The Bohai Bay Basin and the Ordos Basin both belonged to the cratonic basin of the North China in Late Paleozoic, when a set of widely distributed coal-bearing formations deposited^[1,2]. The source rocks in the coal measures have favorable hydrocarbon generation conditions. However, experiencing different tectonic activities later, the Paleozoic of the Bohai Bay Basin and the Ordos Basin differ widely in hydrocarbon accumulation conditions^[2,3,4,5]. The Ordos Basin is a cratonic basin developing successively, where stable structure, good oil and gas preservation condition, widespread source rock, and widely distributed deltaic sand bodies, and the effective near-source continuous hydrocarbon charge provided favorable conditions for the formation of large oil and gas fields. So far, several large gas fields with proven reserves of over 100 billion m³ have been discovered in this basin^[6,7,8,9]. In contrast, transformed by multi-episodic Mesozoic and Cenozoic tectonic activities, the Paleozoic in Bohai Bay Basin formed mountains^{[10,11,12,13]}, then Cenozoic deposited above the Paleozoic, so “new source - old reservoir” type buried-hill gas and oil reservoirs with oil and gas supplied by Paleogene hydrocarbon source rock are likely to develop. In comparison, the “old source - old reservoir” type buried-hill gas and oil reservoirs (primary reservoirs) in Paleozoic inner buried-hills with oil and gas supplied by Paleozoic coal-measures hydrocarbon source rock, have experienced more complicated evolution. With little knowledge on this kind of reservoir, it is difficult to find them. Therefore, it is of great significance for searching this kind of reservoir to study in depth the accumulation theory of it.

In recent years, the Dagang Oilfield Company has explored primary oil and gas reservoirs in inner buried-hill continuously and succeeded in discovering oil and gas in Wumaying buried-hill in the south of the Huanghua Depression of Bohai Bay Basin. In this study, the condition of the primary source rock, the inner buried-hill reservoir-seal assemblages, and timing of hydrocarbon charging have been investigated to find out the formation and deposition characteristics of the Wumaying Paleozoic primary oil and gas reservoir in the inner buried-hill region, in the hope to guide the exploration of primary oil and gas reservoirs in the inner buried-hill in Huanghua Depression and Bohai Bay Basin.

The Wumaying buried-hill is located in the south of the Huanghua Depression, with a prospecting area of approximately 350 km². It is adjacent to Xuhei tectonic belt to the east, is connected with Nanpi sag to the west, and borders with the Dongguang slope zone to the south and the Wangguantun tectonic belt to the north. It is a wide and gentle anticline structure striking NNE on the whole. The Paleozoic anticline of the Wumaying buried-hill is deeply buried under the Paleogene and is a typical low-sequence buried-hill structure^[14]. This buried-hill Permian top is composed of a high and steep thrust-nappe structure on the western side and a wide and gentle anticline structure on the eastern side. Three types of faults, namely, thrust, strike-slip, and extension, separate the buried-hill structure into three local structures; its top Ordovician is folded into a wide and gentle NEE-trending anticline, which is cut by the Wumaying fault formed in Late Neogene into two parts, the east faulted nose and the west anticline (Fig. 1).

