Intercrystalline Pores Research Articles

Characteristics and main controlling factors of intra-platform shoal thin-layer dolomite reservoirs: A case study of Middle Permian Qixia Formation in Gaoshiti–Moxi area of Sichuan Basin, SW China

Based on the study of the distribution of intra-platform shoals and the characteristics of dolomite reservoirs in the Middle Permian Qixia Formation in the Gaoshiti–Moxi area of the Sichuan Basin, SW China, the controlling factors of reservoir development were analyzed, and the formation model of “intra-platform shoal thin-layer dolomite reservoir” was established. The Qixia Formation is a regressive cycle from bottom to top, in which the first member (Qi1 Member) develops low-energy open sea microfacies, and the second member (Qi2 Member) evolves into intra-platform shoal and inter-shoal sea with decreases in sea level. The intra-platform shoal is mainly distributed near the top of two secondary shallowing cycles of the Qi2 Member. The most important reservoir rock of the Qixia Formation is thin-layer fractured-vuggy dolomite, followed by vuggy dolomite. The semi-filled saddle dolomite is common in fracture-vug, and intercrystalline pores and residual dissolution pores combined with fractures to form the effective pore-fracture network. Based on the coupling analysis of sedimentary and diagenesis characteristics, the reservoir formation model of “pre-depositional micro-paleogeomorphology controlling shoal, sedimentary shoal controlling dolomite, penecontemporaneous dolomite benefiting preservation of pores, and late hydrothermal action effectively improving reservoir quality” was systematically established. The “first-order high zone” micro-paleogeomorphology before the deposition of the Qixia Formation controlled the development of large area of intra-platform shoals in Gaoshiti area during the deposition of the Qi2 Member. Shoal facies is the basic condition of early dolomitization, and the distribution range of intra-platform shoal and dolomite reservoir is highly consistent. The grain limestone of shoal facies is transformed by two stages of dolomitization. The penecontemporaneous dolomitization is conducive to the preservation of primary pores and secondary dissolved pores. The burial hydrothermal fluid enters the early dolomite body along the fractures associated with the Emeishan basalt event, makes it recrystallized into medium–coarse crystal dolomite. With the intercrystalline pores and the residual vugs after the hydrothermal dissolution along the fractures, the high-quality intra-platform shoal-type thin-layer dolomite reservoirs are formed. The establishment of this reservoir formation model can provide important theoretical support for the sustainable development of Permian gas reservoirs in the Sichuan Basin.

Reservoir characteristics and formation model of Upper Carboniferous bauxite series in eastern Ordos Basin, NW China

Through core observation, thin section identification, X-ray diffraction analysis, scanning electron microscopy, and low-temperature nitrogen adsorption and isothermal adsorption experiments, the lithology and pore characteristics of the Upper Carboniferous bauxite series in eastern Ordos Basin were analyzed to reveal the formation and evolution process of the bauxite reservoirs. A petrological nomenclature and classification scheme for bauxitic rocks based on three units (aluminum hydroxides, iron minerals and clay minerals) is proposed. It is found that bauxitic mudstone is in the form of dense massive and clastic structures, while the (clayey) bauxite is of dense massive, pisolite, oolite, porous soil and clastic structures. Both bauxitic mudstone and bauxite reservoirs develop dissolution pores, intercrystalline pores, and microfractures as the dominant gas storage space, with the porosity less than 10% and mesopores in dominance. The bauxite series in the North China Craton can be divided into five sections, i.e., ferrilite (Shanxi-style iron ore, section A), bauxitic mudstone (section B), bauxite (section C), bauxite mudstone (debris-containing, section D) and dark mudstone-coal section (section E). The burrow/funnel filling, lenticular, layered/massive bauxite deposits occur separately in the karst platforms, gentle slopes and low-lying areas. The karst platforms and gentle slopes are conducive to surface water leaching, with strong karstification, well-developed pores, large reservoir thickness and good physical properties, but poor strata continuity. The low-lying areas have poor physical properties but relatively continuous and stable reservoirs. The gas enrichment in bauxites is jointly controlled by source rock, reservoir rock and fractures. This recognition provides geological basis for the exploration and development of natural gas in the Upper Carboniferous in the study area and similar bauxite systems.

Pore development characteristics and controlling factors of Lower Cambrian Qiongzhusi shale gas reservoir in Sichuan Basin, China

Recently, the shale gas exploration of Lower Cambrian Qiongzhusi Formation in Sichuan Basin has made a great breakthrough. However, the pore characteristics and controlling factors of the shale-gas reservoir in the Qiongzhusi Formation are not clear, which hinders further exploration and development of shale gas. This paper focuses on the Qiongzhusi Formation in the Lower Cambrian of the Sichuan Basin. On the basis of mineral composition and the lithofacies types of shale-gas reservoirs in Qiongzhusi Formation were classified by microscopic observation. Additionally, focused ion beam-scanning electron microscope (FIB-SEM), advanced mineral identification and characterization system (AMICS) analysis, large-field stitching scanning electron microscope technology (MAPS) and nitrogen adsorption experiment were employed to comprehensively determine the differential characteristics and controlling factors of pore development. The results indicate that the pore types in Qiongzhusi Formation are mainly composed of interparticle pores and intercrystalline pores. The pore structure of Qiongzhusi Formation is complex. Significant differences in pore structure parameters are observed among different lithofacies. Quartz and feldspar contribute to the preservation of primary pores due to their strong compressive resistance. Clay minerals and carbonate minerals impede pore development by occluding pores. The influence of organic matter on the pore development is relatively small. According to the mineral composition and pore parameters, laminated shale is a relatively favorable reservoir facies among different lithofacies types.

