Jiyang Sub-basin Research Articles

Influence of multistage hydrothermal fluids on dolomite reservoirs: A case study from the Lower Ordovician Yeli-Liangjiashan Formation in the Chengdao-Zhuanghai area, Jiyang subbasin, Bohai Bay Basin, China

It remains controversial whether the influence of hydrothermal fluids on dolomite reservoirs is dominated by dissolution or precipitation. In this study, the influence of multistage hydrothermal fluids on the dolomite reservoirs of the Lower Ordovician Yeli-Liangjiashan Formation in the Chengdao-Zhuanghai area was investigated based on petrographic observations and geochemical analyses, with an emphasis on the temporal relationships among dolomitization, pore formation, and multistage hydrothermal fluid activities determined by in situ U-Pb dating. The δ18O, δ13C, and 87Sr/86Sr data indicate that the genesis of pre-hydrothermal dolomites and the earliest-formed dolomite cement (Cd1) closest to the edges of pores were related to coeval seawater. The U-Pb age of pre-hydrothermal dolomites is 427 ± 11 Ma, representing the time when early dolomitization occurred. The original pore formation predates the precipitation of Cd1 with a U-Pb age of 391 ± 12 Ma. Stage I and stage II hydrothermal fluids were derived from the Precambrian basement and the mantle, respectively, as evidenced by differences in δ18Ofluid values and rare earth element characteristics. Stage I and II hydrothermal fluids occurred at 161 ± 12 Ma and 81 ± 16 Ma, respectively. Therefore, early dolomitization and pore formation are not correlated with the two stages of hydrothermal fluids. Stage II hydrothermal fluids contained a relatively higher amount of H2S than stage I hydrothermal fluids. H2S-related dissolution caused by stage II hydrothermal fluids mainly occurred in the dolomite reservoirs adjacent to major faults, and H2S was gradually consumed as the distance from the major faults increased. The two stages of hydrothermal minerals successively filled most of the spaces in pores and fractures. The influence of multistage hydrothermal fluids on dolomite reservoirs was found to be dominated by precipitation.

Storage space development and hydrocarbon occurrence model controlled by lithofacies in the Eocene Jiyang Sub-basin, East China: Significance for shale oil reservoir formation

Complex mineral composition, multiple depositional processes, and strong reservoir heterogeneity cause the complexity and uniqueness of a lacustrine shale oil reservoir. Understanding the storage space development and hydrocarbon occurrence model should facilitate the analysis of the shale oil reservoir and hydrocarbon accumulation in lacustrine shales. In this study, the storage space development model for different lithofacies and the modes of hydrocarbon occurrence in different storage space were analyzed in the Es4s–Es3x shale in the Jiyang Sub-basin, East China, based on thin-section and field emission scanning electron microscopy (FE-SEM) observations, X-ray diffraction (XRD) analysis, physical property testing, and geochemical analysis. Inorganic pores, particularly recrystallization intercrystalline pores in calcite, are the key matrix pores for lacustrine shale oil. However, the types and abundance of storage space are noticeably different in various lithofacies. The hydrocarbon occurrence is mainly manifested in three states: (A) free state in interlaminar fractures, structural fractures, and abnormal pressure fractures; (B) adsorbed state in organic pores, intercrystalline pores in pyrite, and floccule pores; and (C) free state in large pore spaces, including dissolution pores and recrystallization intercrystalline pores, which can form a continuous hydrocarbon accumulation. The hydrocarbon generation potential and thermal maturity are closely associated with the lithofacies and mineral composition. The matching mechanisms of pore formation as well as hydrocarbon generation, migration, and accumulation are favorable. Among the various lithofacies, the organic-rich calcareous shale has abundant storage space, high porosity, total organic carbon content, and hydrocarbon potential, making it a “sweet spot” for shale oil exploration.