In 1999, Well Wushen 1 was drilled in the thrust zone on the west side of the Wumaying buried hill, and oil and gas shows were detected in the Permian and Ordovician of this well. In the test of the Ordovician Fengfeng Formation (5 460 to 5 496 m), a high-yield gas flow of (10-13) × 10⁴ m³ a day was obtained. The gas-source correlation indicated that the gas was derived by Carbonaceous-Permian coal-measure source rock^[15,16], but the well was permanently sealed because high H₂S content was detected in the gas, and hereafter, exploration in this region has ground to a standstill for 18 years. In 2017, the Dagang Oilfield re-started research on the multilayer inner buried-hill accumulation and re-evaluated the Permian buried-hill. The old well, Well Wushen 1, was checked again, in which a layer originally interpreted as water was re-evaluated as 11 gas layers of 76.8 m thick in total. Then Well Yinggu 1 was drilled. With a total depth of 5 045 m, this well had 20 gas layers of 109 m thick in total interpreted in the Permian Lower Shihezi Formation. After fracturing the lower segment of the Lower Shihezi Formation (4 959.4 to 4987.7 m), it produced 30.2 m³ of oil and 80 121 m³ of gas a day with 6-mm choke, and no H₂S was detected in the gas. There is no connection between the buried-hill and the Paleogene hydrocarbon source rock. Correlation between the oil and gas and source rock indicates that the oil and gas of this well are sourced entirely from the Upper Paleozoic coal-measure hydrocarbon source rock, so this reservoir is a primary oil and gas reservoir in the inner buried-hill (Fig. 1), marking a major breakthrough in prospecting non-H₂S-bearing primary petroleum reservoirs in the Paleozoic inner buried-hill. Subsequently, Well Yinggu 2 and Well Wutan 1 were drilled in this area, both of which had oil and gas reservoirs found in the Permian.

The Upper Paleozoic coal-measure hydrocarbon source rock in Huanghua Depression has a large cumulative thickness between 100 and 450 m, and wide distribution area of 9589 km². The source rock has two thickness centers: the south one centered at the Wumaying buried-hill structural zone is larger with coal of up to 450 m thick; the north one is centered at the Qinan-Chenghai area.

The Huanghua Depression contains three sets of coal-measures hydrocarbon source rocks: the Carbonaceous-Permian Benxi Formation, the Taiyuan Formation, and the Shanxi Formation. The source rocks are diverse in lithology, including dark mudstone, coal, and carbonaceous mudstone^[17,18]. The coal seams are 2 to 5 m thick each, and 20 to 45 m thick in total. The coal in the Taiyuan Formation is most stable in distribution, accounting for 65% of the total coal thickness. The carbonaceous mudstone is 40 to 110 m thick, and the dark mudstone is 150 to 350 m thick. The carbonaceous mudstone and dark mudstone correspond well with the coal on the plane.

Statistical analysis of the geochemical parameters show the Huanghua Depression coal-measures hydrocarbon source rocks have abundant organic matter, large hydrocarbon generation potential, and high content of hydrogen-rich components, laying a solid foundation for hydrocarbon generation. The coal samples have a total organic carbon (TOC) of 11.5% to 78.0%, (S₁+S₂) value of 0.50 to 218.56 mg/g, and hydrogen index of 1.0 to 553.0 mg/g. The coal layers of the Shanxi and Taiyuan Formations reach the good-to-premium standard and the coal of the Shanxi Formation is better in quality. The carbonaceous mudstone samples have a TOC of 5.15% to 19.8%, (S₁+S₂) value of 0.18 to 53.64 mg/g, and hydrogen index of 1.0 to 331.0 mg/g, representing good hydrocarbon source rock. The dark mudstone samples have a TOC of 0.54% to 5.95%, greater than 2.0% on average, (S₁+S₂) value of 0.03 to 175.73 mg/g, and hydrogen index of 1.0 to 416.0 mg/g, representing moderate-to-good hydrocarbon source rock.

The Upper Paleozoic coal-measure hydrocarbon source rock in Huanghua Depression is generally at mature-to-highly mature stage, and its degree of thermal evolution degree increases in ladder pattern with the increase of burial depth. At the burial depth of less than 3 000 m, the hydrocarbon source rock has a vitrinite reflectance, R_o of 0.5% to 0.8%, showing no obvious increase with ride of depth, which is likely related to the fact that the hydrocarbon source rock experienced thermal evolution process before the early uplift^[19,20]. At the burial depth of more than 3000 m, the hydrocarbon source rock increases in the degree of thermal evolution gradually, and at approximately 5 200 m, reaches R_o of 1.30%, that is the highly mature stage.

