Therefore, a large part of the gas dissipated vertically, and a part of gas accumulates in the Neogene, forming reservoirs above the cap, but in a smaller scale. Due to dissipation of the gas, large natural gas fields cannot be formed in the gas-depletion sag. A representative of this type is the Yellow River Mouth sag (Fig. 8c).

Bozhong sag is the sag with the thickest Cenozoic in Bozhong depression. The mudstones of Shahejie Formation and Dongying Formation are widely distributed with a thickness of more than 200 m. Since 5.1 Ma ago, the rapid subsidence in large area of Bozhong sag has led to undercompaction and the rapid formation of mudstone overpressure. Currently, the mudstones have a pressure coefficient of generally over 1.6, and can seal most of the natural gas in the deep layers, providing good sealing conditions for the preservation of natural gas.

In Bozhong sag, there are three sets of high-quality source rocks, the third member, the first-second member of Shahejie Formation and the third member of Dongying Formation, which have mainly II₂-II₁ type kerogens. The third member of Shahejie Formation source rock at the largest burial depth has the maximum thermal evolution degree of over 2.0%, and is in the dry gas stage. The source rocks of first-second member of Shahejie Formation have reached the thermal evolution degree of up to 2.0% too. Most of the source rock in the third member of Dongying Formation has a maturity of 0.7%- 0.9%, and reaches 1.0% only near the center of Bozhong sag. Therefore, the large-scale development of high quality source rocks of Shahejie Formation in the center of hydrocarbon generation has laid a foundation for the mass production of natural gas.

The buried hills in Bozhong sag are mainly composed of igneous, carbonate and metamorphic rocks. These rock formations have fractured reservoirs affected by the multi-stage tectonic movements of Indochina, Yanshanian and Himalayan movement, and can form network reservoirs, which provide space for the storage of natural gas.

Therefore, besides Bozhong 19-6 large condensate gas field, other areas in Bozhong sag are also favorable areas for large gas field exploration.

Located in the Liaodong Bay depression in the north of the Bohai sea, with Jinzhou 20-2 gas field discovered, Liaozhong sag is the largest natural gas producing area in the Bohai sea. The Paleogene mudstones in the south and north of Liaozhong sag differ widely in development degree. In the south of Liaozhong sag, the Paleogene strata mainly consist of sand and mudstone interbeds with smaller overpressure degree, so they can hardly seal natural gas; while in north part, the Paleogene is dominated by thick-bedded mudstone with a pressure coefficient of up to 1.8, which provides a good sealing condition for the preservation of large-scale natural gas.

Shahejie Formation of Liaozhong sag has high quality source rock, which has mainly type II₂-II₁ kerogens and high degree of thermal evolution, laying a solid foundation for the generation of large quantities of natural gas. The reservoirs of buried hill in the sag, the slope zone and the adjacent uplift area are in good condition, which can form large-scale reservoirs and provide storage for natural gas.

Therefore, with strong sealing of Paleogene "quilt-like" overpressured mudstone, deep burial source rocks of the Shahejie Formation which can generate abundant gas and large scale buried hill reservoirs, the north part of Liaozhong sag is a favorable area for the exploration of large natural gas fields.

Banqiao sag is one of the important oil-gas rich areas in Huanghua depression. It is located in the west wing of Qikou sag, sandwiched between Cangdong fault and Dazhangtuo fault, with an exploration area of about 700 km².

The third member of Shahejie Formation in Banqiao sag has widespread regional overpressure mudstone. In the lake level rise period of transgression during the depositional period of Shahejie Formation, a set of mudstone in extensive and stable distribution deposited. In the process of compaction and diagenesis, overpressure formed in the mudstone due to undercompaction and hydrocarbon generation. With a pressure coefficient of more than 1.2, the mudstone not only has capillary sealing ability but also pressure sealing ability, so it can seal the natural gas of and under the third member of Shahejie Formation effectively.

In Banqiao sag, there are several hydrocarbon generating strata, including the third member of Shahejie Formation, the second member of Shahejie Formation and the first member of Shahejie Formation, which have mainly type II-III kerogens. When source rock is at the buried depth of about 4100 m, it reaches the Ro value of over 0.7% and the peak of oil generation. When reaching the buried depth of more than 4300 m, the source rock would have Ro value of more than 1.2% and enter the stage of mass gas generation. As the main gas generation layer, the source rock of the third member of Shahejie Formation is thick, rich in organic matter, high in thermal evolution degree and gas generation intensity. Especially in the key reservoir forming period, namely, at the end of deposition of the lower submember of the Minghuazhen Formation, the gas generation intensity of the source rock is generally (20-80)×10⁸ m³/km². The gas generation rate of the source rock increased rapidly in the middle and late sedimentary period of the Minghuazhen Formation, which laid a rich resource foundation for natural gas reservoir exploration in Banqiao sag.

During the sedimentary period of Shahejie Formation, due to the influence of Cangdong fault, the detrital material formed large near-source fan delta sedimentary system along the fault in Banqiao area, meanwhile, lithologic traps are well developed in the sag and slope areas, which provide favorable places for deep gas accumulation.

Based on the above analysis, it is concluded that the deep formation of Banqiao sag should be a gas-rich sag and a favorable area for large-scale natural gas reservoir exploration.

Based on decades of experience in natural gas exploration in the Bohai Bay Basin, this paper summarizes the understanding of large-scale natural gas reservoir exploration in oil type lacustrine basins, that is "gas generation theory of lacustrine basin", and points out that the formation conditions of conventional large natural gas reservoirs in the Bohai Bay Basin mainly include one core element and two key factors. One core element is the strong sealing of Paleogene "quilt-like" overpressure mudstone. The two key factors are the rapid maturation and high-intensity gas generation of source rock in the late stage and the large-scale reservoir.

The large-scale gas accumulation, enrichment and depletion models in the Bohai Bay Basin are established. Four natural gas enrichment and depletion models, including gas enrichment model under the regional overpressure mudstone, gas depletion model under the local overpressure mudstone, gas depletion model in sand-rich sag and gas depletion model under intensive activity of the late-stage faults have been identified, which can guide exploration of large-scale natural gas fields in the Bohai Bay Basin.

It is pointed out that the areas around Bozhong sag, north Liaozhong sag and Banqiao sag which are close to hydrocarbon generation center, have large-scale quality reservoirs in buried hill and thick overpressure mudstones in the Paleogene are promising sites for searching large gas fields.