Zhanhua Sag Research Articles

Effect of laminae development on pore structure in the lower third member of the Shahejie Shale, Zhanhua Sag, Eastern China

Similar to mineral composition and organic geochemical features, laminae development significantly influences pore structure. Taking the lower third member of the Shahejie Shale (Es3l), Zhanhua Sag, Eastern China as the research object, we introduced various methods to analyze the influence of laminae development on pore structure, including thin section observations, field emission scanning electron microscopy, gas adsorption, high-pressure mercury injection, nano-computed tomography (CT), quantitative evaluation of minerals by scanning electron microscopy, and spontaneous imbibition. We draw the conclusions that various minerals present a mixed distribution in nonlaminated shale, whereas laminated shale is characterized by alternating bright and dark laminae. Dark laminae comprise clay and quartz, whereas bright laminae consist of calcite. Microfractures are abundant at the edges of the bright and dark laminae. Nonlaminated shale possesses a pore volume (PV) of [Formula: see text] and a specific surface area (SSA) of [Formula: see text]. In contrast, laminated shale has a PV of [Formula: see text] and an SSA of [Formula: see text] with good reservoir property. Pores, especially macropores and micropores, are much more developed in laminated shale than in nonlaminated shale. Interconnected pores in sheet form are extremely developed in laminated shale, whereas most of the interconnected pores in nonlaminated shale are distributed in isolated spherical and tubular forms. Because of the abundant interconnected pores and throats, laminated shale presents good connectivity. The slopes of the spontaneous imbibition curves in the first and second stages for laminated shale are greater than those for nonlaminated shale. Laminae development could provide microfractures as dominant pathways for fluid migration as well as promote the interconnection of pores, greatly increasing the connectivity of shale reservoirs.

Structural architecture and tectonic evolution of the Cenozoic Zhanhua Sag along the Tan–Lu Fault Zone in the eastern North China: Reconciliation of tectonic models on the origin of the Bohai Bay Basin

The Zhanhua Sag, a major depocenter within the larger Bohai Bay Basin (BBB) along the Tan–Lu Fault Zone (TLFZ) in the eastern North China, developed through two successive phases with different stress regimes during the Paleogene. The kinematics and the slip history of the NW–SE– and N–S–striking normal fault systems strongly controlled the internal structure and stratigraphy of the Paleocene–Lower Eocene strata in the Zhanhua Sag during Phase 1. Isopach maps of the Paleocene–Lower Eocene stratigraphy show ~4000–m–thick clastic rock sequences in these structurally controlled depocenters. The average dip–slip rates of the NW–SE– and N–S–striking faults were 78.96 and 50.82 m/m.y. (respectively) in the Paleocene–Early Eocene, but decreased to 45.87 and 35.81 m/m.y. in the Middle–Late Oligocene. The NE–SW–striking fault systems and their slip history controlled the development of the Middle Eocene–Oligocene depocenters and their strata during Phase 2. The average dip–slip rate of the NE–SW–striking faults was 10.47 m/m.y. in the Paleocene–Early Eocene but increased to 93.49 m/m.y. in the Middle–Late Oligocene. The NE–SW–oriented faults cut across and displace the NW–SE–striking faults, indicating their relatively younger ages. These fault patterns and kinematics recorded in the Zhanhua Sag reflect an abrupt change in extension directions from ENE–WSW to NW–SE before and after the Middle Eocene that was related to a reversal in the slip direction from sinistral to dextral along the TLFZ. This slip reversal was, in turn, a result of a major change in the subduction direction of the Pacific Plate beneath eastern Asia and in the mode of strain partitioning across the TLFZ in the continental upper plate.

Reconstruction of sediment-dispersal patterns using seismic sedimentology in the southeastern Zhanhua Sag, Bohai Bay Basin, China

