Bohai Bay Basin Research Articles

The formation and architecture of Penglai granite intrusion reservoir along the Tanlu strike-slip fault: Implications for basement exploration in the offshore Bohai Bay Basin, North China

The hydrocarbon in the basinal basement mostly occurs in the topmost of buried hills along the unconformity, while we present a special play in architecture with the oil-bearing in the lower geomorphic position at the Penglai buried hill. Based on the cores, thin sections, whole rock X-ray, physical property, well logging, and seismic data, we investigated the characteristics of the reservoirs of the Penglai oilfield and discussed the controlling of strike-slip activity, weathering, and lithology on the reservoir in this paper. The results show that the Penglai buried hill reservoirs were mainly distributed in the fractured and weathered granites. The quality of the reservoir is related to mechanics and weathering dissolution behavior among different rocks. The strong dextral strike-slip activity of the Tanlu strike-slip fault zone during the Paleogene resulted in the strong uplift of the Penglai buried hill and also provided the stress condition for tectonic fractures to develop. The dissolution of atmospheric water along the tectonic fractures further improved the reservoir’s physical properties during the nearly 30Ma exposure interval. Compared to granite rocks, mica quartz schist, with higher content of the femic mineral and lower content of feldspar, presents lower brittleness and less soluble minerals. Therefore, the relatively stronger of weathering resistance of mica quartz schist resulted in a higher ring-landform in buried hill geomorphology which surrounds the granite intrusion, controlling the special oil play in architecture with hydrocarbon accumulated in the lower-seated granite reservoir underlying the unconformity.

Impact of sedimentation and diagenesis on deeply buried sandy conglomerate reservoirs quality in nearshore sublacustrine fan: A case study of lower Member of the Eocene Shahejie Formation in Dongying Sag, Bohai Bay Basin (East China)

The sandy conglomerates of the lower member of Shahejie (Es4x) in the north of Dongying Sag, Bohai Bay Basin, were investigated to understand the formation mechanism of the good reservoir and controlling factors in deeply buried strata. This research is based on seismic, logging, and core data and comprehensively utilizes methods such as core description, petrographic analysis, cathodoluminescence image analysis, SEM-EDS analysis, and mercury intrusion. The sandy conglomerates are predominantly lithic arkose and litharenite. The reservoirs in the Es4x have experienced three diagenetic processes, compaction, dissolution, and cementation. Compaction remains the major factor affecting the reservoir quality, mainly in the progradation sequence, and cementation has a destructive effect on reservoirs, principally in the retrogradation sequence. The quantification of the compaction by the compaction index revealed that mechanical compaction is higher in the progradation sequence (0.68) than in the retrogradation sequences (0.59). Compaction is suppressed by carbonate cementation, but the extensive cementation significantly degrades the reservoir quality, especially in the retrogradation sequence. Dissolution of different rock fragments, feldspar, and the whole dissolution, in general, enhance the reservoir's quality in both intervals but more in the progradation sequence. Three types of reservoirs representing type I, type II, and Type III mercury intrusion capillary pressure curves were encountered in the Es4x. The braided channels in the progradation and retrogradation sets encompass the greatest reservoir quality and are characterized by type I and II reservoirs with good petrophysical properties. The preeminent reservoirs appear in the braided channels of the progradation set. The development of high-quality reservoirs is governed by the depositional subfacies, compaction, and dissolution in the Es4x. This study will contribute to comprehending the origin of the genesis mechanism of high-quality in deep sublacustrine sandy conglomerate and will help the exploration and pre-drill for favorable reservoirs in similar basins around the world.