Dolomitization of early-post rift Lower Jurassic carbonate platforms along the Moroccan Atlantic Margin: Origin and significance

Dolomitization is the most significant diagenetic process to affect Jurassic carbonate reservoirs along the Central Atlantic Margin (CAM). Despite several studies on dolomitization from different parts of CAM, the origin of these dolomites and their influence on the subsequent diagenetic evolution of Jurassic carbonate systems remains enigmatic. In addition, while dolomitization is evident at the surface and in the subsurface of the Moroccan Atlantic Margin, virtually no detailed studies have been conducted to determine the origin, mechanism, and significance of dolomitization in this basin. Therefore, the principal objective of this study is to assess the origin and occurrence of dolomite in the of Upper Sinemurian-Lower Pliensbachian carbonates of the Arich Ouzla Formation in the Essaouira-Agadir Basin by using petrography and geochemistry.The shallow marine carbonates of the Arich Ouzla Formation have been partially dolomitized and are exposed on the salt-cored Amsittene Anticline. The dolomite is stratabound, and predominantly fabric-retentive, although in some parts it is partially replaced by non-stratabound, fabric-destructive dolomites. From petrographic observations and geochemical proxies, the fabric-preserving dolomites show dolomitization by reflux of mesohaline seawater (δ18Odolomite average = −3.5 ‰ VPDB, and δ13Cdolomite average = 2.0 ‰ VPDB). In contrast, petrographic and geochemical characteristics of the fabric destructive dolomites suggest precipitation from modified seawater/formational brines convected along faults and fractures evidenced by depleted δ18O isotopic values (average = −4.1 ‰ VPDB) with high fluid temperatures (average = 78 °C; range = 66–90 °C) where fluids interacted with the basal Triassic evaporites and siliciclastic sediments.Fabric preserving dolomite has higher porosity (average = 6.0 %) than the precursor limestones (average = 0.4 %), whereas permeability in both rock types (average = 0.48 mD, and average = 0.02 mD, respectively) is low. Fabric destructive dolomite has low porosity in proximity to fracture corridors (average = 1.9 %) due to dolomite recrystallization (overdolomitization), whereas porosity increases to an average of 7.4%, away from fracture corridors. The dolomites are post-dated by calcite cement which occludes vugs, intercrystalline pores and fractures. The calcite is interpreted to be meteoric in origin, because of its non-cathodoluminescence and depleted δ18O (average = −4.7 ‰ VPDB) and δ13C (average = −9.3 ‰ VPDB) isotopic values with respect to Jurassic marine carbonates. The meteoric calcites co-exist with bitumen suggesting that hydrocarbon migration in the basin likely occurred at the same time, most likely during basin inversion and exposure. This work considers dolomitization to be a localised process due to salt diapirism and demonstrates that the coincidence of hydrocarbon emplacement with basin inversion results in degradation and probably leakage of hydrocarbons. This emphasises the importance of local and regional tectonics, including salt diapirism, on patterns of diagenetic overprint in sedimentary basins.

Shale Oil–Water Two-Phase Flow Simulation Based on Pore Network Modeling

Abstract With the growing significance of shale oil in the realm of oil and gas resources, there has been a heightened focus on the impact of the indeterminate oil–water two-phase flow behavior in shale reservoirs on the effective exploitation of shale oil. The utilization of FIB–SEM scanning on shale samples enables the establishment of the real pore network structure and facilitates the analysis of pore type, pore throat size and connectivity of shale reservoirs through the implementation of two-dimensional slices. Subsequently, the gridded connectivity-based pore network model is utilized to conduct oil–water two-phase flow simulation, wherein the L–S and N–S mathematical models are incorporated to quantitatively examine the correlation between the displacement pressure and wettability and the recovery degree and remaining oil, as well as the impact of throat size on pressure loss. The research findings indicate the emergence of five distinctive pore types in shale reservoirs, namely intergranular pores, dissolution pores, intercrystalline pores, intracrystalline pores, and microfractures. In shale reservoirs with poor connectivity, a significant quantity of nanometer-scale pores are generated, wherein the seepage capacity is primarily influenced by the size and connectivity of pore throats. The smaller the throat size is, the greater the displacement pressure will be and the greater the pressure drop will be after the throat is passed through. To prevent fingering and excessive pressure drop, it is necessary to maintain reasonable control over the displacement pressure. The displacement efficiency is optimal when the wall surface is in a water-wet state. Therefore, enhancing the wettability of the surface can facilitate the efficient recovery of the remaining oil in the microscopic pore throats. The research findings offer valuable theoretical insights for the efficient exploitation of shale oil resources.

