Search Results (99)

The Permian Xiashihezi Formation in the Ordos Basin is a typical tight sandstone gas reservoir, which is characterized by low porosity and strong heterogeneity. Diagenesis plays a crucial role in controlling reservoir quality. However, the multiple phases and types of diagenetic processes throughout geological history make the compaction mechanisms highly complex. This study employed a high-temperature and high-pressure diagenesis simulation system to conduct geological simulation experiments. Typical reservoir samples from the 2nd Member of the Permian Xiashihezi Formation were selected for these simulations. The experiments replicated the diagenetic evolution of the reservoirs under various temperature, pressure, and fluid conditions, successfully reproducing the diagenetic sequences. The diagenetic sequence included early-stage porosity reduction through compaction, early carbonate cementation, quartz overgrowth, chlorite rim formation, feldspar dissolution, and late-stage illite and quartz cementation. Mechanical compaction is the primary factor reducing reservoir porosity, exhibiting a distinct four-stage porosity reduction pattern: (1) continuous burial stage (>4000 m); (2) stagnation stage of burial (3900 m–4100 m); (3) the secondary continuous burial stage (>5000 m); (4) tectonic uplift stage (3600 m). The experiments confirmed that the formation of various authigenic minerals is strictly controlled by temperature, pressure, and fluid chemistry. Chlorite rims formed in an alkaline environment enriched with Fe²⁺ and Mg²⁺ (simulated temperatures of 280–295 °C), effectively inhibiting quartz overgrowth. Illite appeared at higher temperatures (>300 °C) in platy or fibrous forms. Feldspar dissolution was noticeable upon injection of acidic fluids (simulated organic acids), providing material for authigenic quartz and kaolinite. The key mineral composition significantly impacts reservoir diagenesis. The dissolution released Mg²⁺ and Fe²⁺ ions, crucial for forming early chlorite rims in the overlying sandstones, confirming the importance of inter-strata interactions in “source-facies coupling.” Through physical simulation methods, this study deepened the understanding of the diagenetic evolution and compaction mechanisms of tight sandstones. This provides significant experimental evidence and theoretical support for predicting “sweet spot” reservoirs in the area. Full article

By employing thin section analysis, scanning electron microscopy (SEM), homogenization temperatures of fluid inclusions, and carbon–oxygen isotope analysis of carbonate cements, this study conducted a temporal-quantitative investigation into the porosity evolution of relatively high-quality reservoirs in the Second Member of the Xujiahe Formation (Xu-2 Member) in the Xinchang area of western Sichuan. The analysis focused on quantifying porosity loss due to compaction, cementation, and porosity enhancement from dissolution. Results indicate that compaction exerted the most significant impact on reservoir quality in the Xu-2 Member, causing over 70% of total porosity loss. Cementation processes, including carbonate cements, silica cements, and authigenic chlorite, further degraded reservoir properties. Authigenic chlorite precipitated earliest at burial depths of 600–800 m, while authigenic quartz and carbonate cements persistently affected the reservoir at depths of 2000–5000 m, reducing porosity by at least 10% (up to 21%). Dissolution processes initiated at approximately 3500 m burial depth, generating secondary porosity of ≥2%, with a maximum increase of 16%. Integrating these findings with the natural gas accumulation history, the coupling relationship between pore evolution and gas accumulation was elucidated. The study reveals that reservoir tightness in the Xu-2 Member developed at burial depths of 4050–5300 m, with large-scale gas accumulation predominantly occurring prior to reservoir densification. The findings provide critical guidance for identifying high-quality tight sandstone reservoirs and optimizing exploration targets in the Xu-2 Member of the Xinchang area, Western Sichuan Basin, thereby supporting efficient development of regional tight gas resources. Full article

