Rift Basins In Eastern China Research Articles

Anomalous Post‐Rift Subsidence in the Bohai Bay Basin, Eastern China: Contributions From Mantle Process and Fault Activity

AbstractThe Bohai Bay Basin is the largest Cenozoic rift basin in eastern China, which exhibits a high rate of post‐rift subsidence deviating from the theoretical exponentially decay trend of thermal subsidence. The driving force for this phenomenon remains an outstanding question. Here we quantify the spatial and temporal distribution of the anomalous post‐rift subsidence by removing the thermal subsidence related to earlier stretching events from the observed tectonic subsidence. A multi‐episodic finite extension model is employed to estimate the stretching factors during rifting for nine profiles and 48 wells covering the basin. Our results show that the anomalous subsidence commenced at 12 Ma and the average anomalous subsidence rate accelerated from ∼19 m/Myr during the late Miocene to ∼75 m/Myr during the Quaternary, reaching ∼400 m at present. The anomalous subsidence is compared with published changes in dynamic topography arising from mantle flow. The temporal evolution of the dynamic topography generally fits the evolution of the anomalous subsidence, whereas variation in dynamic topography exhibits smaller amplitude and larger wavelength than our results. Small‐scale convection in the shallow mantle might play a role in generating short‐wavelength topography disturbances in the past tens of million years. Besides, the spatial distribution of the Quaternary anomalous subsidence generally coincides with fault movements in the past ∼2 Myr. We suggest mantle processes as well as fault activities might be possible mechanisms accounting for additional accommodation in the basin.

Impact of input, preservation and dilution on organic matter enrichment in lacustrine rift basin: A case study of lacustrine shale in Dehui Depression of Songliao Basin, NE China

Lacustrine rift basins in Eastern China possess large amounts of petroleum, especially the shale gas and tight sand gas in deep formations, where “sweet spots” are closely related to organic-rich shale intervals. However, the OM enrichment of deep formation shale in lacustrine rift basins and the main controlling factors have not been well-researched in previous studies. The organic matter (OM) enrichment is mainly controlled by the OM input, preservation, and/or dilution. The shales deposited in deep formations of Songliao Basin, including the Jurassic Huoshiling (J3h), Cretaceous Shahezi (K1sh), and Yingcheng (K1yc) formations, are evaluated as fair to excellent source rocks. Taking the shales as an example, the characters and impact of input, preservation, and dilution on OM enrichment in shales are discussed by utilizing the parameters of element concentrations and biomarkers. Results show that the OM sources are mixed sources of aquatic microorganisms and higher plant materials, and the higher plant materials are dominant. The bio-productivity were moderate to high. The paleoclimate was mainly the cold and semi-humid to semi-arid. The lake salinity was fresh-to brackish-water. The redox conditions were sub-reducing to sub-oxidizing. The dilution degree in Yingcheng was the strongest, followed by the Shahezi and Huoshiling. Consequently, the OM enrichment in the J3h shale was called moderate bio-productivity and preservation model; the OM enrichment in the K1sh shale was called high bio-productivity and preservation model; the OM enrichment in the K1yc shale was named high dilution model. This study not only improves the model of OM enrichment but also provide some information for the research of “Deep time” in the Cretaceous of Songliao Basin, guiding the prediction of organic-rich shale distribution in lacustrine rift basin.

Late Cretaceous-Cenozoic intraplate extension and tectonic transitions in eastern China: Implications for intraplate geodynamic origin