Quantitative constraints on hydrocarbon vertical leakage: Insights from underfilled fault-bound traps in the Bohai Bay Basin, China

The mechanism of hydrocarbon leakage in active fault-bound traps is not yet well understood. The underfilled fault-bound traps with different burial depths in the Qinan sag (NDG-3 trap and YN-1 trap) of the Bohai Bay Basin, China, provide a favourable setting to investigate the role of faulting in hydrocarbon leakage. Interpretation of seismic and borehole data, results of rock mechanics testing, and data from quantitative fluorescence analysis are used to evaluate hydrocarbon leakage. The results reveal that the trap-bounding faults display continuous growth faulting throughout the Cenozoic. The shale caprocks for the YN-1 trap and the NDG-3 trap are brittle–ductile transition deformation according to an analysis of brittle–ductile index (BDI). The presence of thick, effective top seals and favourable fault juxtaposition seal and fault membrane seal relationships suggests that vertical leakage caused by fault reactivation is the most viable mechanism for explaining the lost hydrocarbons. A model for the shrinkage of oil column height caused by fault reactivation can be established. According to this model, about 30% of the trapped column height will be lost from structural traps during reactivation in the case of brittle–ductile transition deformation of the shale caprock. These new results have implications for petroleum exploration in fault-dominated petroliferous basins in other areas of China.

Two-step classification of highly mixed oil in the Qikou Sag (Bohai Bay Basin, China): Implications for origin, maturity, and biodegradation

Mixing of crude oil leads to unpredictable changes in the property and composition of mixtures, bringing uncertainties to petroleum production. Therefore, it is crucial to reveal the origin, maturity, and biodegradation of mixed oil during the exploration stage. Here we address these issues using the highly-mixed oil from the Qikou Sag (Bohai Bay Basin, eastern China) after a two-step classification. Firstly, the highly-mixed oil was classified into two groups (groups I and II) according to their physical property (API gravity and sulfur content), molecular composition, and gas chromatograms of saturated hydrocarbons. Secondly, the group I oil was further divided into two sub-groups (groups I-1 and I-2) based on the principal component analysis (PCA) and hierarchical cluster analysis (HCA) of biomarkers (including n-alkanes, isoprenoids, terpenoids, and steroids). The group I oil is non-biodegraded, in which the group I-1 oil from deep reservoirs is a mixture of hydrocarbons from three sets of source rock, whereas the group I-2 oil from shallow reservoirs is mixed by hydrocarbons from five sets of source rock. Maturity parameters of biomarkers suggest that the group I oil is thermally mature. The group II oil was slightly (PM 1–2) to severely (PM 4+) biodegraded, as indicated by low API gravity, high percentages of asphaltenes and NSO, high ratios of Pr/n-C17 and Ph/n-C18, and the assignment of UCM and 25-norhopane. The burial depth of reservoirs and secondary faults (formed after the Neogene) are the primary controlling factors for the biodegradation. Our results demonstrate that the two-step classification facilitates the grouping of highly mixed oil, which provides a basis for indicating their origin, maturity, and biodegradation. The results also provide guidance for regional petroleum exploration and production.

Experimental Study on the Adaptability of Plugging Drilling Fluids to Wellbore Stability in Hard Brittle Shale Formations.

The problem of wellbore stability in hard brittle shale formations is an important research topic in the exploration and development of shale gas. To solve this problem, the adaptability of the plugging drilling fluid to wellbore stability in the hard brittle shale of the tertiary Dongying formation in Bohai Bay Basin, China, was investigated. The results show that the clay content of the hard brittle shale in the study block is as high as 39.2% on average, with great possibility for hydration. The pore structure in the shale is dominated by micron-scale fractures and pores. A dense structure was formed on the surface of the shale after being immersed in plugging drilling fluid, and the matrix permeability of the shale was reduced by 91.1% and the fracture permeability by 98.7%. The water content increment of the shale after immersion was merely 0.75%, which reduced the probability of hydration greatly. Compared with the field-inhibitive drilling fluid, the plugging drilling fluid improved the uniaxial compressive strength of shale by 28%, which is more conducive to maintaining the wellbore stability. The seepage stress aggravates the risk of wellbore instability, while the hydration stress does not, but both increase the risk of rock instability at positions away from the well wall. The plugging drilling fluid affects the seepage stress and hydration stress by reducing the shale permeability and water content. With the decrease of permeability and water content, the potential instability zone of a wellbore becomes smaller.