Nitrogen isotopes as paleoenvironmental proxies in marginal-marine shales, Bohai Bay Basin, NE China

Nitrogen isotopes have been widely used as indicators of depositional conditions in studies of ancient sediments. Bohai Bay Basin is a major continental hydrocarbon province in NE China, but nitrogen isotopic signatures of this lacustrine system (connected with the open ocean occasionally) have barely been investigated. We report a study of paleoenvironmental information recorded by N isotopic variations in the lower third member (Es3L) and upper fourth member (Es4U) of the Eocene Shahejie Formation in the Jiyang Sub-basin, southern Bohai Bay Basin. Paleosalinity (B/Ga, Sr/Ba, and S/TOC) and paleoredox (Corg/P, DOP, UEF, and Fe/Al) proxies and δ13Corg values were used to elucidate paleoenvironmental factors that affect N isotopic signals in the study core. Results indicate that δ15N values are positively correlated with paleosalinity proxies in the Luo-69 and Niu-1 drill-cores. Two possibilities may account for this: (1) a paleosalinity surge caused by marine transgression events led to stratification of the water column, creating an anoxic–suboxic deep-water layer and promoting denitrification in sediments; or (2) strong stratification of the water column impeded the upwelling process that brought nutrients to the upper water layer, weakening nitrogen fixation there. In marginal-marine intervals such as the Eocene Shahejie Formation, water-column salinity may play a critical role in regulating the δ15N signal. This study indicates that nitrogen isotopic compositions may aid paleoenvironmental and paleoclimatic interpretations of marginal-marine shale, particularly when applied together with multiple paleoenvironmental proxies to improve the understanding of ancient sedimentary conditions in marginal-environment.

Burial history and the evolution of hydrocarbon generation in Carboniferous-Permian coal measures within the Jiyang super-depression, China

In the Jiyang Sub-basin, Carboniferous-Permian (C-P) coal-measure source rocks have experienced complex multi-stage tectonics and therefore have a complex history of hydrocarbon generation. Because these coal measures underwent multi-stage burial and exhumation, they are characterized by various burial depths. In this study, we used the basin modeling technique to analyze the relationship between burial history and hydrocarbon generation evolution. The burial, thermal and maturity histories of C-P coals were reconstructed, including primary hydrocarbon generation, stagnation, re-initiation, and peak secondary hydrocarbon generation. The secondary hydrocarbon generation stage within this reconstruction was characterized by discontinuous generation and geographical differences in maturity due to the coupled effects of depth and a delay of hydrocarbon generation. According to the maturity history and the delay effect on secondary hydrocarbon generation, we concluded that the threshold depth of secondary hydrocarbon generation in the Jiyang Sub-basin occurred at 2,100 m during the Yanshan epoch (from 205 Ma to 65 Ma) and at 3,200 m during the Himalayan period (from 65 Ma to present). Based on depth, residual thickness, maturity, and hydrocarbon-generating intensity, five favorable areas of secondary hydrocarbon generation in the Jiyang Sub-basin were identified, including the Chexi areas, Gubei-Luojia areas, Yangxin areas, the southern slope of the Huimin depression and southwest of the Dongying depression. The maximum VRo/burial depth (%/km) occurred in the Indosinian epoch as the maximum VRo/time (%/100Ma) happened in the Himalayan period, indicating that the coupling controls of temperature and subsidence rate on maturation evolution play a significant role in the hydrocarbon generation evolution. A higher temperature and subsidence rate can both enhance the hydrocarbon generation evolution.

Evolution of the Cenozoic thermal lithosphere of the Jiyang Sub-basin, Bohai Bay Basin: Implications for the lithospheric thinning mechanism

The Jiyang Sub-basin is an important sub-tectonic unit of the Bohai Bay Basin (BBB), which thinned by over 100 km during the Meso-Cenozoic. Changes in the thermal lithospheric thickness, of the Jiyang Sub-basin in the Cenozoic, were assessed using data from 131 single boreholes, together with the thermal history and layered crustal structure. From these, basement subsidence, stretching factor and strain rate were calculated for each borehole. The results show that the Jiyang Sub-basin experienced a peak of lithospheric thinning during the Cenozoic. In the early Cenozoic, the thickness of the lithosphere was about 80–100 km, and the strain rate was mostly low, 10−16–10−22s−1, indicating a weak extensional environment. At the peak of the lithospheric thinning (50 Ma to 35 Ma), the thermal lithosphere thickness was 70–55 km, the tectonic subsidence was about 1000 m, the lithospheric strain rate was higher about 10−15–10−16 s−1, the stretching factor had also increased rapidly to 1.1–1.2. The thinning of the lithosphere in the Eocene is interpreted as a product of collective movements of the Indian, Pacific, and the Philippine Sea plates, although the effect of the Pacific plate was small as - the Izanagi-Pacific mid-ocean ridge is still young age (0–10 Ma). Knowledge of the history of the Cenozoic lithosphere in the Jiyang Sub-basin is of great significance for understanding the tectonic evolution of the BBB and of the North China Craton.

