Nanpu Sag Research Articles

Characteristics of middle-deep reservoirs and the genesis of pore development in the southern slope of the Nanpu sag in the Bohai Bay Basin, China

To clarify the distribution of favorable reservoirs in the middle and deep layers of the southern slope of the Nanpu sag and identify relatively high-quality reservoir sections, the characteristics of the reservoirs in the middle and deep layers of the southern slope of the Nanpu sag were studied by means of core analysis, thin section casting, scanning electron microscopy, and analytical tests. Research shows that the middle and deep reservoirs on the southern slope of the Nanpu sag are mainly lithic feldspathic sandstone and that the debris is mainly magmatic rock. The diagenesis processes are mainly compaction, cementation, and dissolution. The deformation of plastic particles, brittle microcracks on the surface of rigid clastic particles, calcareous cementation, quartz overgrowth, and feldspar dissolution are common. The pores are co-developed by primary pores and secondary pores, in which the primary pores are more common in the third member of the Dongying Formation and the first member of the Shahejie Formation, whereas the secondary pores are more common in the first member of the Shahejie Formation. The genesis of pore development is discussed. It is considered that the anticompaction components, chlorite film, and hydrocarbon filling play major roles in the formation of primary pores, whereas atmospheric water leaching, diagenetic evolution, thermal evolution of organic matter, and clay mineral transformation play major roles in the formation of secondary pores.

Evaluation of Lateral Sealing of Faults Based on Porosity: A Case Study of the F1 Fault of the Nanpu No. 5 Structure in the Nanpu Sag

The lateral sealing of a fault plays a pivotal role in the efficacy of a fault trap and its degree of hydrocarbon filling. Nevertheless, the evaluation of this phenomenon remains a challenging task, with evaluation methods either unable to accurately reflect subsurface reality or obtaining the necessary parameters difficult. In light of these considerations, a porosity-based fault laterality evaluation method was proposed, with the F1 Fault of the Nanpu No. 5 structure in the Nanpu Sag serving as the research object. First, the relationship between porosity and the product of burial depth and shale content was established using measured porosity, burial depth, and shale content data of the surrounding rocks in the study area. Subsequently, the reservoir rock porosity was obtained by employing logging data or core samples or by calculating the reservoir rock shale content from natural gamma logging data. Concurrently, the shale content of fault rock was calculated using three-dimensional (3D) seismic data, mud logging data, and natural gamma logging data, thereby enabling its porosity to be determined. Finally, the porosity difference between the two was employed to assess the lateral closure of faults. The results indicate that the porosity difference is less than 0 at lines L1~L3 of the F1 Fault, which suggests that the fault is not laterally closed at this site. Conversely, the F1 Fault is laterally sealed at this location, as indicated by the porosity difference being greater than 0 at lines L4~L10. The findings of this evaluation were found to be in close alignment with the actual distribution of oil and water, indicating that the proposed method can accurately evaluate the lateral closure of faults with developed fault rocks and provide valuable guidance for the exploration of faulted oil and gas reservoirs.

The Application and Effect of AI Fault Interpretation Technology in the Laoyemiao Area

The complex faults, especially mid-deep faults, in the Laoyemiao area of the Nanpu Sag, the Bohai Bay Basin, are unclearly understood for their characteristics, constraining the structural and geological delineation of the area. The hydrocarbon enrichment in the Laoyemiao area is closely related to the faults, and thus the precise identification of mid-deep faults is of great significance for understanding the structural system and reservoir distribution in the area. In the past twenty years, artificial intelligence (AI) scholars developed new technologies and methods to solve engineering problems. Typically, the AI seismic data interpretation technology plays a critical role in improving the accuracy and efficiency of fault interpretation. In order to define the structural characteristics of the Laoyemiao area, the "2W1H" seismic data were processed by fault-constrained structure-oriented filtering, and then interpreted using the EasyTrack module of GeoEast independently developed by BGP. It is found that the imaging quality and accuracy of mid-deep faults are improved effectively. On this basis, the SN-trending strike-slip fault systems were discovered, and the structural pattern and evolution law of mid-deep faults in the Laoyemiao area were re-understood. The results are of great significance for the structural identification, reservoir evaluation and selection of exploration targets in this area.