Reconstruction of sediment-dispersal patterns not only provides insight into paleogeographic evolution, but also sheds light on facies and reservoir prediction. By integrating core, wireline logs, and 3D seismic data, seismic sedimentologists have discovered several advantages to sediment-dispersal reconstruction and paleogeographic-environment analysis. We have subdivided the third member of the Shahejie Formation (Es3) of the southeastern Zhanhua Sag (Sanhecun Block) into three third-order sequences, namely SQ1, SQ2, and SQ3 from bottom to top. The facies architecture was analyzed by using the seismic sedimentology approach based on 3-D seismic data. Seismic RMS-amplitude stratal slices reveal different characteristics of the lobes among the southwest gentle slope, southern slope-break belt and north fault-controlled slope from source to sink. On the basis of an integrated analysis of well log, core data, seismic facies based on RMS attributes, three depositional environments (e.g., “fan-delta”, “turbidite” and “lacustrine” facies) have been recognized. Seismic RMS-amplitude stratal slices indicate that the depositional environments of these sequences evolved from medium-sized gravel-rich fan-delta and turbidite-fan deposits to small-scale mud-rich fan-delta deposits, and lastly to large-scale sand-rich fan-delta deposits. The changes of relative lake level and sediment supply rate control the lithology and size of different depositional systems. This study also suggests that the temporal and spatial evolution and distribution of depositional systems are influenced significantly by paleotopography and slope break in the systems tracts of different sequences of the Es3 in the southeast Zhanhua Sag, Bohai Bay Basin. In addition, fan-delta-front deposits could also be considered as future potential exploration targets because they are relatively sand- or gravel-rich, given the reconstruction of the sediment-dispersal pattern.

Sedimentary characteristics and evolution of the fluvial system of the Neogene Guantao Formation, Zhanhua Sag, Bohai Bay Basin, China

Fluvial facies sand bodies are important reservoirs for oil and gas resources. In this study, the sedimentary characteristics and evolutionary patterns of braided and meandering fluvial deposits in the Neogene Guantao Formation of the Zhanhua Sag, Bohai Bay Basin were analyzed using cores, well logs and three-dimensional seismic data, and the quantitative information of the sand bodies in different fluvial deposits were characterized with dense well-net constraints. Through the calibration between the core and wireline logging data, the developmental characteristics of the sand bodies in the braided and meandering deposits were clearly defined base on single-well sedimentary microfacies interpretation templates. Composite fluvial channel sand body facies associations develop in braided fluvial or high-sinuosity meandering fluvial deposit, and single fluvial channel sand body facies associations develop in a mid-sinuosity meandering fluvial deposit. The fluvial sedimentary evolutionary model was established based on the reconstruction of sedimentary characteristics. Factors such as the relative sea or lake level, climate, and tectonics control the change of the river. During the late period of Guantao Formation deposition, the tectonic movement tended to be stable; thus, climate change was the most important factor in the fluvial style change and the lake level rise also showed an influence. There is a positive correlation between the width and thickness of the braided fluvial channels and the mid-sinuosity meandering fluvial channels; however, this is relatively poor for the high-sinuosity meandering fluvial channels.

Depositional characteristics and reservoir potential of Paleogene sediment gravity flow deposits on a faulted slope of the Zhanhua Sag, Bohai Bay Basin, China

Deep-water sandstones related to sediment gravity flows are becoming an increasingly important exploration domain in continental rift basins in China. However, ambiguities remain regarding the initiation, evolution, and deposition of sediment gravity flows on faulted slopes, where step faults are commonly developed. To address the uncertainties related to these processes, grain size analysis, lithofaciesanalysis, logging interpretation, and seismic interpretation were employed to study Paleogene sediment gravity flow deposits from well cores on a faulted slope in the Zhanhua Sag of the Bohai Bay Basin. Twelve lithofacies related to sediment gravity flows were recognized and attributed to intrabasinalsediment gravity flows (ISGFs) induced by slope failures and extrabasinal sediment gravity flows (ESGFs) generated by floods. Active faults were important triggers for ISGFs on the faulted slope in the Zhanhua Sag. The fault activity rate had a prominent role in determining the volume of re-transported sediments, which further influenced the evolution of the resulting flows. Travelling across multiple faults downslope, ISGFs generally underwent considerable velocity losses, with the result that almost all sediments were deposited in shallow water. Moreover, unconsolidated ISGF deposits could be transported again to generate sediment gravity flows, obscuring the depositional characteristics of ISGFs. ESGFs originated from rivers during floods and involved the transformation of cohesive into turbulent hyperpycnal flows. In humid and warm climates, active sag-border faults favoured the generation of long-lived ESGFs. These flows usually travelled along intra-sag faults and therefore considerable volume of sediments could be transferred into deep water. The resulting thick-bedded hyperpycnites contributed substantially to the reservoirs of Bonan Oilfield. Meanwhile, short-lived ESGFs usually generated in the regions where sag-border faults were extinct. They generally travelled down the faulted slope and deposited sediments in shallow water. These features suggest that the initiation, evolution, and deposition of sediment gravity flows on the faulted slope are mainly controlled by faults and climate. Hyperpycnites can serve as excellent reservoirs for hydrocarbon accumulation and they are probably common in other sags of the Bohai Bay Basin.