Geochemistry and origin of heavy oils in the south part of Zhanhua Depression, east China

Abstract The Sanhecun sag, located in the south of the Zhanhua depression, has yielded a significant amount of heavy oil in recent years and is becoming a new exploration focus in the Jiyang sub-basin. Surprisingly, oils in the deep Shahejie formation have higher densities and viscosities than those in the shallow Guantao formation. In this study, geochemical and fluid inclusion analyses of oils from different depths were carried out to clarify their source and the cause of the high densities. Oils in the Shahejie formation, with high contents of resins and asphaltenes (47%–56%), are generated from the Es4 carbonate source rocks that were deposited in a hypersaline environment with a relatively high terrigenous organic matter contribution. Maturity parameters and fluid inclusions show that the oils with low saturated hydrocarbon contents and high resin and asphaltene contents were charged to the Shahejie formation from 29 Ma to 27 Ma, when the source rocks were in the early oil generation window. Although subsequent biodegradation further increased oil density, the main control on the high densities of oils in the Shahejie formation is the nature of the source kerogen. Oils in the shallow Guantao formation can be divided into two groups: a group from Es3 source rocks in the Gunan sag and a second from Es3 and Es4 source rocks in the Bonan sag. They were expelled within the oil generation window and charged the reservoirs between 3 Ma and the present-day. Oils in the Guantao formation have undergone light to heavy biodegradation, ranging from PM levels 2 to 5. With the increase of biodegradation level, saturated hydrocarbon contents gradually decrease from 50% to 22%, while the contents of resins and asphaltenes increase from 28% to 43%. Biodegradation is the primary cause of oil densification in the Guantao formation. Although oils in the Guantao Formation suffered more severe biodegradation, they still have higher contents of saturated hydrocarbons than those in the Shahejie Formation. Source type, maturity and biodegradation thus combine, in different ways, to control the density of heavy oils in the Sanhecun sag. This study also indicates that low mature oils from Es4 carbonate source rocks can charge reservoirs in the southern part of Zhanhua depression before late Oligocene uplift, and low mature oil may be a potential future target in this area.

High-resolution astrochronological record for the Paleocene-Oligocene (66–23 Ma) from the rapidly subsiding Bohai Bay Basin, northeastern China

The Paleogene succession of the Bohai Bay Basin (BBB), a highly productive petroleum basin in northeastern China with predominantly lacustrine fill, is poorly dated at present. Here, we generated an internal astronomical time scale (ATS) for the Paleogene strata of the Bohai Bay Basin (Dongying Depression, Jiyang Subbasin) using multitaper method (MTM) spectral analysis of high-resolution gamma ray logs. This ATS, which extends from 66 to 23Ma, is anchored to the standard geologic time scale by calibration to the Paleogene/Neogene boundary (23.03Ma) at the top of Dongying Formation. Based on this ATS, we recalibrated the ages of biozones, rifting episodes, and paleoclimate stages within the BBB. The recalibrated ages of rifting episodes and related fluctuations in sedimentation rates exhibit a close relationship to secular variation in production rates of Pacific oceanic plateaus and spreading rates of the Southeast Indian Ridge, confirming that the subsidence history of the BBB was significantly influenced by subduction of the Pacific Plate along the eastern margin of Asia and by collision of the Indian and Eurasian plates. Age recalibration also facilitated re-evaluation of the relationship of sedimentation in the Bohai Bay Basin to the Paleogene climate history of East Asia, e.g., confirming a cooling and drying trend throughout the Eocene.