Petrophysical properties and their influencing factors of carbonates in the fourth member of Sinian Dengying Formation, Sichuan Basin, SW China

Based on the measurements of petrological, petrophysical and elastic properties of the samples of different sedimentary facies in the fourth member of Sinian Dengying Formation (Deng 4 Member) in the Sichuan Basin, the diagenetic processes of reservoirs of different sedimentary facies and their controls on the petrophysical properties were discussed. The results show that cracks and mineral composition jointly control the petrophysical properties, and both are significantly influenced by sedimentary environment and diagenesis. The microbial dolomite of mound-shoal facies mainly experienced multi-stage dolomitization, penecontemporaneous dissolution, tectonic rupture and hydrothermal/organic acid dissolution processes, giving rise to cracks and dissolved pores. The grannular dolomite of inter-mound-shoal bottomland or dolomitic lagoon facies mainly underwent mechanical compaction, burial dolomitization and tectonic-hydrothermal action, creating cracks and intercrystalline pores. The diagenesis related to crack development increases the pressure- and saturation-dependent effects of samples, leading to significant decrease in the compressional wave impedance and Poisson’s ratio. Dolomitization changes the properties of mineral particles, resulting in a Poisson’s ratio close to dolomite. The muddy, siliceous and calcareous sediments in the low-energy environment lead to the decrease of impedance and the differential change of Poisson’s ratio (significantly increased or decreased). The samples with both cracks and dissolved pores show high P-wave velocity dispersion characteristics, and the P-wave velocity dispersion of samples with only fractures or pores is the lowest.

Pore structure of tight sandstones with differing permeability: The He 8 Member of the Middle Permian Lower Shihezi Formation, Gaoqiao area, Ordos Basin

AbstractTight sandstone has strong pore heterogeneity and complex pore structure, and the pore structure of tight sandstone varies with different permeability. To study the differences in the pore structure of tight sandstone with different permeability, this study investigated the tight sandstone of the He 8 Member in the Gaoqiao area of the Ordos Basin. Factors influencing pore formation are analyzed through experiments utilizing methods, such as distinguishing casting thin sections, scanning electron microscopy, high‐pressure mercury intrusion, and low‐temperature nitrogen adsorption. The results indicate that in this area, the pores of the tight sandstone are primarily dissolution and intercrystalline pores, with occasional intergranular pores and microcracks. Type Ⅱ samples (permeability > 0.2 × 10−3 μm2) are primarily composed of dissolution and intercrystalline pores, with a few visible intergranular and microcracks. In contrast, Type Ⅰ samples (permeability < 0.2 × 10−3 μm2) mainly consist of micropores and intercrystalline pores, where microcracks are observed locally. Pore size research demonstrates that Type Ⅰ samples have pore size under 1000 nm, with peaks primarily at 4–5 nm. There are also peaks between 10–1000 nm, but without a consistent pattern. For Type Ⅱ samples with sizes smaller than 1000 nm, pore distribution is evident. Peak values when the pore size exceeds 1000 nm. Type Ⅰ samples have fewer micron‐level pores, while Type Ⅱ samples are primarily dissolution pores. The nanopores of Type Ⅰ samples consist of flat pores with good connectivity, whereas those of Type Ⅱ samples are mainly blind pores with poor connectivity. Type Ⅰ samples have larger pore fractal dimensions, more intricate pore morphology, and rougher and irregular pore throat surfaces compared to Type II samples. Hence, the sedimentary environment, diagenesis, and mineral composition affect the pore distribution in tight sandstone. The research findings highlight the variations in the pore structure of tight sandstone with varying permeability, providing crucial guidance for classifying and assessing pore structure in tight sandstone reservoirs.

Pore-Type Characterization and Reservoir Zonation of the Sarvak Formation in the Abadan Plain, Zagros Basin, Iran

Carbonate reservoirs are characterized by their complex depositional–diagenetic history and, consequently, intricate pore networks. In this study, pore types and reservoir zones of the Sarvak Formation, which is the second most important oil reservoir in Iran, are investigated by integrating core, thin-section, porosity–permeability, scanning electron microscopy (SEM), and mercury injection capillary pressure (MICP) data. Depositional, diagenetic, and fracture pores are identified in macroscopic to microscopic scales. Frequency analysis of pore types revealed that diagenetic pores, including vuggy, moldic, intercrystalline, and fracture pores, formed the majority of pore spaces of the Sarvak Formation. They are dominantly recorded in the regressive systems tracts of both the Cenomanian and Turonian sequences, just below the paleoexposure surfaces. Away from the disconformities, there are two general trends regarding the pore-types distribution: (a) grain-supported facies of shoal and talus settings with dominant primary (depositional) pores including interparticle, intra-skeletal, and growth-framework pores, which are concentrated in the middle part of the formation, and (b) microporous mud-dominated facies with various types of primary and secondary pore types in the TSTs of third-order sequences, especially in the lower part of the Sarvak Formation. Stratigraphic modified Lorenz plot differentiated reservoir, speed, barrier, and baffle zones in this formation. Depositional–diagenetic characters, petrophysical properties, and dominant pore types are defined for each zone in a sequence stratigraphic framework.

