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Siliceous minerals of the Dengying Formation in the Gaoshiti–Moxi area in the central Sichuan Basin exhibit four types of quartz crystals (cryptocrystalline quartz, chalcedony, microcrystalline quartz, and megacrystalline quartz) and three structural types: cryptocrystalline, microcrystalline, and mosaic (laminated mosaic, window-hole interrupted mosaic, and arc-laminated mosaic). Siliceous minerals have a great influence on the storage performance of the reservoirs in the Dengying Formation. According to the petrophysical parameters of the Dengying Formation and porosity intersection diagrams, the siliceous dolomite and the reservoirs have low impedance characteristics, which makes it difficult to distinguish between them and leads to difficulties in the characterization and prediction of the reservoirs. The transverse wave velocity is favorable for reservoir characterization. Currently, the main method used to estimate the transverse wave velocity is petrophysical modeling, which establishes a relationship between the elastic and physical parameters of the reservoir. In this paper, the siliceous minerals in the dolomite in the study area are regarded as solid inclusions, and the calculation method of the rock matrix modulus is improved by using solid replacement. Then, an improved petrophysical model is constructed by combining the KT (Kuster–Toksöz) model, the DEM (Discrete Element Method) model, the Gassmann equation, and the Wood equation. The transverse wave velocity is estimated using the improved model under the constraint of the longitudinal wave velocity. The shapes of the transverse wave velocity curves obtained by the improved model and the deviations from the measured velocities are significantly better than those of the Xu–Payne model and other models. The results show that the improved model can effectively estimate the transverse wave velocity of the reservoir in this area, which provides a basis for future reservoir predictions in this area. [ABSTRACT FROM AUTHOR]

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This paper investigates the origin of ultra-deep dolostone and the factors influencing large-scale dolostone reservoirs, focusing on the Sinian Dengying Formation and the Cambrian Longwangmiao Formation in the Sichuan Basin. The study involves petrology, microscale X-ray diffraction, trace element analysis, and C-O-Sr-Mg isotope experiments to provide a detailed analysis. The research findings indicate that the Dengying and Longwangmiao formations comprise six types of matrix dolostone and four types of cement. The Dengying Formation, which developed under a sedimentary background of a restricted platform, contains special microbial and microcrystalline dolostones. The dolomite grains are small (<30 µm) and have a low order degree (Min=0.55), with large unit cell parameters and an extremely high Na content (Max=788 ppm). The 87Sr/86Sr value of the dolostone is consistent with contemporaneous seawater, while the δ13C and δ18O values are lower than those of the contemporaneous seawater. The δ26Mg value is small (Min=−2.31‰). Powder crystal, fine-crystalline, and calcite dolostones with coarser and more ordered crystals exhibit similar δ13C and 87Sr/86Sr values to microbial and microcrystalline dolostone. During the sedimentary period of the Dengying Formation, ancient marine conditions were favorable for microbial survival. Microorganisms induced the direct precipitation of primary dolomite in seawater, forming microbial and microcrystalline dolostones during the seawater diagenesis period. During the subsequent diagenesis period, dolostones underwent the effects of dissolution-recrystallization, structures, and hydrothermal fluids. This resulted in the formation of dolostone with coarser crystals, a higher degree of order, and various types of cement. The Longwangmiao Formation was developed in an inter-platform beach characterized by special particle dolostone. The particle dolostone has a large grain size (>30 µm), high order degree (Min=0.7), small unit cell parameters, high Na content (Max=432 ppm), and low Fe and Mn content. The δ26Mg and δ13C values are consistent with the contemporaneous seawater, while the δ18O and 87Sr/86Sr values are higher than those of the contemporaneous seawater. There is mutual coupling between multiple-period varying δ26Mg values and sedimentary cycles. The dolostone in the Longwangmiao Formation resulted from the metasomatism of limestone by evaporated seawater. The thickness and scale of the dolostone in the Longwangmiao Formation are controlled by the periodic changes in sea level. The period of dolostone development from the Sinian to the Cambrian coincides with the transition from Rodinia's breakup to Gondwana's convergence. These events have resulted in vastly different marine properties, microbial activities, and sedimentary climate backgrounds between the Sinian and the Cambrian. These differences may be the fundamental factors leading to the distinct origins of dolostone formed in the two periods. The distribution of sedimentary facies and deep tectonic activities in the Sichuan Basin from the Sinian to the Cambrian is influenced by the breakup and convergence of the supercontinent. This process plays a key role in determining the distribution, pore formation, preservation, and adjustment mechanisms of ultra-deep dolostone reservoirs. To effectively analyze the genesis and reservoir mechanisms of ultra-deep dolostone in other regions or layers, especially during the specific period of supercontinent breakup and convergence, it is crucial to consider the comprehensive characteristics of seawater properties, microbial activities, sedimentary environment, and fault systems driven by tectonic activities. This can help predict the distribution of high-quality and large-scale ultra-deep dolostone reservoirs. [ABSTRACT FROM AUTHOR]

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Copyright of Natural Gas Industry is the property of Natural Gas Industry Journal Agency and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Natural Gas Industry is the property of Natural Gas Industry Journal Agency and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Natural Gas Industry is the property of Natural Gas Industry Journal Agency and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Natural Gas Industry is the property of Natural Gas Industry Journal Agency and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Natural Gas Industry is the property of Natural Gas Industry Journal Agency and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Natural Gas Industry is the property of Natural Gas Industry Journal Agency and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Natural Gas Industry is the property of Natural Gas Industry Journal Agency and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Natural Gas Industry is the property of Natural Gas Industry Journal Agency and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The effective mass transfer coefficient of hydrogen ion is an important basic parameter of acid rock reaction kinetics, and it is an essential basic parameter for acid fracturing design of carbonate reservoir. It is usually measured by rotating rock disk instrument or conventional size parallel rock plate flow test, but the rotating rock disk instrument cannot simulate the reaction of acid in the formation, and there is a deviation in the test results of high viscosity liquid. In the parallel plate flow experiment, if the size of rock plate is short, then calculating the hydrogen ion mass transfer coefficient will cause large error by using the difference of the rock plate quality or the difference of acid concentration. In this paper, a new method for measuring H+ effective mass transfer coefficient in fractures by large scale linear dynamic displacement is designed. Three-dimensional laser scanning of large size rock plates before and after peracid is carried out, and the acid etching volume of the rock plates is measured, then the acid consumption rate and H+ effective mass transfer coefficient are calculated. In this paper, the H+ effective mass transfer coefficients of gelling acid and diverting acid in the dolomite of west Sichuan permian Qixia formation are tested, which are between (1.50-2.31)×10-5 m/s. The results provide key dynamic parameters for fracture simulation, acid fracturing design and acid fracturing effect evaluation. [ABSTRACT FROM AUTHOR]