Abstract The geodynamic origin of intraplate extension remains poorly understood. For the case of the Late Cretaceous-Cenozoic intraplate extension in eastern China, its driving mechanism remains highly controversial due to the absence of a comprehensive understanding of significant tectonic transitions. This study uses the well-documented Cretaceous tectonic evolution as a background and integrates extensive multidisciplinary data in the Jianghan Basin, Bohai Bay Basin and Zhejiang area, to develop an integrated view of the Late Cretaceous-Cenozoic geodynamic origin for the first time. During the Late Cretaceous, rifting and depocentres were distributed in the Jianghan Basin and were located in the central Bohai Bay Basin. During the Cenozoic, rifting and depocentres were initially focused in the western basins and then progressively migrated eastward, rather than continuing migrating eastward. The rifting and volcanism during the Middle Cenozoic (50-23 Ma) significantly increased, and all rift basins in eastern China were abandoned at the end of the Middle Cenozoic (~26-23 Ma). The volcanism in the Zhejiang area was markedly enhanced during the Late Cenozoic (23-0 Ma). The asthenospheric mantle sources beneath the two rift basins were isotopically more depleted with an increased extent of melting from the Late Cretaceous (100-65 Ma) to Early Cenozoic (65-50 Ma); however, they became isotopically more enriched and heterogenous with a decreased melting extent (significantly reduced volatile content) from the Early Cenozoic to Middle Cenozoic. The mantle source beneath the Zhejiang area during the Late Cenozoic was isotopically more depleted and heterogeneous than during the Middle Cenozoic, with an increased extent of melting. These three-phase tectonic transitions together with tectonic correlations directly constrain the causal relationships between the intraplate extension in eastern China and the motions of the adjacent plates. Consequently, we propose that the Late Cretaceous-Cenozoic intraplate extension and tectonic transitions in eastern China were successively driven by the Paleo-Pacific slab rollback during the Late Cretaceous, the India-Asia collision during the Early-Middle Cenozoic and by the combination of the subduction of the Pacific Plate and India-Asia collision during the Late Cenozoic. Our study provides an integrated perspective for unraveling the geodynamic origin of intraplate extension.

The sedimentary environment of Early Cretaceous rift basin in eastern China and its response to the Faraoni event

To better understand the sedimentary environment of the rift basin under the influence of a warm global climate, Lingshan Island in eastern Shandong Province (China) was studied. Inorganic geochemical indexes (B, equivalent B, B/Ga, Ga, V, Couch’s palaeosalinity, Adams’s palaeosalinity, Sr/Ba, Sr/Cu, Rb/Sr, Al2O3/MgO, and CaO/MgO·Al2O3) were analyzed and a quantitative calculation of palaeosalinity was carried out based on the Adams and Couch methods. The sedimentary environment of the rift basin at Lingshan Island was determined according to the morphology and distribution of its palaeobiota as well as inorganic geochemical indexes. We demonstrate that eastern China had high-temperature drought-like conditions during the Early Cretaceous, which may have been influenced by the Faraoni event. The dry-hot climate transformed the rift basin at Lingshan Island into saline lacustrine basin. Therefore, the salinization of water was not influenced by seawater but was the response of terrestrial strata to the warm, dry climate. This study helps to understand the sedimentary background of Cretaceous rift basins in eastern China and the influence of a warm climate on China’s terrestrial strata.

Relationships between tectonic activity and sedimentary source‐to‐sink system parameters in a lacustrine rift basin: A quantitative case study of the Huanghekou Depression (Bohai Bay Basin, E China)

AbstractLacustrine rift basins commonly preserve a fairly complete record of the sediment source‐to‐sink (S2S) system, and consequently may form an ideal natural laboratory for establishing quantitative relationships between the various elements within the S2S system. The tectonic‐activity rate in the source (e.g., fault‐growth rate and fault‐activity rate), accommodation space and depositional system in the sink (e.g., areal extent and volume, as well as the depositional dip of the fan‐ and braid‐deltas) are genetically related and their quantitative correlations are explored. The Palaeogene succession on the southwestern margin of the Huanghekou Depressionin the Bohai Bay Basin, one of the largest lacustrine rift basins in eastern China, was chosen to study these relationships, using 3‐D seismic, core and well‐log data. The tectonic activity was strongly related to the sediment supply, accommodation space and morphology of the sink area. Three different rates of tectonic activity are identified; these led to changes in the basic features of the S2S system that influenced each other. In Members 4 and 3 (lower unit) of the Shahejie Fm. (40.44–44.7 Ma), strong tectonic activity led to significant uplift, resulting in the widest exposure of the provenance area to erosion, to a high sediment‐supply rate, to a steep slope and to a large accommodation space which controlled the development of several fan‐deltas with steep progradational angles. In Member 3 (upper unit) of Shahejie Fm. (37.89–40.44 Ma) and Member 3 of Dongying Fm. (30.2–33.28 Ma), decreased tectonic activity led to slower uplifting, resulting in a wider alluvial plain, longer transport distances, a lower sediment‐supply rate and less accommodation space, so that braid‐deltas with larger volumes and a gentler slope developed; In Members 1 and 2 of Shahejie Fm. (33.28–37.89 Ma) and Member 2 of Dongying Fm. (26.71–30.2 Ma), still further decreasing tectonic activity led to a still lower sediment‐supply rate, a more gentle depositional slope, less accommodation space, and the development of several braid‐deltas with a gentle angle. The quantitative relationships established here advance our understanding of the relationships within lacustrine source‐to‐sink systems, especially for tectonically controlled rift basins.