We used the Petromod basin simulation software to simulate the thermal evolution of the coal-measure hydrocarbon source rocks in Well Wushen 1 and Yinggu 1 of the Wumaying buried-hill; the paleo-heat flow parameters used in the simulation process the paleo-heat flow of Bohai Bay Basin from previous studies corrected repeatedly by the measured vitrinite reflectance of Well Wushen 1^[21]. The simulation results indicate that the Wumaying buried-hill generally has two stages of hydrocarbon generation. The first stage of hydrocarbon generation occurred at approximately 132 to 153 Ma (Late Mesozoic); thereafter, the Huanghua Depression experienced tectonic uplift and denudation and the thermal evolution of the hydrocarbon source rock stopped. In the Cenozoic, the Huanghua Depression witnessed rapid deposition of the thick Kongdian Formation, Shahejie Formation and Neogene. After the burial depth exceeding the depth of the primary hydrocarbon generation, the hydrocarbon source rock entered the second stage of hydrocarbon generation. This phase has lasted from approximately 55 Ma to the present, that is from the depositional period of the Kongdian Formation to now. The main period of hydrocarbon generation (with the R_o value of greater than 0.7%) was from approximately 40 Ma to the present, that is from the late depositional period of the third member of the Shahejie Formation to the present (Fig. 2a).

The simulation of gas generation suggests that in Huanghua Depression, the total amount of gas generated by the Paleozoic coal-measures hydrocarbon source rock was approximately 42×10¹² m³, of which coal contributed the largest amount of approximately 19×10¹² m³, followed by carbonaceous mudstone and dark mudstone. Of the two stages of gas generation, more gas, approximately 26×10¹² m³, was produced in the late stage, accounting for 62% of the total amount of generated gas. The coal-measures hydrocarbon source rock has two centers of gas generation. The southern center is in Wumaying-Wangguantun area, and the northern center is in the Qibei-Chenghai area, both with the maximum gas generation intensity of over 200×10⁸ m³/km². In the Wumaying- Wangguantun area, centering at the Wumaying buried-hill, gas generated in large scale, amounting to approximately 12.6× 10¹² m³ in total. Large in gas generation intensity, late in gas generation time and thus conducive to the preservation of natural gas reservoirs in later stage, these regions are the most favorable ones for natural gas exploration (Fig. 2b).

From bottom to top, the Wumaying buried-hill has four sets of favorable reservoir-seal assemblages. The lower reservoir-seal assemblage is composed of Benxi Formation dark mudstone (the cap) and the Ordovician carbonate rock (the reservoir); the middle reservoir-seal assemblage has the Taiyuan Formation mudstone and coal measure as cap and the Taiyuan Formation barrier sand body and platform-facies carbonate rock as the reservoir layers; the upper reservoir-seal assemblage in the middle and lower part of the Permian is made up of the Shihezi Formation red mudstone (regional cap) and the pebbly sandstone on the top of the Lower Shihezi Formation (reservoir layer); the top reservoir-seal assemblage comprises the Shiqianfeng Formation mudstone (the cap) and the Upper Shihezi Formation meandering river-phase sandstone (reservoir layer). To date, the proven oil and gas layers in the Wumaying buried-hill are mainly distributed in the Permian Lower Shihezi Formation sandstone and the Ordovician carbonate rock, and a few in the Shanxi and Taiyuan Formations (Fig. 3).

Reservoir layers of the lower reservoir-seal assemblage are in the Fengfeng Formation and the Upper Majiagou Formation on the top of the Ordovician system, which are a set of carbonate platform deposits (Fig. 3). The imaging logging shows that the reservoir space of Lower Paleozoic carbonate rock includes secondary pores, vugs, and cracks. The cracks are mainly high-angle structural fractures. The dissolved pores and vugs are small and medium-sized largely and distributed along cracks. The reservoirs vary widely in physical properties. The reservoirs in Well Wushen 1 have a porosity of 0.5% to 17.8%, mainly 2.5% to 5.0%, on average 6.15% from logging. The reservoir layer near the unconformity on the top Ordovician is better in physical properties. This set of reservoir belongs to the reservoir-seal assemblage below the source rock.