The lithological dichotomy in the Hammam Faraun Member (Gulf of Suez, Egypt) reveals a stable western flank with Nullipore carbonate deposits, contrasting with the clastic-prone eastern margin influenced by tectonic activity. This study aims to decipher multifactorial controls on spatial lithological variability and reservoir implications through (1) foraminiferal-based paleoenvironmental reconstruction; (2) integrated sequence stratigraphic–petrophysical analysis for sweet spot identification; and (3) synthesis of lateral facies controls. This study uniquely integrates foraminiferal paleoenvironmental proxies, sequence stratigraphy, and petrophysical analyses to understand the multifactorial controls on spatial variability and its implications for reservoir characterization. Middle Miocene sea surface temperatures, reconstructed between 19.2 and 21.2 °C, align with warm conditions favorable for carbonate production across the basin. Foraminiferal data indicate consistent bathyal depths (611–1238 m) in the eastern region, further inhibited in photic depths by clastic influx from the nearby Nubian Shield, increasing turbidity and limiting carbonate factory growth. Conversely, the western shelf, at depths of less than 100 m, supports thriving carbonate platforms. In the sequence stratigraphy analysis, we identify two primary sequences: LA.SQ1 (15.12–14.99 Ma), characterized by evaporitic Feiran Member deposits, and LA.SQ2 (14.99–14.78 Ma), dominated by clastic deposits. The primary reservoir comprises highstand systems tract (HST) sandstones with effective porosity ranging from 17% to 22% (calculated via shale-corrected neutron density cross-plots) and hydrocarbon saturation of 33%–55% (computed using Archie’s equation). These values, validated in Wells 112-58 (ϕe = 19%, S_hc = 55%) and 113M-81 (ϕe = 17%, S_hc = 33%), demonstrate the primary reservoir potential. Authigenic dolomite cement and clay content reduce permeability in argillaceous intervals, while quartz dissolution in clean sands enhances porosity. This research emphasizes that bathymetry, sediment availability, and syn-sedimentary tectonics, rather than climate, govern carbonate depletion in the eastern region, providing predictive parameters for identifying reservoir sweet spots in clastic-dominated rift basins. Full article

In this article, we examine water chemistry and carbonate precipitation in the alkaline waterbody Lake Van in Turkey, analyzing the possible role of microbial communities in stromatolite formation. Lake Van represents a unique environment characterized by high salinity and pH and extensive microbial communities, as revealed by SEM observation. Microbial activity, including that of cyanobacteria, can influence carbonate precipitation processes, leading to the formation of authigenic carbonates through physicochemical or metabolic mechanisms such as photosynthesis or sulfate reduction. In these environments, which are often dominated by cyanobacteria, carbonate precipitation can be influenced by biologically induced processes. This study presents new data on the hydrochemistry of lake water, focusing on the behavior of rare-earth elements (REEs) in this water and the carbon and oxygen isotopic compositions of carbonate microbial mats. The oxygen isotope data suggest that inorganic carbonate precipitation is the dominant process, but a biological influence on inorganic precipitation cannot be ruled out. For a deeper understanding of the role of biological processes in Lake Van, further studies on microbialites are needed. Full article

In order to clarify the pore evolution and coupling characteristics with hydrocarbon charging in the deep-buried ultra-tight sandstone reservoirs of the second member of Xujiahe Formation (hereinafter referred to as the Xu 2 Member) on the eastern slope of the Western Sichuan Depression, this study integrates burial history and thermal history with analytical methods including core observation, cast thin section analysis, scanning electron microscopy, carbon-oxygen isotope analysis, and fluid inclusion homogenization temperature measurements. The Xu 2 Member reservoirs are predominantly composed of lithic sandstones and quartz-rich sandstones, with authigenic quartz and carbonates as the main cementing materials. The reservoir spaces are dominated by intragranular dissolution pores. The timing of reservoir densification varies among different submembers. The upper submember underwent compaction during the Middle-Late Jurassic period due to the high ductility of mudstone clasts and other compaction-resistant components. The middle-lower submembers experienced densification in the Late Jurassic period. Late Cretaceous tectonic uplift induced fracture development, which enhanced dissolution in the middle-lower submembers, increasing reservoir porosity to approximately 5%. Two distinct phases of hydrocarbon charging are identified in the Xu 2 Member. The earlier densification of the upper submember created unfavorable conditions for hydrocarbon accumulation. In contrast, the middle-lower submembers received hydrocarbon charging prior to reservoir densification, providing favorable conditions for natural gas enrichment and reservoir formation. Three sweet-spot reservoir development patterns are recognized: paleo-structural trap + (internal source rock) + source-connected fracture assemblage type, paleo-structural trap + internal source rock + late-stage fracture assemblage type, and paleo-structural trap + (internal source rock) + source-connected fracture + late-stage fracture assemblage type. Full article

