Seismic characterization of ancient lacustrine source-to-sink systems in a rift basin: The Bozhong Depression, Bohai Bay Basin, China

The use of seismic exploration technique to provide reliable reservoir information is a conventional method. However, due to its quality and resolution reasons, it cannot satisfy the detailed research and characterization of reservoirs, especially the clastic reservoir with thin sand body. Diagenesis is a fundamental process in the development and formation of all petroleum reservoirs and is a major contributor to their ultimate physical properties. Based on numerical simulation of diagenesis, a new prediction method called geology prediction techniques is presented to simulate the evolution of the diagenetic stages, diagenetic facies and porosity of clastic reservoirs and ultimately for favorable reservoir prediction. It emphasizes the idea of dynamic quantitative research dominated by process recovery, the most important of which is the establishment of mathematical models, including mineral dissolution models, mineral cementation models and sediment compaction models using the experimental data in study area and the results of previous studies. The essence of this method is illustrated, and its effectiveness is proved using Ed1 clastic sandstones in the Bozhong depression, Bohai Bay Basin, China. At present, the reservoir is in the early diagenetic stage B (IB) and the middle diagenetic stage A1 (IIA1). The major diagenetic processes that influence the porosity of the sandstones in study area are mechanical compaction, carbonate cementation, quartz cementation, clay cementation, feldspar dissolution and carbonate dissolution. There are three types of sandstones including fine sandstone, siltstone, and argillaceous siltstone, and the variation range of primary porosity of these sandstones is from 26% to 38%. Compaction and carbonate cementation are the main reasons for porosity reduction, with porosity loss percentage by compaction (P-Com) and porosity loss percentage by cementation of carbonate (P-C-Car) being 53.1% ~ 7.8% (av. 41.9%) and 53.1% ~ 7.8% (av. 18%), respectively, while carbonate dissolution and feldspar dissolution can greatly improve reservoir physical property, with porosity increase percentage by dissolution of carbonate (P-D-Car) and porosity increase percentage by dissolution of feldspar (P-D-Fel) being 0 ~ 9.9% (av. 8.9%) and 0 ~ 27.8% (av. 9.4%), respectively. The predicted porosities match the measured porosities well. Cited as : Qian, W., Yin, T., Hou, G. A new method for clastic reservoir prediction based on numerical simulation of diagenesis: A case study of the Ed1 clastic sandstones in the Bozhong depression, Bohai Bay Basin, China. Advances in Geo-Energy Research, 2019, 3(1): 82-93, doi: 10.26804/ager.2019.01.07

Geochemical characteristics of He and CO2 from the Ordos (cratonic) and Bohaibay (rift) basins in China

The Shijiutuo uplift is a major uplift to the north of the Bozhong depression, the largest generative kitchen in the Bozhong subbasin, Bohai Bay Basin. Although the N35-2 trap on this uplift contains a medium-size oil accumulation and the Q32-6 trap contains China's third largest offshore oil accumulation, the Q31-1 trap between the N35-2 and Q32-6 traps with very similar evolution history was confirmed to be dry. Biomarker associations of crude oil and source rock samples were analyzed, and three-dimensional migration pathway modeling was conducted to investigate the origin of oils and mechanisms for oil enrichment and depletion on the uplift. Multiple-parameter oil-source correlation and hierarchical cluster analysis using 10 selected biomarker parameters allowed the identification of four source-related oil classes. Almost all oils from the Shijiutuo uplift are derived from the Eocene Shahejie Formation, whereas oils found between the Shijiutuo uplift and the Bozhong depression either are derived from or have important contributions from the Oligocene Dongying Formation. Variations in oil classes and biomarker parameters suggest sequential migration of oil generated from the Shahejie and then Dongying formations in the Bozhong depression, which is reasonably supported by petroleum migration pathway modeling. Oil charge from two oil-prone source rock intervals and, more importantly, focusing of oil originating from a large area of the Bozhong generative kitchen into the same trap accounted for oil enrichment and formation of China's third largest offshore oil field in the Q32-6 structure. The complexity and primary control of the sealing surface (top surface of the carrier bed) morphology on the positions of migration pathways caused the Q31-1 trap to be shielded from migration of oil originating from the Bozhong depression, resulting in oil depletion in this trap. Shadows to petroleum migration may occur because of the three-dimensional behavior of petroleum migration, and two-dimensional migration modeling may be misleading in predicting petroleum occurrences.