Lacustrine sedimentary formations potentially contain hydrocarbons. The lacustrine sedimentary rocks of the Ziliujung and Xintiangou Formations have been investigated for their hydrocarbon potential using low-pressure nitrogen adsorption (LP-N2A), low-field nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), total organic carbon (TOC), rock-eval pyrolysis (Rock-Eval), gas chromatography (GC), gas chromatography–mass spectrometry (GC–MS), X-ray fluorescence spectrometry (XRF), and inductively coupled plasma mass spectrometry (ICP-MS). The results show that the normalized difference of the pore parameters between the two formations is less than 10%, and the pores are mainly slit-like mesopores with high porosity. Macropores and micropores are often developed in the quartz skeleton, while mesopores often occur among organic matter, clay minerals, carbonate minerals, and pyrite particles. The organic matter abundance of the Ziliujing Formation is relatively high. Additionally, the organic matter types of the two formations are mainly type II and type III, and the sources of the organic matter are plankton and bacteria which have reached the mature gas production stage. The palaeoenvironmental differences between the depositional periods of the two formations lie within 10% of each other. The warm and humid climate promotes the development of quartz minerals to further enhance the proportion of both micropores and macropores, and the clay minerals, carbonate minerals, and pyrite carried in the terrigenous detritus are closely associated with the total organic carbon (TOC), which promotes the development of mesopores to enhance the porosity. The reservoir, organic matter, and palaeoenvironmental characteristics of fine-grained sedimentary rocks in the two formations are similar, and both of them have good potential for development. The above results provide a basic geological theoretical basis for unconventional oil and gas exploration in the northeastern margin of the Sichuan Basin. [ABSTRACT FROM AUTHOR]

Pre-stack simultaneous inversion is a seismic exploration technology which is widely applied to recognize lithology and detect oil and gas by analyzing the feature of amplitude variation versus offset (AVO). A gas-bearing reservoir can cause an obvious change in P-impedance and density. To effectively discriminate gas-bearing dolomite within Longwangmiao Fm. in Sichuan Basin of China, the pre-stack inversion method is employed by strict quality procedures. These procedures mainly include rock physics analysis, AVO modeling and pre-stack simultaneous inversion. Using the technology, P-impedance, S-impedance and density are obtained simultaneously. The distribution of gas-bearing dolomite is identified by P-impedance versus density cross-plotting from well data. The final prediction result indicates the existence of gas-bearing dolomite in the upper part of Longwangmiao Fm. in the study area, which is consistent with well data. Generally, pre-stack simultaneous inversion represents an effective method to predict gas-bearing dolomite reservoir and should be applicable in other study areas. [ABSTRACT FROM AUTHOR]

The Middle Permian Maokou Formation in the Sichuan Basin has huge resources and is an important target for natural gas exploration. In recent years, significant exploration breakthroughs have been made in the dolomite field of member Mao-2 in central Sichuan, and the gas production of several wells has exceeded 1 × 106 m3/d, indicating promising prospects for exploration. However, the origin of the dolomite reservoir in member Mao-2 remains ambiguous, which restricts the accurate prediction of favorable reservoirs. This study focuses on drilling in the Hechuan area as its research object, by using a detailed description of the cores from member Mao-2 of seven wells; samples were selected for tests of the degree of dolomite cation ordering, stable carbon and oxygen isotopic compositions, strontium isotopic composition, rare earth elements, LA-ICP-MS element mapping and U-Pb dating. It is clarified that: (1) The crystalline dolomite of member Mao-2 in the Hechuan area is the main reservoir rock, and the heterogeneous vugs and fractures are the main reservoir space. The dolomite in member Mao-2 has been characterized by a low degree of cation ordering value (avg. 0.59), with values of δ13C (avg. 3.87‰), δ18O (avg. −7.15‰) and 87Sr/86Sr (avg. 0.707474) having similar geochemical characteristics to Middle Permian seawater; the REEs normalized distribution patterns have similar characteristics to limestone; and the U-Pb age (261.0~262.0 Ma) corresponds to the age in the Capitanian stage of the Permian Guadalupian Series. (2) Petrological studies show that member Mao-2 has vertical karstification zonation characteristics; syngenetic karstification controls the formation of a large-scale fracture-cave system in the phreatic zone; the dolomitization of sediment in the fracture-cave system occurred during the penecontemporaneous period with locally restricted seawater. (3) The main controlling factors of the reservoir were syngenetic karstification, early dolomitization and hydrothermal dissolution related to Emei taphrogenesis. The research results are of great significance for dolomite reservoir prediction; the highlands of paleogeomorphology with syndepositional faults are favorable areas for dolomite reservoirs. [ABSTRACT FROM AUTHOR]