The Lower Shihezi Formation fluvial-facies sandstone, generally thick medium-coarse sandstone, is the reservoir of the upper reservoir-seal assemblage. The well logging data of Well Yinggu 1 indicates that the reservoir layers of this set are 10 to 22 m thick each, 130 m thick combined, and higher than 30% in sand-to-ground ratio. The rocks are high in compositional maturity, composed of mainly quartz sandstone and a small amount of lithic quartz sandstone, with a quartz content of higher than 90% in general. The reservoir layers are fairly tight, with a porosity of 0.15% to 10.16%, mainly 3.26% to 7.89%, and 5.79% on average; and a permeability of (0.01- 6.47)×10^-3 μm², largely (0.12-3.17)×10^-3 μm², and 1.28×10^-3 μm² on average. This set of the reservoir has hardly primary pores, and micropores and secondary dissolved pores as reservoir space, and a small number of fractures in local parts. The micropores are mainly kaolinite intercrystalline pores, and the secondary pores are dissolved pores of fillings and a small number of dissolved pores at particle edges. This reservoir-seal assemblage is above the source rock (Fig. 3).

If natural gas accumulates in a late stage, less will be lost, and it is more likely to form large scale accumulation^[22]. Therefore, it is very important to define the time of large-scale oil and gas filling. In this study, the fluid inclusions were analyzed comprehensively to figure out the times and stages of oil and gas charging in Wumaying buried-hill. The fluid inclusions are the fluid samples enclosed in the lattice defects of diagenetic authigenic minerals or diagenetic healed cracks of clastic minerals and can provide direct evidence of the oil and gas filling and accumulation history^[23]. Samples were taken systematically from the Permian Lower Shihezi Formation, the major pay, from Well Yinggu 1 in Wumaying buried-hill. Through petrographic observation of fluid inclusions, homogeneous temperature measurement of hydrocarbon-associated brine inclusions, and simulation of the thermal history of Well Yinggu 1, it is concluded that the Wumaying inner buried-hill oil and gas were accumulated in two stages, the early oil stage and late gas stage, and the late gas stage takes dominance.

The first stage of oil and gas filling happened in the Early Cretaceous. Observation of the Permian Lower Shihezi Formation samples from Well Yinggu 1 shows that a large amount of carbonaceous bitumen fills the intergranular pores, which is dark brown under transmitted light and black under fluorescent light and mixed with blue-and-white fluorescent light oil (Fig. 4a-4b). By combining the observation results with the simulation of thermal evolution of Well Yinggu 1, we think that the Carbonaceous-Permian coal-measure hydrocarbon source rock generated hydrocarbon once in the early Cretaceous (Fig. 2a). During this period, the hydrocarbon source rock entered the threshold of hydrocarbon generation, but low in thermal evolution degree, and generated oil largely. After this period, the area subjected to uplift and denudation in Mesozoic, and oil and gas filling in the pores were oxidized to form carbonaceous bitumen^[24,25,26].

The second stage of oil and gas filling lasted a longer time, from the early deposition of the third member of the Shahejie Formation to the deposition of the middle Minghuazhen Formation (between 6 and 43 Ma). The main period of filling was from the late deposition of the third member of the Shahejie Formation to the late deposition of Guantao Formation (approximately 15 to 40 Ma). Microscopic observation shows that a large number of hydrocarbon-bearing inclusions are distributed in the secondary enlargement edge of quartz or appear in line or band along the cracks through quartz particles. The abundance of the inclusions is 65% to 93%. Gas-liquid hydrocarbon-bearing inclusions take the majority at 85%, gas hydrocarbon inclusions account for 10%, and liquid hydrocarbon inclusions 5%. The liquid hydrocarbon in these hydrocarbon-bearing inclusions is pale yellow under mono-polarized light and gives out blue-and-white fluorescence under ultraviolet light, indicating that light oil once charged and accumulated at a large scale in the Lower Shihezi Formation sandstone reservoir layer of the Wumaying buried-hill^[23] (Fig. 4c-4f). We used a LabRAM HR800 laser Raman probe to measure the gas composition captured in the gas-liquid two-phase inclusions from Well Yinggu 1 samples, and we selected two wavelengths, 532 nm and 633 nm, to test the different fluid inclusions. The results show that there is an obvious characteristic peak at 2 912 cm^-1 in the Raman spectrum. The characteristic Raman spectrum peak of the main components of the fluid inclusions at room temperature correspond to methane^[27], which indicates that the gas hydrocarbon that filled the rock during this time period was mainly methane (Fig. 4g). The homogeneous temperature of the Permian Lower Shihezi Formation hydrocarbon-bearing associated brine inclusions in Well Yinggu 1 is 110 to 149 °C. By combining these observations with the simulation of the thermal history, we conclude that this period of filling occurred between 6 and 43 Ma, between the early deposition of the third member of the Shahejie Formation and the deposition of the Minghuazhen Formation. The main homogenous temperature interval is 130 to 139 °C, corresponding to the period between the late deposition of the third member of the Shahejie Formation and the deposition of the late Guantao Formation (15 to 40 Ma), which is the main period of oil and gas filling (Fig. 5).