An overview of types and characterization of hot fluids associated with reservoir formation in petroliferous basins

Increasing petroleum explorations indicate that the formation of many reservoirs is in close association with deep hot fluids, which can be subdivided into three groups including crust-derived hot fluid, hydrocarbon-related hot fluid, and mantle-derived hot fluid. The crust-derived hot fluid mainly originates from deep old rocks or crystalline basement. It usually has higher temperature than the surrounding rocks and is characterized by hydrothermal mineral assemblages (e.g. fluorite, hydrothermal dolomite, and barite), positive Eu anomaly, low δ18O value, and high 87Sr/86Sr ratio. Cambrian and Ordovician carbonate reservoirs in the central Tarim Basin, northwestern China serve as typical examples. The hydrocarbon-related hot fluid is rich in acidic components formed during the generation of hydrocarbons, such as organic acid and CO2, and has strong ability to dissolve alkaline minerals (e.g. calcite, dolomite, and alkaline feldspar). Extremely 13C-depleted carbonate cements are indicative of the activities of such fluids. The activities of hydrocarbon-related hot fluids are distinct in the Eocene Wilcox Group of the Texas Gulf Coast, and the Permian Lucaogou Formation of the Jimusaer Sag and the Triassic Baikouquan Formation of the Mahu Sag in the Junggar Basin. The mantle-derived hot fluid comes from the upper mantle. The activities of mantle-derived hot fluids are common in the rift basins in eastern China, showing a close spatial relationship with deep faults. This type of hot fluid is characterized by high CO2 content, unique gas compositions, and distinct noble gas isotopic signatures. In the Huangqiao gas field of eastern China, mantle-derived CO2-rich hot fluids have created more pore spaces in the Permian sandstone reservoirs adjacent to deep faults.

Hydrothermal indications of Early Cretaceous red beds in lacustrine successions, North Yellow Sea Basin, eastern China

Controversies over the origin of globally distributed Cretaceous red beds have mainly focused on climate vs. weathering with potential hydrothermal influence. In comparison to Cretaceous Oceanic Red Beds, Cretaceous lacustrine red beds exhibit chronological inconsistencies and their diverse tectonic settings suggest differing genetic mechanisms. In this paper, Early Cretaceous red beds in the North Yellow Sea Basin, one of Mesozoic rift basins in eastern China that are associated with multi-phase magmatic activities, was chosen to document potential hydrothermal impact. Geological and geochemical analyses show that these red beds were syndepositional products developed in shallow to moderately deep lacustrine environments and that they were influenced by hydrothermal activity. Major and trace elements, including enhanced Al2O3-normalized Fe2O3 and Fe2O3 (total) values, Al/(Al + Fe + Mn) and (Fe + Mn)/Ti ratios, Ni-Co-Zn and Fe-Mn-(Ni + Co + Cu)*10, ∑REE-La/Yb, the concentrations of hydrothermally-related ions, invalid redox indicators of trace elements, abnormally high Fe2O3 (total) content, positive Eu anomaly, correlation of Fe2O3 (total) and Eu anomaly, and vitrinite reflectance (Ro) vs. total organic carbon (TOC), all show strong hydrothermal fluid influence and heating effects. By integrating and comparing data from previous studies, we propose a hydrothermally affected sedimentation model for Cretaceous red beds and associated sediments in magma-activated lacustrine basins, and conclude that the enriched metallic ions introduced by the hydrothermal fluids may be the most significant reason for the deposition of the red beds, and the flushed ferric ions, accompanied by hydrothermal fluid, are the main ions that are hosted inside hematite causing the red sediment. This model can be applied to similar lacustrine basins that experience strong magmatic activity.