The evolutionary history of methane adsorption capacity with reference to deep Carboniferous-Permian coal seams in the Jiyang Sub-basin: Combined implementation of basin modeling and adsorption isotherm experiments

Carboniferous-Permian (C-P) coal seams experienced multiple burial and uplift events, which result in complex gas adsorption and desorption processes. Therefore, the understanding of the adsorption history and its dominant factors contribute to determining present-day coalbed methane (CBM) storage and enhance the development of deep CBM. In this paper, we first reconstructed the burial history using the basin model and synchronously conducted the isothermal adsorption experiment. Then an adsorption model was achieved from regression analysis about Langmuir parameters, and therefore, the adsorption history was rebuilt, accompanied by temperature, pressure, and maturity. Lastly, incorporated with the hydrocarbon generation history and adsorption history, we divided the actual adsorbed stages and further analyzed the dominant factors controlling the actual adsorption amount. The results indicate that: (1) based on the adsorption model calculated using regression analysis, a real critical depth of adsorption capacity (approximately 2000 m) is obtained; (2) coal-bearing sequences experience three temperature, pressure, and hydrocarbon generation processes, and the Himalayan epoch is the main stage for producing coal-derived gas; (3) the actual adsorption history contains a maximum depth stage (250–208 Ma), reburial stage (208–110 Ma), re-uplift stage (110–50 Ma), and present stage (50 Ma–present); (4) the critical event (208 Ma) represents that generating the amount of gas is equal to the adsorption capacity in the actual adsorbed amount, and before and after the event, the dominant factors controlling the actual adsorbed amount are maturity and temperature, respectively.

Evolution of Meso-Cenozoic lithospheric thermal–rheological structure in the Jiyang sub-basin, Bohai Bay Basin, eastern North China Craton

The Meso-Cenozoic lithospheric thermal–rheological structure and lithospheric strength evolution of the Jiyang sub-basin were modeled using thermal history, crustal structure, and rheological parameter data. Results indicate that the thermal–rheological structure of the Jiyang sub-basin has exhibited obvious rheological stratification and changes over time. During the Early Mesozoic, the uppermost portion of the upper crust, middle crust, and the top part of the upper mantle had a thick brittle layer. During the early Early Cretaceous, the top of the middle crust’s brittle layer thinned because of lithosphere thinning and temperature increase, and the uppermost portion of the upper mantle was almost occupied by a ductile layer. During the late Early Cretaceous, the brittle layer of the middle crust and the upper mantle changed to a ductile one. Then, the uppermost portion of the middle crust changed to a thin brittle layer in the late Cretaceous. During the early Paleogene, the thin brittle layer of the middle crust became even thinner and shallower under the condition of crustal extension. Currently, with the decrease in lithospheric temperature, the top of the upper crust, middle crust, and the uppermost portion of the upper mantle are of a brittle layer. The total lithospheric strength and the effective elastic thickness (T e) in Meso-Cenozoic indicate that the Jiyang sub-basin experienced two weakened stages: during the late Early Cretaceous and the early Paleogene. The total lithospheric strength (approximately 4–5 × 1013 N m−1) and T e (approximately 50–60 km) during the Early Mesozoic was larger than that after the Late Jurassic (2–7 × 1012 N m−1 and 19–39 km, respectively). The results also reflect the subduction, and rollback of Pacific plate is the geodynamic mechanism of the destruction of the eastern North China Craton.

Formation Conditions and Sedimentary Model of Over‐Flooding Lake Deltas within Continental Lake Basins: An Example from the Paleogene in the Jiyang Subbasin, Bohai Bay Basin

AbstractA large quantity of drilling core, paleontology, geochemistry and geophysics data revealed several features of the Jiyang subbasin during the deposition of the Ek1‐Es4x members: (1) the paleotopography of the gentle slope belt had an extremely low gradient; (2) the paleoclimate frequently alternated between dry and wet periods in a generally arid setting; (3) there was strong weathering around the periphery of the basin; (4) the lake was very shallow; (5) the lake level frequently rose and fell; and (6) the sedimentary environment of the gentle slope belt was an over‐flooding lake. All of these factors provided favorable geological conditions for the development of an over‐flooding lake delta. The lithologies of the continental over‐flooding lake delta deposits are complex and diverse. The compositional maturity is moderate to low, and the grain size distribution curves and sedimentary structures indicate the presence of both gravity and traction currents. The sedimentary microfacies associations consist of a combination of ordered superposition of flood channels, distributary channels and sheet sands. The delta exhibits a weak foreset seismic reflection. The over‐flooding lake delta deposits are laterally extensive. The sandstone content is high, and the individual sandstone beds are thin. The flood channel and distributary channel deposits exhibit evidence of bifurcation and lateral migration. The distribution of the sandbodies and the oxidation color of the mudstones provide evidence of cyclic deposition. The paleoclimate was the dominant factor controlling the development of the over‐flooding lake delta. Due to the frequently alternating wet and dry paleoclimates, the over‐flooding lake delta is characterized by the development of a broad upper plain and a lower delta plain. The upper delta plain is characterized by flood channel deposits, whereas the lower delta plain is represented by distributary channel deposits. The transition zone is characterized by the interaction of flood channels and distributary channels. Due to fault activity, the sandbodies of the over‐flooding lake delta were juxtaposed against hydrocarbon source rocks, which was favorable for the development of lithologic reservoirs or structural‐lithologic reservoirs. The lower delta plain deposits comprise the most favorable reservoirs.

