The Bohai Bay Basin experienced multi-phase tectonic activities after Paleozoic sedimentation. And then Paleozoic sediments were uplifted and denuded, forming a tectonic framework characterized by horsts and grabens and a series of buried hills. Due to weathering and leaching processes, high quality reservoirs were created at the structural highs of buried hills. Such Paleozoic reservoirs usually directly contact with Cenozoic high quality source rocks, which are favorable for forming oil pools with young source rocks and ancient reservoirs^[1,2,3,4,5]. Guided by this idea, Paleozoic hydrocarbon exploration focused on the structural highs around buried hills in the past. However, hydrocarbon shows and commercial oil- gas flows were also discovered in some wells targeting Paleozoic reservoirs which do not directly contact with Cenozoic source rocks. This means that the Paleozoic has its own independent petroleum system. The Paleozoic petroleum system with Carboniferous-Permian coal-measure source rocks has been discovered in the Ordos Basin, which is also a part of the North China Craton^[6]. In recent two years, by re-discovering conditions of Paleozoic primary hydrocarbon accumulation in the Bohai Bay Basin, we have made major breakthroughs in petroleum exploration in the Qibei and Wumaying buried hills of the Huanghua Depression. Preliminary assessment shows that the Paleozoic gas resources exceed a hundred billion cubic meters, suggesting huge potential of Paleozoic hydrocarbon exploration in the Huanghua Depression and even the Bohai Bay Basin.

The Huanghua Depression lies in the hinterland of the Bohai Bay Basin, where the Cenozoic depositional basin overlies residual Mesozoic and Paleozoic strata. More than 20 buried-hill traps in the Dagang oilfield mining right area in the central and southern depression have been confirmed based on 3D seismic data of 7 654 km² (Fig. 1). Despite early outset, the exploration of buried-hill reservoirs in the Huanghua Depression has been in the exploratory stage. In a 23-year history from 1963 to 1986, hydrocarbon exploration concentrated on Ordovician carbonate buried hills at the structural highs of large uplifts. Among 64 wells drilled, only one well which yielded oil of 6.65 t and gas of 11 275 m³ a day targeted the northern Dagang buried hill. Because of the small reservoir size, the reserves are too low to be booked. From 1987 to 2014, guided by the accumulation model of young source rock and age-old reservoir rock, the exploration direction shifted to Ordovician buried hills at structural lows, where the source rock and reservoir rock are in direct contact. Among 103 wells deployed, 44 wells obtained industrial hydrocarbon flow, leading to the discovery of Qianmiqiao, Chenghai, and Wumaying buried hill reservoirs consecutively ^[7,8,9,10]. But except the Qianmiqiao buried hill, it is challenging to develop these reservoirs due to high H₂S content. Consequently, buried hills in the Huanghua Depression haven’t been taken as major targets of reserves and production increase.

The basement structure of the Bohai Bay Basin is complex, with alternated sags and bulges and large tectonic relief, and varing residual pre-Paleogene formations. The Paleozoic is thickest, so buried-hill traps with different reservoir-seal assemblages are likely to exist (Fig. 2). These Paleozoic traps are likely to be close to Paleogene source rock, making it possible to form buried-hill oil reservoirs. In the Jizhong Depression and Liaohe Depression in the vicinity of the Huanghua Depression, some major discoveries have been made in this system^{[11,12,13,14,15]}, especially the large condensate field with gas resources of over a hundred billion cubic meters has become a major contributor to oil and gas reserves^[16,17]. Until the end of 2015, cumulative 16×10⁸ t of oil reserves within buried-hill reservoirs, Bohai Bay Basin, have been discovered. Among these oil reserves, only 4 021×10⁴ t are from the Dagang oilfield, accounting for 3% of the total reserves. How to evaluate and find Paleozoic buried-hill reservoirs in the Huanghua Depression is not only a theoretical question for geologists but also an urgent need for sustainable development of this mature field.