The Upper Triassic Xujiahe Formation in the western Sichuan Basin is rich in natural gas resources and is one of the main tight sandstone gas-producing layers in the Sichuan Basin. Taking the tight sandstone of the second member of the Xujiahe Formation (Xu-2 Member) in the western Sichuan Basin as the study target, based on the analysis of the rock sample, a thin section, scanning electron microscopy, inclusion, the carbon and oxygen isotope, the petrological characteristics, the reservoir properties, the diagenetic sequences, and the pore evolution processes were revealed. The tight sandstones are composed of litharenite, sublitharenite, and feldspathic litharenite with an average porosity of 3.81% and a permeability mainly ranging from 0.01 to 0.5 mD. The early to late diagenetic stages were revealed, and the diagenetic evolution sequence with five stages was constructed. The Xu-2 sandstones were subdivided into three different types, and each type has its own tightening factors and processes. In the quartz-rich sandstone, the compaction and pressure solution were the primary causes of reservoir tightening, while late fracturing and dissolution along fractures were the main factors improving reservoir properties. In the feldspar-rich sandstone, early dissolution was a primary factor in improving porosity, while carbonate and quartz cements generated by dissolution contributed to a decrease in porosity. In the rock-fragment-rich sandstone, chlorites formed in the early stage and dissolution were the main factors of reservoir quality improvement, while the authigenic quartz formed in the middle diagenetic stage was the primary cause of reservoir tightening. Four major source-to-sink systems were identified in the western Sichuan Basin and they have different reservoir characteristics and reservoir quality controlling factors. This study will contribute to a deeper understanding of the characteristics, diagenetic evolution, and tightening process of tight sandstone reservoirs, effectively promoting scientific research and the industrial development of tight sandstone gas in the Xu-2 Member of the Sichuan Basin. Full article

The identification and prediction of effective reservoir rocks are important for evaluating the energy production potential of ultra-deep tight sandstone reservoirs. Taking the Keshen gas field, Tarim Basin, as an example, three distinct petrofacies are divided according to petrology, pores, and diagenesis. Petrofacies, well logs, and factor analysis are combined to predict effective reservoir rocks. We find that petrofacies A has a relatively coarse grain size, moderate mechanical compaction, diverse but low-abundance authigenic minerals, and well-developed primary and secondary pores. It is an effective reservoir rock. Petrofacies B and petrofacies C are tight sandstones with a poorly developed pore system and almost no dissolution. Petrofacies B features abundant compaction-susceptible ductile grains, intense mechanical compaction, and underdeveloped authigenic minerals, while petrofacies C features pervasive carbonate cementation with a poikilotopic texture. We combine well logging with gamma ray, acoustic, bulk density, neutron porosity, resistivity, and factor analyses to facilitate the development of petrofacies prediction models. The models reveal interbedded architecture where effective reservoir rocks are interbedded with tight sandstone, resulting in the restricted connectivity and pronounced reservoir heterogeneity. Classifying and combining well logs with a factor analysis to predict petrofacies provide an effective means for evaluating the energy potential of ultra-deep reservoirs. Full article