Geothermal evidence of Meso-Cenozoic lithosphere thinning in the Jiyang sub-basin, Bohai Bay Basin, eastern North China Craton

The Bohai Bay Basin is at the center of lithosphere destruction and thinning in the eastern North China Craton (NCC). In this paper, the thermal lithospheric thicknesses of the Meso-Cenozoic Jiyang sub-basin in the Bohai Bay Basin were calculated by reconstructing the thermal history of the sedimentary basin using apatite fission track and vitrinite reflectance data. The results show that the Jiyang sub-basin experienced two heat flow peaks in the late Early Cretaceous and in the Middle to Late Paleogene, with heat flow values of 84~88mW/m2 and 85~88mW/m2, respectively. The thermal lithosphere thicknesses of the Jiyang sub-basin, calculated from the modeled thermal histories, experienced two thinning stages in the Cretaceous and in the Paleogene. The lithosphere began with an initial thickness of 150km in the Early Mesozoic and reached a thinning peak of 51km in the late Early Cretaceous. The thickness then increased to approximately 80km at the end of the Cretaceous. A second thinning peak occurred in the Middle and Late Paleogene, with a thickness of only 48km, which corresponds to a rift phase in the Bohai Bay Basin. The lithosphere thickened thereafter and is 78km at present. The North China Craton now has a thin lithosphere. Our research provides new geothermal evidence for the lithospheric thinning of the NCC.

Applications of molecular fossils in lacustrine stratigraphy

Biomarkers, or the so-called molecular fossils, are used tentatively in the Eogene lacustrine stratigraphy study in the Jiyang Sub-basin. Notwithstanding the fact that unidentified microfossils or amorphism and acritarchae are widely distributed in lacustrine source rocks, molecular fossils are useful to identify the sources. It is helpful to reconstruct the palaeo-enviroment, palaeo-ecosystem and compartmentalize the stratigraphic sequence by using molecular fossils with which the existence and types of microbes including bacteria, archaea and certain algae can be identified.

Compound-specific C- and H-isotope compositions of enclosed organic matter in carbonate rocks: Implications for source identification of sedimentary organic matter and paleoenvironmental reconstruction

The Bohai Bay Basin is one of the most important oil-producing provinces in China. Molecular organic geochemical characteristics of Lower Paleozoic source rocks in this area have been investigated by analyzing chemical and isotopic compositions of solvent extracts and acid-released organic matter from the Lower Paleozoic carbonate rocks in the Jiyang Sub-basin of the Bohai Bay Basin. The results indicate that enclosed organic matter in carbonate rocks has not been recognizably altered by post-depositional processes. Two end-member compositions are suggested for early organic matter trapped in the Lower Paleozoic carbonate rocks: (1) a source dominated by aquatic organisms and deposited in a relatively deep marine environment and (2) a relatively high saline, evaporative marine depositional environment. In contrast, chemical and isotopic compositions of solvent extracts from these Lower Paleozoic carbonate rocks are relatively complicated, not only inheriting original characteristics of their precursors, but also overprinted by various post-depositional alterations, such as thermal maturation, biodegradation and mixing. Therefore, the integration of both organic matter characteristics can provide more useful information on the origin of organic matter present in carbonate rocks and the environments of their deposition.