In the past exploration targeting the buried hills with young source rock and age-old reservoir rock combination, the principal bases for well deployment were buried-hill trap and its contact with Paleogene source rock; thus, whether the buried-hill structure was certain was crucial to the success of exploration. The northern Dagang buried hill at the upthrown wall of the Gangdong fault in the Qikou sag, for example, is in direct contact with Paleogene Shahejie source rock at the downthrown side, hence, it has the conditions to form hydrocarbon reservoirs with young source rock and age-old reservoir rock. Since 1964, structural highs of carbonate buried hills had been the targets of exploration. Among more than 40 exploratory wells, only G1 and T4 yielded low production oil flow and industrial oil flow, respectively, because it was difficult to locate the structural highs of buried hills with small area. Consequently, the Gangbei buried-hill exploration had been stagnant for 30 years^[18]. In recent years, through overall 3D seismic data processing and interpretation, the shape of the Gangbei buried hill has been made clear (Fig. 3)^[19]. Well GG1507 deployed in 2015 tapped an oil equivalent of about a hundred tons (Tables 1 and 2), discovering 3 new clastic oil-bearing sequences in the Cretaceous System, Permian System (the Lower Shihezi Formation, P₁x), and Carboniferous System (the Taiyuan Formation, C₃t). From lab tests of oil and gas and rock samples acquired from the buried hill, it is found that the Carboniferous-Permian and Paleogene Systems all have effective source rocks; but the oil and gas sources differ greatly from northeast to southwest^[20]. The 20 exploratory wells drilled subsequent all obtained oil and gas, indicating that there is an independent petroleum system in the Paleozoic.

The Wumaying buried hill lies in the southern Cangdong sag, where deeply buried reservoir rocks are far away from Paleogene source rock. Prospecting Ordovician buried hills with oil and gas supplied by Carboniferous-Permian coal- measure source rock is a main direction to search Paleozoic self-sourced reservoir. Through detailed analyses of 3D seismic data at 20th century, it is found that traps within Carboniferous-Permian source rock which are in direct contact with Ordovician reservoir rock are about 20 km² in area. Well WS1 drilled in 1998 in one of the traps detected active oil and gas shows in the Paleozoic, of which, more than 10 points were fluorescent shows. After acidizing of Ordovician, this well tested a high production oil and gas flow of daily gas output of 137 279 m³. But due to high H₂S content of 6.27%, this well has been plugged permanently. Most researchers held the opinion that TSR (thermochemical sulfate reduction) was likely to occur in deep Ordovician buried hills to produce H₂S; in addition, deeply buried tight Upper Paleozoic clastic rocks were too poor in petrophysical properties to be effective reservoirs. Consequently, Paleozoic hydrocarbon exploration had been stagnant in a long period of time. Thanks to the breakthrough in Permian buried-hill exploration in Gangbei, samples were taken in 2017 for lab tests from the Permian Lower Shihezi Formation in Well WS1, which wasn’t a zone of interest at the time it was drilled. The results showed that there are effective clastic reservoirs with the porosity as high as 10% at 5 000 m deep. The previously interpreted water layers with abnormal gas shows at 4 820-5 100 m were reinterpreted as 11 gas layers of 77 m thick. After reinterpretation, the trap area at the top of the gas layers was confirmed to be 77 km², and the resources were estimated to be 530×10⁸ m³. Wells YG1, YG2, and WT1 were drilled later in this trap, all of them found thick gas layers in the Permian Shihezi Formation. In Well YG1, gas layers of 109 m thick combined were found in the Permian Shihezi Formation; after fracturing, the well section of 4 959-4 988 m in the lower part of the Lower Shihezi Formation yielded an oil output of 24.26 t and gas output of 80 122 m³ a day, without H₂S. Well YG2 revealed pure gas layers of nearly 100 m thick in total in Lower Shihezi Formation, which was tested a gas output of 178 897 m³ and condensate output of 18.8 t a day in oil testing (Tables 1 and 2). The breakthrough in Wumaying buried-hill exploration proved again the existence of large primary hydrocarbon reservoir without H₂S in the Upper Paleozoic Permian System.