Sedimentary and crustal structure of eastern North China from joint inversion of Rayleigh wave phase velocity and ellipticity and its implication for magmatism

Understanding the tectonic evolution of the North China Craton and its destruction mechanism requires the knowledge of eastern North China's fine crustal velocity structure. Using the two-year record of 342 ChinArray stations deployed in eastern North China, we obtain the Rayleigh wave phase velocity via ambient noise interferometry and Rayleigh wave ellipticity from ∼1000 teleseismic events. High-resolution sedimentary thickness and crustal 3D S-wave velocity structure are obtained by jointly inverting these observations. The results show that the Bohai Bay Basin is infilled with thick sediments and the Central Bohai Sea is the current depocenter with sedimentary thickness up to about 6 km. The crust and uppermost mantle beneath the Bohai Bay Basin are characterized by low velocity, indicating a high internal temperature of the crust. An upwelling of mantle thermal material under the crust of the Bohai Bay Basin continuously modifies the crust, suggesting that bottom-up thermochemical erosion is likely the primary control factor of crustal modification and thinning in this area. The uplift areas around the basin generally exhibit high velocity with weaker crustal modification, showing the heterogeneity of craton modification. Moreover, localized low-velocity zones observed beneath the Chifeng Volcanos, Dalainor Volcanos, and Kuandian Volcanos indicate the residual magmatism beneath these Quaternary volcanos. Furthermore, the magmatism beneath the Chifeng and Dalainor Volcanos shares a common magmatic source.

Evolution Characteristics through Thermo-Rheological Lithosphere of the Liaonan Metamorphic Core Complex, Eastern North China Craton

Metamorphic core complexes are developed in crustal activity belts at the continental margins or within continents, and their main tectonic feature is that the ductile middle crust is exhumed at the surface. The deformation properties are closely related to the geodynamic process affecting the continental crust. However, the evolution of the metamorphic core complexes after their formation is still unclear. The Cretaceous Liaonan metamorphic core complex developed in the eastern North China craton provides an ideal environment to study its evolution. In this study, we estimate the paleo-temperature and paleo-stress at the time of formation of the metamorphic core complex dynamical recrystallization of quartz and calculate the thermo-rheological structure of the present Liaonan metamorphic core complex by one-dimensional steady-state heat conduction equation and power-creep law. The results show that compared with the Cretaceous period, the geothermal heat flow value of the present Liaonan metamorphic core complex decreases from 70–80 mW/m2 to 49.4 mW/m2, the thermal lithosphere thickness increases from 59–75 km to 173 km, and the brittle transition depth increases from 10–13 km to about 70 km, showing coupling of the crust–mantle rheological structure. We speculate that the evolution of the thermo-rheological structure of the Liaonan metamorphic core complex is possibly caused by rapid heat loss or lithospheric mantle flow in the Bohai Bay Basin.

Tectono-sedimentary evolution of the Paleogene Qikou Sag, Bohai Bay Basin, NE China

The Qikou Sag is a petroliferous tectonic unit within the Bohai Bay Basin, northeast China. The depositional system evolved temporally, spatially, and episodically in five rifting stages (stages 1–5) during the Paleogene and Neogene, with sedimentation responding to regional subsidence. This study focuses on the tectono-sedimentary evolution through stages 2–4, while the main oil-bearing sedimentary rocks were deposited in the Bohai Bay Basin. The depositional system exhibited retrogradation and then progradation during the rifting stage 2 (43–36.7 Ma), retrogradation in stage 3 (36.7–32.8 Ma), and upward coarsening and progradation in stage 4 (32.8–24.6 Ma). This vertical superposition corresponded to three episodic stages of rising and falling lake levels during the Paleogene, which were equivalent to a transgressive with regressive (stage 2), transgressive (stage 3), and then regressive (stage 4) cycle. The sediment fills coincided with the tectonism and the regional subsidence history of the Qikou Sag, which is composed of uplift areas and sub-sags. In this respect, the boundary fault activity around the sub-sags decreased from stages 2–4, leading to the migration of sub-sag depocenters from near faults to the center of sub-sags, which suggests that faults released control over the sedimentation. Secondly, tectonism and the total subsidence rate decreased from SQ1 to SQ7 (stages 2–3) and then accelerated at the end of stage 4 (SQ10 to SQ11). The tectonic subsidence rates decreased quicker than the total subsidence rate; this demonstrates that major rift faulting was the dominant geological process in the early Paleogene, whereas overall subsidence was dominant in the late Paleogene.

