Shulu Sag Research Articles

Heterogeneity of organic-rich lacustrine marlstone succession and their controls to petroleum expulsion, retention, and migration: A case study in the Shulu Sag, Bohai Bay Basin, China

Abstract We documented a new type of self-sourced unconventional reservoir consisting of interbedded organic-rich marlstone and calcareous conglomerates developed in lacustrine carbonate successions in the Eogene Shahejie formation (Es33) of Shulu Sag, Bohai Bay Basin. The Es33 interval used to be regarded as a set of homogeneous source rock for oil plays in the upper strata due to their high abundance of organic-matter. However, the Es33 interval actually has very strong heterogeneity in their organic richness and hydrocarbon generation ability among different stratigraphic sequences (the Es33 interval can be divided into sequence SQ1 to SQ5 from bottom to up). To better understand the heterogeneity in the unconventional reservoir and its effect on the hydrocarbon retention and migration, thin sections and scanning electronic microscopes were used to characterize the pore system in the reservoir, together with a amount of collected well database including wireline logging, gas logging, and physical property data (e.g. porosity and permeability). Intensive sampling from the Es33 interval of Well ST1H was performed and samples were measured with Rock-Eval and TOC analysis. The rock extracts were analyzed with SARA and gas chromatography - Mass spectrum (GC-MS) analysis to examine the fluid geochemical characteristics and molecular biomarkers composition. The results show that although the whole Es33 are characterized by low and ultra-low porosity and permeability, but the pore types in different lithology are different. The organic-rich marlstones develop large amount of organic matter pore and dissolved mineral pores, while the conglomerates developed inter-partical or inter-crystalline mineral pores dominantly. The sequence SQ2 has high quality of source rock for hydrocarbon generation, possessing moderate to high content of organic matter (TOC range 2–5 wt %) with Type I-II kerogen predominantly, basically in the thermal maturity stage of the peak oil window. OM and lithology play significant roles in controlling hydrocarbon expulsion and retention. Oil retained in the organic-rich marlstone was observed to absorb on the surface of OM and clay-rich matrix, which is also supported by the positive correlation between retained oil (represented by Rock-Eval S1) and TOC. Organic-lean conglomerates in the sequence SQ3 obtain abnormal high content of low mature oil determined by biomarker composition, which suggests the oil were derived from oil migration and mixture. Thermal maturity profile based on hydrocarbon biomarkers composition of rock extract provides a method to determine the effect of the hydrocarbon migration or retention within source rock. The organic-lean intervals adjacent or close to organic-rich source rock can be favorable targets for oil exploration.

Genesis and implications of the composition and sedimentary structure of fine-grained carbonate rocks in the Shulu sag

Fine-grained carbonate rocks, which extensively occur in the Eocene strata in the Shulu sag, Bohai Bay Basin, North China, represent an unconventional, fine-grained carbonate reservoir. However, previous studies have ignored the complexity of the lithofacies components and their formation mechanisms. Fine-grained carbonate rocks are typical reservoirs in which hydrocarbons form and gather. A better understanding of the nature of these rocks is extremely important for evaluating the quality of unconventional, fine-grained carbonate reservoirs. Various lithofacies components were discriminated in this study with a combination of petrographic observations and carbon isotope analyses. These fine-grained carbonate rocks comprise terrigenous, biogenic and diagenetic materials. Terrigenous input and biologically induced precipitation are the main sources of the materials in the lake. Five lithofacies were identified based on the observations of sedimentary features (core and thin section) and mineralogical data: (1) varve-like laminated calcilutite, (2) graded laminated calcilutite, (3) interlaminated calcisiltite-calcilutite, (4) massive calcilutite, and (5) massive calcisiltite-calcarenite. Their origins were recorded by various lithofacies components, which are controlled by the interactions of physical, chemical and biological processes. This study indicated that the lithology of the bedrocks was the key factor controlling carbonate accumulation. The tectonics and climate can influence the weathering and types of lithofacies. Primary productivity controlled the precipitation of the endogenic calcite. These factors jointly determined the abundant fine-grained carbonate rocks that have accumulated in the Shulu sag.