Hydrocarbon phase limit and conversion process in the deep formation from Qikou Sag of Bohai Bay rift lacustrine basin, China

It is an important research direction for the prediction of hydrocarbon phase in the reservoir during the deep exploration of rift basins in eastern China. The typical lacustrine crude oil of Shahejie Formation in Qikou Sag was used for oil cracking to gas simulation experiment by the gold tube in this paper. Then, the kinetic parameters obtained from the experimental data were studied during the cracking reactions and the hydrocarbon phase conversion process under geological conditions. The results showed that the activation energy of the oil cracking to gas from the BH-28 lacustrine crude oil ranged from 250 kJ/mol to 270 kJ/mol with an average of EO = 255.47 kJ/mol, and the frequency factor was about A = 1014 s−1. It is higher than that of typical marine oil EO = 246.97 kJ/mol. Using these kinetic parameters, the simulated cracking process of the lacustrine oil of Shahejie Formation was far different from that of marine oil from western China under the same geological heating rate (3 °C/Ma). And the simulated oil cracking degree in Well Niudong-1 is 73.9%, almost the same with the conversion cracking degree (60%–75%) by concentrations of 3,4-dimethyldiamantane. The kinetics of oil cracking gas could be used to investigate the oil cracking degree easily, then to get the theoretical separate oil phase depth limit. By using this model, oil in Qikou Sag, as a theoretical separate phase destruction, occurs above 5700 m depth limit (RO is about 2.0%, reservoir temperature = 209 °C, with cracking transition C of 62.5%), and condensate gas destruction occurs above 6700 m depth limit (RO is about 3.2%, reservoir temperature = 240 °C, with C to 99%) under the geological conditions from Qikou Sag. Actually, the hydrocarbon phase is affected by many factors. For example, migration and changes of temperature and pressure conditions have great influence on phase conversion behavior. The injection of gas, originated from kerogen cracking gas and oil cracking gas in deeper, could increase GOR and the real separate phase depth would reduce.

Neogene residual subsidence and its response to a sinking slab in the deep mantle of eastern China

Mantle convection could have a significant effect on basin evolution; however, research quantifying this relationship is controversial. To understand the formation mechanism and evolution of the Cenozoic rift basins in eastern China, we applied the back-stripping technique and strain rate inversion modeling to 119 wells from Sangjiang Basin, Bohai Bay Basin, North Yellow Sea Basin, South Yellow Sea Basin, East China Sea Shelf Basin, and four basins within the northern South China Sea margin. The modeled results can be used to reconstruct the tectonic subsidence history and further assess the potential subsidence mechanisms of eastern China. Residual subsidence is defined as the difference between the theoretical and observed tectonic subsidences. Our results show that the residual subsidence since 20Ma in eastern China generally increases from ∼100 to 300m in terrestrial areas to ∼1.2–1.8km onthe continental shelf. Our observed residual subsidence is generally consistent with the present-day dynamic topography that is predicted from mantle flow models, and it is associated with a stagnant slab within the mantle that has been observed by seismic tomography. The migration pathway of the residual subsidence since 20Ma is consistent with the movement direction of the Pacific and Philippine plates. Therefore, we suggest that the residual subsidence might be a dynamic subsidence induced by a negative buoyancy of the sinking slab in the deep mantle beneath eastern China.

Influence of normal fault growth and linkage on the evolution of a rift basin: A case from the Gaoyou depression of the Subei Basin, eastern China

ABSTRACT It is widely acknowledged that the growth and linkage of a boundary fault affects the evolution of the accompanying sedimentary basin and exerts a strong influence on its sedimentary characteristics. In this study, we attempt to discuss the evolution of a boundary fault and rift basin by using seismic interpretation, incremental fault throw, and growth rate–distance (G–d) profile, which are more suitable parameters than the expansion index in rift basins where footwall strata are missing because of erosion or a depositional hiatus. Therefore, the Gaoyou depression is taken as an example. The Gaoyou depression is the largest hydrocarbon-bearing depression in the Subei Basin, which is a representative rift basin in eastern China. The depression is bounded to the south by the Jiangdu–Wubao fault zone, which includes the Zhen 1–Wu 1, Zhen 2, and Wu 2 faults. Using high-resolution, three-dimensional seismic data, the Zhen 1–Wu 1 fault can be subdivided into four segments based on its G–d profile: the Shaobo, Fanchuan, Liuwushe, and Liulu segments. Growth rates indicate that the Zhen 1–Wu 1 fault and the Zhen 2 and Wu 2 faults developed successively from the Late Cretaceous to the Miocene during rifting of the Gaoyou depression and that development of the Zhen 2 and Wu 2 faults was controlled by the Zhen 1–Wu 1 fault. The four segments of the Zhen 1–Wu 1 fault were linked with each other by the time the Zhen 2 and Wu 2 faults formed in the Late Paleocene. An evolution model of the Gaoyou depression is set up in this study. Integrating the fault activity with previous sedimentological studies, we suggest that the evolution of the Gaoyou depression was controlled by the Jiangdu–Wubao fault zone rather than by a single fault. Successive slip of the Zhen 1–Wu 1 fault and the Zhen 2 and Wu 2 faults caused subsidence and migration of the depocenters. This study presents a case investigation of the effects of boundary faults on rift basins in eastern China.