The Qibei buried hill lies in the hinterland of the southwestern Qikou sag. It is a faulted nose buried hill, with an exploration area of 200 km², under the Paleogene slope. Previous study showed Mesozoic and Paleozoic buried hills in Qibei were in good contact with the Paleogene source rock, making it possible to form buried-hill reservoirs with young source rock and age-old reservoir rock. Three exploratory wells including QG1 were then drilled, all of them had active oil and gas shows in the Paleozoic and Mesozoic, and small amount of H₂S detected in the Ordovician System. In recent years, following the idea of Paleozoic petroleum system dominated by Carboniferous source rock, favorable traps of buried hills with age-old source rock and reservoir rock were reinterpreted to be 24 km². Well QG8 was drilled in 2017, revealing Paleozoic and Mesozoic oil and gas layers of 121.2 m thick. Among them, Ordovician oil and gas layers were interpreted to be 64.9 m. After acidizing and fracturing of Fengfeng Formation, the well obtained a daily gas output of 162 800 m³ and daily oil output of 46.3 t; with no H₂S (Tables 1 and 2). Due to the success of QG8, the resources in gas traps with age-old source rock and reservoir rock were estimated to be 600×10⁸ m³, demonstrating primary reservoirs with Carboniferous-Permian coal-measure source rock in the Huanghua Depression have large potential resources.

In addition to the Wumaying and Qibei buried hills, thick oil and gas layers have also been found in the Nandagang and Chenghai buried hills, and prolific gas and condensate oil were yielded in oil testing. Chenghai buried-hill structural belt in the south margin of the Qikou sag is an anticlinal buried-hill structure caused by bedrock faulting that finally fixed in shape in the Paleogene Period. Due to rift faulting at basin margin since the Paleogene Period, the Paleozoic and Mesozoic are overlain by Oligocene, Miocene, and Pliocene formations of 3 400 m thick; thus, it is a typical buried-hill structure inside the deep zone. Early stage evaluation showed that this buried hill was characterized by well-preserved Mesozoic and Carboniferous formations, underdeveloped karstic reservoirs, and poor hydrocarbon accumulation conditions in Ordovician buried hill. Thus, the degree of exploration in the Chenghai buried hill was extremely low. Since 2008, a new hydrocarbon accumulation model of “dark caprock and lateral migration” has been proposed based on systematic structural interpretation, in which Paleogene and Carboniferous-Permian source rocks are in direct contact with Mesozoic and Paleozoic buried hills in general, with big oil and gas supply window. Thus, the structures connected with the Zhangbei and Qidong faults should be the exploration targets of first choice. The risk exploratory well HG1 was drilled and obtained prolific gas flow from the Ordovician System, with a daily gas output close to 30×10⁴ m³, but the content of H₂S is high. Oil and gas shows were also detected in the Permian System. Well HG102 deployed in 2018 had an industrial oil output of 8.19 t and gas output of 1 905 m³ a day from the Permian System. In addition, Well QG6 drilled in the Nandagang buried hill found thick oil and gas layers of 110 m in the Ordovician Fengfeng and Majiagou Formations and Permian Shihezi Formations, and gas shows were active during well drilling. All these demonstrate large exploration potential of the Paleozoic.