Formation mechanisms of rudstones and their effects on reservoir quality in the Shulu sag, Bohai Bay Basin, Eastern China

Based on observations made on cores and cuttings from several wells in the lowermost part of the third member of the Shahejie Formation, several rock types, specifically clast-supported rudstone, matrix-supported rudstone, mixed-source rudstone, calcisiltite/calcarenite, massive calcilutite and laminated calcilutite, have been identified in the Shulu sag. According to the sedimentary structures and distribution characteristics of these rocks, the carbonate breccias fall into two categories, based on their origins: one formed by fan-delta channel sedimentation, whereas the other formed by earthquake-induced slump fan deposition. Clast-supported rudstone and matrixsupported rudstone are the main lithologies deposited by braided rivers in the fan delta plain and front, of which the pore space is mainly dissolution pores within gravels and tectonic fissures. Clastsupported rudstone, matrix-supported rudstone and mixed-source rudstone are the main lithologies of the earthquake-induced slump fans. These carbonate breccias developed along with soft-sediment deformation structures, which are interpreted as seismites and are widely distributed in the sag, in which intercrystalline pores, intergranular pores and fissures created from diagenetic shrinkage are developed. The two kinds of rudstones have different reservoir characteristics and oil/gas testing results. The rudstones generated in the fan delta have higher porosity and permeability, as well as better oil/gas testing results. Thus, they are key targets for petroleum exploration.

A type of continuous petroleum accumulation system in the Shulu sag, Bohai Bay basin, eastern China

ABSTRACT This study analyzed crude oils from the lower part of the third member of the Eocene Shahejie Formation (Es 3 L ) and three prospective source rocks from the Shulu sag, Bohai Bay basin, eastern China, using a variety of organic geochemical methods. Biomarker characteristics were used to interpret source rock organic matter input and depositional environment, and oil–source rock correlation. The biomarker data indicate that the crude oils originated from the Es 3 L source rock, which contains a mixture of plankton and land plant organic matters deposited in brackish–fresh water under reducing conditions. The oil in the Es 3 L is self-sourced instead of migrated from the overlying source rocks. The petroleum generation potential of the Es 3 L source rock was evaluated using organic geochemistry. Total organic carbon (TOC) values for approximately 100 samples are between 1.02 and 4.92 wt. %, and hydrogen indices range from 285 to 810 mg hydrocarbons/g TOC. The Es 3 L source rock contains mainly type II and III kerogen, and most of the samples are thermally mature. The data show that the Es 3 L source rock has good potential for liquid hydrocarbon generation. The Es 3 L rock also acts as the oil reservoir, having very low bulk porosity and permeability. Various types of storage space in the marlstone and carbonate rudstone in the Es 3 L of the Shulu sag include (1) fractures, (2) intergranular pores, (3) dissolution pores, (4) organic matter pores, (5) intragranular pores, and (6) seams around gravels. Pore size ranges from nanometers to millimeters. Because the oil was generated and stored in Es 3 L strata, which lack any obvious trap and seal and have low permeability, the unit represents a continuous petroleum accumulation.

Tight oil accumulation of the redeposited carbonates in the continental rift basin: A case study from Member 3 of Shahejie Formation in Shulu sag of Jizhong depression, North China

Tight oil in the redeposited carbonates was mainly distributed in the Lower Submember of Member 3 of Shahejie Formation in Shulu sag of Jizhong depression, North China. Through high-resolution 3D seismic data, well logging data and drilling data, the Lower Submember of Member 3 of Shahejie Formation was divided into 5 third-order sequences and 15 parasequence sets. The redeposited marl and rudstone were major reserving horizons of tight oil, and ten reserving space types were developed and could be classified into two main categories, i.e., pores and fractures. Two types of tight oil reservoirs were established, i.e., the marl hydrocarbon reservoir of the source-reservoir integration and the rudstone hydrocarbon reservoirs of the source-reservoir paragenesis. The assemblage relationship among the high-quality source rocks, system tracts with the source-reservoir configuration was the major control factor for tight oil accumulation in the redeposited carbonates. The lacustrine transgressive system tracts and highstand systems tracts in SQ1 to SQ5 were the favorable horizons for development of the marl hydrocarbon reservoir, the lowstand system tracts in SQ1 to SQ3 were the favorable horizons for development of the rudstone hydrocarbon reservoir.