Igneous intrusion contact metamorphic system and its reservoir characteristics: A case study of Paleogene Shahejie Formation in Nanpu sag of Bohai Bay Basin, China

Taking the Paleogene Shahejie Formation in Nanpu sag of Bohai Bay Basin as an example, this study comprehensively utilizes seismic, mud logging, well logging, physical property analysis and core thin section data to investigate the metamorphic reservoir formed by contact metamorphism after igneous rock intrusion. (1) A geological model of the igneous intrusion contact metamorphic system is proposed, which can be divided into five structural layers vertically: the intrusion, upper metamorphic aureole, lower metamorphic aureole, normal sedimentary layers on the roof and floor. (2) The intrusion is characterized by xenoliths indicating intrusive facies at the top, regular changes in rock texture and mineral crystallization from the center to the edge on a microscopic scale, and low-angle oblique penetrations of the intrusion through sedimentary strata on a macroscopic scale. The metamorphic aureole has characteristics such as sedimentary rocks as the host rock, typical palimpsest textures developed, various low-temperature thermal metamorphic minerals developed, and medium-low grade thermal metamorphic rocks as the lithology. (3) The reservoir in contact metamorphic aureole has two types of reservoir spaces: matrix pores and fractures. The matrix pores are secondary “intergranular pores” distributed around metamorphic minerals after thermal metamorphic transformation in metasandstones. The fractures are mainly structural fractures and intrusive compressive fractures in metamudstones. The reservoirs generally have three spatial distribution characteristics: layered, porphyritic and hydrocarbon impregnation along fracture. (4) The distribution of reservoirs in the metamorphic aureole is mainly controlled by the intensity of thermal baking. Furthermore, the distribution of favorable reservoirs is controlled by the coupling of favorable lithofacies and thermal contact metamorphism, intrusive compression and hydrothermal dissolution. The proposal and application of the geological model of the intrusion contact metamorphic system are expected to promote the discovery of exploration targets of contact metamorphic rock in Nanpu sag, and provide a reference for the study and exploration of deep contact metamorphic rock reservoirs in the Bohai Bay Basin.

New model of linkage evolution for the transtensional fault systems in the Nanpu Sag of Bohai Bay Basin: Insight from seismic interpretation and analogue modelling

The evolution of faults within the same stress field is frequently influenced by numerous factors, involving the reactivation of pre-existing structures, stress transmission through ductile detachment layers, and the growth, interaction, as well as linkage of new fault segments. This study analyses a complex multi-phase oblique extension fault system in the Nanpu Sag (NPS) of the Bohai Bay Basin (BBB), China. High-resolution three-dimensional (3D) seismic data and analogue modelling indicate that the oblique extensional reactivation of pre-existing structures governs the sequential arrangement of fault segments in the caprock, and they dip synthetically to the reactivated fault at depth. During the NW–SE extension in the Eocene, the predominant movement of the pre-existing fault is strike-slip. Subsequently, during the N–S extension since the Oligocene, inclined at 20° to the pre-existing fault, forming splay fault segments and ultimately creating large en-échelon arcuate faults linked by relay ramps. Using fault throw-distance (T-D) and laser scanning, we reconstructed the fault evolution model of oblique extension reactivation in the presence of a ductile detachment basement. Our study illustrates that the arcuate faults can be categorized into linear master fault segments controlled by pre-existing structures, bending splay faults in the termination zone, and normal fault segments responding to the regional stress field. The interaction between faults occurs among normal faults and strike-slip faults, and the kinematic unification of the two fault systems is accomplished in the intersection zone. As the faults continue to evolve, the new fault segments tend to relinquish the control of pre-existing structures and concentrate more on the development of planar and continuous major faults. The ductile detachment layer significantly contributes to the uniform distribution of strain, resulting in narrow shear zones and discontinuous normal faults in its absence.