In this paper, the genesis of dolomite in Longwangmiao Formation is analysed through field investigation and geochemical methods, and dolomization model is established, which is very important for the next reservoir prediction in Longwangmiao Formation. According to the crystal size, the Longwangmiao Formation dolomite can be divided into two types: microcrystalline dolomite (MID) and macrocrystalline dolomite (MAD). Both types of dolomite have higher Al content than lime mudstone, indicating that during the deposition process of the Longwangmiao Formation, terrigenous materials are continuously mixed into the sediment, which significantly affects the geochemical characteristics of the Longwangmiao Formation. The rare earth element (REE) composition of the two types of dolomite is consistent with that of lime mudstone (LM), with an obvious δCe positive anomaly and no δEu anomaly, which is a typical distribution pattern of marine carbonate sediments. The microcrystalline dolomite has a higher Na element content, lower δ13C value, and higher δ18O value, suggesting that it is formed in a highly saline evaporation environment. Most of the macrocrystalline dolomite has a residual grain texture. The elements, as well as the isotopic geochemical characteristics of the dolomite, are consistent with those of lime mudstone, indicating that macrocrystalline dolomite is formed early and inherits the geochemical characteristics of the precursor limestone. We believe that the dolomitization model of the Longwangmiao Formation is an evaporative concentration + reflux seepage model. In the late sedimentary period of the Longwangmiao Formation, the sea level declined and a large area of lagoon appeared in the restricted and semi-restricted platform. As the evaporation and concentration continued, precursor minerals such as high-magnesium calcite and aragonite mud experienced chemical precipitation. In this process, a large amount of Ca2+ was consumed, resulting in an increase in the Mg2+/Ca2+ ratio. Then, the precursor minerals underwent dolomitization, and microcrystalline dolomite began to form. At the same time, owing to the difference in density between the high-salinity concentrated seawater and the pore water existing in the early limestone, the concentrated seawater flowed back and penetrated downward, causing the dolomitization of the underlying strata, and then the formation of macrocrystalline dolomite. The degree of dolomitization was mainly affected by the development degree of the granule beach and the thickness of the strata. • The highlights of this article are as follows: • This paper provides a case study of terrigenous clastic dolomite. • This paper analyses the genesis of the dolomites of Longwangmiao Formation and establishes a dolomitization model. • We analysed a large number of samples and provided basic geochemical data for Longwangmiao Formation. [ABSTRACT FROM AUTHOR]

The thick Upper Cambrian Xixiangchi dolostones, developed in the Sichuan Basin, are an important deep exploration target, but their genesis is still controversial, which hinders predicting the porous dolomite distribution and related potential hydrocarbon play. Herein, based on the observation and sampling of field outcrops, combined with a microscopic thin section analysis, cathodoluminescence analysis, and geochemical study, their characteristics and genesis were investigated. The results showed that there are mainly three types of dolomite that can be distinguished: (1) fine crystalline dolomite with a low crystallinity (Type 1); (2) granular dolomite with coarse grains, maintaining the original particle structure (Type 2); and (3) grain-texture relict dolomite with a higher degree of crystal form and obvious recrystallization (Type 3). The Type 1 dolomite with a common lamina structure and the highest 87Sr/86Sr ratio implies the most continental-influenced seawater in a supratidal environment around paleouplift, where there is an evaporation pump effect in its formation. The Type 2 dolomite recorded a slightly higher diagenesis temperature and slightly lower brine salinity, which would be formed in a seepage-reflux model in the beach environment of the platform. The lowest REY content and higher dolomite temperature with structural residuals indicate that the Type 3 dolomite is the result of further burial dolomitization during the diagenetic process. [ABSTRACT FROM AUTHOR]

Marine organic-rich shale is widely distributed in the Upper Ordovician Wufeng Formation (WF-F) and Silurian Longmaxi Formation (LMX-F), making it an important target for shale gas exploration and development. In order to clarify the paleoenvironment evolution characteristics and the effect of depositional environment on organic matter (OM) accumulation of the marine shale in the Wufeng and Longmaxi Formations, a series of geochemical and petrological experiments were carried out, including TOC, Rock-Eval pyrolysis, XRD, and major and trace element analyses. Research results show that based on the variation characteristics of TOC, mineral composition, and paleoenvironment evolution characteristics, four units can be identified from bottom to top: Wufeng Formation (WF-F), Lower Longmaxi Formation (L-LMX-F), Middle Longmaxi Formation (M-LMX-F) and Upper Longmaxi Formation (U-LMX-F). The high-quality marine shale developed in WF-F and LMX-F in the Zigong area (TOC: 0.65–4.56%, avg. 2.15%) contains type I kerogen (kerogen type index: 86.0–98.3, avg. 92.7) and OM in mature stage (average of Rb and Tmax are 2.94%, 646 °C, respectively). Clay minerals (avg. 42.5%) and quartz (avg. 37.7%) dominate the mineral compositions, with subordinated dolomite (avg. 6.3%), feldspar (avg. 6.0%), calcite (avg. 4.0%), and pyrite (avg. 3.5%). Paleoenvironment indicators suggest that during the sedimentary period of WF-F and L-LMX-F, the paleoclimate condition was humid; the weathering condition, paleosalinity, and redox conditions were the strongest; and there was a relatively high level of paleoproductivity and a relatively low level of terrigenous detritus influx. However, during the period of M-LMX-F and U-LMX-F, the climate gradually changed from warm and humid to hot and dry; the intensity of weathering conditions, paleosalinity, and redox conditions was relatively reduced; terrigenous detritus influx increased; and the paleoproductivity decreased. Relationships between TOC and paleoclimate condition, paleosalinity, redox condition, paleoproductivity, and terrigenous detritus influx suggest that redox condition is most important controlling factor for OM enrichment. A combination of anoxic bottom water conditions and high primary productivity and a relatively low terrigenous input resulted in the enrichment of OM in the WF-F and L-LMX-F, making it a potential exploration and development target. The research can provide scientific guidance for the selection of potential shale gas development targets in the Zigong area. [ABSTRACT FROM AUTHOR]

Microbialite textures, such as microbial mats and biofilms, were observed in the Middle Triassic dolomite in the Sichuan Basin, western China, using core examination, thin section petrography, scanning electron microscopy (SEM), and geochemical analyses. The dolomite texture, consisting of fibrous and spherulitic structures, is similar in morphology and size distribution to those observed in microbial culture experiments. Extracellular polymeric substances (EPS) were identified based on the occurrence of fibers forming a reticular pattern and nanometer-sized spheroids. The rare earth element (REE) and stable isotope (C, O, and Sr) compositions of the Middle Triassic dolomite were measured to determine their geochemical characteristics. Using seawater as a standard, the dolomitic microbialites (MD) exhibited significantly positive La and Eu anomalies and higher REE concentrations and (Nd/Yb)sn values than associated limestones, and these patterns are inferred to be related to initial complexation on organic ligands in the biofilm, as proposed by previous researchers. The ambient temperature during dolomite precipitation was estimated to be within the 23 °C to 50 °C range, as indicated by the δ18O values of the dolomite. This study suggests that various microbial effects can significantly affect diagenetic processes in the Middle Triassic dolomite. [ABSTRACT FROM AUTHOR]