Dynamic Evaluation of Flow Unit Based on Reservoir Evolution: A Case Study of Neogene Guantao Ng3 Formation in M Area, Gudao, Bohai Bay Basin

To clarify the dynamic evolution characteristics of reservoir flow units during water injection development, the upper member of the Neogene Guantao formation in Block M of Gudao Oilfield is taken as a case study. Based on logging data, water injection profile test data, subwell data, and production performance data, among others, the flow zone index (FZI static) was proposed as the static evaluation parameter of the flow unit. The relationship between cumulative water injection (WT) and FZI change ( △ FZI ) was fitted. Hence, the △ FZI caused by water injection is combined with the static parameter of flow unit evaluation (FZI static) as the dynamic parameter of flow unit evaluation (FZI dynamic). The comprehensively evaluated reality of flow units in different periods is characterized by formula FZI static + △ FZI = FZI dynamic . The study shows that as the division standard of the flow unit, the FZI has a good correlation with the test results of the water absorption profile, such as water absorption intensity and relative water injection volume. Using the FZI as the static parameter of flow unit evaluation, four types of flow units were divided as follows: type I flow unit, FZI ≥ 4.1 ; type II flow unit, 4.1 > FZI ≥ 2.4 ; type III flow unit, 2.4 > FZI ≥ 1.7 ; type IV flow unit, 1.7 > FZI . The reservoir porosity and permeability characteristics of different flow units are highly correlated. Moreover, the relative permeability curve confirms that different flow units have different seepage capacities. Though, comparing characteristic reservoir parameters in different periods, the reservoir’s physical parameters became more conducive to fluid flow with the water injection development. The increase in the same water injection rate for type I and II flow units was greater than that for type III and IV flow units. Furthermore, when type I flow units were continuously distributed in a large area, high water consumption bands were formed, absorbing most of the water injection in the water injection wells. Hence, the waterflood efficiency was low. The change in different flow units was mainly controlled by the injection production well pattern and WT. Combined with the relative change characteristics of interlayer flow units, the changes can be divided into increasing type change and decreasing type changes. Finally, according to the distribution characteristics of different flow units, oil saturation, and water flooding results, strategies for tapping the potential of the remaining oil from three aspects (plane, interlayer, and inner layer) were formulated.

Division and identification of vertical reservoir units in Archaeozoic metamorphic buried hill of Bozhong Sag, Bohai Bay Basin, East China

Based on the data associated with cores, sidewall cores, casting thin sections, reservoir physical properties, conventional logging and imaging logging, the classification schemes of vertical reservoir units are proposed for the two types of Archaeozoic buried hills (exposed and covered ones) in the Bozhong Sag, Bohai Bay Basin. The geological characteristics and storage spaces of these reservoir units are described, and their identification markers in conventional and imaging log curves are established. The Archaeozoic metamorphic buried hills can be vertically classified into two primary reservoir units: weathering crust and inner buried hill. The weathering crust contains four secondary units, i.e., the clay zone, weathered glutenite zone, leached zone, disaggregation zone; and the interiors contain two secondary units, i.e., interior fracture zone and tight zone. In particular, the inner fracture zone was further divided into cataclasite belts and dense-fracture belts. It is proposed that the favorable reservoirs of exposed Archaeozoic metamorphic buried hills are mainly developed in four parts including weathered glutenite zone, leached zone, disintegration zone superposed with the cataclasite belt and the cataclasite belt of inner fracture zone, and are controlled by both weathering and tectonic actions. Favorable reservoirs in covered Archaeozoic metamorphic buried hills are mainly developed in the weathering crust superposed with the cataclasite belts and the cataclasite belts of inner fracture zone, and are mainly controlled by tectonic actions.