Segmented growth of reactivated major bounding faults and their control on basin structures: Insights from the Nanpu Sag, Bohai Bay Basin, eastern China

AbstractA thorough insight into the initiation, segmentation, propagation and interaction of multitrend basin‐bounding faults is crucial to restoring the growth history of the faults and clarifying the fault growth pattern and its influence on the structures developed along the margin due to the growth of the basin‐bounding faults, but systematic studies on the individual influence of the evolution of each fault segment on the present structure are lacking. Based on 3D seismic data, the timing and growth of multitrend basin‐bounding faults were analysed using T‐z plots and throw backstripping, allowing us to determine the individual effects that each fault segment evolution exerteds on the present‐day configuration of the northern margin of the Nanpu Sag. The basin‐bounding fault is composed of the Xinanzhuang and Baigezhuang faults, and the Xinanzhuang fault comprises three linked segments with varying orientations (i.e., NE–SW, E–W, and NNE–SSW). In comparison, the Baigezhuang fault comprises only two linked NW–SE‐oriented fault segments. The evolution process can be divided into three stages. (1) During the early synrift I stage, namely, the isolated fault stage, five isolated multitrend basin‐bounding segments were active. (2) During the late synrift I stage, namely, the hard‐linkage stage, the five segments propagated laterally and linked with each other, behaving as a single fault. Meanwhile, the NE‐trending No. 5 Fault bifurcated upward from the basin‐bounding fault to accommodate local stress, and the NW‐trending Gaobei Fault deviated from the basin‐bounding fault controlled by local stresses induced by differential activities of the multitrend fault segments under the same far‐field stress. (3) During the synrift II to postrift linkage development stage, the extension orientation changed from NW–SE‐ to N–S, and additional displacement accumulated along the basin‐bounding fault without further lateral propagation. Newly formed E–W‐trending faults developed orthogonal to the extension orientation and linked with preexisting NE‐ or NW‐trending faults, forming a complex fault zone. In addition, influenced by the geometry of the basin‐bounding fault, the Laoyemiao Anticline formed by gravitational collapse under the dual action of a rollover anticline and transverse anticline. Furthermore, the evolution of the basin‐bounding faults played an important role in controlling the source‐to‐sink system, and the transition zone was the main provenance channel formed by the segmented growth of the faults. This study provides new insight into multitrend large fault evolution, and their impact on basin development provides a comprehensive explanation of the later structures developed in polyphase rifts.

A new nuclear magnetic resonance-based permeability model based on two pore structure characterization methods for complex pore structure rocks: Permeability assessment in Nanpu Sag, China

The nuclear magnetic resonance (NMR) estimate of permeability is a fundamental method that has numerous applications in reservoir engineering and petrophysics. To improve the accuracy of the NMR-based permeability model, many variables are introduced into NMR-based permeability prediction models due to geometric complexity and pore structure heterogeneity. In this paper, two pore structure characterization methods are investigated based on the Kozeny-Carman model and equivalent component model. Furthermore, an NMR-based permeability model accounting for the effect of pore structure is developed based on the analysis of the relationship between two pore structure parameters, and it is applied to practically predict permeability. Results indicate that the new model-calculated permeability has good agreement with experimental data; moreover, the adaptability of the new NMR-based permeability prediction model is highly improved through reducing undetermined variables, and key parameters can be measured directly using NMR. The new model provides a valuable scientific resource and assists in the evaluation of hydrocarbon-bearing reservoirs with complex pore structure, such as tight sandstone, shale, and carbonate rock.

Quantitative characterization of critical reservoir physical properties of tight oil charging in the third member of the Shahejie Formation in the Gaobei Slope of Nanpu Sag, Bohai Bay Basin