Rock mechanical properties and fracability of continental shale in Zhanhua Sag, Bohai Bay Basin

Rock mechanical properties and fracability of continental shale in Zhanhua Sag, Bohai Bay Basin

Effect of lithofacies on shale reservoir and hydrocarbon bearing capacity in the Shahejie Formation, Zhanhua Sag, eastern China

By using microscope, nuclear magnetic resonance (NMR), hydrocarbon generation and expulsion and nitrogen adsorption, reservoir properties and oil-gas bearing capacity of different shale lithofacies were examined with the continental shale of Zhanhua Sag in the Jiyang Depression of the Bohai Bay Basin as the examples. The results showed that the organic-rich laminar lithofacies have the best oil and gas bearing properties, while the organic-rich layer lithofacies take second place. The laminar lithofacies mainly developed mesopores and macropores, even microcracks, and the porosity connectivity is better than that of the layer lithofacies. The micropores and mesopores are mainly developed in the layer lithofacies, so the reservoir space is small. Therefore, the lithofacies with laminar not only increases the storage space of reservoir but also can improve the mobility of oil and gas. By the actual oil production comparison of different lithofacies, the hydrocarbon generation quantity of the organic-rich lithofacies is 336.68t, otherwise, the organic-contain lithofacies only 40.49t. Therefore, the organic-rich laminar lithofacies have the best oil production effect and can be used as a favorable target for oil and gas exploration in shale.

Lacustrine carbonates of Paleogene in Zhanhua Sag, Bohai Bay Basin, East China

The lacustrine carbonate has become an important exploration target, as the carbonates in half-graben Zhanhua Sag, a typical petroliferous lacustrine deposit, are not well-documented. Here the depositional characteristics, distribution patterns and controlling factors of the lacustrine carbonate deposition in Zhanhua Sag were studied, using seismic, well logging and core data associated with optical petrography, XRD and geochemical analysis. We identified 11 microfacies and 5 facies associations. The distributions of each facies associations were clarified, with results of carbonate-developed high-gradient margins and carbonate-restricted low-gradient margins, which could be concluded into two depositional models of different structural settings in half-graben Zhanhua Sag. In addition, climate, the outcropped lithology in the catchment area and structural settings are the main factors controlling the lacustrine carbonate microfacies types and their distribution here. These results could serve as important supplements and references to other lacustrine carbonate researches.

Lower Es3 in Zhanhua Sag, Jiyang Depression: a case study for lithofacies classification in lacustrine mud shale

Oil and gas exploration in lacustrine mud shale has focused on laminated calcareous lithofacies rich in type I or type II1 organic matter, taking into account the mineralogy and bedding structure, and type and abundance of organic matter. Using the lower third member of the Shahejie Formation, Zhanhua Sag, Jiyang Depression as the target lithology, we applied core description, thin section observations, electron microscopy imaging, nuclear magnetic resonance, and fullbore formation microimager (FMI) to study the mud shale lithofacies and features. First, the lithofacies were classified by considering the bedding structure, lithology, and organic matter and then a lithofacies classification scheme of lacustrine mud shale was proposed. Second, we used optimal filtering of logging data to distinguish the lithologies. Because the fractals of logging data are good indicators of the bedding structure, gamma-ray radiation was used to optimize the structural identification. Total organic carbon content (TOC) and pyrolyzed hydrocarbons (S2) were calculated from the logging data, and the hydrogen index (HI) was obtained to identify the organic matter type of the different strata (HI vs Tmax). Finally, a method for shale lithofacies identification based on logging data is proposed for exploring mud shale reservoirs and sweet spots from continuous wellbore profiles.