Clumped isotope thermometry is an effective tool which allows to understand the post-depositional alteration, including recrystallization and solid-state reordering, of carbonate rocks. However, as both processes are controlled by elevated temperatures, it is frequently a challenge to distinguish between the relative importance of these two processes in deeply buried carbonate successions (depth > 5000 m or formation temperature > 150°C). In this paper, we have established a clumped isotopic framework of paired dolomite and calcite in order to decipher the relative influence of solid-state reordering versus recrystallization and consequently offer an improved understanding of the mechanisms involved. The material studied was derived from two deeply buried successions (Jurassic-Ediacaran) drilled in the central Sichuan Basin of South China. The calcite and dolomite minerals from these cores were separated using buffered acetic acid, and based on a two-component mixing model, the carbon, oxygen and clumped isotopic (Δ 47) compositions of the two endmembers were determined. Based on an evaluation of solid-state reordering models, we conclude that micritic carbonates (dolomite and calcite) from the Jurassic to the Cambrian have been altered during burial by the geothermal gradient as determined by vitrinite reflectance. The Ediacaran micritic dolomite records a hydrothermal event in the transition between the Ediacaran and Cambrian. In contrast, the clumped isotopic temperature of Ediacaran saddle dolomites reflects subsequent hydrothermal activity during the Silurian and Devonian and shows compositional variations controlled by cooling rates. This study highlights the utility of the theoretical framework based on paired dolomite and calcite records to untangle overprinted signals of clumped isotopes in ancient carbonate rocks and gives insight to better understand the complexity of alteration processes in deeply burial carbonate successions. [ABSTRACT FROM AUTHOR]

Hydrothermal dolomite paleokarst reservoir is a type of porous carbonate reservoir, which has a secondary porosity and can store a large amount of oil and gas underground. The reservoir is formed by magnesium-rich hydrothermal fluids during the karstification and later stages of the transformation. Due to the strong heterogeneity and thin thickness of hydrothermal dolomite paleokarst reservoirs, it is a real challenge to characterize the spatial distribution of the reservoirs. In this paper, we studied the hydrothermal dolomite paleokarst reservoir in the Wolonghe gasfield of the eastern Sichuan Basin. First, based on detailed observations of core samples, the characteristics and storage space types of the dolomite reservoir were described. Secondly, the petrophysical parameters of the paleokarst reservoirs were analyzed, and then the indicator factor for the dolomite reservoirs was established. Thirdly, using the time–depth conversion method, the geological characteristics near boreholes were connected with a three-dimensional (3D) seismic dataset. Several petrophysical parameters were predicted by prestack synchronous inversion technology, including the P-wave velocity, S-wave velocity, P-wave impedance, and the hydrothermal dolomite paleokarst reservoir indicator factor. Finally, the hydrothermal dolomite paleokarst reservoirs were quantitatively predicted, and their distribution model was built. The 3D geophysical characterization approach improves our understanding of hydrothermal dolomite paleokarst reservoirs, and can also be applied to other similar heterogeneous reservoirs. [ABSTRACT FROM AUTHOR]

The Middle-Upper Cambrian Xixiangchi Formation in the Sichuan Basin is regarded as an important reservoir with great potential for hydrocarbon exploration. It is previously indicated that the Xixiangchi carbonates have experienced extensive dolomitization, however, the origin of dolomitizing fluids and the dolomitization mechanism still remain uncertain. In this study, a set of petrographic and geochemical examinations, including rare earth elements (REE) and isotopic (C, O, and Sr) compositions were used to trace the origins of dolomitizing fluids and associated diagenetic processes. The petrographic examination revealed three types of matrix dolomites (D1, D2, D3) and one cement saddle dolomite (SD). These phases have crystal size ranges of less than 30 μm (very fine to fine crystals, D1), 30–100 μm (fine to medium crystals, D2), 100–300 μm (medium to coarsely crystalline dolomite, D3), and 0.3–4 mm (fracture filling cements, SD), respectively. D1 is characterized by non to very weak luminescence, weakly negative Ce anomalies (Ce/Ce* = 0.84 ± 0.02), strongly negative Eu anomalies (Eu/Eu* = 0.65 ± 0.03), and high 87Sr/86Sr ratios (0.71062 ± 0.00122). In combination with δ13C (−1.5‰ ± 0.2‰) and δ18O (−9.7‰ ± 0.5‰) compositions, D1 is interpreted to be formed by penecontemporaneous dolomitization in the near-surface environment with seawater as the dolomitizing fluid. In contrast, D2 is characterized by intercrystalline pores, dirty crystal surfaces, similar δ13C (−1.4‰ ± 0.4‰) compositions but higher δ18O (−8.9‰ ± 0.7‰) compositions, and lower 87Sr/86Sr ratios (0.70992 ± 0.00035), similar Ce anomalies (Ce/Ce* = 0.87 ± 0.04) and higher Eu anomalies (Eu/Eu* = 0.85 ± 0.04). The coarser D2 is regarded to be formed by the post-penecontemporaneous seepage-reflux dolomitization or by recrystallization of D1 dolomite in a near-surface or shallow burial environment. D3 is distinguished by a cloudy core with clear rims, showing slightly higher Eu anomalies (Eu/Eu* = 0.88 ± 0.02) and similar Ce anomalies (Ce/Ce* = 0.88 ± 0.02) than those of D1 and D2. Combined with the δ18O compositions (−10.4‰ ± 0.4‰) and 87Sr/86Sr ratios (0.70989 ± 0.00048), D3 is thought to be formed by the overgrowth or recrystallization of D1 and D2 dolomites in a shallow to moderate burial environment. The fractures filling SD dolomite consists of nonplanar and much coarser crystals with undulatory extinctions and brighter red luminescence. The lower δ18O (−11.1‰ ± 0.3‰) compositions, lower negative Eu anomalies (Eu/Eu* = 0.70 ± 0.01) of SD than the matrix dolomites, and similar Ce anomalies (Ce/Ce* = 0.83 ± 0.01) are indicative of hydrothermal dolomitization, with possible fluids associated with the magma during the period of Emei taphrogenic movement. In addition, the 87Sr/86Sr ratios (0.70941 ± 0.00003) of SD suggest probable origin from the coeval seawater partially. Therefore, SD dolomite is interpreted to be formed by hydrothermal dolomitization with mixed dolomitizing fluid of seawater and hydrothermal fluids. In summary, all the matrix dolomites have almost the same ΣREE concentrations and exhibit similar near-flat REE partition patterns with weak LREE enrichments, weakly negative Ce anomalies, and negative Eu anomalies. Such characteristics of REE compositions are indicative of similar evolved dolomitizing fluid, such as seawater or seawater- derived fluids. By contrast, SD dolomites have a different REE partition pattern with left-leaning characteristics, LREE depletions, and negative Eu anomalies, thus suggesting a different dolomitizing fluid source from the matrix dolomites. In addition, the development of intercrystalline pores associated with D2 dolomite makes it more likely to be a potential reservoir, indicating that the dolomitizing history of carbonate has a strong influence on the quality of potential dolomite reservoirs. [ABSTRACT FROM AUTHOR]