Cenozoic tectonic evolution of the Bohai Bay Basin and its coupling relationship with Pacific Plate subduction

The Bohai Bay Basin is a Mesozoic–Cenozoic rift basin in eastern China. Based mainly on a balanced-section analysis, this study compares the spatio-temporal differences of tectonic evolution in relation to strike-slip faults among different depressions within the basin. In combination with the analysis of subsidence characteristics, the study also attempts to clarify the Cenozoic tectonic evolution of the basin and its coupling relationship with the subduction of the Pacific Plate. It was found that: (1) the strike-slip faults were activated generally from south to north and from west to east during the Cenozoic; (2) there is a negative correlation between the intensity of tectonic activity in the Bohai Bay Basin and subduction rate of the Pacific Plate; and (3) the migration direction of the basin depocenters is consistent with the direction of Pacific Plate subduction.

Sedimentary characteristics and seismic geomorphology of gravity-flow channels in a rift basin: Oligocene Shahejie Formation, Qinan Slope, Huanghua Depression of Bohai Bay Basin, China

Deep-water gravity-flow sandstones are important hydrocarbon exploration and production targets in the Bohai Bay Basin, a Paleogene intra-continental rift basin in eastern China. In this paper, the seismic-sedimentology techniques are used to characterize, in plan view, the temporal and spatial evolution of a gravity-flow-channel complex of the Paleogene Shahejie Formation (Es) on the Qinan faulted-monoslope (Qinan Slope), Bohai Bay Basin. The results show that two or three gravity-flow channels, 9–12 km long and 0.5–2 km wide, were successively developed in later Es (Es1z–Es1s). The channels initially experienced westward migration and then shifted eastward. The corresponding wireline logs of the channel-fill sequences mainly present blocky-shaped or bell-like configurations, whereas their seismic profile features are characterized by strong amplitude reflections, such as U-shaped, plate-like, spindle-shaped and lenticular configurations.The syndepositional activity of three normal faults, i.e., the Nandagang Fault to the northwest, the Zhangbei Fault to the northeast and the Zhaobei Fault to the east led to gradient changes of the Qinan Slope, which have controlled the plan morphology (width, curvature, and bifurcation) of the gravity-flow channels. In the medium-late period of Es1z, triggered by intensive faulting on the three faults, the gradient of the Qinan Slope was steepened abruptly, resulting in an increase of flow velocity and erosion amplitude to underlying deposits. As a result, channels exhibiting narrow and straight configurations in plan view were formed. During the stage of early Es1z and Es1s, tectonic activity intensity was relatively low and the gradient of the Qinan Slope was gentle, so channels with great width and curvature were bifurcated and merged downstream.Comparison of the faulting amplitude of the three syndepositional faults suggests that the Nandagang and Zhaobei faults were inversely strengthened in the Es1z and Es1s. The Nandagang Fault to the west was found to be more active than the Zhaobei Fault to the east in the Es1z stage. This condition was reversed in Es1s. For that reason, the channels migrated to the west in the Es1z stage and then went back to the east during Es1s.Core analysis shows that the channel fills are mainly composed of sandy-debrites, slumps and turbidites. Among them, sandy debrites dominate deposition in terms of reservoir volume and hydrocarbon potential. These units primarily consist of sandstones and gravel-bearing sandstones, with bed thicknesses ranging from 10 to 40 m, an average porosity of 11% and a permeability of 25 mD. Being mostly encased in organic-rich dark mudstones, these sandy debrites are significant hydrocarbon exploration targets.The results of this study are not only useful to the hydrocarbon exploration and development planning for the Qinan Slope, but also helpful when considering other faulted-depressions in the Bohai Bay Basin and other intra-continent rifted basins around the world, particularly in terms of gravity-flow hydrocarbon exploration and research.

