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The unconventional tight oil and gas resources in the Xinzhao East belt of the Hangjinqi Prospect area in the Ordos Basin of China are abundant. However, the reservoir's internal storage space is complex, and the microscopic pore throat structural features are not well recognized, which has led to some trouble in the deployment of oil and gas exploration. To reveal the microscopic characteristics of the dense sandstone gas layer in the first member of the Lower Stone Box Formation of the D-well Zone in the Xinzhao East belt of the Hangjinqi Prospect area, a three-dimensional space digital core was built, and the stored set spatial data were extracted, based on rock sheet and coring data and X-CT scanning technology. Quartz grain size was segmented and analyzed based on an adaptive approach. The microscopic characteristics of the gas layer in the studied section and the factors influencing its development were studied, combining the use of a field emission scanning electron microscope, helium porosimeter, and gas permeability meter. We found that in the studied section, the porosity is relatively high, the pore throat size is large, and the pore permeability correlation is good. The reservoir space, which consists of intergranular pores, intragranular pores, and microcracks at the grain edges in the study area, is characterized by a complex distribution pattern. Within the gas layer, isolated pores are connected by microcracks to form a network of reservoir spaces, which increases the pore throat size, enhances the connectivity of the pore throat, and makes the microscopic characteristics of the reservoir space better. The first member of the Lower Stone Box Formation could be an advantageous reservoir. Hole–throat connectivity is poor because of the gas layer having underdeveloped primary pores, the blockage of pores by unstable minerals (kaolinite, etc.), and poorly connected pore throats based on insoluble mud cementation. The high content of quartz brittle minerals and the development of natural microcracks within the gas formation are favorable conditions for fracking development. The quartz grain size within the gas layer is positively correlated with the pore throat size, which suggests that the quartz grain size somewhat influences the microscopic characteristics of the reservoir space. This comprehensive study shows that the methodology of the study is more advantageous than traditional methods in the fine and three-dimensional spatial characterization of the microstructure of dense sandstone reservoirs. The research results of this paper have certain guiding significance for further reservoir evaluation and advantageous reservoir prediction in the Hangjinqi Prospect area in the Ordos Basin. We also provide the basis for the subsequent efficient development of the gas reservoir. [ABSTRACT FROM AUTHOR]

The tight sandstone of Lower Shihezi Formation of Permian in Northwestern Ordos Basin is taken as the research object, based on the observations of cast thin slice, scanning electron microscopy, nuclear magnetic resonance and nitrogen adsorption porosity permeability test, the characteristics of tight sandstone reservoirs in Lower Permian Shihezi Formation and the factors affecting reservoir quality were analyzed. The findings indicate that the Lower Shihezi Formation reservoir of Permian in the study area predominantly comprises feldspar lithic sandstone and lithic sandstone, representing a typical example of tight sandstone. Rapid burial is the primary cause of reservoir densification, while compaction plays a crucial role in reducing reservoir quality, resuming in a significant loss of 58% in primary porosity. Cementation further compacts the sandstone reservoir, resulting in a secondary reduction of reservoir physical properties and loss of 24% in primary porosity. Other factors diminishing reservoir quality include the occlusion of intergranular pores through diagenetic processes such as calcite and iron calcite formation. The factors influencing the improvement of reservoir quality include extensive intergranular porosity provided by the dissolution of feldspar and the protection of some primary pores by the resistance of rigid quartz particles to compaction. These phenomena can serve as a valuable reference for reservoir research in other fluvial tight sandstone basins. [ABSTRACT FROM AUTHOR]