The wettability of shale by NMR measurements and its controlling factors

In this study, shale samples from the Zhanhua Sag in the Jiyang Depression of the Bohai Bay Basin were used to test the wettability of shale by nuclear magnetic resonance (NMR). The organic matter abundance, movable hydrocarbon content, clay content and carbonate content of shale samples with different wetting characteristics were compared. The results demonstrated that the TOC content of shale samples with mixed wettability was greater than that of water-wettability samples. This finding indicates that with the increase in TOC content, pores can be changed from water-wetting to oil-wetting, and the inorganic pores become organic. The above analysis demonstrates that the existence of organic matter is the basic reason that shale porosity exhibits oil wettability. Further, the shale samples with mixed wettability were higher in mobile hydrocarbon content (S1) than were those with only water wettability. The shale samples with mixed wettability showed higher clay content than did those with water wettability, indicating that organic matter is associated with clay minerals. However, the samples with water wettability had greater carbonate content than those with mixed wettability, indicating that inorganic pores are formed primarily by carbonate rock. In summary, shale with mixed wettability in the Zhanhua Sag can be used as an exploration target favorable for shale oil enrichment.

The effects of shale pore structure and mineral components on shale oil accumulation in the Zhanhua Sag, Jiyang Depression, Bohai Bay Basin, China

The pore structure and mineral components of shale have different effects on the enrichment of different phases of shale oil. Thus, research on the relationships between shale's pore structure and mineral components and shale oil enrichment is particularly important. Using core observations, scanning electron microscopy, whole rock mineral diffraction analysis, CO2 and N2 low-temperature and -pressure adsorption experiments and nuclear magnetic resonance experiments, this study analyzed the effects of the pore structure characteristics and mineral components of shale on shale oil accumulation in the lower segment of the third member of the Shahejie Formation in the Zhanhua Sag, Jiyang Depression, Bohai Bay Basin, China. This shale segment primarily contains inorganic pores and fractures. The inorganic pores include intergranular, intragranular, intercrystalline and dissolution pores. Structural microfractures and bedding fractures are the main types of fractures in the connected network of the shale rocks. Organic pores are not fully developed in the segment, and a few organic materials with inter-bedded clay minerals form the intergranular and dissolution pores with calcite. The pore structures in the study area are very complex. Micropores, mesopores and macropores are all present, and micropores account for 19% of the pores, mesopores account for 31%, and macropores account for 50%. We compared the results of CO2 and N2 adsorption experiments performed before and after chloroform extraction of shale samples and found that adsorbed-phase shale oil is primarily stored in micropores, which have diameters less than 2 nm, whereas free-phase shale oil is stored in mesopores, macropores and microfractures, which all have diameters greater than 2 nm. The carbonate content ranges from 46% to 88% and primarily comprises calcite, which favors the formation of intergranular and dissolution pores. The pyrite content is positively correlated with the pore volume, which is conducive to the development of pores. However, the clay content is negatively correlated with the pore volume, which is not conducive to the development of larger pores or the enrichment of movable shale oil.

Mesozoic to Cenozoic tectonic transition process in Zhanhua Sag, Bohai Bay Basin, East China

The Zhanhua sag is part of the Bohai Bay intracontinental basin system that has developed since the Mesozoic in East China. The timing of this basin system coincides with the final assembly of East Asia and the development of Western Pacific-type plate margin. Here we use 3-D seismic and core log data to investigate the evolution of this basin and discuss its broad tectonic settings. Our new structural study of Zhanhua sag suggests that there are four major tectonic transitions occurred in the Bohai Bay Basin during Mesozoic and Cenozoic: (1) The first tectonic transition was from stable Craton to thrusting during the Triassic, mainly caused by the South China Block's subduction northward beneath the North China Block, which induced the formation of the NW-striking thrust faults. (2) The second tectonic transition was mainly characterized by a change from compression to extension, which can be further divided into two-stages. At the first stage, two episodes of NW-SE shortening occurred in East Asia during Early-Middle Jurassic and Late Jurassic-earliest Cretaceous, respectively. At the second stage, the extension and left-lateral shearing took place during Early Cretaceous while compression occurred during Late Cretaceous. The NW-striking thrust faults changed to normal faults and the NNE-striking left-lateral strike-slip faults started to influence the eastern part of the basin. (3) The third transition occurred when the NW-SE extension and NNE-striking right-lateral shearing started to form during Paleogene, and the peak deformation happen around 40 Ma due to the change of the subduction direction of Pacific Plate relative to Eurasia Plate. The NE-striking normal faults are the main structure, and the pre-existing NNE-striking strike-slip faults changed from left-lateral to right-lateral. (4) The fourth transition saw the regional subsidence during Neogene, which was probably caused by the India-Asia “Hard collision” between 25 and 20 Ma.