The Mesozoic and Cenozoic tectonic evolutionary history of the East Asian continental margin has been the focus of many researchers because of the overprinting of multiple tectonic domains. Previous studies have suggested that the westward subduction of the Paleo-Pacific Plate and the Pacific Plate resulted in the deconstruction of the North China Craton and controlled the formation of the related basins on the continental margin of East Asia. However, controversy remains regarding the tectonic transition processes and mechanisms that occurred from the Mesozoic to the Cenozoic. Since the Mesozoic, the Bohai Bay Basin on the eastern margin of the North China Craton of East China has been influenced by multiple tectonic domains of the Paleo-Tethys, Paleo-Pacific, and Pacific oceans, and there are complete records of these tectonic transition processes. The Bozhong Depression is a subbasin in the Bohai Bay Basin, which is a crucial area for researching the tectonic evolution of the Bohai Bay Basin throughout the Mesozoic–Cenozoic and the regional tectonic evolution of the eastern continental margin of China. Based on 3-D seismic data, logging core data, and a balanced cross section in the Bozhong area, combined with data from the apatite fission-track inversion model, we reconstructed the tectonic evolutionary history of central Bohai Bay Basin and established a three-cycle and eight-stage tectonic model of the central Bohai Bay Basin during the Mesozoic–Cenozoic. The three cycles are the Indosinian, the Yanshinian, and the Himalayan. (1) The Indosinian was marked by two stages. During the early Indosinian, NW-trending thrust faults were formed due to the collision and northward subduction of the South China Block underneath the North China Block. In the late Indosinian, the tectonic stress in the central Bohai Bay Basin shifted from compression to extension. Consequently, the thrust faults reversed, leading to the deposition of Early–Middle Jurassic strata. (2) The Yanshanian cycle comprises three main phases. Early Yanshanian transpressional shearing led to the formation of a NE/NNE-trending, left-lateral strike-slip fault due to NWW-directed subduction of the Paleo-Pacific Plate. Middle Yanshanian transtensional shearing was driven by Paleo-Pacific Plate rollback and resulted in regional extension and the negative inversion of previous compressive faults. Late Yanshanian compression gave rise to the basin reversion, which resulted from an increased subduction speed of the Paleo-Pacific Plate and a transition from a high angle to a low angle. (3) The Himalayan cycle was marked by three phases. During the early Paleogene, the region was characterized mainly by extension, and NE-trending, right-lateral strike-slip normal faults began to form. This coincided with a decrease in the Pacific Plate’s subduction speed. In the late Paleogene, the subduction rate of the Pacific Plate increased, resulting in the change of the central Bohai Bay Basin from an extensional environment to one marked by regional differential compression. In the Neogene, regional thermal subsidence and depression sedimentation occurred, which were probably induced by the increasing subduction speed and rollback of the Pacific Plate. The Bozhong Depression has experienced multiple stages of tectonic evolution, which indicates the concurrent and superimposed effects and transition of multiple tectonic domains.