The sedimentary environment and organic matter enrichment relationship of the Upper Ordovician Wufeng Formation black shale, Guanyinqiao mudstone, and Lower Silurian Longmaxi Formation black shale in the Sichuan Basin of Weiyuan are analyzed using geochemical methods such as organic carbon, sulfur, major elements, and trace elements. The experimental results illustrate that the upper section of the Wufeng Formation and the lower section of the Longmaxi Formation are organic matter enrichment layers. The presence of P indicates a high productivity level in the Sichuan Basin from the Late Ordovician to the Early Silurian. In addition, indicators such as V/Cr, Ni/Co, and S/C suggest that the Wufeng Formation was deposited under anoxic reductive conditions, that the ice age Guanyinqiao segment was in an oxygen-rich to oxygen-poor environment, and that the Longmaxi Formation was in a sulfidic environment. Mo/TOC indicates that the Wufeng Formation shale was controlled by a restricted basin and that the Guanyinqiao segment and the Longmaxi Formation were in a medium-to-weak retention environment. The weak correlation of TOC with P/Al and Al indicates that the level of primary productivity and terrigenous detritus had a minor effect on the organic matter enrichment of the Wufeng and Longmaxi Formation black shale. Conversely, the positive correlation of TOC with V/Cr and Ni/Co illustrates that the anoxic reductive sedimentary environment is the main factor affecting the organic matter enrichment of the Wufeng and Longmaxi Formation black shale. Based on these studies, the development model of organic-rich shales of the Ordovician–Silurian in Weiyuan, Sichuan Basin is proposed. This paper may provide a reference for shale gas exploration in the Wufeng–Longmaxi Formation and a sedimentary response to the major geological events of Ordovician–Silurian. [ABSTRACT FROM AUTHOR]

Coal is a complex, porous medium with pore structures of various sizes. Therefore, it is difficult to accurately describe the characteristics of pore structure by using the traditional geometry method. The results from the present investigation suggest that the porous media system of the coal reservoir has obvious fractal characteristics at different scales. To study the complexity of the pores in the coal reservoir, 27 coal samples from Guizhou, SW China were studied. The fractal dimensions of coal pores were calculated, and the fractal dimension of a pore in a coal reservoir can be classified into two types: percolation and diffusion. The comprehensive fractal dimension can be obtained using the weighted summation method and the pore volume fraction of different fractal segments as the weight. The percolation fractal dimensions (Dp) of coal samples are between 2.88 and 3.12, the diffusion fractal dimensions (Dd) are between 3.57 and 3.84, and the comprehensive fractal dimensions (Dt) are between 3.05 and 3.63. The Dd values of all coal samples are all larger than the Dp values, which indicates that the random distribution and complexity of diffusion pores in coal are stronger than those of the percolation pores. The percolation fractal dimension decreases as the maturity degree increases, whereas the diffusion and comprehensive fractal dimensions increase. The diffusion pore volume fraction and total pore volume are all highly correlated with the comprehensive and diffusion fractal dimensions, respectively. The correlation between the comprehensive fractal dimension, diffusion pore volume fraction, and coal reservoir porosity is negative exponential, whereas the correlation between the total pore volume and coal reservoir porosity is positive linear. In comparison with the percolation and diffusion fractal dimensions, the comprehensive fractal dimension is better suited for characterizing the permeability of coal reservoirs. The fractal analysis of this paper is beneficial for understanding the relationship between the fractal characteristics of coal pores and properties. [ABSTRACT FROM AUTHOR]

In order to evaluate reservoir characteristics and gas potential of organic-rich shale in Bottom-Lower Cambrian Hetang Formation and Upper Ordovician Xinkailing Formation, Xiuwu Basin, Jiangxi Province, Lower Yangtze Plate of China, researches regarding reservoir characteristics of organic-rich shale thickness, moderate burial depth, abundance and maturity of organic matter, fracturability, organic-matter pores and absorption performance have been conducted to Hetang Formation and Xinkailing Formation through field survey, shallow drilling, analysis to hydrocarbon-generation potential, scanning electron microscopy, measuring specific surface area and isothermal adsorption experiments. The results show that (1) Hetang Formation reservoir, although with extremely rich material basis, is of unfavorable hydrocarbon generation conditions due to late-stage thermal maturity, besides of undeveloped micropore structure, reservoir space development mainly by macroscopic crack, so further exploration should focus on locating favorable areas of low maturity and developed cracks, which is considered distributing in the core of Yanshan-Lingangshan-Shiziyan Mountains syncline; (2) Xinkailing Formation reservoir is of better organic matter abundance, pore development and absorption performance than Xinkailing Formation and hence further exploration should focus on surveying favorable areas of better sedimentary facies belt and stable structure for major breakthrough in shale-gas exploration, which is considered distributing in Wuning-Lixi and Jinkou-Tuolin areas. [ABSTRACT FROM AUTHOR]