Effect of pore structure on shale oil accumulation in the lower third member of the Shahejie formation, Zhanhua Sag, eastern China: Evidence from gas adsorption and nuclear magnetic resonance

As shale oil occurs primarily in micro–nano pores and fractures, research about the effect of pore structure on shale oil accumulation has great significance for shale oil exploration and development. The effect of pore structure on shale oil accumulation in the lower third member of the Shahejie formation (Es3l), Zhanhua Sag, eastern China was investigated using gas adsorption, soxhlet extraction, nuclear magnetic resonance (NMR) analysis, and field emission scanning electron microscope (FE-SEM) observation. The results indicated that the samples contained a larger amount of ink-bottle-shaped and slit-shaped pores after extraction than before extraction. The pore volume and specific surface area of the samples were approximately 2.5 times larger after extraction than before extraction. Residual hydrocarbon occurred primarily in the free-state form in pores with diameters of 10–1000 nm, which can provide sufficient pore volume for free hydrocarbon accumulation. Therefore, pores with diameters of 10–1000 nm were regarded as “oil-enriched pores”, which are effective pores for shale oil exploration, whereas pores with diameters smaller than 10 nm were regarded as “oil-ineffective pores”. Samples with only well-developed small pores with diameters smaller than 1000 nm showed high oil saturation, whereas samples with both small pores and also relatively large pores and micro-fractures presented low oil saturation. As the minimum pore size allowing fluid expulsion is 1000 nm, pores with diameters greater than 1000 nm were considered as “oil-percolated pores”. Large pores and micro-fractures are generally interconnected and may even form a complex fracture mesh, which greatly improves the permeability of shale reservoirs and is beneficial to fluid discharge.

Effect of sedimentary environment on shale lithofacies in the lower third member of the Shahejie Formation, Zhanhua Sag, eastern China

Research on shale lithofacies is important for shale oil and gas production. This study focused on the lower third member of the Shahejie Formation ([Formula: see text]) in the Luo-69 well in the Zhanhua Sag, Jiyang Depression, Bohai Bay Basin, eastern China. Several methods, including thin section observations, total organic carbon (TOC) analysis, X-ray diffraction analysis, quantitative evaluations of minerals by scanning electron microscopy, major and trace-element analyses, and field emission-scanning electron microscopy, are used to investigate the effect of sedimentary environment on the type and distribution of shale lithofacies. Our research indicates that 36 types of shale lithofacies can be classified based on the TOC content, mineral composition, and sedimentary structure, of which five types are identified in the study area. The [Formula: see text] shale has a high calcareous mineral content (average of 49.64%), low clay and siliceous minerals contents (averages of 19.54% and 19.02%, respectively), a high TOC content (average of 3.00 wt%), and well-developed horizontal bedding. The sedimentary environment during the deposition of the [Formula: see text] shale in the Zhanhua Sag had a warm and moist climate, limited provenance, saline water, and strong reducibility. The sedimentary environment in the early stage had a drier climate, more limited provenance, higher salinity, and stronger reducibility than that in the later stage. Shale lithofacies can reflect a certain sedimentary environment and depositional process; similarly, a depositional environment controls the type and distribution of shale lithofacies. Due to the characteristics of the [Formula: see text] sedimentary environment, organic-rich massive mixed shale, organic-rich bedded mixed-calcareous shale, organic-rich laminated calcareous shale, and organic-fair laminated calcareous shale are developed in the [Formula: see text] formation from top to bottom.

Characteristics and controlling factors of reservoir space of mudstone and shale in Es3x in the Zhanhua Sag