Origin and diagenetic evolution of dolomites in Paleogene Shahejie Formation lacustrine organic shale of Jiyang Depression, Bohai Bay Basin, East China

The origin of dolomite in Shahejie Formation shale of Jiyang Depression in eastern China were studied by means of petrologic identification, compositional analysis by X-ray diffraction, stable carbon and oxygen isotopic composition, and trace element and rare earth element analyses. The results show that the development of dolomite is limited in the lacustrine organic rich shale of Shahejie Formation in the study area. Three kinds of dolomite minerals can be identified: primary dolomite (D1), penecontemporaneous dolomite (D2), and ankerite (Ak). D1 has the structure of primary spherical dolomite, high magnesium and high calcium, with order degree of 0.3-0.5, and is characterized by the intracrystalline corrosion and coexistence of secondary enlargement along the outer edge. D2 has the characteristics of secondary enlargement, order degree of 0.5-0.7, high magnesium, high calcium and containing a little iron and manganese elements. Ak is characterized by high order degree of 0.7-0.9, rhombic crystal, high magnesium, high calcium and high iron. The micritic calcite belongs to primary origin on the basis of the carbon and oxygen isotopic compositions and the fractionation characteristics of rare earth elements. According to the oxygen isotopic fractionation equation between paragenetic dolomite and calcite, it is calculated that the formation temperature of dolomite in the shale is between 36.76-45.83 °C, belonging to lacustrine low-temperature dolomite. Based on the maturation and growth mechanism of primary and penecontemporaneous dolomite crystals, a dolomite diagenetic sequence and the dolomitization process are proposed, which is corresponding to the diagenetic environment of Shahejie Formation shale in the study area.

Reconstruction of Paleoenvironment and Paleoclimate of the Neogene Guantao Formation in the Liaodong Sub-Uplift of Bohai Bay Basin in China by Sedimentary Geochemistry Methods

The paleosedimentary environment and paleoclimate of the Neogene Guantao Formation in the Liaodong sub-uplift of Bohai Bay Basin in China, which is an important oil and gas exploration horizon, are not clear owing to the lack of quantitative analysis. The paleosedimentary environment and paleoclimate can be qualitatively or semi-quantitatively reconstructed by sedimentary geochemical indicators sensitive to the environment and climate. Based on the evaluation of whether the elements and isotopes can effectively record paleosedimentary environment and paleoclimate information or not, the paleoclimate (temperature and humidity), paleoenvironment (salinity, water depth, redox conditions), and evolution of the paleoenvironment and paleoclimate are studied by analyzing the trace elements, carbon and oxygen isotopes, strontium isotopes, whole-rock mineral compositions, and clay minerals of mudstones of the Guantao Formation in the Liaodong sub-uplift, Bohai Bay Basin. The study results show that (1) according to the trace element distribution patterns, high concentrations of continental elements (Ti, Zr, Th), clay minerals, and detrital content, the Guantao Formation is a product of proximal deposits, and the provenance mainly originates from the intermediate–acidic magmatic rocks near surrounding uplifts. (2) The paleoclimate during the sedimentation of the Guantao Formation was mainly semi-arid and semi-humid, as revealed by the Sr/Cu ratio. However, the high Rb/Sr and 87Sr/86Sr ratios demonstrate that the paleoclimate is relatively warm and humid. (3) The paleosedimentary temperature calculated by the Sr content and δ18O is roughly 30 °C, indicating that the Guantao Formation was deposited in a subtropical to tropical climate. (4) The Guantao Formation was deposited in continental freshwater according to the Li, Ni, and Sr content, Sr/Ba ratios, 87Sr/86Sr ratios, and salinity index Z. (5) The water depth of the paleoenvironment of the Guantao Formation is mainly in nearshore shallow water, reflected by the Mn/Ti ratio and calculated by the Co content, and the oxidizing condition of the paleoenvironment of the Guantao Formation, reflected by the U/Th, V/Cr, and Ni/Co ratios, is in a good coupling relationship with this environment. (6) As revealed by the variations in the geochemical data in the section, the temperature, humidity, and water depth of the Guantao Formation gradually increased from the early to late period, while the salinity gradually decreased, and the chemical weathering effect gradually increased.