Rock physics characteristics and their control factors of carbonate in different sedimentary microfacies of the Yingshan Formation, Gucheng Area, Tarim Basin

Abstract Understanding the influence of geological characteristics on rock physics properties is crucial for accurately recognizing the relationship between rock physics variation and reservoir characteristics. Unlike the conventional rock species, the rock physics properties of the deep carbonate rocks in the third member of Yingshan Formation (Ying-III Member) in Gucheng area, Tarim Basin are relatively more complex. To address this problem, we investigated the rock physics characteristics and controlling factors of different sedimentary microfacies samples, combined with sedimentological analysis and rock physics experiments. The results show that the sedimentary environment affects the lithology and pore structure by controlling the properties of the primitive rock and early diagenesis. Dolomitized shoal microfacies and shoal top dolomitic flat microfacies primarily form crystalline dolomite and siliceous dolomite, with pores consisting of inter-crystalline pores, dissolution pores, and cracks. Inter-shoal dolomitic flat microfacies develops silty dolomite, with only a few inter-crystalline pores and cracks. Middle- to high-energy shoal microfacies and inter-shoal sea microfacies develop tight calcarenite and micritic limestone. Samples with similar mineral composition have relatively consistent density values and acoustic properties. Soft pores, such as micro-cracks, have a significant impact on the effective pressure and acoustic wave velocity, velocity and velocity ratio, and velocity and porosity relationships. The research shows a new approach for the rock physics characteristics of deep carbonate reservoirs under geological background constraints, as well as the rock physics basis for seismic prediction of the Ying-III Member reservoir.

Organic Matter Source, Fluid Migration, and Geological Significance of Stylolites in Organic-Lean Carbonate Rocks: A Case from the Sichuan Basin

Carbonate rocks exhibit significant heterogeneity as both a source rock and reservoir. Stylolite formation plays a crucial role in the enrichment of organic matter and the migration of geofluids within carbonate rocks. In order to study the enrichment mechanism of organic matter and the geofluid migration mode in the stylolites developed in carbonate rocks, stylolite-bearing core samples from the Dachigan structural belt in the eastern Sichuan Basin were collected. The stylolites and matrix were subjected to the total organic carbon (TOC) test and Rock-Eval pyrolysis, thin-section observation under fluorescent light, whole-rock X-ray diffraction, carbon and oxygen isotope analysis, and scanning electron microscopy. The organic matter occurring in the stylolites is mainly in the form of three types: soluble organic matter, pyrobitumen, and bitumen. This suggests that the organic matter within the stylolites mainly consists of secondary migrated organic matter. The stylolites also exhibit well-developed secondary dolomite and pyrite resulting from late-diagenetic recrystallization. These minerals contribute to the preservation of intercrystalline pore spaces and fractures, providing favorable conditions for oil and gas accumulation and migration. The strong cementation observed at the contact between the stylolites and matrix, along with the presence of secondary minerals nearby, may be attributed to the fractionation of light and heavy oil components during the migration of hydrocarbon fluids from the matrix to the stylolites. The thicknesses of the stylolites vary within the bulk, indicating severe diagenesis in thinner areas. Consequently, this leads to significant fractionation effects. The fractionation of crude oil components by stylolites poses challenges for the study of definitive oil–source correlations. To overcome these challenges, future research could investigate biomarker compounds to attempt oil–source correlations. Additionally, future efforts should take into consideration the spatial variation in the crude oil properties. Understanding the role of stylolites in organic matter enrichment and geofluid migration is crucial for optimizing exploration strategies in the Sichuan Basin, a region of growing importance in the energy industry. Moreover, our findings shed light on the complex interactions within stylolite-bearing rocks, which are not limited to this specific basin. These insights offer valuable contributions to the broader field of geology and reservoir characterization, enhancing our ability to predict and interpret similar geological formations globally.

Differential Diagenesis and Pore Evolution Characteristics of Different Sedimentary Microfacies Reservoirs of Bioclastic Limestone: A Case Study of MB2-MC1 Section in X Oilfield in The Middle East

In order to clarify the differential diagenesis of different sedimentary microfacies of bioclastic limestone in Cretaceous M Formation in the Middle East and systematically analyze the characteristics of reservoir pore evolution, the data of regional geology, well logging, core, cast thin section, scanning electron microscope, three-dimensional CT and conventional porosity and permeability test are comprehensively used, taking MB2-MC1 section in X oilfield as an example, starting from the diagenesis and pore evolution of bioclastic-arene shoal, the characteristics of diagenesis of each sedimentary microfacies in different environments are analyzed, and the differential diagenesis, pore evolution process and its influence on physical properties are studied. It is showed that the reservoirs of MB2-MC1 section are mainly developed in the ramp shoal and platform margin shoal. The shoal facies can be further subdivided according to the type of bioclastic debris. The sedimentary microfacies determine the original fabric and bioclastic type of M Formation, and then control the diagenetic sequence and strength. Each sedimentary microfacies mainly experiences different degrees of mud crystallization and burrows in seawater environment, dissolution and cementation in atmospheric freshwater environment, compaction, pressure solution and dolomitization in burial environment. High energy microfacies have the characteristics of non-selective dissolution, mainly developing intergranular pores and intergranular dissolved pores. Medium energy microfacies have the characteristics of selective dissolution, mainly developing mould pores and intragranular pores. The medium-low energy microfacies have the characteristics of moderate cementation, moderate compaction and moderate-weak dolomitization, mainly developing intragranular pores and visceral foramen pores. The low energy microfacies have the characteristics of strong cementation, strong compaction and moderate dolomitization, mainly developing micropores, intercrystalline pores and visceral foramen pores, which are non-reservoirs.