Zhanhua Sag is a widely accepted target zone with huge exploration and development potential for shale oil and shale gas resources. Many detailed studies have been undertaken around the geochemistry of the lower section of the third member of the Shahejie Formation (Es3x), while few studies have focused on the reservoir. In this study, based on the mineralogical features and geochemical characteristics, and by using statistical methods, the characteristics and controlling factors of reservoir space of mudstone and shale in Es3x in the Zhanhua Sag are explored through field-emission scanning electron microscopy (FE-SEM), high pressure mercury injection capillary pressure (MICP), and nuclear magnetic resonance (NMR) techniques. Three major findings were obtained. ① There are micropores and microfractures in the reservoir space, which include intergranular pores, clay intercrystal pores, pyrite intercrystal pores, dissolved pores, structural microfractures, and bedding microfractures. ② According to the features of pore size distribution (PSD), the pore distribution can be divided into the following three categories: 0–50 nm, 50 nm–2 μm, and >2 μm; the average volumes of these components are 0.01079 mL g−1, 0.00361 mL g−1, and 0.00355 mL g−1, respectively, thus showing that the pores whose radii are distributed at 0–50 nm form the most important reservoir space (though those with the 50 nm–2 μm and >2 μm radii are also important and cannot be ignored). ③ There are different controlling factors when it comes to different scale pores. Based on statistics and FE-SEM results, the dissolved pores in calcite were determined to be the controlling factor for the 0–50 nm portion, the intercrystalline pores in clay and pyrite, and intergranular pores between authigenic minerals (calcite, dolomite, and pyrite) and clastic minerals (calcite and dolomite) were determined to be the controlling factors for the 50 nm–2 μm portion, and the structural microfractures and bedding microfractures were determined to be the main factors for the >2 μm portion. Furthermore, it is the brittle minerals content and bedded structure that control the microfractures. This study thus clarifies the types and characteristics of reservoir space and identifies pore structure controlling factors of mudstone and shale in Es3x in the Zhanhua Sag; this information has important significance for future reservoir evaluations.

Continental shale pore structure characteristics and their controlling factors: A case study from the lower third member of the Shahejie Formation, Zhanhua Sag, Eastern China

Since shale oil and gas primarily occur in micro-nano pores and fractures, research on pore structure characteristics is important to understand shale oil and gas accumulation mechanisms. Studies on the pore structure of the lower third member of the Shahejie (Es3l) Formation in Zhanhua Sag, Jiyang Depression, Bohai Bay Basin, Eastern China, are quite limited, thereby, restricting further investment on shale oil and gas exploration. We studied continental shale pore structure characteristics and their controlling factors by using a series of laboratory experiments on core samples taken from the Luo-69 Well. The results demonstrate that the Es3l continental shale has a high proportion of calcareous minerals, low siliceous minerals, and low clay minerals. It is characterized by low total organic carbon (TOC) content, low thermal maturity, and Type I and II organic matter. Five different kinds of pore spaces are developed in the shale samples: organic matter-hosted pores, intergranular pores, intercrystalline pores, dissolution pores, and micro-fractures, with the majority of slit-shaped and ink-bottle-shaped pores. Macropores make the greatest contribution to the total pore volume, whereas micropores provide the major surface area. As the organic-rich laminated calcareous shale (ORLCS) presents a larger pore volume, surface area, and mercury withdrawal efficiency, it is a favourable shale lithofacies for shale oil and gas storage and flow. Compared with the Longmaxi marine shale, macropores well-developed in the Es3l shale make great contribution for free hydrocarbon accumulation. Samples with different TOC contents, mineral compositions, thermal maturities, and lamellar layers exhibit distinct differences in pore structure. Mineral composition and thermal maturity primarily affect micropores and mesopores. Macropores are influenced by mineral composition, TOC content, thermal maturity, and lamellar layers.

A new method for continental shale oil enrichment evaluation

We have evaluated continental shale oil enrichment via experiments. Rock pyrolysis, nuclear magnetic resonance, and pulse permeability tests were conducted to establish the pore saturation index (PSI), which comprehensively evaluates the enrichment of shale oil features using the characteristics of self-generation and self-preservation, the parameters of which include the pyrolysis-free hydrocarbon ([Formula: see text]) and total organic carbon of source rocks as well as the porosity and permeability of reservoir rocks. The correlation of the oil content ([Formula: see text]) and PSI values indicated that PSI values greater than 50 generally indicate good oil enrichment plays, whereas those smaller than 50 imply poor conditions for oil enrichment. This conclusion was successfully applied to the Zhanhua Sag with shale oil plays in the Jiyang Depression, Bohai Bay Basin, China. In addition, the shale oil plays of the Dongying Sag in the Jiyang Depression were investigated to verify the effectiveness and reliability of the new method.