The new discoveries in the Gangbei, Qibei, Wumaying, and Chenghai buried hills prove that there may be several petroleum systems in the Ordovician System and Permian System in the Huanghua Depression, with high oil and gas production. Most of the clastic buried hills discovered are in the central and southern Huanghua Depression, where thick Carboniferous-Permian formations have been preserved, and Permian Upper (P₁s) and Lower Shihezi sandstone has high oil content. In the Ordovician Fengfeng and Upper Majiagou Formations in Chenghai, Wumaying, Wangguantun, and Gangbei buried hills where deposit Carboniferous-Permian formations, oil-bearing carbonate reservoirs were found. The Ordovician reservoirs have higher gas-oil ratio and oil maturity than Permian reservoirs. The analyses of properties of oil and geochemical properties of gas produced from the buried hills show the crude oil has a density of 0.801 9-0.832 4 g/cm³ at 20°C and viscosity of 1.38-1.96 mPa•s at 50 °C. The crude oil features light weight, high freezing point (above 0 °C), and medium to high wax content (generally above 10%). The Ordovician oil has higher maturity than Permian oil, and decreases in density from south to north. Paleozoic buried hills have higher gas-oil ratio of above 4 000 m³/t in general, the gas produced in these buried hills is mainly methane, with a drying coefficient of over 0.90. Gases from Ordovician buried hills often contain acidic gases, e.g. H₂S and CO₂. Gases from Permian buried hills have lower content of non-hydrocarbon components. Different buried hills and layers differ widely in oil and gas production. For example, thick oil and gas layers have been found in the Lower Paleozoic Ordovician System and Upper Paleozoic Carboniferous-Permian Systems in the Gangbei buried hill, but production layers in different wells vary greatly, and the oil production varies from less than ten tons to dozens of tons; this is related to hydrocarbon accumulation conditions.

Paleozoic buried-hill reservoirs in the Huanghua Depression are mostly in direct contact with Paleogene source rock and coal-measure source rock; this means both of them could supply oil and gas for the Paleozoic reservoirs. Oil and source rock correlation shows that the oil from the Wumaying buried hill in the south has nearly L-type C₂₇-C₂₉ steranes and higher content of tricyclic terpane; the oil from the Qibei buried hill in the north has inverse L-type C₂₇-C₂₉ steranes and lower content of tricyclic terpane. But all oils from buried-hills have biomarkers similar with the Carboniferous-Permian coals, i.e. high content of gammacerane, high pristane to phytane ratio (Pr/Ph above 2.5), and relatively low content of homohopane, which are typical features of coal-measure organic matter (Fig. 4). This indicates that the crude oil originated from Carboniferous dark mudstone^[20,21]. In addition, Permian condensate oil from Well Z1502 has a Pr/Ph of 1.0-2.8, namely more pristine, this may be related to oxidation, decarboxylation, and hydrogenation of phytol in terrigenous plant materials under oxidizing condition, reflecting the typical features of coal-genetic oil (Fig. 5). Tricyclic terpane, highly resistant to degradation, can indicate the origin of hydrocarbon. High prepeak of tricyclic terpane biomarkers, high content of C₂₄ tetracyclic terpane, and low content of C₂₆ tricyclic terpane of the oil samples may be attributed to the bacteria action in the sedimentary environment where the continental generative kerogen is. Pregnane is also related to terrigenous higher plants. From the distribution chart of sterane biomarkers, pregnane distribution has some dominance in all samples, indicating the oils from the wells are coal-genetic oil generated by terrigenous organic matter. Gammacerane is resistant to biodegradation, and its content is dependent on high-salinity sedimentary environment. The ratio of gammacerane to C₃₁ hopane is an indicator of water salinity. All the samples have this ratio values below 1.5, suggesting the sedimentary environment was brackish environment. In conclusion, the sedimentary environment in the Wumaying and Qibei buried hills was an oxidizing environment with low water salinity.

The natural gas features high methane coefficient (generally above 0.8) and heavy isotopic values (Table 2). Except the Qibei buried hill with oil-type gas from Paleogene source rock, most gases from the buried-hills differ distinctively from Paleogene oil-type gas in isotopic values (Fig. 6), indicating the gases are also from Carboniferous-Permian coal-measure source rocks.