The Sanandaj–Sirjan Zone and Zagros Fold–Thrust Belt in Iran host numerous Mediterranean-type karst bauxite deposits; however, their formation mechanisms and critical raw material potential remain ambiguous. This study combines mineralogical and geochemical analyses to explore (1) the formation of authigenic minerals, (2) the role of microbial organic processes in Fe cycling, and (3) the assessment of their critical raw materials potential. Mineralogical analyses of the Late Cretaceous Daresard and Middle–Late Permian Yakshawa bauxites reveal distinct horizons reflecting their genetic conditions: Yakshawa exhibits a vertical weathering sequence (clay-rich base → ferruginous oolites → nodular massive bauxite → bleached cap), while Daresard shows karst-controlled profiles (breccia → oolitic-pisolitic ore → deferrified boehmite). Authigenic illite forms via isochemical reactions involving kaolinite and K-feldspar dissolution. Scanning electron microscopy evidence demonstrates illite replacing kaolinite with burial depth enhancing crystallinity. Diaspore forms through both gibbsite transformation and direct precipitation from aluminum-rich solutions under surface conditions in reducing microbial karst environments, typically associated with pyrite, anatase, and fluorocarbonates under neutral–weakly alkaline conditions. Redox-controlled Fe-Al fractionation governs bauxite horizon development: (1) microbial sulfate reduction facilitates Fe³⁺ → Fe²⁺ reduction under anoxic conditions, forming Fe-rich horizons, while (2) oxidative weathering (↑Eh, ↓moisture) promotes Al-hydroxide/clay enrichment in upper profiles, evidenced by progressive total organic carbon depletion (0.57 → 0.08%). This biotic–abiotic coupling ultimately generates stratified, high-grade bauxite. Finally, both the Yakshawa and Daresard karst bauxite ores are enriched in critical raw materials. It is worth noting that the overall enrichment appears to be mostly driven by the processes that led to the formation of the ores and not by the chemical features of the parent rocks. Divergent bauxitization pathways and early diagenetic processes—controlled by paleoclimatic fluctuations, redox shifts, and organic matter decay—govern critical raw material distributions, unlike typical Mediterranean-type deposits where parent rock composition dominates critical raw material partitioning. Full article

The Dolomite reservoir of the Maokou Formation is rich in gas resources in the central Sichuan Basin. Acid fracturing is an important technical means to improve reservoir permeability and productivity. The interaction mode of the dolomite and limestone acid system will affect the effect of reservoir reconstruction. In order to clarify the influence of complex structure on fracture morphology, we explore the fracturing effect of different acid systems. Physical simulation experiments of true triaxial acid fracturing were carried out with two acid systems and downhole full-diameter cores. The experimental results show: (1) After the carbonate rock is subjected to acid fracturing using a “self-generated acid + gel acid” system, the fracture pressure drops significantly by up to 60%. The morphology of the acid-eroded fractures becomes more complex, with an increase in geometric complexity of about 28% compared to a single acid solution system. It is prone to form three-dimensional “spoon” shaped fractures, and the surface of the acid-eroded fractures shows light yellow acid erosion marks. Analysis of the acid erosion marks indicates that the erosion depth on the fracture surface reaches 0.8–1.2 mm, which is deeper than the 0.2 mm erosion depth achieved with a single system. (2) Acid solution is difficult to penetrate randomly distributed calcite veins with a low porosity and permeability structure. When the fracture meets the calcite vein, the penetration rate of acid solution drops sharply to 15–20% of the initial value, resulting in a reduction of about 62% of the acid erosion area in the limestone section behind. And the acid erosion traces in the limestone behind the calcite vein are significantly reduced. The acid erosion cracks are easy to open on the weak surface between dolomite and limestone, causing the fracture to turn. (3) The results of field engineering and experiment are consistent, and injecting authigenic acid first in the process of reservoir reconstruction is helpful to remove pollution. The recovery rate of near-well permeability is more than 85% with pre-generated acid. Reinjection of gelled acid can effectively communicate the natural weak surface and increase the complexity of cracks. The average daily oil production of the completed well was increased from 7.8 m³ to 22.5 m³, and the increase factor reached 2.88. Full article