The Bohai Bay Basin, located in northeast China, is a Meso-Cenozoic strike-slip extensional basin. A lot of research work carried out in Bohai Bay Basin, has shown that there is a huge potential of oil and gas resources. However, the proportion of known oil and gas reserves to the total estimated resources is not high, which means that this area still has broad exploration prospects. Although oil and gas resources are mainly distributed in sedimentary basins, their enrichment degree is largely influenced by the structure and development of the lithosphere. Based on lithospheric local isostasy theory and thermal conduction principle linked to temperature dependence of rock densities, the three-dimensional deep structure of the lithosphere under the Bohai Bay Basin is calculated by using geoid and gravity anomalies, topographic and existing geological-geophysical data. The results show that the lithosphere-asthenosphere boundary of Bohai Bay Basin gradually rises from the western onshore to the eastern offshore area from 90 to 110 km. The thinnest lithosphere is found under the Bozhong Depression in the southeast of the Bohai Bay Basin. It is concluded that the thinning of the lithosphere in the Bohai Bay Basin is closely related to the subduction of the Meso-Cenozoic Pacific plate, which led first to thickening followed by delamination of the North China Craton lithosphere, and then the magma upwelling led to slow uplift of the Earth’s surface and continuous stretching of the lithosphere. At the same time, favorable conditions of temperature, pressure, chemistry and structure were provided for the formation of oil and gas. In this wqy, the Bohai Bay basin developed into the present oil-rich basin. This study provides a new perspective for understanding the deep structure and hydrocarbon resource control mechanisms of Bohai Bay Basin.

The Bohai Bay and Hehuai (southern North China) rift basins in the eastern part of the North China Craton are south-north-adjacent. They have shown synchronous evolutionary processes, and possess generally identical superficial and shallow structural characteristics as well as similar basin areas. However, there is a large difference in the richness of oil resources between the two basins. The Bohai Bay Basin has extremely abundant oil reserves, while commercial oil reserves have not been found in the Hehuai Basin. The deep tectonic structures, magmatic activities, and modern and paleo-geothermal fields of the two basins are significantly different. Compared with the Hehuai Basin, the Bohai Bay Basin has a thinner crust and more complex structure with multiple low-velocity layers. It is also characterized by intense magmatic activity, high modern and paleo-geothermal fields, frequent seismic activity, and active deep interactions, small effective elastic thickness of the isotropic lithosphere, and shorter balanced transformation wavelength of the lithosphere with a high likelihood of local compensation. The Hehuai Basin has a simple deep structure and homogeneous crustal composition, with a high likelihood of regional compensation. The characteristics of the deep structures mentioned above are generally similar to those of the southern part of the stable Ordos Basin, except for the smaller crust thickness. This indicates the presence of differences in Mesozoic destruction between the southern and northern zones in the eastern part of the North China Craton. The northern zone was subjected to significant destruction, while the southern zone was subjected to modifications, primarily in the form of local changes in the structures and/or properties of the crust or lithospheric mantle, with the overall structure and stability of the craton kept intact. The formation of high-quality source rock is primarily influenced by the abnormal flourishment of organisms in water bodies during the syndepositional period, and is also strongly associated with the high geothermal setting of basins and nutrients from hydrothermal solutions and volcanoes. In other words, it is mainly controlled by deep processes and deep-major fault activity. The differences in the deep structures and modern and paleo geothermal fields of the two basins correspond to the difference in richness of oil resources, suggesting that there is an important internal or causal relationship between the two aspects. This viewpoint coincides with the conditions and environments required for the development of high-quality source rock in hydrocarbon-rich basins (sags) in China and other countries, and is evidenced by the modern lake basin of the East African Rift. A new hydrocarbon generation model is proposed in this work: petroleum is a comprehensive product of the integration of bioenergy, thermal energy, and other related energies (such as chemical and kinetic energy) and their interactions; the degree of richness of petroleum is generally controlled by the regional tectonic structure, thermal environment, and deep processes; nonmarine basins or depressions with abundant resources are closely related to active deep processes, intense exchange of material between the deep and shallow layers, participation of external hydrocarbons, and energy integration and conversion.