Three-dimensional facies architecture analysis using sequence stratigraphy and seismic sedimentology: Example from the Paleogene Dongying Formation in the BZ3-1 block of the Bozhong Sag, Bohai Bay Basin, China

The Bohai Bay Basin is a classic non-marine rift basin in eastern China. The Paleogene Dongying sequences are the main hydrocarbon-bearing stratigraphic unit in the basin. Using three-dimensional (3-D) seismic data and one well control in the BZ3-1 Block in the western slope of the Bozhong Sag, we analyzed 3-D facies architectures of the Dongying sequences. The Dongying Formation, a second-order sequence, can be subdivided into four third-order sequences (from base to top: SQ1, SQ2, SQ3, and SQ4). The facies architecture was analyzed by using the seismic sedimentology approach based on 3-D seismic data. Sediment of the Dongying sequences was derived from the northern Shijiutuo Uplift via four major configurations of incised valleys, namely “V”, “U”, “W”, and composite shaped incised valleys. Seismic stratal slices reveal branching and converging characteristics of the channels from upstream to downstream. On the basis of an integrated analysis of well log, core data, seismic facies based on multi-seismic attributes, three sedimentary facies (e.g., “delta”, “fan-delta”, and “shore” or “shallow lacustrine” facies) have been recognized. The four types of incised valleys and their evolution control the sedimentary systems in the sedimentation area. The numbers and sizes of the fans are controlled by the sedimentary systems at various scales. Incised valley-fill and deltaic sand bodies are excellent hydrocarbon reservoirs and potentially good exploration targets for the study area. The reservoir quality of sequences SQ1, SQ2, and SQ3 become better gradually from base to top. The proposed sediment dispersal patterns may aid in the prediction of potential reservoir distribution. This study also demonstrates that facies architecture analysis using sequence stratigraphy and seismic sedimentology may serve as an effective approach for constructing 3D facies models for petroleum exploration in areas lacking of well or outcrop data.

Seismic interpretation of tectonic and paleogeomorphologic controls on sediment dispersal patterns in a continental rift basin: A case study from the Bohai Bay Basin, China

Tectono-paleogeomorphologic frameworks exert a major control on sediment dispersal patterns and types of depositional systems, both of which influence reservoir geometry and quality. In this study, we analyze the framework of the [Formula: see text] Member of the Dongying Formation in QHD29-2 Block, Bohai Bay Basin, China. The framework is constructed through (1) conventional sedimentologic study of single-well facies, cumulative frequency curves, and thin section analysis, (2) seismic stratigraphic interpretation of sectional seismic facies, and (3) seismic sedimentologic mapping through stratal slicing and seismic multiattribute analysis. The structural system is marked by a master boundary fault in the Shijiutuo Uplift that has greater displacement at its center than at the propagating fault tips. The resulting paleogeomorphologic architecture is characterized by a steep slope in the central part of the boundary fault and two gentle slopes at fault tips. Sediment dispersal patterns indicate that fan-shaped and lobate proximal fan deltas were rapidly deposited in the central steep slope and belt-shaped distal river deltas were deposited in the gentle slopes after long-distance transport. Proposed sediment dispersal patterns for the central fault segment and the propagating fault tips should serve as a useful reference for the stratal geometry, depositional patterns, distribution of depositional facies and of potential reservoir and source rocks, and evaluation of hydrocarbon reservoirs in extensional rift basins in eastern China and similar basins around the world.