Unconventional petroleum occupies an important position in the world energy structure. Tight oil is an important field of unconventional oil and gas exploration and development, and the physical property condition of a tight oil reservoir is an important determinant of its commercial production. The application of different geological and engineering strategies to quantitative characterization of the aspect of hydrocarbon charge vis-à-vis reservoir physical properties is critical and essential to thoroughly understand the formation and accumulation mechanism and help in achieving the optimum producibility of tight oil resources. This study focuses on the tight oil of the third member of the Shahejie Formation (Es3) region in the Gaobei Slope of the Nanpu Sag. The upper and lower physical limits of charging were quantitatively deduced and explained characterized by a combination of geological analysis, analytical testing, and numerical simulation of force balance; the formation mechanism of the tight oil reservoir is primarily constrained by its permeability. The inversion phenomenon of oil and water in the tight oil Es3 reservoir occurs at a burial depth of 3900 m. The pore type of the reservoir is mainly primary in nature, with a positive correlation between the porosity and the permeability. The upper limit of tight oil charging is quantitatively characterized by porosity approximately equal to 10%, the corresponding permeability being 1 × 10−3 μm2. When the reservoir's physical properties exceed the upper limit of tight oil charging, the reservoir mainly develops a water layer. This is mainly caused by the boundary effect of tight reservoirs, where the pore throat radius corresponding to the critical physical property is equivalent to the boundary layer thickness of oil in the reservoir pore, making it difficult for boundary fluids to flow in the pore space at this time. On the other hand, based on comprehensive analysis of the minimum flow hole roar radius, the irreducible water saturation, and drilling result statistical methods, the lower limit of the tight oil charging in Es3 reservoir is characterized quantitatively by the porosity of about 2%–4%, wherein the pore throat radius is approximately equal to 0.04 μm. The lower limit of tight oil charging varies from one place to another, mainly attributed to factors such as interfacial tension, hydrocarbon generation dynamics, kerogen type, and basin temperature. The charging process of tight oil complies with the principle of force balance, whereas the pressure exerted the generated hydrocarbon is counterbalanced by the capillary force of the pore space.

Logging evaluation of petrophysical facies based reservoir quality prediction method for the volcanic rocks in the deep depth of Nanpu Sag, China

The volcanic rocks in the deep depth of Nanpu sag is the main formation of oil and gas enrichment. It has huge resource potential and prospects for exploration and development. Accurate characterization of lithology and pore space is the basis of reservoir quality prediction. Through petrography studies of cores and thin sections, explosive and sedimentary facies represent the dominant oil and gas-bearing reservoirs, and intergranular pores and fractures are major pore and seepage space. In order to predict favorable reservoirs and improve the efficiency of fracturing, according to composite well loggings and electrical images, logging response features of different lithofacies are characterized and five types of pore structure facies are classified. Further, five types of petrophysical facies are proposed. Among them, Type I and Type II are the most favorable petrophysical facies in the study area. The petrophysical facies typing approach show consistency with oil test data, which verifies that this approach can well predict reservoir quality and provide technical support for the exploration and development of the oil and gas-bearing volcanic rocks.

Diagenetic controls on pore structure variations in fan deltaic sandstones of the Eocene Shahejie Formation, Bohai Bay Basin, China: Implications for reservoir quality and oiliness heterogeneities

The fan deltaic sandstones in the third section of the third member of the Eocene Shahejie Formation (Es33) are important hydrocarbon exploration targets in the northern Nanpu sag, Bohai Bay Basin, China. The Es33 sandstones are characterized by highly variable pore structures. A total of 364 Es33 sandstone samples were measured by a series of petrological and petrophysical analyzes to investigate these variations and their controls on reservoir quality and oiliness heterogeneities. The heterogeneity of Es33 sandstone can be manifested by type I, type II and type III pore structures. Primary intergranular pores and dissolution pores were the main storage spaces for type I sandstones, while dissolution pores and intercrystalline pores were the main storage spaces for type II and type III sandstones, respectively. These results indicate that compaction is the main factor in the loss of primary intergranular porosity, with an average loss of 78%. The grain size controlled the preservation of primary pores in Es33 sandstones with a carbonate cement content of less than 6%. The different types of clay minerals have various effects on the pore structure. Chlorite plays an active role in the preservation of intergranular pores and the improvement of reservoir quality, while mixed layers of illite and smectite greatly reduced the pore size and connectivity and lowered the reservoir quality. The Es33 sandstones have undergone a more open diagenetic system. Dissolution significantly increased the porosity of type I and type II sandstones with more primary intergranular pores, but exerted a minor role in contributing to the permeability. Furthermore, a pore‐throat radius of approximately 0.4 μm is considered to be the threshold for oil accumulation in Es33 fan deltaic sandstones.