In order to investigate the gas generation from the pyrolysis of the crude oil, the thermal pyrolysis simulation experiment of the crude oil under the different geological conditions are carried out by using high temperature and high pressure closed system. The cumulative amounts of gaseous hydrocarbons from the pyrolysis of crude oil are almost the same but the generating activation energy are different under the different media (limestone, mudstone, sandstone). The limestone has the greatest impact on the pyrolysis, followed by the mudstone and the sandstone. In addition, pressure and heating rate have an inhibitory effect on the pyrolysis of crude oil. The experimental results can provide an evidence for source judgment and resource evaluation of natural gas in marine strata in China. [ABSTRACT FROM AUTHOR]

Fractures and vugs were used as the primary dolomite reservoir space in the study area, which improved the reservoir’s physical properties and influenced the distribution pattern of oil and water that were developed in the Upper Tenger Formation, Lower Cretaceous, Baiyinchagan sag, Erlian Basin. Based on the core observation data, thin slice analysis, and image logging, the cave-fractured reservoir was primarily developed in the actic region near the side of a depositional fault. Tectonic and diagenetic fractures were the primary types of fractures. High-angle tectonic fractures in the closure and open status were developed well in dolomitic mudstone, while a low-angle fracture and reticulate fractures were developed in the tectonic-hydrothermal hybrid process and were filled with hydrothermal minerals. The reservoir was located in low-lying land; therefore, detrital grain pressolution fractures were formed by burial diagenesis. Intergranular and intrangranular dissolved pores, which were formed by hydrothermal activity, were developed primarily in the highly dolomite lithology. According to the diagenesis temperature of dolomite from a carbon-oxygen isotope analysis, multiple hydrothermal fluid injection events occurred, and dolomitization ensued before massive hydrocarbon migration. The dissolution of organic acids formed by hydrocarbon charges after dolomitization and inorganic acid produced by burial di agenesis for soluble minerals effectively promoted the reservoir properties. Strong heterogeneity due to lithology and fracture-vug systems led to intricate oil-water relationships. Four statuses occurred, including cave-fracture oil, isolated oil, stored oil, isolated water, and stored water. The oil-water distribution patterns were established based on the core observation. [ABSTRACT FROM AUTHOR]

This paper investigates volcaniclastic fan facies and reservoir characteristics of Guantao Formation in the No. 1 and No. 2 structures in the Nanpu Sag, Bohai Bay basin, Eastern China. Core and log data from the Guantao 3 and 4 Formations were analyzed in order to characterize volcaniclastic fan facies. Comprehensive analyses show that volcaniclastic fan sediments have been characterized as rapid proximal deposition of well-rounded and poorly sorted large-grain sediments. During the developmental processes of the volcanic fans, shallow to moderately deep lacustrine mudstones are deposited on the volcanoes. They vertically form superimposed sedimentary bodies that were characterized by laterally alternating fingers of volcaniclastic fan sediments and interchannel mudstone. Differences in pyroclastic supply and the volcanic terrain during the deposition account for the variations in volcanic rocks distribution. Distribution of volcaniclastic fans also exhibited characteristic features. Volcaniclastic fans around the volcanic highlands show variable distribution of volcanic rocks. The deep valley sizes of the debris flows and streams indicate variable sediment supply. The extent and thickness of the lahar deposits from the volcaniclastic debris flows indicate variable size of sedimentation processes. Most volcaniclastic fans are composed of debris-transported deposits having small clasts. The bed thickness increases with increasing distance from the volcanic source. The volcaniclastic rocks show high content of intermediate-basic volcanic rocks debris. The original and dissolved intergranular and dissolved intragranular porosities in the volcaniclastic rocks provide the best reservoir properties. Identification of lithofacies and their assemblages, and volcaniclastic sedimentary facies buried within volcanos and adjacent regions provide a better understanding that can be applied to similar rift basins. A method to identify depositional geometry and accumulation patterns of volcaniclastic fans was employed. This method assisted in constructing depositional models, reconstructing the depositional history of volcaniclastic fans and the related history of volcanic activity based on geological and geophysical data within volcanic areas in rift basins. [ABSTRACT FROM AUTHOR]