Pore Characteristics, Oil Contents and Factors Influencing Laminated Shale in the First Member of the Qingshankou Formation in the Gulong Sag, Northern Songliao Basin

To clarify the reservoir characteristics of laminated shale, the occurrence mechanism of shale oil and its influencing factors in the Gulong Sag, northern Songliao Basin, are studied to better guide the exploration and development of shale oil there. First, X-ray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM) are used to characterize the pore types, pore geneses and factors influencing the pore volume in the study area. Second, the organic matter of the samples is extracted with a mixture of dichloromethane and methanol. Total organic carbon (TOC), nitrogen adsorption and Rock-Eval tests are performed on the samples before and after extraction to reveal the pore size distribution after extraction. The factors influencing free and adsorbed shale oil and the lower limit of pore size are discussed in detail. The results show that interparticle pores (interP pores), intraparticle pores (intraP pores), organic matter pores (OM pores) and microfractures can be found in the laminated shale (Q1) in the Gulong Sag, Songliao Basin, and that the interP pores and intercrystalline pores in clay minerals are the main pores. The FE-SEM results show that the diameters of interP pores vary from several hundred nanometers to several microns, and their morphologies are mainly triangular, strip-shaped or irregular. The morphology of the intercrystalline pores in the clay minerals is generally irregular, depending on the crystal type and arrangement of clay minerals. According to the characteristics of the nitrogen adsorption and desorption curves, the pore morphologies are mainly slit-shaped pores, parallel-plate-shaped pores and ink-bottle-shaped pores. The pore size distribution is mostly bimodal, and the pore volume contribution is the greatest in the pore size range of 10~20 nm. Before and after extraction, the overall characteristics of the pore size distribution change only slightly, but the number of micropores increases significantly. Different minerals have different degrees of influence on the proportions of micropores, mesopores and macropores. Quartz mainly inhibits the formation of micropores, while the overall effect on mesopores and macropores is positive depending on the diagenetic period. Feldspar has a strong positive correlation with the micropore and mesopore proportions but is not highly correlated with the macropore proportions. The influence of the carbonate mineral content on the pore volume is not obvious because of its complex composition. The TOC content and vitrinite reflectance (Ro) are the two most important factors controlling free oil and adsorbed oil, and the contents of mineral components, such as felsic minerals, carbonate minerals and clay minerals, have no obvious correlation with shale oil content. With increasing pore volume, the contents of free oil and adsorbed oil increase, but the proportion of adsorbed oil decreases gradually. The correlation between the specific surface area and adsorbed oil content is poor. At normal temperatures and pressures, the lower limit of the pore diameters that can contain free oil is 4 nm, and the lower limit of the pore diameters that can contain movable oil is 10 nm.

Development characteristics and genetic analysis of dense limestone reservoirs in the Taiyuan Formation of the Ordos Basin, China

Tight limestone reservoirs in the Permian Taiyuan Formation of the Ordos Basin have garnered increasing attention in recent years, emerging as a pivotal domain in the quest for natural gas exploration. This research delves into the comprehensive investigation of petrology, reservoir space, reservoir physical properties, and microscopic characteristics through the examination of field outcrops, core observations, thin section identification, scanning electron microscopy, stable carbon and oxygen isotope analysis, and testing. The systematic analysis focused on elucidating the development characteristics of these reservoirs and identifying the controlling factors governing favorable reservoir conditions. Based on our research analysis, the specific rock types with the potential to serve as excellent reservoirs, includes bioclastic silty limestone, bioclastic micrite limestone, and algal-rich limestone. The predominant reservoir spaces within these formations were found to consist of dissolved pores, residual bioclastic cavity pores, intercrystalline pores, and microfractures. These reservoirs exhibit an average porosity of 2.1%, and an average permeability of 0.22 × 10−3 μm2, indicating their classification as low porosity and low permeability reservoirs. The formation of favorable reservoirs in the Taiyuan Formation limestone was determined to be influenced by many factors; notably, the favorable sedimentary microfacies associated with bioclastic shoals and bioherms provided the foundational material basis for the formation of reservoirs, influencing the type of reservoir space and its extensive planar distribution. Penecontemporaneous karstification, guided by high-frequency cycles, was favorable for the formation of dissolution holes, effectively improving reservoir performance and facilitating the development of thick limestone reservoirs. Furthermore, fractures were identified as crucial agents in improving the seepage capacity of these tight limestone reservoirs. Drawing from our research results, this study offers valuable guidance for the future exploration of limestone formations in the Taiyuan Formation in the Ordos Basin. Additionally, these findings hold considerable significance as a reference point for research and exploration endeavors focused on bioclastic limestone reservoirs in the North China Platform.