Factors controlling the development of carbonate reservoirs of Ordovician Yingshan formation in the Gucheng area, Tarim Basin

Deep carbonate rocks are important targets for oil and gas exploration in China. In recent years, significant oil/gas discoveries have been made in the deep carbonate sequences in the Tarim, Sichuan, Ordos, and Bohai Bay basins. Despite significant oil/gas discoveries, large-scale exploration has not been conducted in the Gucheng area in the Tarim Basin. To break the bottleneck restricting the petroleum exploration in the Gucheng area, this study analyzed the factors controlling the formation of carbonate reservoirs of the Yingshan Formation in the Gucheng area in detail based on the basic geological conditions of the study area and the data from cores, thin sections, well logging, testing, and 3-D seismic survey. The inner shallow-ramp in the Gucheng area acts as the main sedimentary facies zone for the development of high-quality reservoirs. The grainstones formed in the high-energy environment of the inner shallow-ramp laid the foundation for subsequent reservoir development in the study area. The dolomitized shoal grainstones in the inner shallow-ramp have well-developed intercrystalline pores and intercrystalline dissolved pores due to later dolomitization, thus serving as high-quality reservoirs. Strike-slip faults are crucial to reservoir reformation and determine whether high production can be achieved in oil and gas exploitation in the study area. Moreover, later reformation by hydrothermal solutions also plays a constructive role in reservoir formation.

An indication of salt caprock failed integrity in inclusion records: Dongpu depression, bohai bay basin, China

Salt rocks are widely distributed in the Paleogene strata of the Dongpu Depression, China, as major caprocks of the Shahejie Formation. Previous studies have suggested that the decline of formation pressure during the Dongying movement is accompanied by petroleum leakage, but they lack any direct evidence of failed integrity of the salt caprocks. Therefore, fluid inclusion analysis under both incident ultraviolet (UV) and transmitted light (TR) modes on halite samples were conducted in this study, supported by basin and fluid inclusion PVTX modeling to prove lack of integrity in the caprock, periodically. Our observation showed that various types of hydrocarbon inclusions exist in halite crystals, revealing multi-stage fracturing and healing processes inside the salt caprocks. Coupling the PVTX and basin modeling results, revealed that oil inclusions were trapped at around 6.6 Ma during the secondary hydrocarbon generation of the source rocks. Moreover, salt caprocks failed due to the overpressure that was formed during hydrocarbon generation. This was exacerbated by the following uplifting which ultimately led to the present-day normal reservoir pressure in the sampled depth. In addition, the fluorescence of oil inclusions in the same fracture gradually changed from orange to blue. This proved that analyzing the thermal maturity of trapped hydrocarbons within the inclusions, solely based on the fluorescence color, could be misleading and other submicron scale analytical methods should be employed to examine the inclusions. Overall, this study enabled us to investigate the caprock integrity via optical methods, which can eventually help us to avoid regions where hydrocarbon is lost vs. prolific areas in the formation understudy.