The Cambrian strata of Sichuan Basin, China, present many dolomite-evaporite paragenesis systems (DEPS), which contain oil and gas resources for industrial exploitation. However, despite their wide distribution, the formation models and influencing factors of DEPS remain poorly understood. By analyzing an abundance of literature from China, this study comprehensively summarizes the sedimentary characteristics, combination patterns of rocks, and formation models of DEPS. The research significance, research questions, and future research directions of DEPS are also clarified. Firstly, the distribution of DEPS is clarified based on published studies: DEPS are widely distributed in the Longwangmiao Formation, the Gaotai Formation, and the Xixiangchi Formation of the eastern and southern Sichuan Basin. Evaporite and dolostone combination patterns in DEPS include the following modes: interbedding of dolostone and evaporite, evaporite overlying thick dolostone, thick dolostone mixed with thin evaporite, and thick evaporite mixed with thin dolostone. Secondly, five types of evaporites and five types of dolostones have been identified. Thirdly, spatiotemporal distribution, influencing factors, and developmental models are summarized. Studies on DEPS provide new insight into the origin of dolomite. Such research also provides theoretical guidance for the exploration of oil and gas resources in DEPS. [ABSTRACT FROM AUTHOR]

High-quality reservoirs in the third member of Lower Triassic Feixianguan Formation in the Jiannan area are exclusively restricted to dolostone, which makes it essential to understand origin of porosity in dolostone. However, the studies on the porosity origin of the dolostone in the Jiannan area are rare, and its origin is still controversial. In the paper, the formation mechanisms of porosity in replacement dolostones ever proposed worldwide are summarized, and they can be divided into two main categories based on the formation time of porosity relative to the dolomitization process: syndolomitization and postdolomitization origin. Studies on well cores that have been dolomitized to varying degrees indicate that the porosity in dolostones in the Jiannan area is a syndolomitization origin which is intimately associated with the dolomitization process, depending mainly on the degrees of dolomitization. Four stages of porosity evolution, each with distinctive petrographic and petrophysical properties, were distinguished. In initial stage of dolomitization (stage I), dolomite was formed mainly by volume for volume replacement with porosity constant or slight decrease due to external carbonate source added. In the next stages of progressive dolomitization (stage II), only minor intercrystalline pores were developed and the porosity slightly increased. In more advanced stage (stage III), abundant intercrystalline, as well as some vuggy pores, were formed by selectively dissolution of residual microcrystalline calcites and micritized grains, and the porosity remarkably increased with increasing dolomite content. In the most advanced stage (stage IV), dolomite cements were precipitated in pre-exiting pores in dolostone, resulting in varying decrease in porosity and permeability. Carbonates that must proceed at least in the stage III have the high porosity, while the dolostones in the stage II-2 that display similar rock frameworks only have a minor increase in porosity. [ABSTRACT FROM AUTHOR]

Thermochemical sulfate reduction (TSR) commonly occurred in the dolostone reservoirs of the Upper Permian Changxing Formation (P3c) in the Yuanba (YB) gas field, Sichuan Basin, yet controversy remains regarding the source of sulfate required for TSR. To trace the source of the sulfate, sulfur and strontium isotopic compositions were measured for three diagenetic celestite (SrSO4) samples found in the P3c dolostone reservoirs in the YB gas field. In addition, the sulfur isotopic compositions of Carbonate-associated sulfates (CAS) from the P3c carbonates and spheroidal pyrites in dolomicrites from the P/T boundary (PTB) in the YB gas field were measured for comparative studies. The results show that the sulfur isotopic compositions of celestites are significantly heavier than those of the contemporaneous seawater, and these celestites have strontium isotopic values consistent with those of the host dolostones and contemporaneous seawater. The −33.68‰ fractionation in average δ34S values between pyrites and celestites indicated that their formation was likely to be related to bacterial sulfate reduction (BSR). During the P/T extinction event that accompanied the end-Permian regression, the reflux of brine directly precipitated 34S-rich celestites in the fractures and vugs of the dolostone reservoirs due to the BSR and the evaporation of seawater. These 34S-rich celesites and associated pore fluids provide a new explanation for the source of sulfate required for the late TSR. [ABSTRACT FROM AUTHOR]

This paper focuses on celestite-carbonate-cemented veins that developed in the Jialingjiang Formation of the Lower Triassic carbonates from the Libixia anticline in eastern Sichuan Basin, China. Comparative petrography and geochemistry of the host rock and vein minerals provide clues to the nature, origin, and evolution of the vein-cementing fluids. The host rocks are composed mainly of micrite dolomite which yields δ18O, δ13C and 87Sr/86Sr ratios close to the values that characterize contemporaneous seawater. Two generations of vein-mineralization were recognized based on textural relationships, fluid inclusion data and isotope geochemistry. The early generation is dominated by celestite and saddle dolomite, formed from hypersaline, Sr-enriched, hydrothermal fluids. The later generation is represented by calcites, precipitated from less saline fluids at lower temperatures. The 87Sr/86Sr ratios of vein dolomite and celestite vary from 0.7076 to 0.7081, significantly lower than those of host carbonate counterparts (0.7078 to 0.7081; average 0.7080). This isotopic discrepancy suggests that the majority of Sr was sourced from deeper sequences of the lower Triassic carbonates, probably associated with thermochemical sulfate reduction for Sr accumulation. It is argued that the fracture systems associated with the Huayingshan fault provided the heat source and served as conduits for migration of hydrothermal fluids through several hundred meters, and the final mineralization occurred as a result of mixture of Sr-enriched brines with sulfate-bearing solutions. The large celestite ore bodies in the studied area are proposed to form related partially to these hydrothermal fluids. [ABSTRACT FROM AUTHOR]