The Wumaying buried-hill was formed in the Mesozoic era. The Yanshanian-Himalayan episodic tectonic activities (especially the fault structures) had a strong influence on the large-scale accumulation and preservation of natural gas. In contrast, the Neogene tectonic activity was weak, during this period, the large-scale gas generation of coal-measure hydrocarbon source rock and the large-scale filling of natural gas matched well, laying foundation for the late preservation and large-scale accumulation of natural gas.

The primary oil and gas reservoir in Wumaying inner buried-hill has three major favorable accumulation conditions, i.e., the large-scale secondary gas generation of coal-measure hydrocarbon source rock, the multiple sets of reservoir-seal assemblages, and the late stage hydrocarbon filling, which gave rise to the compound hydrocarbon accumulation of above-source sandstone and under-source carbonate rock in the Paleozoic inner buried-hill (Fig. 6).

The Ordovician carbonate rock of the Wumaying buried-hill has a large number of high-angle cracks, connecting karst fissures and pores, so the blocky oil and gas accumulation area is likely to form. The overlying coal-measure hydrocarbon source rock not only supplies a large amount of hydrocarbon, but also acts as the overpressure cover on top of the reservoir layer, providing effective sealing for the accumulation and preservation of the gas reservoir. The main hydrocarbon-producing layer of natural gas is distributed near the top of the Ordovician weathering crust and occurs as massive gas-condensate reservoir with bottom water. The gas reservoir has a high point burial depth of 5 150 m, gas-water contact at 5 800 m, gas-bearing area of 17.9 km², gas column height of 650 m (Fig. 6), pressure of 56.89 MPa, and pressure coefficient of 1.04, representing water-soluble wet gas reservoir under normal pressure system. In the natural gas, methane takes the majority of about 86.96%, heavy hydrocarbons account for about 4.77%, and nonhydrocarbon components (mainly CO₂) make up approximately 8.27%. In the low yield section between 5618 and 5637 m, gas test detected a small amount of methane and H₂S as high as 16%.

The Carboniferous-Permian coal-measure hydrocarbon source rock is the source rock supplying oil and gas for the Permian Lower Shihezi Formation reservoir in the Wumaying buried-hill. The oil and gas can migrate vertically or laterally along faults and sand bodies to the Lower Shihezi Formation sandstone reservoir layer above the source rock, forming primary oil and gas reservoir in the inner buried-hill. This gas reservoir comes in layers in the Lower Shihezi Formation sandstone. With a burial depth of structural high at 4 450 m, gas-water interface depth of 5 150 m, cumulative thickness of 700 m (Fig. 6), gas-bearing area of 9.6 km², pressure of 43.36 MPa, pressure coefficient of 0.87, and the surface condensate oil content of 156.508 g/m³, this reservoir is a gas-condensate reservoir of low pressure. The crude oil from this reservoir has a relative density of 0.809 0 g/cm³ at 20 °C, viscosity of 1.36 mPa·s at 50 °C, freezing point of 1 °C, paraffin content of 8.04%, sulfur content of 0.01%, the colloid content of 3.32%, and gas-oil ratio of 5 255 m³/m³. The natural gas has a methane content of 80.24%, heavy hydrocarbon content gas of 9.99%, nonhydrocarbon gas content (mainly CO₂ and N₂) of 8.94%, relative density of 0.69, and no H₂S.