The study investigated depositional model for mud-diapir controlled intra-slope basins, deepwater Niger delta, Nigeria using conventional 3D seismic data and an enhanced detection of geologic events volume. The results of the study revealed three types of seismic facies namely accretionary channel, distributary channel and lobe and drape complexes. Channels varied in both morphology and associated depositional elements and their sinuosity was found to generally reduce with decreasing age of occurrence. Six different types of reservoir elements were identified in the study area. Three reservoir elements were associated with the channel axis and the other three elements were associated with sheet deposits. Knowledge of these variations as well as identification of the different elements is necessary to a successful exploration and development plan. The depositional model generated for the study area showed repeatable and cyclic depositional processes involved in sedimentation in this type of basin. The ideal major depositional cycle was made up of mass transport deposits at the base, overlain by a distributary channel and lobe complex, overlain by an amalgamated channel complex, and capped by a drape complex. A modified depositional model referred to as the deflect-model applies to shale-diapir sub-basins in Nigeria. [ABSTRACT FROM AUTHOR]

3-D seismic data obtained from Joint Development Zone (JDZ) Licensing Round, 2004, was used to investigate the structural and stratigraphic characteristics on distal parts of the outer fold and thrust belt of southern Niger Delta, Nigeria. Five main seismic reflectivity units (I, II, III, IV and V) were identified and interpreted based on seismic reflection patterns to represent the prograding cyclic sequence of sedimentation in the study area. These units record the episodic gravity spreading that is responsible for thrusting in the toe of the delta. Faults in the central parts of the study area exhibit hard linkage by the gradational propagation of segments into an overlap that developed into soft-linked relay structures. Thrust faults in the study area can be classified into two groups based on the width of the individual thrust faults and their extension and the geometry of the hanging wall folds. These are E-W south-verging thrusts and NE-SW SE-verging thrusts. In the proximal parts of the JDZ, thrust faults are widely spaced with a large structural closure of the hanging wall folds. Faults in the eastern part are complex and typically verge in the opposite direction. In the distal parts of the study area, thrust faults are regularly spaced with hanging wall folds that occasionally created bathymetric expressions on the seabed. The high concentration of pockmarks in the proximal JDZ and the large structural closure of the hanging wall folds characterized by stacked amplitude anomalies and discernable flat spots imply the possible existence of an active hydrocarbon system in the proximal JDZ. Several lenses of sills observed in unit III may impact on hydrocarbon generation and expulsion in the study area. If hydrocarbon migration into traps is occurring after the intrusion of magma, then the sills could act as a barrier to fluid flow, depending on post-emplacement tectonics. Seismic cross sections show that faults in the study area have variable cross-sectional geometries but possess scaling properties typical of thrust faults. [ABSTRACT FROM AUTHOR]

This article prognosticates the hydrocarbon generation potential of core samples from fields A, B, C and D in Niger delta, Nigeria. The objectives of this study are to characterize the quality of these core samples by organic geochemical analyses. A total of ten core samples collected from fields A, B, C and D in Niger delta were analyzed using total organic carbon (TOC) content analysis, rock-eval pyrolysis technique. The analytical results of the studied core samples reveal that they have generally high total organic carbon contents (TOC), suggesting that conditions in the Niger delta favour organic matter production and preservation. There is a variation in the kerogen types and this may be attributed to the relative stratigraphic positions of the core samples within the Niger delta. The rock-eval results indicate that the samples from fields C and D contain predominantly Type II kerogen with a capacity to generate oil and gas and hence have good generative potential. The samples from fields A and B contain mainly Type III kerogen and are gas-prone with moderate generative potential. [ABSTRACT FROM AUTHOR]