A methodology for estimating the organic porosity of the source rocks at the mature stage: example from the marlstone in the Shulu Sag, Bohai Bay Basin

In recent years, organic-matter pores have been identified as an important storage space type in unconventional reservoirs. Accurate estimation of the organic porosity is critical for unconventional petroleum evaluation and exploration. Organic-matter pores account for an important proportion of the pore network in the Shulu organic matter-rich marlstone. This study determined the characteristics of organic pores in this tight reservoir and provided a method to restore the original hydrogen index (HIo) of samples within the oil window zone, based on the relationship between the hydrogen index (HI) and temperature of the maximum yield of pyrolysate (Tmax). The original total organic carbon (TOCo) and the organic porosity (Φo) were estimated based on data from the pyrolysis experiment using a mass-balance relation. The results show that the value of HIo is in the range of 64 to 1048 mg HC/g TOC, with an average of 605 mg HC/g TOC. The value of TOCo is between 0.12 and 9.7 %,with an average of 1.9 %. The value of Φo is mainly from 0 to 2.5 %, with an average of 0.67 %,which is approximately 16.8 % of the total porosity. The calculated value of Φo is consistent with that obtained by SEM image-based organic porosity measurements. The approach outlined in this article involves a relatively simple methodology for estimating the organic porosity of kerogen-rich source rocks within the oil window zone, which is useful in the exploration and development of unconventional petroleum resources.

Characteristics of solid residue, expelled and retained hydrocarbons of lacustrine marlstone based on semi-closed system hydrous pyrolysis: Implications for tight oil exploration

The Paleogene lacustrine marlstone is an important source rock for giant oil fields found in Eastern China, which presently has become a main exploration target for tight oil resources. However, seldom studies have been conducted on possible variations in geochemical features of lacustrine marlstones during on-going burial process as well as the thermal evolution. Hydrous pyrolysis experiments were performed using organic-rich thermally immature marlstone samples from the Shulu Sag, Bohai Bay Basin, in order to have a better understanding in the formation process and accumulation mechanism for tight oil retained within the source rock. Laboratorial data of samples including solid residues, retained hydrocarbon (extracted bitumen) and expelled hydrocarbons were analyzed. The results indicate that quantity and composition of retained hydrocarbon are controlled by thermal maturity of the source rocks. Mass balance of organic matter provides a clue for recovering the amount of gaseous/light hydrocarbon retained in the solid residues. The formation and accumulation of tight oil mostly takes place in the oil window–wet gas window, which consists of four stages: (1) In the thermal maturity range of 0.5–0.8% Ro, the amount of retained and expelled hydrocarbons appears to increase synchronously; (2) At 0.8–1.0% Ro, the retained hydrocarbons reach maximum amount while the expelled hydrocarbons keep constant; and the gas to oil ratio of retained hydrocarbon (GORR) in the above two stages keeps at a low level; (3) Over the maturity of 1.0–1.1% Ro, the amount of retained hydrocarbons decreases while expelled hydrocarbons increases, during which GORR increases significantly and retained hydrocarbons characterized by more lighter hydrocarbon composition; (4) At about 1.3–1.5% Ro the amount of retained hydrocarbons is less than that of expelled hydrocarbons.