The Meso-Cenozoic Wumaying buried-hill experienced the middle-late Triassic Indosinian movement, the Jurassic-Cretaceous Yanshanian movement, and the Himalayan movement since the Eocene^[28,29,30]. The multistage tectonic deformations affected the migration and accumulation of oil and gas, leading to the compound hydrocarbon accumulation of primary oil and gas in Wumaying inner buried-hill. Based on the studies on the primary oil and gas source condition, inner buried-hill reservoir-seal assemblages, and the hydrocarbon accumulation time, we reconstructed the hydrocarbon accumulation process, which showed that the filling and accumulation process of primary oil and gas in Wumaying inner buried-hill is characterized by “mixed oil and gas charge in local parts in early stage, adjustment accumulation due to structural high migration in middle stage, and natural gas scale charge and compound accumulation in late stage”. The oil and gas accumulations in different stages have quite different features.

During the Indosinian stage, the overall Wumaying area uplifted, leading to weak faulting and folding deformation, but forming mainly large-scale and gentle uplifts. During early Yanshanian movement, based on the inherited Indosinian Paleotectonic framework, the study area was strongly compressed, forming a thrust-nappe structural trap^[28]. During the relatively continuous settlement of the research area in the early Cretaceous, hugely thick Mesozoic system deposited, causing rise of thermal evolution degree of the coal-measure hydrocarbon source rock. When reached R_o value of 0.7% at the burial depth of approximately 2 700 m, the source rock passed the hydrocarbon generation threshold. But low in thermal evolution degree, the source rock mainly produced oil and gas of small amount, and the oil and gas accumulated at the structural highs of the thrust-and-fold belt where Well Yinggu 1 is located, and vertically filled the above-source Lower Shihezi Formation sandstone reservoir and the under-source Ordovician carbonate rock reservoir (Fig. 7).

In the late Cretaceous, affected by the regional uplift during the Yanshanian Movement, the Wumaying buried-hill was subjected to large-scale denudation and turned into a large- scale buried-hill^[29]. During this period, the hydrocarbon source rocks were shallower in burial depth on the whole and stopped generating hydrocarbons. Due to the weathering and leaching of the overlying strata, the oil and gas accumulated early were oxidized, forming the residual carbonaceous bitumen in the pores (Fig. 7).

During the Himalayan movement, the Huanghua Depression entered into fault depression stage, and very thick Kongdian and Shahejie Formations deposited on the residual Mesozoic buried-hill. Simulation of the thermal evolution of the hydrocarbon source rock indicates that during deposition of the third member of the Shahejie Formation (approximately 45 Ma), the Carboniferous-Permian coal-measure hydrocarbon source rock began to generate hydrocarbon for the second time as the burial depth increased. The source rock reached R_o value of 0.7% at the burial depth of approximately 2 900 m and R_o value of 1.0% at the burial depth of 3 650 m. During this period, the source rock, higher in thermal evolution degree, produced largely gas and a small amount of oil. The oil and gas charged and accumulated in the Lower Shihezi Formation sandstone reservoir and the Ordovician carbonate rock reservoir nearby. First, the gas and oil reservoir with “gas above oil” came about, later on, as the source rock increased in thermal evolution degree further, the gas-to-oil ratio rose, and the liquid oil evaporated into gas under high-temperature and high-pressure, forming the gas-condensate reservoir (Fig. 7). During this period, the Wumaying buried-hill experienced strong uneven subsidence and faulting activity; the strong dextral strike-slip of the Xuxi Fault since the deposition of the first member of the Shahejie Formation caused the change of local stress field and the migration of the structural high of the Wumaying buried-hill from the northeast corner of the buried-hill to the southeast corner^[30]. During the late sedimentary stage of the Dongying Formation, the structural high gradually migrated from the location of the Well Yinggu 1 to that of the Well Wushen 1. The migration of the structural high transformed the oil and gas accumulation zone from nearly south-north strike to nearly north-east-east strike, and the structural trap where Well Wushen 1 is located became the main site for scale accumulation of natural gas.