GC–MS and ESI FT-ICR MS characterization on two type crude oils from the Dongying depression

Molecular-level characterization of sulfur-containing compounds has aroused much concern due to their relatively high abundance, sedimentary environment judgment and application at maturity. In this paper, saline lacustrine immature oils and freshwater lacustrine mature oils from the Dongying Depression (Bohai Bay Basin) were characterized by positive-ion electrospray ionization [(+) ESI] Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The saline lacustrine oils are dominated by C20-DBE1, C30-DBE1, and C40-DBE3 which contain carbon skeletons of phytol, squalene and β-carotane respectively, denoting a genetic relationship between organosulfur compounds (OSCs) and their precursors. The precursors of C19-C22 and C26-C30 with DBE = 5 are derived from short-chain steranes and regular steroids, and their different contents can reflect the biological sources. The ratios of C20/(C19 + C21)-DBE1-S1, C40/(C39 + C41)-DBE3-S1, C30/C10-55-DBE1-S1 and C27-29/C10-55-DBE5-S1 are sensitive to organofacies, which are the first time used for oil-oil correlation. The novel maturity indices are based on the general process of thermally induced cyclization and aromatization of hundreds to thousands of OSCs from immature to mature oils. The high coefficient of correlations between the newly developed maturity parameters (DBE6-16/DBE1+2+5-S1, C35-60/C10-35-DBE1-17-S1) with Ts/(Ts + Tm) and C29-sterane αββ/(ααα + αββ) show that these utilities for assessing precisely the thermal maturity of lower maturity oils. The positive-ion ESI FT-ICR MS index has essential application prospects for organic input analysis, maturity evaluation, and oil-oil correlation.

Influence of Paleosedimentary Environment on Shale Oil Enrichment in the Raoyang Sag, Bohai Bay Basin

Influence of Paleosedimentary Environment on Shale Oil Enrichment in the Raoyang Sag, Bohai Bay Basin

Lithofacies Characteristics and Sweet Spot Distribution of Lacustrine Shale Oil: A Case Study from the Dongying Depression, Bohai Bay Basin, China

Abstract Lacustrine shale is characterized by rapid lithofacies transformation and compositional heterogeneity, which present challenges in shale oil sweet spot evaluation and distribution prediction and should be systematically studied. Field emission-scanning electron microscopy (FE-SEM), low-pressure adsorption isotherm analysis, mercury intrusion porosimetry (MIP), and triaxial compression testing were employed to comprehensively analyze the oil-bearing capacity, reservoir properties, fluidity, and frackability of different lithofacies. Via analyses of mineral composition, total organic carbon (TOC) content, and sedimentary structure, seven lithofacies were identified: organic-rich calcareous shale (L1), organic-rich laminated calcareous mudstone (L2), organic-rich laminated carbonate-bearing mudstone (L3), intermediate-organic laminated calcareous mudstone (L4), organic-poor laminated calcareous mudstone (L5), organic-poor thin-bedded calcareous mudstone (L6), and organic-rich laminated silty mudstone (L7). Considered together, the oil-bearing capacity, reservoir properties, fluidity, and frackability suggested that the L1 and L7 lithofacies were high-quality sweet spots, with satisfactory oil-bearing capacity (TOC>3.5%; S1>10 mgHC/grock), well-developed pores and microfractures, notable fluidity (as indicated by a high oil saturation index value), and suitable brittleness. The sweet spot distribution was predicted according to multiresolution graph-based clustering analysis of well logs. The results indicate that comprehensive research of the key factors for shale oil and lithofacies prediction can promote sweet spot prediction and enhance shale oil exploration.

Effect of faulting on diagenetic processes in the silicate-sulfate-carbonate system: A case study from the Bonan Sag of Jiyang Depression, Bohai Bay Basin