Through recent studies on buried-hill reservoirs in Gangbei, Wumaying, and Qibei, Paleozoic petroleum system has Upper Carboniferous Taiyuan and Lower Permian Shanxi source rocks; the hydrocarbon generated by these source rocks flowed upward and downward to form the Upper Paleozoic petroleum system and Lower Paleozoic petroleum system, respectively. The former has Lower Shihezi and Upper Shihezi sandstones as major reservoirs and Upper Shihezi-Shiqianfeng mudstone as regional caprock. The latter has Ordovician Fengfeng and Upper Majiagou carbonate rocks as major reservoirs and Taiyuan and Benxi (C₂b) mudstone and coal beds as caprocks.

The basement of the Huanghua Depression is an ancient syncline, where Carboniferous-Permian formations have been well preserved. Source rocks, including coal seams, carbonaceous mudstone, and dark mudstone, mainly occur in the Upper Carboniferous Taiyuan and Lower Permian Shanxi Formations. Although distributed across the whole depression, due to denudation, the source rocks are thin in the north and thick in the south, with a thickness changing between 100 and 300 m^[7]. The source rock is only 150 m thick in Beitang, 200-300 m thick in GG1505 and HG1 well areas in the central part, and thickest (300-450 m) in Wangguantun-Nanpi in the south. Organic maceral in Upper Paleozoic coal seams has 56.7-67.0% vitrinite, 19.2-24.5% inertinite, and 12.3-34.0% exinite + sapropel; the content of hydrogen-rich exinite is relatively high (Table 3). The abundance of organic matter in Taiyuan coal seams is generally above 60%, higher than that in the Shanxi Formation; so does carbonaceous mudstone. Different from coal seams, the mudstones have big differences in organic matter abundance. The concentration of TOC varied from 0.25 to 14.83% with the average of 4.32%; the (S₁+S₂) is 0.45-20.14 mg/g with the average of 7.91 mg/g. Most samples (more than 50%) show medium to low (S₁+S₂) values. In general, the Shanxi Formation has slightly better hydrocarbon generation conditions than the Taiyuan Formation.

Coal-measure source rocks in the central and southern Huanghua Depression vary greatly in thermal maturity, with R_o values generally ranging between 0.5% and 1.5%. In some regions, due to the impact of igneous intrusion (e.g. the southern Cangxian Uplift and Dongguang area), R_o may exceed 2.0% and reach 3.73% at most. The thermal maturity of source rock in this area is mainly dependent on the process of burial. Through restoration of single-well denudation thickness, burial history, and thermal history, it is concluded that the Upper Paleozoic Erathem in the central and southern Huanghua Depression may have four types of burial history, i.e. sustained deep burial, early uplifting and later burial, periodic burial, and early burial and later uplifting^{[7, 22-25]}, and different types of burial history have diffrent processes of hydrocarbon generation. For example, the study of Well WS1 drilled in Wumaying area shows that the Carboniferous-Permian source rocks in this region were buried at 2 812 m during the Upper Jurassic-Lower Cretaceous which have experienced the first stage of hydrocarbon generation. The R_o value is over 0.5%. When this region was uplifted and denuded at the end of the Mesozoic Era, hydrocarbon generation suspended. Until the end of the depositional stage of the Paleogene Kongdian Formation, the second stage of hydrocarbon generation started (Fig. 7).

Most buried hills in Qi'nan, Qibei, and Chenghai experienced secondary hydrocarbon generation in a later deep burial, where the organic matter has higher thermal maturity and the value of R_o more than 1.5%. In contrast, the Kongdian buried hill, Gangxi, and the Xuhei buried hill were uplifted and then buried shallow after a long period of evolution, where the duration of secondary hydrocarbon generation was short, and the source rock has lower thermal maturity. The R_o value is usually 0.6-1.0%. The other regions didn’t reach the threshold buried depth of petroleum generation in the late Mesozoic, and didn’t have secondary hydrocarbon generation, and the source rock R_o value is less than 0.5%. In summary, Carboniferous-Permian source rocks in most areas of southern Huangye Depression experienced secondary hydrocarbon generation locally in the Paleogene-Neogene Periods. There are two great gas reservoirs, one in the Qibei-Chenghai area with the maximum gas generation ability of 190×10⁸ m³/km², the other one is in Wumaying-Wangguantun area, with the maximum gas generation ability of 180×10⁸ m³/km², and the total area is 5600 km², which are favorable exploration areas for primary hydrocarbon reservoirs (Fig. 8).