Core evidence of paleoseismic events in Paleogene deposits of the Shulu Sag in the Bohai Bay Basin, east China, and their petroleum geologic significance

The Shulu Sag, located in the southwestern corner of the Jizhong Depression, Bohai Bay Basin of east China, is a NE–SW trending, elongate Cenozoic half-graben basin. The lowermost part of the third member of the Shahejie Formation in this basin is characterized by continental rudstone and calcilutite to calcisiltite facies. Based on core observation and regional geologic analysis, seismites are recognized in these lacustrine deposits, which include soft-sediment deformation structures (sedimentary dikes, hydraulic shattering, diapir structures, convolute lamination, load-flame structures, ball-and-pillow structures, loop bedding, and subsidence structures), synsedimentary faults, and seismoturbidites. In addition, mixed-source rudstones, consisting of the Paleozoic carbonate clasts and in situ calcilutite clasts in the lowermost submember of Shahejie 3, appear in the seismites, suggesting an earthquake origin. A complete representative vertical sequence in the lowermost part of the third member found in well ST1H located in the central part of the Shulu Sag shows, from the base to the top: underlying undeformed layers, synsedimentary faults, liquefied carbonate rocks, allogenetic seismoturbidites, and overlying undeformed layers. Seismites are widely distributed around this well and there are multiple sets of stacked seismites separated by undeformed sediment. The nearby NW-trending Taijiazhuang fault whose fault growth index is from 1.1 to 1.8 and the NNE-trending Xinhe fault with a fault growth index of 1.3–1.9 may be the source of the instability to create the seismites. These deformed sedimentary layers are favorable for the accumulation of oil and gas; for example, sedimentary dikes can cut through many layers and serve as conduits for fluid migration. Sedimentary faults and fractures induced by earthquakes can act as oil and gas migration channels or store petroleum products as well. Seismoturbidites and mixed-source rudstones are excellent reservoirs due to their abundant primary or dissolved pores.

Characteristics and formation mode of salt-bearing series in typical rift valley basin, Eastern China

The Shulu Sag is a typical rift valley basin in Eastern China, where the formation mechanism of Cenozoic salt-bearing series has been poorly understood. In this paper, petrologic analysis, X-ray powder Diffraction (XRD) analysis and the analysis of halite ion concentrations of the first member of the Shahejie Formation from two drilling cores were conducted to interpret the formation mechanism of these salt-bearing series in Shulu Sag. The results indicated that the salt-bearing series is mainly composed of calcilutite or mudstone at top and bottom, and anhydrite-bearing halite and thin interbeds of salt-bearing calcilutite and dark grey shale in the middle part. These thin interbeds with porphyritic or petalshaped glauberite crystals or some thinner dark grey shale interbeds separated halites into several members with the average thickness of only 9~12 cm in the two drilling holes. This typical rhythmic pattern due to frequent alternation of concentration was evidenced by the variation of chemical composition and hydrogen isotopes of halite fluid inclusions. The presence of large number of primary fluid inclusions within chevron crystals and vertically oriented bottom-growth crystals showed that the halite were precipitated in shallow water (<60 cm). These features are direct evidence for this frequent alternation of dissolution and concentration of brines and terrigenous detrital-chemical shallow-water sedimentary model of these small rift-valley basins in Eastern China.

Organic geochemistry and reservoir characterization of the organic matter-rich calcilutite in the Shulu Sag, Bohai Bay Basin, North China