Fractal-Based Approaches to Pore Structure Investigation and Water Saturation Prediction from NMR Measurements: A Case Study of the Gas-Bearing Tight Sandstone Reservoir in Nanpu Sag

Pore space of tight sandstone samples exhibits fractal characteristics. Nuclear magnetic resonance is an effective method for pore size characterization. This paper focuses on fractal characteristics of pore size from nuclear magnetic resonance (NMR) of tight sandstone samples. The relationship between the fractal dimension from NMR with pore structure and water saturation is parameterized by analyzing experimental data. Based on it, a pore structure characterization and classification method for water-saturated tight sandstone and a water saturation prediction method in a gas-bearing sandstone reservoir have been proposed. To verify the models, the fractal dimension from NMR of 19 tight sandstone samples selected from the gas-bearing tight sandstone reservoir of Shahejie Formation in Nanpu Sag and that of 16 of them under different water saturation states are analyzed. The application result of new methods in the gas-bearing tight sandstone reservoir of Shahejie Formation in Nanpu Sag shows consistency with experimental data. This paper has facilitated the development of the NMR application by providing a non-electrical logging idea in reservoir quality evaluation and water saturation prediction. It provides a valuable scientific resource for reservoir engineering and petrophysics of unconventional reservoir types, such as tight sandstone, low porosity, and low permeability sandstone, shale, and carbonate rock reservoirs.

Sedimentary environment controls on the lacustrine shale lithofacies: A case study from the Nanpu depression, Bohai Bay Basin

Organic-rich shale of the Paleogene Shahejie Formation in the Nanpu Sag Bohai Bay Basin was analyzed using petrographic and elements analysis. These analyses were used to identify the lithofacies, reconstruct the sedimentary paleoenvironment and their relation. This paper adopted the division scheme and naming method of “Total organic carbon (TOC) + sedimentary structure + mineral composition” to carry out lithofacies identification and division. The petrographical characteristics indicated that there are 15 types of lithofacies. According to the lithologic changes, we divided the study interval from the fourth sub-member of the third member of the Shahejie formation into three parts from the bottom up. The lower part has an evident fluctuation in paleoclimate, salinity, productivity, and redox condition. The middle part has cold and arid climate conditions, the weakest weathering, the highest paleo-productivity, and anoxic conditions. The upper part was deposited under humid and warm climate conditions, strongest weathering, brackish-fresh water, lowest paleo-productivity, and dominant oxic condition. Paleo-productivity and redox conditions are the basic conditions for the enrichment and preservation of organic matter, the paleo-climate and paleo-salinity were the critical factors controlling the mineral composition and laminae development. Under warm and humid climate conditions, the shallow lake mainly develops organic-poor massive felsic shale and siltstone; the semi-deep lake is mainly developed with organic-medium laminated/layered felsic shale; the deep lake mainly develops organic-rich laminated/layered clayey shale and mixed shale. Under dry and cold climate conditions, the shallow lake area mainly develops organic-poor felsic shale, and the semi-deep lake mainly develops organic-rich layered carbonate shale and organic-rich massive mixed shale; the deep lake mainly develops organic-rich laminated/layered clayey shale.

Quantitative calculation of upper limit of hydrocarbon charging resistance in sandstone carrier bed: A Case Study of the northern reservoir of the 4th structural belt, Nanpu Sag

In order to evaluate the controlling effect of sandstone heterogeneity on oil-bearing difference in sandstone carrier bed, from the point of view of hydrocarbon accumulation dynamics, using the empirical relationship between porosity, permeability and displacement pressure, the displacement pressure of different sand bodies of the sandstone carrier bed in reservoir conditions is calculated. By comparing with the oil-water interpretation, the upper limit of reservoir displacement pressure in the north of Nanpu 4 structure is finally determined.