3.2.1. Features of Permian clastic reservoirs

Upper Paleozoic Permian buried-hill reservoirs in the Huanghua Depression are mainly clastic rock ones, including Lower Shihezi and Upper Shihezi sandstone.

Lower Shihezi sandstone is continental fluvial facies depositing after marine regression. With wide range of river beds, well-developed sandbodies, and stable lateral variation, it is the most promising reservoir series in the Upper Paleozoic of the Huanghua Depression. The sandstone layers are more than 70 m thick combined in general and 170 m thick at most. They coarsen upward, and have a sandstone-formation thickness ratio of more than 30% and up to 40%. Vertically, the upper part of the Lower Shihezi Formation contains more sandstone than the lower part; the sandstone layers are over 20 m thick at most and generally 10 m thick each. The Lower Shihezi sandstone mainly consists of lithic quartz sandstone and lithic feldspar sandstone (with quartz, feldspar, and clastic debris accounting for 54%, 24%, and 22% of the clastic components, respectively) and some feldspathic quartz sandstone. The clastic particles are medium-good in sorting, subangular, in contact and pore types of cementation.

The Upper Shihezi sandstone has big lateral variation, lower sandstone-formation thickness ratio, distinct zonation and sand content decreasing from east to west. The sandstone in this formation is 30-70 m thick, and thicker in the Chenghai and Kongdian-Wumaying buried hills. Its sandstone-formation thickness ratio is high in the north and low in the south and below 20% in general. Sandstone distribution is stable at the top of the formation. The lower part of the formation mainly consists of mudstone, with no sandstone. The Upper Shihezi formation consists of feldspar lithic sandstone and lithic feldspar sandstone (with quartz, feldspar, and clastic debris accounting for 42%, 25%, and 33% of clastic components, respectively) and some lithic sandstone. Compared with the underlying Lower Shihezi Formation sandrocks, the Upper Shihezi Formation has lower quartz content. The clastic particles of this formation are well to moderately sorted, subangular, in pore type cementation, point-line contact, and grain support, and generally smaller than 0.5 mm in size.

These two packages of sandstone reservoirs have similar petrophysical properties, an average porosity of 8% and 10%, and average permeability of 0.15×10^-3 μm² and 0.13×10^-3 μm², respectively, representing extremely low-porosity low- permeability reservoirs (Fig. 9). Observation and analysis of cast thin sections show that they have different types of pore space due to different grain size. But their pore space is of secondary genesis universally, and there are no primary pores found in them. The pore space is composed of secondary intergranular dissolved pores, intragranular dissolved pores, and intercrystalline micropores. Fractures are rare and only seen in a limited number of samples. Secondary intergranular dissolved pores and intragranular dissolved pores were generated by dissolution of feldspar particles and magmatic debris.