CCUS (Carbon Capture, Utilization and Storage) is one of the key technologies to cope with global climate change, and an effective way to reduce greenhouse gas emissions on a large scale and mitigate global warming in the future. Therefore, the evaluation and target selection of favorable areas for CO2 sequestration are crucial. Based on the five primary geotectonic units in the Bohai Bay Basin, i.e., Canning Rise, Liaodong Bay Depression, Bohai Central Depression, Jiyang Depression, and Huanghua Depression, the entropy-weighted TOPSIS method was used to evaluate the conditions for CO2 storage based on the conditions of the Bohai Bay Basin, such as sedimentation (sedimentary thickness, sedimentary phases), tectonics (fracture, seismicity, etc.), storage-cover assemblage, geothermal temperature, heat flow, etc. The entropy-weighted TOPSIS method was used to evaluate the conditions for CO2 storage. Considering the factors of geological safety, storage scale and economic appropriateness, a CO2 storage appropriateness evaluation index system consisting of 3 first-level indexes and 11 second-level indexes is constructed, and the establishment of this index system is of reference significance for screening and determining the favorable areas of CO2 storage in the Bohai Bay Basin. The evaluation results show that the order of CO2 storage suitability in the Bohai Bay Basin is as follows: Huanghua depression, Bozhong depression, Liaodong Bay depression, Jiyang depression, and Canning uplift.

The Bohai Bay Basin (BBB) is the most petroliferous Cenozoic basin in the east of China. It consists of seven depressions. Each depression has been subjected to different stress states and then has experienced varying faulting processes since the Neogene, especially during the Neotectonism (from the Pliocene to the present). On the basis of the investigation of fault patterns, fault densities and fault activity rates (FARs) for each depression, this paper demonstrates the discrepancy of faulting development and evolution across the BBB. The dynamic mechanism for the differences in faulting is also discussed by the analysis of the regional stress state. The Bozhong Depression is just situated in the transtensional zone induced by the two active strike-slip faults, namely Yingkou-Weifang and Beijing-Penglai. In this depression, the major faults which cut through the Paleogene or the Cenozoic have had higher than 10 m/Ma FARs since the Neogene, and the highest FARs have reached or exceeded 25 m/Ma during the Neotectonism. As a result, most of the petroleum has migrated along these major faults and accumulated within the Neogene. In contrast, in the other depressions of the BBB away from the Bozhong Depression, the FARs of the major faults were decreased to lower than 10 m/Ma since the Neogene, and tended to be zero during the Neotectonism. Therefore, the major faults could not serve as vertical conduits for petroleum migration, and the petroleum was entrapped in the Paleogene. Consequently, the faulting since the Neogene, especially during the Neotectonism, controlled the petroleum richness in vertical strata.

Multiple metamorphic buried-hill hydrocarbon reservoirs have been recently discovered in the Bozhong Depression, Bohai Bay Basin. Among them, the buried-hill reservoirs in the BZ13, BZ19 and BZ26 regions have the largest condensate gas reserve in eastern China. The hydrocarbon-bearing reservoir has a large thickness with a maximum of ∼1 km, making the oil-equivalent reserve exceed 5 × 108 tons. This study used petrography, isotopes, gas composition, zircon U–Pb geochronology, X-ray diffraction, electronic probe and hydrothermal dissolution experiments to discuss the formation mechanisms of the metamorphic buried-hill reservoirs. The results indicate that the reservoirs 500 m below the surface of the buried hill have been significantly altered by the mantle-derived CO2. This is supported by the carbon isotope of CO2, He isotope of the helium gas, strong negative excursions in δ18OPDB, positive excursions in δ13CPDB, Co/Ni ratio greater than 1 in pyrite and secondary characteristic minerals such as dawsonite. Hydrothermal dissolution experiments using CO2 result in the precipitation of kaolinite, ankerite and quartz cements, which is consistent with petrographic observation. The mantle-derived CO2 reacts with feldspars, hornblende, biotite and carbonates, creates secondary dissolution pores and significantly improves reservoir quality. Moreover, partial carbonate cements exhibit negative excursions in δ13CPDB and the depletion of light rare earth elements, indicating the influence of organic acids-bearing low-temperature hydrothermal fluids. Therefore, three development models illustrating the formation of metamorphic buried-hill reservoirs were proposed. This study may serve as a reference for the hydrocarbon exploration in metamorphic buried-hill reservoirs.