The role of algal blooms in the formation of lacustrine petroleum source rocks — Evidence from Jiyang depression, Bohai Gulf Rift Basin, eastern China

Algal blooms are responsible for the laminae in the lacustrine Shahejie Formation in the Jiyang depression, one of the sub-basins of the Bohai Gulf Rift Basin in eastern China. About 65 oil fields have been discovered in this depression during the past 40years and the oil is mainly derived from three types of finely laminated lacustrine rocks of Shahejie Formation (Eocene–Oligocene). These lacustrine petroleum source rocks are oil shales, calcareous shales and calcareous laminated mudstones. Optical, SEM and BSEI analyses of these source rocks reveal that there are algal (coccoliths and dinoflagellates) laminae in the rocks. The individual algal lamina is generally composed almost wholly of one species, such as Reticulofenestra bohaiensis in coccolith laminae, and Fromea as well as Bohaidina in dinoflagellate laminae. This distribution suggests that the algal laminae formed from algal blooms in the ancient lake. Other components of source rocks included organic matter laminae, carbonate laminae and clay-rich laminae. We also discuss the sedimentation of organic matter laminae and carbonate laminae which were related to algal blooms. We conclude that the laminated lacustrine petroleum source sediment of Jiyang depression formed during episodes of algal blooms alternating with chemical and detrital sedimentation, and this represents the primary mechanism responsible for the formation of lacustrine petroleum source rocks.

A quantitative simulation study of asymmetrical tectonic subsidence control on non-synchronous sequence stacking patterns of Eocene lacustrine sediments in Bohai Bay Basin, China

Compared with large-scale passive margin marine basins, continental basins are susceptive to local or regional tectonic, geomorphologic and climatic influences and can form complicated sequence stratigraphic architectures, configurations and sedimentary facies. We report herein a quantitative simulation study on the influences of asymmetrical tectonic subsidence on sequence stratigraphic stacking patterns in rift basins by varying tectonic subsidence, sediment supply, and intra-rift lake levels. It has been demonstrated that isochronous sequences within a rift basin can exhibit both synchronous and non-synchronous stacking patterns as a result of differential asymmetric tectonic subsidence. In sequence stratigraphy, synchronous stacking patterns refer to isochronous sequences at any parts of a basin, which are either all retrogradational or all progradational. We propose the concept of non-synchronous stacking patterns of isochronous sequence stratigraphy for rift basins. Non-synchronous stacking patterns refer to the fact that, within the same basin, some parts may concomitantly exhibit stacking patterns that are exactly opposite to that of other parts. Non-synchronous stacking patterns can form when the accommodation space increment (∆A) is less than that of sedimentary supply (∆S) in one area, but larger than or equal to that of ∆S in another. Moreover, the duration of non-synchronous stacking patterns is controlled by both ∆A and ∆S. Recognition of non-synchronous stacking patterns within isochronous sequence stratigraphy provides a conceptual model for correlation of sequence stratigraphic units in tectonically asymmetric rift basins in eastern China and similar basins around the world.

Orderliness of Hydrocarbon Accumulation Distribution in Rift Basins of Eastern China

Abstract:The Cenozoic rift basins in eastern China show a clear temporal and spatial zonation and episodic tectonic evolution, which control their episodic hydrocarbon generation and zonal accumulation. In this paper, based on the study of depositional architecture, hydrocarbon migration system and dynamic evolution in the rift basins, combinations of hydrocarbon accumulation elements were analyzed using sequence stratigraphy. Hydrocarbon distribution in system tracts with different sequence orders was further studied. And we summarized stacking patterns and horizontal combination relationships for different types of reservoirs, such as lithological, tectonic‐lithological, tectonic and stratigraphical reservoirs which can be observed from depression center to basin margin. The result reveals that various scales of pools exhibit significant distribution and evolution orderliness in different pool‐forming units, i.e., depositional systems, plays and depressions. The regular distribution of various scales of pools is closely related to tectonic evolution and depositional filling in the rift basins. The result can be applied to the fine petroleum exploration in rift basins in eastern China. It will promote the scientific prediction and evaluation of reservoir types and their spatial distribution, lead to the active shifts of exploration targets in different zones, and thus support the stable progress of fine exploration in mature exploration areas.