3.2.2. Features of Ordovician weathering crust buried-hill reservoirs

The upper series and part middle series of the Ordovician System in the Huanghua Depression were denuded, and only the Yeli and Liangjiashan Formations in the lower series and the Lower Majiagou, Upper Majiagou, and Fengfeng Formations in the middle series have been preserved. These formations are mainly composed of limestone and dolomitic limestone. Karst reservoirs are most developed in the Upper Majiagou and Fengfeng Formations. The Upper Majiagou Formation is a marine regression cycle, the upper section is composed of limestone sandwiched with micritic and powder-scale crystalline dolostone, and lower in shale content. The thickness of reservoirs is about 300 m in general. The Fengfeng Formation whose residual thickness is 0-200 m, is composed of dolostone, dolomitic limestone, and tuff. And shale content increases from top to bottom. After regional uplifting and karstification in the Indosinian, uplifting and denudation in the Caledonian, and late burial and dissolution for a long period of time, these two sets of carbonate have a mass of dissolved cavities and pores. Cellular karstic caverns are largely distributed at the edge of major extensional faults and quickly decrease in the slope zones of half-grabens. The ancient karstic landform had strong control on the development of reservoir. Residual hills, platforms, dissolved ridges, and slopes experienced intense karstification, where effective pores and cavities below the weathering crust appear 150 m. Karst depressions were filled with sediments with less pores and cavities. Besides karstic caverns, there are fractured Ordovician carbonate reservoirs generated by tectonic reversal and basement rifting since the Mesozoic Era and Cenozoic Era. The well sections in Ordovician with high production all have rich karst fractures and caverns. Fractures and dissolved pores and cavities related to fractures are the main factors affecting high porosity, high permeability, high yield, and stable production of weathering crust buried-hill reservoirs.

The formation process of clastic buried-hill traps in the Upper Paleozoic was closely related to the structural evolution in the Bohai Bay Basin. The Upper Paleozoic buried hills in the central and southern Huanghua Depression experienced isostatic subsidence in the Paleozoic Era. And then inner traps formed in the Jurassic-Cretaceous Periods. The tectonic reversal occurred in the Paleogene Period (Fig. 10). Inner trap forming in the Mesozoic Era and structure activity since the Paleogene Period had important control on hydrocarbon accumulation^[26,27,28].

Due to 3 phases of tectonic deformation and 2 phases of tectonic reversal, Upper Paleozoic buried-hill structures in the central and southern Huanghua Depression show regular distribution pattern. And the Indosinian paleohighs and related structures controlled the distribution of different types of structures. Structure groups with superimposed residual hills and fault blocks could be seen in the denudation areas at both flanks of Indosinian paleohighs. Complicated reverse nappe structures and anticlinal buried hills occur in the southern slopes of Indosinian paleohighs. Due to the impacts of Yanshanian strike-slip faults, the Upper Paleozoic inner structures in this region are very complex. The southeastern Huanghua Depression was a syncline in the Indosinian, where the residual formation is thicker because of weak Mesozoic tectonic deformation. Fault blocks and faulted nose structures forming in the Oligocene Epoch have been preserved. This region is characterized by epigenetic buried hills. Early structures inside buried hills provided traps for hydrocarbon accumulation.

The Carboniferous-Permian fluid inclusions range from 85°C to 170 °C in homogenization temperature and are mostly higher than 120 °C. A small number have homogenization temperature between 100 °C and 120 °C or below 100 °C. Based on burial history and paleo-geotherm history, two phases of hydrocarbon accumulation occurred in the central and southern Huanghua Depression (Table 4).

The first stage occurred at the end of the Yanshanian (Late Jurassic-Early Cretaceous), when some small oil and gas reservoirs formed due to small volume of hydrocarbon generation with the highest temperature of 120 °C (Table 4). These reservoirs were prone to damage in the later stage. The second stage happened in the Himalayan (since the middle Paleogene Period), when the organic matter in Carboniferous-Permian coal-measure source rocks was mature enough to generate large volume of oil and gas. This is the major stage of hydrocarbon accumulation in the Paleozoic buried hills of the Huanghua Depression. Coal-derived gas generated in this period migrated vertically along high-angle faults inside buried hills and then accumulated to form primary Paleozoic gas reservoirs above the source bed. In those areas close to source rocks, Paleogene oil may also travel along permeable Permian sands into buried hills and then travel laterally to structural highs of intra-buried hill traps to form reservoirs of mixed oil and gas sources. For deeply buried Ordovician reservoir-seal assemblages, there are no enough faults to connect source rocks and buried-hill traps. But natural gas generated by coal-measure source rock may travel downward into Ordovician intra-buried hill fractured-vuggy carbonate reservoirs to form large gas pools (Fig. 11). In the middle and late Neogene Period, organic matter in the Carboniferous-Permian Systems became more mature due to increase of geothermal temperature. Thus, the source rock of Carboniferous-Permian entered the peak of hydrocarbon generation and expulsion. Structure activities in this period guaranteed primary hydrocarbon accumulation in the buried hills with age-old source rock and age-old reservoir rock in the central and southern Huanghua Depression^[29].