Thermal anomalies can be caused by fast and intensive tectonism in sedimentary basins. So such thermal anomalies should be eliminated when the thermal history is used to reveal the geodynamic process at depth. In this work, based on the dynamic‐instantaneous heat conduction theory, geothermal correction of 3 typical wells in the Bozhong Depression was carried out. The results indicate that the Paleogene rapid subsidence in the Bozhong depression leads to the present geothermal disequilibrium, where average heat flow at present is about 60.9 mW·m−2, but a higher heat flow 67.4 mW·m−2 is yielded after correction of the low thermal anomalies. The average thermal lithosphere thickness in the Bozhong depression is about 70 km, calculated by using the corrected heat flow, which is 13∼28 km thinner than before correction (82∼100 km), consistent with that from geophysical and xenoliths analyses.

Neotectonism occurred intensively in the Bozhong depression in the Bohai Bay Basin, which was reflected vertically by dramatic subsidence and a number of uplifts and laterally by notable fault movements. This particularity has resulted in the special petroleum geological conditions of the Bozhong depression which are different from those of adjacent lands. For example, the source rocks of the Shahejie Formation were overpressured and hydrocarbon generation occurred in the late stage; the Dongying Formation was deeply buried below the hydrocarbon‐generating threshold, therefore there were sufficient oil sources. The rapid subsidence led to starved sedimentation of the Guantao Formation fine sandstone and the regional Minghuazhen Formation lacustrine shale, which formed the Neogene regional reservoir‐caprock association. The active faults formed in the neotectonism became passages for oil to migrate from the Paleogene to Neogene. The traps formed by late fault activity and accompanied anticlines provided spaces for the formation of reservoirs. All the above factors match well with one another in the Bozhong depression, providing favorable conditions for the formation of a series of large oilfields in the region

Seismic attributes, such as P- and S-wave velocity, Poisson’s ratio, and acoustic impedances, all generally can be used for distinguishing different rock types. The non-uniqueness can be largely reduced using Lame impedances instead of acoustic impedances as additional constraints. We have followed this method to constitute a petrologic composition model of the upper crust in the Bohai Bay basin, China. We briefly review the seismic parameters used for discrimination of rock types and focus our attention on the sensitivity of different combinations of parameters to determine the composition of materials. Corrections for pressure and temperature are performed in order to compare elastic wave velocities and densities measured at room temperature and surface pressure in laboratory with those for representative rock parameters. In a second step, we find the rock classes in the tested area by contrasting known data to laboratory measurements on a variety of rock samples extracted in the area. The basic field data are P-wave velocity values collected along a seismic profile conducted in the Bozhong Depression. The different rock types belonging to a particular rock class are finally constrained by the seismic velocities, Poisson’s ratio, density, acoustic impedance, and Lame impedance related to the topmost 10 km of the Bohai Bay crust.

The Bohai Bay Basin is a Cenozoic extensional basin along the eastern aspect of Asia. Whether the Bohai Bay Basin is a pull-apart or rift basin is controversial. The Bohai Bay Basin exhibits a high density of extensional faults and records destruction of the North China Craton. Many structural analyses have been performed on the Bohai Bay Basin, especially the Tan-L and Taihang Mountain fault systems which control its boundary. The initial deposition of Kongdian Formation was mainly distributed along the boundary of Bohai Bay Basin during the Palaeocene–early Eocene. Subsequently, tectonic activity migrated toward the interior of the basin during deposition of Shahejie Formation in the middle Eocene–early Oligocene. Bohai Bay Basin crust was thickened in early Mesozoic time and has thinned since late Mesozoic time. The crustal strength profile of Bohai Bay Basin is characterized by very weak lower crust, which differs from that of adjacent crust. In regard to the crustal structure, lithospheric thickness, and extensional style, an alternative rift model is proposed. Initial Bohai Bay Basin rifts were characterized by metamorphic core complexes affecting the North China Craton, which reflects collapse of parts of the early Mesozoic intra-plate orogen. Furthermore, westward subduction of the Palaeo-Pacific Plate led to upwelling of asthenosphere mantle. Persistent upwelling of mantle decreased the strength of lower crust and led to the warm heat-flow regime and generation of a lower crustal fluid layer and wide rifting. Outward flow of ductile lower crust following late Cretaceous extension thinned the lower crust and generated the overall sag appearance of the basin in early Cenozoic time. The model supports a model whereby a wide rift narrows with time. For the Bohai Bay Basin, extension and strike-slip faulting were two independent deformation systems superimposed on each other.