Characteristics and controlling factors of pore structure of shale in the 7th member of Yanchang Formation in Huachi area, Ordos Basin, China

To clarify the influence of organic matter and mineral composition on the pore structure of shale reservoirs in the 7th member of the Yanchang Formation (Chang 7 Member) in the Ordos Basin, we characterized the pore structure of Chang 7 shale reservoirs using argon ion polishing-field emission scanning electron microscopy (FE-SEM) and low-pressure nitrogen adsorption (LP-N2A). This characterization was combined with whole-rock mineral composition and organic geochemical experiments to analyze the main controlling factors of the pore structure of Chang 7 Member shale. The results reveal the presence of various pore types in shale, including organic matter pores, intergranular pores, intercrystalline pores, dissolved pores, and micro-cracks. The LP-N2A isotherms of shale consistently exhibit type Ⅱ isotherms with H3 and H4 hysteresis loop characteristics, indicating the relatively developed nature of mesopores and a pore morphology characterized by parallel lamellar and “ink bottle” shapes. The primary determinants of shale pore structure are identified as organic matter, clay minerals, quartz, feldspar, and pyrite. Among these factors, clay mineral phase transformation generates a substantial number of micropores and mesopores within the mineral crystal layers, serving as the main source of shale pores in the study area. Additionally, liquid hydrocarbons generated, solid bitumen, and euhedral pyrite fill inorganic mineral pores, thereby reducing the pore space of Chang 7 shale to a certain extent. These results provide a new cognition into understanding the pore structure characteristics and controlling factors of Chang 7 shale.

Research on Fractal Characteristics and Influencing Factors of Pore-Throats in Tight Sandstone Reservoirs: A Case Study of Chang 6 of the Upper Triassic Yanchang Formation in Huaqing Area, Ordos Basin, China

In recent years, tight sandstone oil and gas have been an important area for unconventional oil and gas exploration and development in China. It is of great significance to clarify the pore-throat structure characteristics of tight sandstone reservoirs to guide production practices. This study takes the tight sandstone of the sixth member of the Yanchang Formation in the Huaqing area, Ordos Basin, as an example, based on experimental methods such as high-pressure mercury intrusion, cast thin sections and scanning electron microscopy. At the same time, the pore-throat structure of tight sandstone reservoirs is divided into three types using the tube-bundle and spherical fractal models. The corresponding pore and throat radius distribution, pore-throat combination mode and influencing factors of various pore-throats are studied. The results show that the fractal dimension of type I pore-throats is the smallest, and the distribution of their pore-throat radii is the most uniform. They are dominated by intercrystalline pores and dissolution pores with tube-bundle throats and small pores with small throats. Type II pore-throats have the largest fractal dimension and the worst pore-throat uniformity. They are dominated by residual primary intergranular pores with necked throats and large pores with small throats. The type III pore-throat fractal dimension is in the middle, mainly composed of residual dissolved intergranular pores with pore-reduced throats, sheet-like and curved sheet-like throats, and large pores with large throats. The influence of different pore-throat combinations on the reservoir is reflected in the different characteristics of mercury injection parameters. The main influencing factors for the differences in the fractal dimensions of different pore-throats are diagenesis, rock composition and pore-throat combination type. Diagenesis and rock composition, in turn, affect the type and development degree of pore-throats, as well as the combination of pore-throats. The purpose of this study was to clarify the internal connection modes of different homogeneous pore-throats and their influencing factors, enrich the theoretical basis for the study of tight sandstone reservoirs and provide theoretical guidance for their exploration and development.

Microscopic oil occurrence in high-maturity lacustrine shales: Qingshankou Formation, Gulong Sag, Songliao Basin