The Cambrian Longwangmiao gas reservoirs in the Moxi area of the Sichuan Basin have a complex associated gas accumulation history. Based on core and thin section examination, fluid inclusion analyses and 1-D burial-thermal modelling, the diagenetic evolution and hydrocarbon accumulation processes in the Longwangmiao Formation have been reconstructed. Various diagenetic events were identified, making up a complete pore fill sequence as follows: solid bitumen/dolomite→ nonfluorescent solid bitumen → dolomite → quartz/yellow fluorescent oily bitumen → residual hole. Analyses of oil inclusions and bitumen-bearing inclusions are key to the understanding of the hydrocarbon accumulation processes. The Th values of the aqueous inclusions that are contemporaneous with hydrocarbon inclusions range from 74.3 to 214.3°C. In conjunction with burial-thermal history modelling results, the results indicate that there were two stages of oil charge and three stages of natural gas accumulation in the Longwangmiao carbonate reservoirs. The two stages of oil charge occurred in the Late Silurian and Middle Triassic, respectively. Three gas accumulation events occurred in the Middle to Late Triassic, Middle Jurassic to Early Cretaceous, and Late Cretaceous, respectively. [ABSTRACT FROM AUTHOR]

In the Triassic marine sediments, an obvious enrichment of lithium has been found. The source and enrichment mechanism of lithium is unknown. Here, we report trace and rare earth element and isotope analyses for Triassic sedimentary samples from core ZK601, recovered from the Huangjinkou anticline in the Xuanhan basin. Lithium concentrations from the Leikoupo and Jialingjiang formations are much higher than the average concentrations in the crust of eastern China and in other marine sediments. Lithium concentrations are highest at depths of 3300–3360 m (in argillaceous marine sediments), and Li is positively correlated with Rb, Ga, Zr, Nb and other trace elements. The range of δ7Li values in our samples is consistent with that in other Triassic marine carbonate rocks. Lithium concentrations and isotope ratios are negatively correlated in the argillaceous dolomite samples at depths of 3300–3360 m. We compared the results in this study with trace and rare earth elements in the clay from Sichuan and Chongqing, and propose that the clay in the argillaceous marine evaporites from Huangjinkou formed via the hydrolysis of volcanic ash during Early–Middle Triassic volcanic eruptions into brine basins, during which clay adsorbed Li from the brine and formed Li‐rich argillaceous dolomites. The addition and hydrolysis of volcanic ash in the evaporative brine is also related to the formation of a new type of polyhalite. [ABSTRACT FROM AUTHOR]

The ultra-deep dolomite reservoir of the Permian Qixia Formation in northwest Sichuan is important for hydrocarbon reservoirs, but the mechanism of dolomite genesis is complex and has long been controversial, thus requiring further elucidation. By studying the dolomite with different types in the Permian Qixia Formation, northwest Sichuan Basin, this paper, based on outcrop sections observation and core description, seeks to find out the dolomitizing fluids' source and the dolomite's genetic mechanism by analyzing the dolomite's rock-mineral and geochemical characteristics. Various technologies are applied, including rock thin section observation, cathode luminescence, carbon and oxygen isotope analyses, laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), and inclusion homogenization temperature and salinity. Results show that the dolomite is mainly classified into silty-fine dolomite (Md1), fine-medium dolomite (Md2), medium-coarse dolomite (Md3), and saddle dolomite (SD) filled in the pores. Md1 is formed in the early shallow-burial stage; the dolomitizing fluids are mainly derived from the early seawater sealed in carbonate sediments; and the penesaline seawater is formed by the seepage reflux under a moderate evaporitic environment. Md2 and Md3 are formed in the middle and late shallow-burial stages. Of these two types, Md2 is formed earlier, and the dolomitizing fluids are primarily derived from the seawater sealed in the early stage and partially derived from the residual evaporated seawater and hydrothermal fluids. In contrast, Md3 is formed later, and the dolomitizing fluids are the mixture of the seawater sealed in the early stage and the hydrothermal fluids formed in the late stage. In addition, Md3 was affected by the hydrothermal process more significantly during the formation. SD is deposited and formed when the saturated hydrotherm upwells along with the fracture system in the late stage. This study provides a significant reference for understanding the source and action mechanism of multi-stage dolomitizing fluids and expands the exploration field of the deep dolomite reservoir. • The precise microanalysis is conducted on the dolomite of different grain sizes by the LA-ICP-MS technology. • The geochemical characteristics of dolomites show the trend of continuous and progressive. • Dolomite fluids come from the normal seawater, penesaline seawater. and hydrothermal fluids. [ABSTRACT FROM AUTHOR]

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Based on comprehensive analyses of occurrence, petrological observation, pore structure and geochemistry, the different reservoir characteristics and reservoir evolutionary pathways between different oolitic shoal reservoir types of the Feixianguan Formation on the west side of the Kaijiang‐Liangping Trough have been studied. There exist three stages of high‐energy slope break belts in the Feixianguan period, the corresponding three stages of oolitic shoals gradually migrating in the direction of the trough. Three types of oolitic shoal reservoirs, namely, residual‐oolitic dolomite, mold‐oolitic dolomite and sparry oolitic limestone, were formed during sedimentary‐diagenetic evolution, the pore types being intergranular dissolved pore, mold pore (or intragranular dissolved pore) and residual intergranular pore, respectively. The petrology, physical properties and pore structure of the different types of oolitic shoal reservoirs are quite different. Residual‐oolitic dolomite reservoirs have the best quality, while sparry oolitic limestone reservoirs have the poorest. Combined with analyses of trace elements, rare earth elements and carbon‐oxygen isotopes, it is suggested that the formation of residual‐oolitic dolomite reservoirs is jointly controlled by penesaline seawater seepage‐reflux dolomitization and hydrothermal dolomitization. Mold‐pore oolitic dolomite reservoirs are controlled by penesaline seawater seepage‐reflux dolomitization and meteoric water solution. The burial dissolution of organic acid not only further improves the reservoir qualities of previously formed oolitic dolomite reservoirs, but also preserves residual intergranular pores in the sparry oolitic limestone reservoirs. [ABSTRACT FROM AUTHOR]