Calcite in hydrocarbon reservoirs records abundant information about diagenetic fluids and environments. Understanding the formation mechanisms of calcite is crucial for predicting reservoir characteristics and hydrocarbon migration. This study identifies the types of authigenic calcite present in the Lower Paleozoic carbonate reservoirs of the Bohai Bay Basin through petrographic analysis, cathodoluminescence, and other experimental methods. By integrating electron probe microanalysis, in situ isotopic analysis, and fluid inclusion studies, we further constrain the source of the diagenetic fluids responsible for the authigenic calcite. The results show that there are at least three types of authigenic calcite in the Lower Paleozoic carbonate reservoirs of the Bohai Sea. Calcite cemented in the syn-depositional-to-early-diagenetic stage displays very weak cathodoluminescence, with δ¹³C and δ¹⁸O and paleo-salinity distributions similar to those of micritic calcite. These features suggest that the calcite was formed during burial heating by sedimentary fluids. Calcite filling fractures shows heterogeneous cathodoluminescence intensity, ranging from weak to strong, indicating multiple stages of cementation. The broad elemental variation and multiple cementation events suggest that the diagenetic fluid sources were diverse. Isotopic data show that samples with carbon isotope values greater than −2.9‰ likely formed through water–rock interaction with fluids retained within the strata, whereas samples exhibiting more negative δ¹³C were formed from a mixed-source supply of strata and mantle-derived fluids. Calcite that fills karst collapse pores exhibits alternating bright and dark cathodoluminescence, strong negative δ¹⁸O shifts, and variability in trace elements such as Mn, Fe, and Co. These characteristics indicate a mixed origin of diagenetic fluids derived from both meteoric freshwater and carbonate-dissolving fluids. Full article

The successful exploration and development of shale oil in the clay-rich Gulong shale have sparked increased research into the influence of clay minerals on shale reservoirs. However, compared to chlorite in sandstones, limited studies have focused on the occurrence of chlorite in continental shales and its effects on shale reservoir properties. This study offers a comprehensive analysis of chlorite in Gulong shale samples from three wells at different diagenetic stages. Four primary chlorite occurrences are identified in the Gulong shale: Type I, which is chlorite filling dissolved pores in carbonate; Type II, which is isolated chlorite; Type III, which is chlorite filling organic matter; and Type IV, which is chlorite filling authigenic microquartz. Types I and III chlorites exhibit higher porosity, offering more storage space for shale reservoirs. Chlorites of Types I, III, and IV, filled with other substances, display higher fractal dimensions, indicating more complex pore structures. These complex pores are favorable for oil adsorption but hinder oil seepage. The processes of organic matter expulsion and dissolution, which intensify with increasing diagenesis, promote the development of Types I and III chlorites, thereby positively influencing the shale reservoir porosity of Gulong shale. This study underscores the influence of chlorite occurrences on shale reservoir properties, providing valuable insights for the future exploration and development of shale oil and gas. Full article

Tuffaceous fillings are a significant component of the Permian Kuishan sandstone in the North China Platform, and their complex diagenetic processes have a notable impact on the development of clastic rock reservoirs. This study, based on microscopic analysis of reservoirs and combined with quantitative analytical techniques such as electron probe microanalysis, homogenization temperatures of fluid inclusions, micro-area carbon-oxygen isotope analysis, and laser Raman spectroscopy, investigates the influence of tuffaceous interstitial material dissolution on reservoir development in the Permian Kuishan sandstone of the Gaoqing buried hill in the Jiyang Depression, Bohai Bay Basin. The results indicate that the dissolution intensity of tuffaceous interstitial materials can be classified into three levels: strong, moderate, and weak. In the strong dissolution zone, associated fractures and dissolution pores significantly contribute to reservoir porosity, with a positive correlation between dissolution plane porosity and total plane porosity. The reservoir space is characterized by a network of dissolution pores and fractures. The moderate dissolution zone is marked by the development of authigenic quartz, feldspar, and clay minerals, which do not effectively enhance porosity and permeability. The weak dissolution zone contains well-preserved volcanic glass shards, crystal fragments, and clay minerals, representing non-reservoir development sections. Lithology, sedimentary facies, diagenesis, and fractures collectively control the quality of the Permian Kuishan sandstone reservoir in the Gaoqing buried hill of the Jiyang Depression, Bohai Bay Basin. The advantageous zones for reservoir development in this area can be effectively predicted using thickness maps of the Kuishan sandstone, planar distribution maps of sedimentary facies, and fracture prediction maps derived from ant-tracking and coherence algorithms. Full article