Paleogene-Neogene Cap Rocks and its Relationship with Hydrocarbon Accumulation in the Zhanhua Sag

To analyse the Zhanhua Paleogene–Neogene cap rocks and its relationship with hydrocarbon accumulation, the seal lithology, the relationship between compaction of argillite rock and its sealing capacity, and its destruction by faults and fractures were studied. The results indicate that there are four types of cap rocks: argillite rock and silty mudstone, microcrystalline carbonate, dense cemented sandstone and dense cemented carbonate. Among these cap rocks, argillite rock is the main type in the Zhanhua Sag. According to the evolutionary characteristics of the argillite rock and its destruction by fractures and faults, the argillite cap can be classified into three categories: porosity cap, fracture transformation cap and the fault transformation cap. Among their sealing capacities, the porosity cap is the best, followed by the fracture transformation cap, and the fault transformation cap is the worst. Through the analysis of the relationship between existing oil & gas reservoirs and the distribution characteristics of the Paleogene–Neogene cap rocks in the Zhanhua Sag, it was found that the cap combination which was below or above the reservoir together controlled the hydrocarbon accumulation and preservation. It means that the destruction of the cap below or down-dip the reservoir is a necessary condition for hydrocarbon accumulation, and only when the sealing capacity of the cap rock above or up-dip the reservoir is better than that of below or down-dip the reservoir, hydrocarbon could be efficiently stored in reservoirs, thus could be effectively enriched.

Three-dimensional forward stratigraphic modelling of the gravel-to mud-rich fan-delta in the slope system of Zhanhua Sag, Bohai Bay Basin, China

An important hydrocarbon reservoir is hosted by the third member of the Shahejie Formation (Es3) in the Zhanhua Sag, Bohai Bay Basin. Seismic stratal slices reveal different characteristics of channels and fan-delta lobes between the south (slope break belt) and southwest (gentle slope) areas combined with lithology, wire-line logs and three-dimensional (3-D) seismic data in the southern slope of Zhanhua Sag. And an excellent analogue has been provided for understanding various key depositional evolution of fan-deltas in the slope system (from base to top: Es3L, Es3M and Es3U). The Sedsim, a three-dimensional stratigraphic forward modelling programme, is applied to simulate the evolution of fan-deltas in the southern slope break systems and southwestern gentle slope systems of the Zhanhua Sag by considering a number of key processes and parameters affecting the fan-deltaic deposition from 43 Ma to 38.2 Ma. Modelling results indicate that depositional types and scales evolved from the thickest medium-scale gravel- or sand-rich fan deltas (43 Ma ∼41.4 Ma, Es3L) to the thinnest small-scale mud-rich fan deltas and lacustrine mud (41.4 Ma ∼39.8 Ma, Es3M), and lastly to less thicker larger-scale mixed sand-mud fan deltas (39.8 Ma ∼38.2 Ma, Es3U). The types of slope system, sediment supply and lake-level change are three controlling factors for determining the source-to-sink architecture of the gravel-to mud-rich fan-deltas and sediment-dispersal characteristics. This study has demonstrated that the process-based modelling approach can be effectively used to simulate complex geological environments and quantify controlling factors.

An evaluation workflow for shale oil and gas in the Jiyang Depression, Bohai Bay Basin, China: A case study from the Luojia area in the Zhanhua Sag

AbstractShale oil and gas plays in continental rift basins are complicated and have not been reported elsewhere. In the Luojia area of the Jiyang Depression, an evaluation workflow for shale oil and gas in this continental rift basin is proposed. Based on analysis of oil- and gas-related geological conditions, a favorable area of shale oil and gas can be identified, and a high-frequency sequence stratigraphic framework of the target area can be established, therefore, the spatiotemporal distribution of shale has been elucidated in the Luojia area. According to the rock texture, structure, composition and color, petrographic classification criteria for shale are determined, and well log data are used to demarcate, track and predict high-quality lithofacies. Based on geochemical analyses and physical simulations of hydrocarbon generation, abundance, types and maturity of organic matter are analyzed, furthermore, geochemical parameters criteria of hydrocarbon generation and the characteristics of oil and gas occurrence in shales can be determined. Storage space types, assemblages and evolution characteristics of shale reservoirs are studied through core observation, thin-section analysis, electron microscopy examination and fluorescence spectrometry. Combined with analysis of reservoir physical properties, the reservoir performance is evaluated. A saturation model is established based on core analysis, well-log interpretation and well-test production data. The model is further used for evaluation of the movable hydrocarbon contents and integrated assessment of the oil potential. Finally, the shale oil and gas production capacity and exploration prospects in the Luojia area are forecasted based on the analyses of factors controlling production capacity and the rock fracability. Through an integrated analysis of multi-factors (including the lithofacies, source rocks, reservoir properties, oil saturation, and production capacity), the shales in the Luojia area can be divided into three categories, i. e., Class I (high porosity - high resistivity), Class II (medium porosity - medium resistivity), and Class III (low porosity - medium resistivity).