The Penglai 19-3 (PL19-3) oil field, the largest offshore oil field in China, was found in shallow reservoirs (700–1700 m, 2297–5577 ft) within the most active fault zone in east China. The PL19-3 anticline was not finally formed until about 2.0 Ma and is cut by densely distributed faults. Source rock and crude oil samples from the PL19-3 field were analyzed to determine the origin and formation mechanisms of this large oil field. Three organic-rich, oil-prone source rock intervals exist in the Bozhong subbasin, each of which has a distinct biomarker assemblage. Oil samples from different wells have different biomarker associations, and three source-related oil classes were identified within the PL19-3 field based on biomarker compositions and multivariate analysis of the data. The PL19-3 field displays considerable compositional heterogeneity. The compositional heterogeneity within the field and comparison between oil samples from the PL19-3 field and those from nearby structures suggest three field-filling directions, which is consistent with the results of migration pathway modeling. The PL19-3 field was charged in the north by oil generated from Dongying Formation source rocks in the eastern Bozhong depression and Bodong depression, in the southeast by oil generated from Shahejie Formation source rocks in the Miaoxi depression, and in the northwest by oil generated from Shahejie Formation source rocks in the central Bozhong depression. Oil charge from multiple source rock intervals and multiple generative kitchens and focusing of oil originating from a large area of the Bozhong depression into the same trap resulted in rapid oil accumulation in the PL19-3 structure and the formation of this large oil field in a very young trap within an active fault zone.

Archaean metamorphic basement reservoirs, characterized by the development of natural fractures, constitute the primary target for oil and gas exploration in the Bozhong Depression, Bohai Bay Basin, Eastern China. Based on analyses of geophysical image logs, cores, scanning electron microscopy (SEM), and laboratory measurements, tectonic fractures are identified as the dominant type of natural fracture. Their development is primarily controlled by lithology, weathering intensity, and faulting. Fractures preferentially develop in metamorphic rocks with low plastic mineral content and are positively correlated with weathering intensity. Fracture orientations are predominantly parallel or subparallel to fault strikes, while localized stress perturbations induced by faulting significantly increase fracture density. Open fractures, constituting more than 60% of the total reservoir porosity, serve as both primary storage spaces and dominant fluid flow conduits, fundamentally governing reservoir quality. Consequently, spatial heterogeneity in fracture distribution drives distinct vertical zonation within the reservoir. The lithological units are ranked by fracture development potential (in descending order): leptynite, migmatitic granite, gneiss, cataclasite, diorite-porphyrite, and diabase. Diabase represents the lower threshold for effective reservoir formation, whereas overlying lithologies may function as reservoirs under favorable conditions. The large-scale compressional orogeny during the Indosinian period marked the primary phase of tectonic fracture formation. Subsequent uplift and inversion during the Yanshanian period further modified and overlaid the Indosinian structures. These structures are characterized by strong strike-slip strain, resulting in a series of conjugate shear fractures. During the Himalayan period, preexisting fractures were primarily reactivated, significantly influencing fracture effectiveness. The development model of the fracture network system in the metamorphic basement reservoirs of the study area is determined by a coupling mechanism of dominant lithology and multiphase fracturing. The spatial network reservoir system, under the control of multistage structure and weathering, is key to the formation of large-scale effective reservoirs in the metamorphic basement.