Interactions between basalts and oil source rocks in rift basins: CO2 generation

Basalts interbedded with oil source rocks are discovered frequently in rift basins of eastern China, where CO2 is found in reservoirs around or within basalts, for example in the Binnan reservoir of the Dongying Depression. In the reservoirs, CO2 with heavy carbon isotopic composition (δ13C>-10‰ PDB) is in most cases accounts for 40% of the total gas reserve, and is believed to have resulted from degassing of basaltic magma from the mantle. In their investigations of the Binnan reservoir, the authors suggested that the CO2 would result from interactions between the source rocks and basalts. As the source rocks around basalts are rich in carbonate minerals, volcanic minerals, transition metals and organic matter, during their burial history some of the transition metals were catalyzed on the thermal degradation of organic matter into hydrocarbons and on the decomposition of carbonate minerals into CO2, which was reproduced in thermal simulations of the source rocks with the transition metals (Ni and Co). This kind of CO2 accounts for 55%–85% of the total gas reserve generated in the process of thermal simulation, and its δ13C values range from-11‰–-7.2‰ PDB, which are very similar to those of CO2 found in the Binnan reservoir. The co-generation of CO2 and hydrocarbon gases makes it possible their accumulation together in one trap. In other words, if the CO2 resulted directly from degassing of basaltic magma or was derived from the mantle, it could not be accumulated with hydrocarbon gases because it came into the basin much earlier than hydrocarbon generation and much earlier than trap formation. Therefore, the source rocks around basalts generated hydrocarbons and CO2 simultaneously through catalysis of Co and Ni transition metals, which is useful for the explanation of co-accumulation of hydrocarbon gases and CO2 in rift basins in eastern China.

THE LACUSTRINE LIANGJIALOU FAN IN THE DONGYING DEPRESSION, EASTERN CHINA: DEEP-WATER RESERVOIR SANDSTONES IN A NON-MARINE RIFT BASIN

A lacustrine fan covering an area of about 175sq. km has been identified in the Liangjialou area in the SW of the Dongying Depression, a Tertiary non-marine rift basin in eastern China. Fluvial and deltaic sandstones are established reservoir rocks in the basin, and the deep-water sandstones of the fan succession, which are assigned to Member 3 of the lower Tertiary Shahejie Formation, are also thought to have important reservoir potential. Available data for this study included some 800m of core from 16 wells, well-log data from 426 wells, and 220 sq.km of 3D surveys together with well-test and other production data. From geomorphological reconstructions of the fan, we distinguish first-order (major) fan channels from second-order branched and more distal tip channels. Crevasse splays and overbank shales occur between channels, and sandstone lobes were deposited at channel mouths. Conglomeratic sandstones deposited in major channels are probably the most promising reservoir facies (average porosity c. 20%; average permeability > 1D). Fan construction took place during a single complete cycle of lake level variation which was composed of several sub-cycles. During initial highstand conditions, the fan was dominated by small-scale branched and tip channels and intervening sandy lobes. Fan size increased rapidly during the following lowstand, and then decreased during the ensuing highstand.

Origin of crude oils in the Jinhu Depression of North Jiangsu-South Yellow Sea Basin, eastern China

The chemical compositions of 20 crude oils and the extracts of potential source rocks from the Jinhu Depression of the North Jiangsu-South Yellow Sea Basin, eastern China were studied in detail by geochemical methods in order to understand their genetic relationship. The oil samples were collected from the major producing fields in the Western Slope (WS) and the Biandong-Yangjiaba-Minqiao uplift belt (BMY). The rock samples studied, selected after Rock-Eval pyrolysis screening of a large suite of samples, consist of 20 shale samples in the E2f2 member of the Paleocene Funing Formation from the oilfield areas (WS and BMY) and the Sanhe sag all within the Jinhu Depression. Geochemical characteristics indicate only one single family of oils. A number of biomarker features support deposition of the source rocks for these oils under conditions of enhanced salinity/stratified water columns. A wide thermal maturity range for the oils as indicated by sterane isomerization ratios was not substantiated by other maturity parameters. The genetic relationships between the oils and the E2f2 member of the Paleocene Funing Formation present in the deeper parts of the Sanhe and Longgang sags were suggested using a wide range of molecular parameters. Variation in the sterane concentrations in source rocks with different maturity suggests that migrational contamination and/or in-situ extractions of organic matter from caprocks or intra-reservoir shales by mature oils could potentially produce oils with apparent immaturity at the molecular level. Secondary alteration (e.g. biodegradation) of oils in the reservoir could also have a similar effect on the bulk oil compositions. Although we do not discount the possible presence of minor amounts of “immature oils”, exploration focus in the Jinhu Depression, as well as many other rift basins in eastern China, should be on mature oils derived from the deep source kitchens.