Occurrence and mobility of shale oil are prerequisites for evaluating shale oil reserves and prioritizing exploration targets, particularly for heterogeneous lacustrine shales. The Qingshankou Formation in the Gulong Sag, Songliao Basin is a classic lacustrine pure shale reservoir that contains abundant shale oil resources. The predicted geological reserves of the shale are 1.268 × 109 t. In this study, field emission scanning electron microscope (FE-SEM), the modular automated processing system (MAPS), pyrolysis-gas chromatography (Py-GC), low-pressure nitrogen gas adsorption (LPNA), Soxhlet extraction, pyrolysis, and 2-D nuclear magnetic resonance (NMR) were integrated to describe the shale oil components, microscopic occurrence, mobility, and the effective pore size distribution. Meanwhile, the related controlling factors are discussed.The shale oil in the Qingshankou Fm exists dominantly in the matrix pores of the clay minerals, with small amounts distributed in the intergranular pores of terrigenous clastic grains, intercrystalline pores of pyrite, intragranular pores of ostracod shells, and micro-fractures. Shale oil is distributed in the pore spaces of variable sizes in different lithofacies. The clay mineral-laminated shales are characterized by the broadest range of pore size and largest volume of pore spaces with shale oil distribution, while the ostracod-laminated shales have limited pore spaces retaining oil. Furthermore, the proposed integrated analysis evaluates the shale oil molecules existing in two states: movable, and adsorbed oil, respectively. The result illustrates that movable oil takes up 30.6%–79.4% of the total residual oil. TOC, mineral composition, and pore structures of the shale joint together to control the states and mobility of the shale oil. TOC values are positively correlated with the quantities of shale oil regardless of the state of oil. The mineral components significantly impact the state of shale oil. Noticeable differences in the states of oil were observed following the changing types of minerals, possibly due to their difference in adsorption capacity and wettability. Clay minerals attract more adsorbed oil than movable oil. Felsic minerals generally decrease the occurrence of total and adsorbed oil. Carbonate plays a positive role in hydrocarbon retention of all the shale oil states. As for the pore structure, the average pore size exerts a critical impact on the total, movable, and adsorbed oil content. The total pore volume and specific surface area of shales play a principal role in controlling the total yields and amounts of adsorbed oil. This research improves the understanding of the occurrence characteristics and enrichment mechanisms of shale oil in terrestrial pure shales and provides a reference for locating favorable shale oil exploration areas.

Geothermal carbonate reservoirs and their sustainability: what can natural hydrothermal systems tell us?

Minerals that make up carbonate rocks (i.e. calcite) have very high chemical reactivity, allowing for various types of porosity-altering deformation processes such as dissolution and recrystallization (e.g. dolomitization, silicification), that produce secondary porosity. Even though water enriched with CO2 may quickly equilibrate by dissolving calcite, carbonate is also very dynamic, and cemented veins may still act as planes of weakness compared to the host rock allowing fluids to open up and then seal these fractures again. Therefore, natural carbonate rocks often show not only the numerous deformation stages overprinting one another, but also the same veins opening up again and again forming crack-seal structures.Natural fossil hydrothermal systems were investigated within outcrops at Nowa Wioska, Czatkowice, Pomorzan, Trzebionka, Orzel Bialy mines, Dziura cave and Hucianski Klin crest in the south of Poland to evaluate the influence of hydrothermal activities on the rock pore evolution. We present in more detail the Triassic carbonate sequence near Nowa Wioska village in the Silesian-Kraków region, where hydrothermal water had been flowing through an open fault zone, forming various hydrothermal products: secondary intercrystalline pores, vugs, breccias, metal ore and mineral precipitates. The impact of these products on the permeability of the geothermal system was analyzed in comparison with the adjacent host rock unaffected by tectonic and hydrothermal processes. We observed that at least three successive episodes of fracturing occurred during the Tertiary demonstrating that fluids were introduced episodically in the Triassic rocks. Fracture- and vug-filling calcite, saddle dolomite, pyrite, sphalerite, galena, quartz and clays were preferentially observed on the downthrown side of the fault that underwent a more intense tectonic deformation that had a decisive influence on the migration of mineralizing hydrothermal solutions. Even though mineral and metal-ore precipitates, sediments and solution / collapse breccias fill most of these karsts and fractures, the porosity and permeability of the hydrothermally altered rocks remain quite high compared to the host rock. Moreover, secondary dissolution forming vugs increase this porosity and permeability even further. The studied examples suggest that once geothermal system has been active, it stays as a preferential zone for the following deformation events, making it sustainable for tens of thousands or even millions of years.

Logging evaluation of pore structure and reservoir quality in shale oil reservoir: The Fengcheng Formation in Mahu Sag, Junggar Basin, China

The Permian Fengcheng Formation in the Mahu Sag of the Junggar Basin is characterized by frequent changes of mineral compositions and lamina structure. The alkali lacustrine shale reservoir shows strong heterogeneity and anisotropy, it is particularly important to clarify the relationship between pore structure and reservoir quality. The pore characteristics were investigated by using thin sections, Scanning Electron Microscope (SEM), and Nuclear Magnetic Resonance (NMR) analyses. The pore types are mainly composed of inorganic pores, organic pores, and microfractures. The storage space is dominated by inorganic pores and microfractures. The pore structure was divided into four types by NMR parameters including T2 spectral morphology, T2gm, total porosity, and movable fluid porosity. Combined with core NMR experiments and two-dimensional (2D) NMR logs, 21 ms was adopted as the T2cutoff value to represent the demarcation between movable and bound fluids. From Type I to Type IV, the energy clusters reflect the movable fluids including movable oil and movable water gradually decreasing in the upper right corner. The energy clusters reflect the bound fluids including clay-bound water and bitumen gradually increasing in the lower left corner in the T1-T2 map. The results show that the heterogeneity of mineral compositions and lamina structures are the key factors directly leading to the complexity and distribution of pore structure. Quartz with strong compaction resistance helps keep a large number of primary interparticle pores, and feldspar dissolution pores caused by organic acids are present. Dolomite intercrystalline pores and dissolution pores provide a large amount of storage space for the shale, while the cementation of calcite reduces pore spaces. The increase of felsic mineral content is conducive to improving the pore structure and oil-bearing potential of oil shale reservoir, while clay minerals and calcite are contrary. There are abundant primary interparticle pores and dissolution pores in the layered siliceous (felsic) shales, and they consist of the upper sweet spot in the Fengcheng Formation. The longitudinal time (T1) and transverse time (T2) spectra are bimodal with high total porosity and movable oil content, reflecting high reservoir quality and oil-bearing property. Laminated dolomite shales contain dolomite intercrystalline pores and dissolution pores, and they consist of the lower sweet spot in the Fengcheng Formation. The results above provide a new perspective on the evaluation of pore structure and reservoir quality of shales in the context of an alkaline lake.