Although it is practically impossible to find locations without a massive flux of tourists, few beach destinations present a great attraction due to their privileged natural characteristics. This is often the case for sites that show splendid beach sands. To maintain their tourist attraction and related economic income, it is essential to know sediment characteristics such as their mineralogical composition, particle size, and colour. This paper presents a textural, chromatic, and mineralogical database of 90 beaches in Cuba. The composition of sediments was identified by stereomicroscopy, their texture by digital image analysis, sand colour according to the CIE space and X-ray diffraction, and fluorescence and electron microscopy were used to determine sediment mineralogy. Two main groups of beaches were identified: the lighter and brighter beaches of the cays are dominated by the association of authigenic carbonates (aragonite, kutnohorite, and calcite) while the south and northeastern coasts of eastern Cuba are dominated by darker sediments with larger grain sizes composed of amphibole, pyroxene, serpentines, chlorites, quartz, and plagioclase of detrital origin. The data obtained will allow the design of proper management actions of coastal resources, i.e., the maintaining of beaches’ sediment quality after nourishment works and, at the same time, the promotion and development of new, presently undervalued areas. Full article

Rough surfaces known as stylolites are common geological features that are developed by pressure solution, especially in carbonate rocks, where they are used as strain markers and as stress gauges. As applications are developing in various geological settings, questions arise regarding the uncertainties associated with quantitative estimates of paleostress using stylolite roughness. This contribution reports for the first time a measurement of the temperature at which pressure solution was active by applying clumped isotopes thermometry to calcite cement found in jogs linking the tips of the stylolites. This authigenic calcite formed as a redistribution of the surrounding dissolved material by the same dissolution processes that formed the extensive stylolite network. We compare the depth derived from these temperatures to the depth calculated from the vertical stress inversion of a bedding parallel stylolite population documented on a slab of the Calcare Massiccio formation (early Jurassic) formerly collected in the Umbria-Marches Arcuate Ridge (Northern Apennines, Italy). We further validate the coevality between the jog development and the pressure solution by simulating the stress field around the stylolite tip. Calcite clumped isotopes constrain crystallization to temperatures between 35 and 40 °C from a common fluid with a δ¹⁸O signature around −1.3‰ SMOW. Additional δ¹⁸O isotopes on numerous jogs allows the range of precipitation temperature to be extended to from 25 to 53 °C, corresponding to a depth range of 650 to 1900 m. This may be directly compared to the results of stylolite roughness inversion for stress, which predict a range of vertical stress from 14 to 46 MPa, corresponding to depths from 400 to 2000 m. The overall correlation between these two independent depth estimates suggests that sedimentary stylolites can reliably be used as a depth gauge, independently of the thermal gradient. Beyond the method validation, our study also reveals some mechanisms of pressure solution and the associated p,T conditions favouring their development in carbonates. Full article

The Coqueiros Formation, a strategic stratigraphic unit within the Lagoa Feia Group (LFG) in the Campos Basin offshore Brazil, is known for its lacustrine carbonate deposits, which include both organic-rich shales and economically important “coquina” reservoirs. While coquina facies are widely recognized as reservoirs, the source-rock potential of the intercalated shales remains relatively underexplored. This study aims to characterize the mineralogy and thermal maturity of the Coqueiros Formation to assess its potential as a source rock, using a multi-technique approach integrating X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Raman spectroscopy analyses of shale samples from two wells: 3-BP-11-RJS and 6-DEV-18P-RJS. XRD analyses revealed a heterogeneous mineralogy dominated by carbonates (calcite and dolomite) and quartz, with significant contributions from clay minerals and trace minerals such as pyrite and barite. SEM imaging revealed a heterogeneous fabric with grain size, morphology, and porosity variations, reflecting a dynamic lacustrine depositional setting influenced by storm events and fluctuations in terrigenous input. The presence of authigenic minerals, as reported in other studies, such as saddle dolomite, mega-quartz, and various sulfides, provides evidence for hydrothermal alteration, likely related to Late Cretaceous magmatic activity in the Campos Basin. Raman spectroscopy yielded equivalent vitrinite reflectance (Ro%) values consistently exceeding 1.00, ranging from 1.03 to 1.40, indicating that the organic matter in the Coqueiros Formation shales has attained a high thermal maturity level, surpassing the oil window and reaching the condensate wet gas zone. The mineralogical and equivalent maturation data presented herein provide a valuable foundation for future studies, highlighting the complexity and heterogeneity of the Coqueiros Formation and its potential significance as a source rock within the Campos Basin petroleum system. Full article