Many extraordinary cements, including mosaic dolomite, poikilitic celestine–barite, anhydrite and chemically zoned saddle-formed dolomite–ankerite, were observed in a reservoir sandstone unit underlying an evaporite sequence in the northern Bonan Sag, which is a saline lacustrine rift basin in the Jiyang Depression of Bohai Bay Basin, China. A suite of petrologic and chemical methods, including optical observation with cathode luminescence, fluid inclusion with laser Raman spectroscopy, X-ray fluorescence, and stable isotopes of carbon, oxygen and sulfur, were carried out. The results suggest that these extraordinary cements are mainly distributed near a fault zone and related to episodes of faulting events. Fault rupturing at shallow burial depths resulted in the dislocation of the evaporites and sandstones and made it possible for an influx of alkaline fluid from evaporites into the fault zone, where mosaic dolomite precipitated, and near fault reservoir units, such as where celestine–barite precipitated. Within the ‘oil window’, the reactivated normal faults, which induced a pressure drop in the reservoir unit, may have resulted in gas influx (e.g., CO2 and H2S) and hydrocarbon charging, enhancing K-feldspar dissolution and cementation of assemblages of quartz, albite and illite. The cyclic fault rupturing and sealing in this setting probably resulted in repeated ‘openness’ of the diagenetic system, chemical perturbation of the pore fluids (e.g., pH dropping and returning), and cyclic dissolution–regrowth within the silicate–sulfate–carbonate diagenetic system. This study may highlight the awareness of diagenetic complexity in fault zones and shed light on reservoir evaluation in rift settings.

Geochemical behavior of aliphatic hydrocarbons during primary migration and implication for hydrocarbon expulsion: A case study in Qikou Sag, Bohai Bay Basin, eastern China

Lipid biomarker parameters and molecular carbon isotope compositions of n-alkanes were used as tracers to determine geochemical behavior of aliphatic hydrocarbons during their migration from two sequences of source rock-type mudstones (4050.8–4062.2 and 4067.4–4073 m depth, respectively) to the interbedded reservoir sandstones (4062.6–4066.3 m depth) in the Qikou Sag, Bohai Bay Basin, eastern China. Abnormally high values of the production index (0.69–0.90) and total extract/total organic carbon (TOC) ratios (0.53–1.21) in the sandstone samples indicated that the hydrocarbons in the sandstones were not produced in situ but migrated from the adjoining mudstones. A comparison of values in lipid biomarker parameters (oleanane index, gammacerane index, C23 tricyclic terpane/C30 αβ-hopane, and C29/C27 ααα20R sterane ratios) in the sandstone with the mudstone samples indicated steroids and hopanoids in the upper section of the sandstone layer (4062.6–4065.2 m depth) were transported from the overlying mudstones, while those in the lower sandstone layer (4066.3 m depth) were transported from the underlying mudstones. Molecular carbon isotope data suggest that all n-alkanes in the upper section of the sandstone layer were transported from the overlying mudstones. However, in the lower sandstone layer, most of the C15–C17n-alkanes were transported from the overlying mudstones, while the C18–C31n-alkanes were transported mainly from the underlying mudstones. These combined results suggest the short-chain n-alkanes (<C17) were expelled preferentially relative to other compounds (probably via molecule diffusion) while the long-chain n-alkanes (>C18) and steroids/hopanoids may have been expelled in a separate oil phase.

Implication Linkage among Microfacies, Diagenesis, and Reservoir Properties of Sandstones: A Case Study of Dongying Formation, Nanpu Sag, Bohai Bay Basin

The reservoir quality of sandstones is significantly impacted and transformed by sedimentation and diagenesis. It is necessary to clarify the internal relationship among them to precisely predict the sweet reservoir. In this study, five types of sedimentary microfacies are recognized through core observation and logging data: submerged distributary channel (fan delta), submerged interdistributary bay, submerged distributary channel (braided delta), distal bar, and turbidite fan. The major diagenetic processes, including compaction, cementation, and dissolution, have been analyzed based on petrography, scanning electron microscopy, and X-Ray diffraction. The dominant diagenetic cement includes calcite, smectite, kaolinite, illite, and I/S mixed-layer minerals, with small quantities of chlorite, pyrite, siderite, feldspar, and quartz cement. The reservoir quality is best in the submerged distributary channel (fan delta) sandstones, followed by submerged distributary channel (braided delta). Submerged interdistributary bay, distal bar, and turbidite fan are of poor reservoir quality. The grain size is the primary reservoir quality controlling factor, highly affected by sedimentary microfacies. Subsequent controls are diagenetic processes such as mechanical compaction, clay minerals formation, grain replacement, and dissolution that collectively influence the porosity and permeability.