Characteristics and Controlling Factors of Crystalline Dolostone Reservoirs of the Sanshanzi Formation, Central-Eastern Ordos Basin, Northwestern China

In recent years, Ordos Basin has been the largest petroleum field in China and represents significant potential for gas exploration in the Lower Paleozoic carbonate reservoirs. Taking the central-eastern Ordos Basin as an example, this work studies the reservoir characteristics, origin, and controlling factors of the crystalline dolomites of the Sanshanzi Formation by means of petrological and geochemical studies. The Sanshanzi Formation dolostones comprise three kinds: crystalline dolostone with grain-shadow (Rd1), very fine–fine crystalline dolostone (Rd2), and medium-coarse crystalline dolostone (Rd3). The pore spaces include intercrystalline pores and intercrystalline solution pores, residual intergranular pores, vugs, and fractures. The medium-coarse crystalline dolostone is the best reservoir. The average porosity is 4.61%, and the average permeability is 0.91 mD. The sedimentary environment in the upper Cambrian was a large area of tidal flats. Under the influence of penecontemporaneous meteoric water leaching, dissolution developed at the top of the dolomite flat–lagoon depositional cycle, with vugs developing in the very fine–fine crystalline dolostone. Penecontemporaneous dolomitization enhanced the compaction resistance and effectively preserved the pore space of the reservoir. In addition, recrystallization improved the crystalline texture, turning the intergranular pores into residual intergranular pores and intercrystalline micropores into intercrystalline pores. In this case, the very fine–fine crystalline dolostone was transformed into medium-coarse crystalline dolostone. The results of this study provide a case for the in-depth understanding of the development and preservation mechanism of ancient dolomite reservoirs and also point to the research pathways for oil and gas exploration in the Cambrian in the Ordos Basin in the future.

Early diagenetic REE migration from Fe-Mn nodules to fish teeth in deep sea sediments

In deep sea sediments, rare earth elements and yttrium (REYs) are predominantly carried by bioapatite and Fe-Mn (oxyhydr)oxides. According to recent studies, REYs can migrate into bioapatite after the dissolution of Fe-Mn (oxyhydr)oxides. However, the REY migration mechanism from Fe-Mn (oxyhydr)oxides to bioapatite are still unclear. Here, we integrated in situ trace element and Nd isotope analyses to elucidate the geochemical characteristics of fish tooth-bearing Fe-Mn nodules from the Northwest Pacific Ocean. Our Fe-Mn nodule samples can be classified into different morphological and geochemical types of the different periods in early diagenesis: (I) completely overgrown around bioapatite; II) overgrown around half of the bioapatite. Type I nodules grow around a bioapatite core, which has high LaN/SmN (0.41–0.50, avg. 0.46) and LaN/YbN (0.49–0.90, avg. 0.62). The ΣREY content is non-uniform in the two bioapatite types (WP-01 & WP-02) in Type I nodules, i.e., 475–2150 ppm and 272–431 ppm, respectively. The REY results show that the bioapatite is mainly adsorbed REY in early diagenesis. Type II nodules were covered both inside and at the tip of the tooth. The LaN/SmN (0.34–0.38, avg. 0.36) and LaN/YbN (0.26–0.61, avg. 0.37) ratios of the bioapatite samples are relatively low. Meanwhile, the ΣREY content is very uneven (229–3521 ppm). The REY data show that the bioapatite is mainly substituted during strong early diagenesis. The intercrystalline pores in some authigenic apatite may have closed upon recrystallization, causing REY redistribution and fractionation. The Y/Ho vs. ΣREE/Th plot shows a higher REY proportion from pore water in type II nodules than in type I ones. The widespread distribution of high Mn content and the gradual decrease of REY content from the nodule rim to core reflect the transport of REY from dissolved Fe-Mn (oxyhydr)oxides to bioapatite during early diagenesis under suboxic conditions. From type I to type II nodules, the mean εNd value of bioapatite increases from −6.69 to −5.56. The comparison of εNd values between REY-rich mud and bioapatite indicate that the REYs were derived from the bottom water and pore water in deep sea sediments. Our findings comprehensively document the transport and enrichment mechanisms of REY in early diagenetic processes from Fe-Mn (oxyhydr)oxides to bioapatite, which we argued to be relatively localized.