Prediction of hydrocarbon source rock distribution using logging curves: A case study of Es32 source rock in Nanpu Sag, Huanghua depression, Bohai Bay Basin

The Es3 is the main hydrocarbon source rock system in the Nanpu Sag. Finally, the TOC and hydrocarbon potential of each sub depression of Es3were predicted. The study shows that the hydrocarbon source rocks of Es32 and Es34 sections are mainly of type II2 and type II1 respectively, with good organic matter type and high maturity. Biomarker compound parameters indicate that the Es32 section hydrocarbon source rocks developed in a semi-saline, low to medium terrestrial source organic matter supplied reduction environment with a high algal contribution; the Es34 section hydrocarbon source rocks formed in a freshwater, low terrestrial source supplied reduction environment with a medium-high algal contribution. The multiple linear regression method is more effective than the ΔlgR method in predicting hydrocarbon source rocks in the Nanpu Sag, and the prediction accuracy is higher; the correlation between TOC and S1 + S2 is the best in the model for predicting hydrocarbon potential. The TOC and hydrocarbon potential of the hydrocarbon source rocks in Es31 are generally low; the high value area of TOC and hydrocarbon potential of the hydrocarbon source rocks in Es32 is partly between the No. 1 tectonic zone and No. 5 tectonic zone in Linque sub depression, and the TOC and hydrocarbon potential of the hydrocarbon source rocks in Liunan sub depression are larger; the high value area of TOC and hydrocarbon potential of the hydrocarbon source rocks in Es34 is mainly concentrated in Shichang sub depression.

Analysis of fault characteristics and oil and gas enrichment differences in the No. 2 structural belt, Nanpu Sag, Bohai Bay Basin, East China

There are great differences in oil and gas enrichment at different locations in petroliferous basins, and faults are an important factor affecting the differential enrichment of oil and gas. In this paper, taking the No. 2 structure in the Nanpu Sag, Bohai Bay Basin, East China, as an example, based on the analysis of fault geometry and kinematics characteristics, the determination of oil source faults, the analysis of fault trap origin and effectiveness, and the analysis of fault caprock combinations are carried out to study the differences and mechanisms of oil and gas enrichment east and west of the Nanpu No. 2 structure. The results show that the fault-source contact relationship affects the direction of oil and gas migration. The No. 2 structure in the Nanpu Sag has a dual-source hydrocarbon supply. The western structure supplies hydrocarbons from the north and east, with oil and gas migrating along the direction parallel and perpendicular to the fault strike. The east structure supplies hydrocarbons from the south and north, with oil and gas migrating along the direction perpendicular to the fault strike. The effectiveness of fault traps affects the scale of oil and gas accumulation. Most of the traps of the Nanpu No. 2 structure formed before hydrocarbon expulsion, which are all effective traps and are conducive to oil and gas accumulation. The relationship between fault and caprock determines the vertical accumulation layers of oil and gas: The faulted thickness of the east structure is large, the fault is vertically closed, and oil and gas are enriched mainly in the deep layer. The faulted thickness of the west structure is small, and oil and gas are enriched in both deep and shallow layers.

Investigation of volcanic rock quantitative fracture evaluation and reservoir classification based on the logging method - an example from Nanpu Sag

Investigation of volcanic rock quantitative fracture evaluation and reservoir classification based on the logging method - an example from Nanpu Sag

Investigation of volcanic rock quantitative fracture evaluation and reservoir classification based on the logging method - an example from Nanpu Sag

Multi-stage development results in complex lithology and variable pore structure in volcanic reservoirs. Therefore, it is difficult to calculate fracture porosity and determine the volcanic reservoir type. In this study, we use the deep volcanic rock of the Nanpu No. 5 structure as an example to use a matrix porosity logging evaluation model in different lithology reservoirs. Dual Laterolog has a large detection range and is sensitive to fractures. Therefore, the relationship between igneous fracture and logging response characteristics is analysed via numerical simulation. Then, the method used for calculating fracture parameters was verified by combining formation MicroScanner image (FMI) logging technology with Dual Laterolog. For accurate reservoir classification, a fracture porosity less than 0.0005 is ineffective since the formation fluid cannot be transported. The quantitative fracture evaluation of volcanic rock reservoir was completed based on calculation model. The agreement with the actual production proved the effectiveness of the method. [Received: September 9, 2022; Accepted: December 30, 2022]