The distribution of indigenous microorganisms was surveyed in Block 1 of Daqing Oilfield. Based on this survey, the indigenous microorganisms in the formation water were activated with different activator systems at the simulated stratum ecological environment. The changes of the number of bacteria of various physiological groups were determined during the process of activation. Also changes of pH value and composition of gas productions were analyzed at the end of culturing. The results showed that the selected block formation water contained a great number of saprophytic bacteria, hydrocarbon-oxidizing bacteria, fermentative bacteria, methane-producing bacteria and sulfate-reducing bacteria. Under the conditions that the growth of sulfate-reducing bacteria was controlled the block had the potential to enhance oil recovery by activating beneficial bacteria. The growth of sulfate-reducing bacteria can be inhibited through the activation of nitrate-reducing bacteria. The number of nitrate-reducing bacteria reached 106–107 cells/mL, but sulfate-reducing bacteria reached only 0–45 cells/mL in A system. Methane-producing bacteria can be activated by C, D activators. The relative content of biological methane in the light hydrocarbon gas reached 80% in C, D systems. B activator was conducive to the propagation of acid-producing bacteria, so that the pH value of the culture medium decreased from 7.5 to around 5.0. Hydrocarbon-oxidizing bacteria can be activated by various activator systems. There was low molecular light hydrocarbon in gas production according to the analysis of gas chromatograph. According to the content of methane and the number of methane-producing bacteria, methane only can be generated through activating methane-producing bacteria. By choosing different activator systems, various populations of indigenous microorganisms can be activated accordingly. [ABSTRACT FROM AUTHOR]

Different pore types and variable pore structures can determine reservoir potentials of volcanic rocks. Therefore, it is of great importance to understand the pore structures of volcanic reservoirs. With the Lower Cretaceous Yingcheng volcanic rocks, this study attempts to investigate and evaluate the micro pore structures of a volcanic reservoir through: (1) measuring porosity and permeability of core samples; (2) identifying pore types within thin sections; (3) characterizing pore structures with pressure-controlled mercury injection. The Lower Cretaceous Yingcheng volcanic rocks exhibits low porosities (on average 7.39%) and very low permeability (on average 0.283 × 10−6 m2). Three major pore types were identified from the Lower Cretaceous Yingcheng volcanic reservoir, including primary pores, secondary pores and fractures. Additionally, the pore structures of the Lower Cretaceous Yingcheng volcanic reservoir can be classified into three types. Type I pore structure primarily occurs in the volcanic breccia, crystal tuff and tuffites. The corresponding pore system is dominated by primary intergranular pores, intragranular dissolved pores and devitrified micropores with porosity and permeability higher than 5.4% and 0.05 × 10−6 µm2, respectively. Corresponding mercury intrusion curves are characterized by low displacement pressure, high maximum mercury saturation and mercury withdrawal efficiency. Type II pore structure is commonly observed in the tuff and tuffites, while primary intergranular pores and secondary micropores dominate pore system with porosity of 2.5-5.4% and permeability of 0.012-0.05 × 10−6 m2. The mercury intrusion curves are similar to the type I. Type III pore structure is also pervasively developed in the crystal tuff and tuffites. Intragranular dissolved micropores are most important ones with porosity and permeability less than 2.5% and 0.01 × 10−6 m2, respectively. Different from type I and II pore structure, the mercury intrusion curves of the type III have high-displacement pressure, low maximum mercury saturation and mercury withdrawal efficiency, indicating poor reservoir quality. [ABSTRACT FROM AUTHOR]