Although extensive studies have been conducted on unconventional mudstone (shales) reservoirs in recent years, little work has been performed on unconventional tight organic matter-rich, fine-grained carbonate reservoirs. The Shulu Sag is located in the southwestern corner of the Jizhong Depression in the Bohai Bay Basin and filled with 400–1000 m of Eocene lacustrine organic matter-rich carbonates. The study of the organic matter-rich calcilutite in the Shulu Sag will provide a good opportunity to improve our knowledge of unconventional tight oil in North China. The dominant minerals of calcilutite rocks in the Shulu Sag are carbonates (including calcite and dolomite), with an average of 61.5 wt.%. The carbonate particles are predominantly in the clay to silt size range. Three lithofacies were identified: laminated calcilutite, massive calcilutite, and calcisiltite–calcilutite. The calcilutite rocks (including all the three lithofacies) in the third unit of the Shahejie Formation in the Eocene (Es3) have total organic carbon (TOC) values ranging from 0.12 to 7.97 wt.%, with an average of 1.66 wt.%. Most of the analyzed samples have good, very good or excellent hydrocarbon potential. The organic matter in the Shulu samples is predominantly of Type I to Type II kerogen, with minor amounts of Type III kerogen. The temperature of maximum yield of pyrolysate (Tmax) values range from 424 to 452 °C (with an average of 444 °C) indicating most of samples are thermally mature with respect to oil generation. The calcilutite samples have the free hydrocarbons (S1) values from 0.03 to 2.32 mg HC/g rock, with an average of 0.5 mg HC/g rock, the hydrocarbons cracked from kerogen (S2) yield values in the range of 0.08–57.08 mg HC/g rock, with an average of 9.06 mg HC/g rock, and hydrogen index (HI) values in the range of 55–749 mg HC/g TOC, with an average of 464 mg HC/g TOC. The organic-rich calcilutite of the Shulu Sag has very good source rock generative potential and have obtained thermal maturity levels equivalent to the oil window. The pores in the Shulu calcilutite are of various types and sizes and were divided into three types: (1) pores within organic matter, (2) interparticle pores between detrital or authigenic particles, and (3) intraparticle pores within detrital grains or crystals. Fractures in the Shulu calcilutite are parallel to bedding, high angle, and vertical, having a significant effect on hydrocarbon migration and production. The organic matter and dolomite contents are the main factors that control calcilutite reservoir quality in the Shulu Sag.

Source‐controlled carbonates in a small Eocene half‐graben lake basin (Shulu Sag) in central Hebei Province, North China

AbstractCarbonate deposits, which unconformably overlie the Palaeozoic bedrocks, extensively occur in the base of the Tertiary lake succession in the half‐graben Shulu Sag, central Hebei Province, North China. This study focuses on the basal carbonate successions on the hinged western slope. Based on seismic, borehole and core data, nine facies are identified in the carbonate successions, and are further grouped into five facies associations: mid‐proximal alluvial fan, distal alluvial fan, fan fringe, moderately deep lake and deep lake. The first two facies associations constitute alluvial fans formed by debrisflows at the edge of lake and are dominated by mounded‐ to lobate‐shaped, matrix‐ to clast‐supported carbonate rudstones with minor calcretes in the lowermost rudstone units and basinward increase in interfingering with lacustrine carbonate facies. The fan fringe, moderately deep lake and deep lake associations are dominated by pebbly carbonate arenites (or rare carbonate arenites), calcisiltite‐calcilutites, and varve‐like calcilutites, calcareous shales and oil shales, respectively. Widespread occurrences of fine‐grained limestone packages containing varve‐like organic‐rich laminations, minor authigenic glauconite and pyrite, and planktonic and plant fossils suggest a meromictic, anoxic deep lake under a semi‐humid to humid climate, probably with a connection to marine basins. Similarities in lithology and fossil assemblages (e.g. trilobites) of lithoclasts with those of the Mid‐Upper Cambro‐Ordovician bedrock carbonates suggest that the clastic and dissolved carbonate loads were sourced from this Lower Palaeozoic catchment, and shed off the surrounding highlands into the basin. These carbonate facies associations represent the lake lowstand and transgressive deposits of the basal third‐order sequence (Ia) in which the highstand deposits are composed of lacustrine siliciclastics. During the lake lowstand stage (or initiation of basin‐filling) under an intermediate climate, carbonate alluvial fans occurred mostly subaerially at the bottom of the hinged slope with a narrow, shallow lake zone basinwards, and locally were perched within the palaeovalley on the mid‐upper slope. During the transgressive (deepening) stage under a semi‐humid to humid climate, carbonate alluvial fans became smaller in size and episodically stepped backwards upon the slope, with greatly expanded and deepened lake. Nevertheless, the carbonate system was switched to an exclusively siliciclastic system during the highstand stage. The exhumation and erosion of the Mid‐Lower Cambrian bedrock dominated by siliciclastics was probably the cause due to further uplift of the drainage basin. All these facts indicate that the carbonate deposition in the Shulu Sag was mostly controlled by the interactions of tectonics, climate and provenance.