Middle Eocene terrestrial paleoweathering and climate evolution in the midlatitude Bohai Bay Basin of eastern China

The middle Eocene climatic optimum (MECO, ca. ∼42 Ma) is a key time period for understanding Cenozoic cooling of the global climate. Still, midlatitude terrestrial records of climate evolution during MECO epoch are rare. In this study, continuous high-resolution record of shale sediments in mid-Eocene Shahejie Formation (MES shales) in the Bohai Bay Basin were performed with major-element and wavelet analysis. The midlatitude paleoweathering and paleoclimatic evolution during MECO epoch were analyzed in this study. The MES shales experienced weak-moderate paleoweathering under a subtropical monsoon paleoclimate with mean annual temperature of 8.3–12.9 °C and mean annual precipitation of 685–1100 mm/yr. The MES shales record a mixed provenance involving intermediate igneous rocks, and low compositional maturity. The nutrient-rich environment led to enrichment in organic matter in the MES shales. Wavelet analysis revealed good periodicity about the paleoclimate and weathering during MECO epoch. In the stage I of MES shales depositional process, the paleolake was high in nutrients, and the MES shales experienced high chemical weathering due to a relatively warmer and more humid climate. In contrast, the climate in stage II was relatively cold and dry, and the maturity of the MES shales was relatively high during this stage, suggesting a relatively stable tectonic background. This work provides more terrestrial records of MECO epoch for midlatitude region, and is benefit for better understanding of the palaeoenvironment when MES shales formed. The implication of organic matters enrichment in this study is meaningful for the shale oil/gas exploration in Nanpu Sag.

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.

Evolution and controls of high reservoir quality in Oligocene sandstones from moderate to deep burial, Nanpu Sag, Bohai Bay Basin, China

The Oligocene sandstones characterized by highly variable reservoir quality are the main hydrocarbon exploration and development targets in the Nanpu Sag, Bohai Bay Basin, China. In this study, the evolution and controls of reservoir quality in Oligocene sandstones from moderate to deep burial were investigated using petrographic and petrological data. The results show that the Oligocene sandstones are mostly fine-to coarse-grained, subangular to subrounded roundness, moderately sorted feldspathic litharenites and lithic arkoses with an average Q/F/R distribution of 48.4: 29.1: 22.5. Three anomalously high porosity/permeability zones (AHPZs, hereinafter named AHPZ-I, AHPZ-II, and AHPZ-III, respectively) are developed at depth ranges of 2300–3100 m, 3200–3550 m, and 3700–4200 m, respectively. Notably, the three zones are featured by distinctly different pore types: mainly primary pores in AHPZ-I; pore combination (average primary pore volume > average secondary pore volume) in AHPZ-II; primary pores with a few microfractures in AHPZ-III. The pore-throat sizes controlling permeability in anomalously high porosity sandstones (AHPS) decrease steadily from AHPZ-I to AHPZ-III-1 and then increase significantly at AHPZ-III-2. The AHPS reservoirs are characterized by low displacement pressure, large pore throat and mercury intrusion saturation, and high free fluid index (FFI) and reservoir quality index (RQI).Compaction greatly degraded the reservoir quality of Oligocene sandstones; carbonate cementation further restricts the formation of high-quality reservoirs. The sandstones with low content (<10 %) of carbonate cements tend to have larger primary porosity and secondary porosity, hence a larger RQI. The formation of AHPZ-I and AHPZ-II is attributed to higher content of rigid particle components, lower carbonate cements, and strong dissolution. Moreover, good sorting, responsible for a higher original porosity, also plays a positive role in the cause of AHPZ-I. The coarse-grained particles and early petroleum emplacement are the keys to the preservation of primary porosity in AHPZ-III. The petroleum emplacement effectively inhibits the feldspar dissolution and kaolinite precipitation in the deeply buried sandstones with depths larger than 4000 m. In addition, the generation of crushed microfractures in coarse grains leads to relatively low porosity and high permeability.