Effect of igneous activity on hydrocarbon source rocks in Jiyang sub-basin, eastern China

A 50 m thick diabase sill was found in the Paleogene black mudstone hydrocarbon source rocks in Xia38 well. Within heating aureola of the sill, optical changes of kerogen and signatures of extractable bitumens in the mudstones were investigated. Under effects of high heating rate associated with sill intrusion, the vitrinite reflectance (Ro) has an increasing tendency with decreasing distance to the sill both below and above the sill. However, only within a zone of about 15 m to the diabase sill, the vitrinite reflectance shows significant increase. Above the sill, the Ro values increase gradually from 0.6% to 1.0%, whereas below the sill, the Ro values increase dramatically from 0.8% to 3.8%. The contrasting Ro increasing patterns are attributed to the different heat transferring conditions, the relatively open system above the sill and the relatively close system below. The more mature signatures below the sill are also demonstrated by the n-alkane distribution, carbon predominance index and odd-to-even carbon number predominance of the extracted bitumens. Below the sill, the proportion of the saturated hydrocarbons in the extracted bitumens increases from 34% to 79% towards the sill contact. The bitumens in the two highest rank samples which are closest to the sill are dominated by saturated or saturated + aromatic hydrocarbons. The %20S, %αββ and %Ts biomarker parameters of the extracted bitumens are 46%, 58% and 54%, respectively, at Ro = 1.5%. At Ro = 2.6% or 3.0%, they reach to 52%, 71%, and 71%, respectively, still under or close to their corresponding equilibrium end-points. These samples of high rank, established on the basis of optical assessment (vitrinite reflectance), have less mature biomarker signatures. The remarkable disparities between optical and biomarker parameters can be ascribed to that the aromatization of kerogen to increase Ro was more favored than the isomerization of biomarker in the rapid heating scenario.

Formation mechanism and model for sand lens reservoirs in the Jiyang Sub-basin, East China

The Bohai Bay basin comprises some very important and well documented subtle traps known in China, which have been the major exploration focus and have become a major petroleum play since the 1990s. However, recent exploration showed that the oil-bearing properties of some sand lens reservoirs may vary significantly and the accumulation mechanisms for these lithological subtle traps are not well understood. Based on statistical analysis of oil-bearing properties for 123 sand lens reservoirs in the Jiyang Sub-basin and combined with detailed anatomy of typical sand lens reservoirs and NMR experiments, it has been shown that the structural and sedimentary factors, hydrocarbon generation and expulsion conditions of the surrounding source rocks, as well as the petrophysical properties of sand lens reservoirs are the main controlling factors for the formation of sand lens reservoirs. The formation of a sand lens reservoir depends on the interaction between the hydrocarbon accumulation driving force and the resistance force. The driving force is made up of the differential capillary pressure between sandstones and sources rocks and the hydrocarbon diffusion force, and as well as the hydrocarbon expansion force. The resistance force is the friction resistance force for hydrocarbons and water to move through the pore throats of the sand lens. The sedimentary environment, source rock condition and sand reservoir properties can change from unfavorable to favorable depending on the combination of these factors. When these three factors all reach certain thresholds, the sand lens reservoirs may begin to be filled by hydrocarbons. When all of these conditions become favorable for the formation of sand lens reservoirs, the reservoir would have high oil saturation. This approach has been applied to evaluating the potential of petroleum accumulation in the sand lens reservoirs in the third member of the Neogene Shahejie Formation in the Jiyang Sub-basin.

The application of an integrated approach in exploration of lacustrine turbidites in Jiyang Sub-basin, Bohai Bay Basin, China

This study presents an integrated approach that has been successfully used in delineation of turbidite reservoirs in the Jiyang Sub-basin, Bohai Bay Basin, China. The techniques involve an integration of multi-disciplinary investigations, which comprise firstly the determination of depositional areas of turbidites using a sequence stratigraphic approach, then mapping the spatial distribution of turbidites using seismic velocity data, and finally, the selection of drilling prospects by an optimization process using available well logs and core data. Interpreted turbidites with a moderate burial depth and thickness of at least 10 m were targeted for drilling. The integrated approach has been proved to be quite successful in recent exploration activities with approximately 60 million tonnes (438 million bbls) of proven in place oil reserves had been discovered for the 6-year period of 1996 to 2001 in turbidite reservoirs, highlighting the importance of turbidite plays in current and future exploration.

ABSTRACT: Petroleum system and large oilfields formation in Jiyang Sub-basin

ABSTRACT: Petroleum system and large oilfields formation in Jiyang Sub-basin