In addition, the active faults provided paths for the vertical migration of oil and gas, allowing the diversion of the early accumulated oil and gas in the buried-hill to the favorable traps in Paleogene reservoir. During the late sedimentary stage of the Dongying Formation, the whole area of the Bohai Bay Basin experienced tectonic uplift, the Dongying Formation of the Wumaying area in Huanghua Depression suffered slight denudation, and the hydrocarbon generation of the hydrocarbon source rock paused for a short period.

During neotectonic movement, the basin entered the depression period, the Wumaying buried-hill tectonic activity almost ceased, and the overlying strata deposited stably. The coal-measure hydrocarbon source rock reached the R_o value of 1.0% overall and over 1.3% in some parts after deep burial, entering the large-scale gas generation stage. During this period, part of the coal-measure gas filled into the Lower Shihezi Formation sandstone reservoir overlying the hydrocarbon source rock and accumulated in the high portion of the structural trap; part of the gas entered the Ordovician carbonate rock reservoir layer underlying the hydrocarbon source rock, leading to the compound accumulation of primary oil and gas in the Wumaying inner buried-hill (Fig. 7).

The Paleozoic coal-measure hydrocarbon source rock in Huanghua Depression has a large cumulative thickness, wide distribution range, high organic matter content, high hydrocarbon generation potential, and high hydrogen-rich component content, showing good material base for hydrocarbon generation. The hydrocarbon generation simulation indicates that the coal-measure hydrocarbon source rock experienced two stages of hydrocarbon generation. The late stage was high in gas generation intensity and large in gas generation scale, providing 62% of the total gas generated. Located at the southern hydrocarbon generation center of the Upper Paleozoic hydrocarbon source rock, the Wumaying buried-hill has favorable oil and gas source condition.

The Wumaying inner buried-hill has four sets of favorable reservoir-seal assemblages. The thick-layer quartz sandstone reservoir in Lower Shihezi Formation has mainly micropores and secondary dissolved pores as storage space, and a small number of cracks in local parts. The reservoirs in the Fengfeng Formation and the Upper Majiagou Formation carbonate rock at the top of Ordovician with medium and small dissolved pores, dissolved vugs, and high-angle fractures, are better in physical properties and main hydrocarbon-producing layers of the Wumaying inner buried-hill.

The primary oil and gas reservoirs in Wumaying inner buried-hill experienced two stages of accumulation, the early oil stage and late gas stage, and the late stage is the dominant. The late stage accumulation taking place between 6 and 43 Ma, was long in continuous charging time, and the tectonic activity was weak during this period, the large-scale gas generation of coal-measure hydrocarbon source rock and the large-scale filling time period of natural gas matched well, laying solid foundation for the late scale accumulation and preservation of natural gas.

The Wumaying buried-hill experienced multi-stages of tectonic deformation and reformation in the Mesozoic and Cenozoic eras, accordingly, the migration and accumulation of primary oil and gas in its inner buried-hill feature “mixed oil and gas charge in local parts in early stage, adjustment accumulation due to structural high migration in middle stage, and natural gas scale charge and compound accumulation in late stage”, giving rise to the compound hydrocarbon accumulation zones in multiple types of traps, diversely lithological and multiple reservoir layers.

The Wumaying inner buried-hill in the Huanghua Depression has abundant primary oil and gas resource and huge potential for hydrocarbon exploitation; the exploration, discovery, and theoretical understanding of this field will provide a reference for the exploration and reservoir accumulation study of primary oil and gas in the inner buried-hills of Bohai Bay Basin.