To solve the problem of multi-resulted in seismic responses brought by dual impacts of the thickness and the lithologic association for further research of seismic response characteristics of hydrothermal dolomite, significant efforts have been made, which were based on the study of dolomite, the Maokou formation in the eastern Sichuan Basin, to conduct an analysis of mineral composition, strata distribution and dolomitization in its area, to develop layered medium models in accordance with different dolomitization degrees and thicknesses, and to build standardized wedge models for the full display of that in binary changes. Moreover, an essential application of the pre-stack and post-stack seismic forward modeling have been performed for analyzing how different dolomitization degrees and thicknesses had influences on the seismic response characteristics of hydrothermal dolomite. Given this, the sensitive seismic attributes of the target interval in the study area have been selected preferentially by analyzing the seismic response characteristics of hydrothermal dolomite, which plays a critical role in the entire process. The results show that?1. The response characteristic of amplitude is greatly influenced by the dolomitization degree and bed thickness?2. The dolomitization degree constitutes a transformation of AVO response characteristics from II to IV, while the variation of thickness leads to a transformation of AVO response characteristics from IV to II. 3. The actual results indicate that the relative fluid factor FF is one of the most sensitive AVO attributes, and the predicted results are in par with the actual results. [ABSTRACT FROM AUTHOR]

Neoproterozoic marine dolomite cements represent reliable, albeit complex, archives of their palaeoenvironment. Petrological and high‐resolution geochemical data from well‐preserved fibrous dolomite and pyrite in the upper Ediacaran (ca 551·1 to 548·0 Ma) Dengying Formation in south‐west China are presented and discussed here. The aim of this research is to reconstruct the redox state of late Ediacaran shallow seawater and porewater in the Sichuan Basin using early marine diagenetic fabrics. Based on crystalline texture and axis, four basic types of fibrous dolomite cements formed penecontemporaneously in a microbialite reef setting at the platform margin: (i) bladed dolomites (replacement from a high‐Mg calcite precursor); (ii) fascicular fast dolomites (replacement from an aragonitic precursor); (iii) fascicular slow dolomites; and (iv) radial slow dolomites. The latter two fabrics are considered direct marine porewater precipitates due to their length‐slow character, cathodoluminescent zonation, and enriched copper and cobalt concentrations. Marine cements yield rare earth element and yttrium patterns comparable to modern seawater and represent a refined set of archive data relative to previously published bulk dolostones. Redox‐sensitive elements and cathodoluminescence indicate that the fascicular fast dolomites formed in suboxic seawater, while fascicular slow and radial slow dolomites formed in euxinic marine porewaters. Microbial sulphate reduction during the formation of fascicular slow and radial slow dolomites is recognized by nanometre‐scale spheroidal ankerite and sulphur‐containing dolomite, and intergrown pyrite grains with U‐shaped δ34S transects. Data shown here suggest predominantly suboxic shallow late Ediacaran seawater and euxinic marine porewaters, with microbial activity promoting the direct precipitation of dolomite. [ABSTRACT FROM AUTHOR]

Several types of dolomite had occurred pervasively in the lower Cambrian Qingxudong Formation in the Sichuan Basin. However, the origin of these dolomites remains particularly controversial until now. Here, we describe the petrographic and geochemical characteristics of dolomites from the lower Cambrian Qingxudong Formation in the southeastern Sichuan Basin, which can potentially provide useful information to constrain the origin and evolution of dolomitization fluid. Three fabric types and five dolomite generations were identified in this study: fabric-retentive dolomites (dolomicrite, Dol-1, and dolomitic microbialite, Dol-2), fabric-destructive dolomites (sucrosic dolomite, Dol-3) and pore-filling dolomites (medium- to coarse-crystalline dolomite, Dol-4, and saddle dolomite, Dol-5). The δ13C and δ18O values, 87Sr/86Sr ratios and rare earth element (REE) pattern of Dol-1, Dol-2 (except the δ13C values) and Dol-3 are approximately consistent with original seawater values, suggesting that the dolomitization fluids were mediated by seawater. The calculated dolomitization temperature (25.66 °C) and Z values (125.2) of Dol-1 with evaporite minerals indicate that the dolomitization fluid was high-salinity concentrated seawater derived from evaporation. The calculated dolomitization temperature (24.20 °C) and Z value (121.8) of Dol-2, and no traces of evaporite minerals in Dol-2 imply that the dolomitization fluid was slightly hypersaline seawater. The more negative δ13C and abundant microbial features of Dol-2 indicate that microbial mats were involved in dolomite formation. The dolomitization temperature (30.55 °C) and Z value (125) of Dol-3 also indicate that the dolomitization fluid was high-salinity concentrated seawater. However, Dol-3 is commonly distributed in reef and shoal facies (high porosity and permeability) throughout the Qingxudong Formation. Moreover, the abundant organic matters in the dolomitization fluid likely overcame the kinetic and thermodynamic barriers in the process of dolomitization. The geochemical data (except the δ18O values) of Dol-4 are approximately consistent with fabric-retentive dolomites, suggesting that the Dol-4 had been inherited from precursor carbonates. The negative δ18O values of Dol-4 revealed a further burial environment. High LREE/HREE ratios, positive europium anomalies and negative δ18O values of Dol-5 suggest that these dolomites were affected by hydrothermal fluids under deep burial conditions. Thus, Dol-1, Dol-2 and Dol-3 are likely attributed to sabkha, organogenic (microbial) and seepage-reflux dolomitization models during early-stage diagenesis, respectively. Dol-4 and Dol-5 are commonly attributed to burial and hydrothermal dolomitization models during deep burial diagenesis. This study provides new insights into the origin of dolomites and is especially useful for the further understanding of dolomite reservoir formation in the Qingxudong Formation. [ABSTRACT FROM AUTHOR]