Petroleum exploration for more than 50 years demonstrated that Upper Paleozoic coal-measure source rocks are of great importance to the Bohai Bay Basin, especially to the Huanghua Depression. The Carboniferous-Permian strata within the depression are well preserved, and they contain thick and widespread coal-bearing source rocks, indicating strong potential of hydrocarbon generation. Hence, there are geological conditions for forming primary hydrocarbon reservoirs. It has been confirmed by Paleozoic oil and gas discoveries in the Huanghua Depression since 2017 that the Paleozoic petroleum system is a new domain of reserves and production increase in the Bohai Bay Basin. According to preliminary evaluation, Carboniferous-Permian coal measures in the Bohai Bay Basin have a residual area of 84 795 km² and residual thickness up to 800 m (Fig. 12). Based on geologic conditions in the Bohai Bay Basin, the basin-wide gas resources were estimated at over 1×10¹² m³ at a conservative gas accumulation factor of 2% for secondary hydrocarbon generation and 0.5% for primary hydrocarbon generation. For primary hydrocarbon reservoirs inside Paleozoic buried hills, the degree of exploration is very low at present. According to preliminary estimation for the primary petroleum system, gas reserves in the Wumaying, Qibei, Qi'nan, and Chenghai buried hills may reach 1 027.2×10⁸ m³ and condensate oil reserves may reach 3 383.1×10⁴ t. Paleozoic buried hills in the Huanghua Depression thereby have the potential resources of hundreds of billion cubic meters. Although depressions in the Bohai Bay Basin have big differences in residual thickness and evolution history of Upper Paleozoic formations, based on in-depth examination of Paleozoic source conditions, reservoir-seal assemblages, and structural evolution in the Bohai Bay Basin, Paleozoic petroleum system may become an important resources replacement domain in the mature oilfields.

In recent years, major breakthroughs have been made in Paleozoic hydrocarbon exploration in the Gangbei, Wumaying, and Qibei buried hills of the central and southern Huanghua Depression. Oil and source rock correlation shows that the oil and gas originated from Carboniferous-Permian source rocks, demonstrating that the Paleozoic is an independent petroleum system with promising geologic conditions to form primary oil and gas reservoirs. There are two types of source-reservoir assemblages, i.e. Upper Paleozoic clastic assemblage and Lower Paleozoic carbonate assemblage, in the well-preserved Paleozoic formations of the Huanghua Depression. These reservoir-caprock assemblages and the relatively independent Carboniferous-Permian high-abundance source rocks comprise independent Paleozoic petroleum systems. Multiphase tectonic activities gave rise to two stages of hydrocarbon generation of coal-measure source rocks. The second stage began at the depositional stage of the Kongdian Formation and has lasted to nowadays. The match of the second stage accumulation, reservoir-seal assemblage, and tectonic activity determined the hydrocarbon accumulation scale. The Carboniferous-Permian coal-measure formations in Bohai Bay Basin have wide residual area and large residual thickness. According to features of primary hydrocarbon accumulation in the Huanghua Depression, the resources were estimated at around 1×10¹² m³ preliminarily. With huge exploration potential, the Paleozoic petroleum system may become an important replacement of deep hydrocarbon exploration.