Dolomitization of reservoir reefs is common at platform margins of the Changxing Formation in northeastern Sichuan Basin, southwestern China. We carried out a research on these variously dolomitized reef microfacies based on sedimentology and petrology theories and geochemical technologies. The results show that the reservoir reefs at the platform margins of the formation can be divided into several microfacies: reef flats/caps, reef cores, reef bases, etc. The reef flats contain mostly crystal grain dolomites, the reef cores are made up of largely (calcite) reef dolomites, and the reef bases are generally dolomite limestone/calcite dolomites. Samples taken from different microfacies of the reefs were measured and showed a relatively high content of Fe and Sr and low content of Mn, with Mn/Sr ranging between 0.057 and 2. Samples from the east flank of the Kaijiang–Liangping trough and the Exi–Chengkou trough have higher Fe and Mn contents and Mn/Sr ratios than those from the west flank of the Kaijiang–Liangping trough. Dolomites in reef flats have the highest Fe and Mn contents and Mn/Sr ratios, followed by dolomites from reef cores, and those from reef bases are found to have the lowest Fe and Mn contents. Isotopic analyses of the samples from different microfacies of the reefs indicate more negative δ13C and δ18O values than that of limestone in seawater during the Changxing deposition period. The δ18O values were increasingly more negative in the order of reef flat–reef core–reef base. Based on the study, we propose that the dolomitization processes the reefs at different platform margins experienced have the following features: (1) Meteoric water affected dolomites at the eastern flank of the Kaijiang–Liangping and Exi–Chengkou troughs more than those of the west flank of the Kaijiang–liangping trough. From the microfacies perspective, the influence of meteoric water decreases from reef flats to cores and then to bases. Apart from meteoric water, the dolomitization was also under control of seawater at the same period. (2) Meteoric water dolomitization is closely linked to sedimentary microfacies. Reef flats and cores with better porosity and permeability are more dolomitized than reef bases containing mostly micrite with bioclastics. (3) Dolomitization processes driven both by early mixing of water and "juiced" marine fluids during the shallow burial and compaction stage were all completed before the first stage of hydrocarbon emplacement. Hydrothermal dolomitization occurred locally and its contribution to the whole reef dolomitization of the formation may be negligible. [ABSTRACT FROM AUTHOR]

Combined elemental and isotopic geochemistry of dolomites in the lower Cambrian Longwangmiao Formation, eastern Sichuan Basin with petrographic observations has been investigated to determine the origin of these dolomites. Our data show that these dolomites are mostly located in the middle and upper parts of the Longwangmiao Formation, and most of them commonly occur as crystalline dolostones, whereas they less commonly occur as a dolograinstone. The dolomite contents of the dolostones range from 64 to 97%. The Mg/Ca ratios of these dolomites vary from 0.6 to 1.2, with an average of 0.8. These criteria divide the Longwangmiao dolomites into two types, according to either a positive correlation (type-A) or negative correlation (type-B), which exist between the MgO and CaO contents. Type-A dolomite consists of micritic-to-fine crystals (< 50 μm) of dull-to-no cathodoluminescence. It occurs in association with evaporites and relicts, and has no direct or indirect evidence of formation from carbonate precursors. These features suggest that the type-A dolomite has a parasyngenetic, evaporitic, and near-surface origin. Type-B dolomite consists mainly of medium-to-coarse (50–250 μm) crystals of zoned to uniform, red, or brown luminescence associated with relict textures. High Na concentrations, low Sr concentrations, and Sr/Ca ratios lower than those of type-A seem to indicate that the salinity of the dolomitizing fluid is similar to that of hypersaline seawater. The strongly positive correlation between δ13C and δ18O values, with the occurrence of karst caves and vugs, shows that the Type-B dolomite has been modified by meteoric water. Estimates from δ18O values suggest precipitation at depths of approximately 308 m. These proxies demonstrate that type-B dolomite formed during burial from seepage–reflux of hypersaline seawater, with a minor influence of meteoric water. Type-A dolomite is characterized by micropores and has a low permeability ranging from 0.00195 to 45.00262 × 10−3 μm2, with an average of 0.03730 × 10−3 μm2. It is volumetrically abundant throughout the region, and thus, it represents a potential gas reservoir. High-quality hydrocarbon reservoirs in the region are closely related to type-B dolomite, which is likely to occur in banks at the margins between open and restricted platforms. [ABSTRACT FROM AUTHOR]

The Upper Permian Changxing dolomite reservoirs serves as one of the most important gas and oil reservoirs in the NE Sichuan Basin. Determining the dolomitizing fluid's pathway is regarded as the key to solve the 'dolomite problem' and further petroleum exploration. Outcrop samples from Upper Permian Changhsingian Panlongdong Section were studied using oxygen isotopic analysis, cathodoluminescence (CL) and major element analysis, in an attempt to determine the migration path way and properties of the dolomitizing fluid. Of the Changxing dolomite, the δ O values ranged from -3.494‰ to -5.481‰, which decreased from the top layer to the bottom in the section; the MgO contents varied from 9.24% to 21.43%, CaO contents from 28.65% to 39.87%, the CaO/MgO ratio from 1.40 to 4.31 and the MnO contents from 0.004% to 0.009 8%. The MgO contents showed a downwardly decreasing trend in the section, while the CaO/MgO showed an opposite rule. All of the dolomites looked dull or dark when they were exposed to the electron beam of the cathodoluminescence device. None of the fine- to medium grained dolomite showed a banded structure. Given that dolomitizing fluid's salinity decreased during the dolomitization process in its pathway, we concluded that the dolomitizing fluid migrated downwardly in Changxing Formation after excluding the possibility of deep burial or meteoric-marine mixing-water influences. As the dolomitizing fluid's pathway has always been difficult to be determined in highly dolomitized Formation, this study showed an important application of oxygen isotope values in resolving this problem. [ABSTRACT FROM AUTHOR]

Resulting in dense lithology and poor physical properties, dedolomitization has detrimental effects on reservoir characteristics, so there has been little systematic research reported on the dedolomitization in the Carboniferous Huanglong Formation from the Eastern Sichuan basin. Through well drilling, well logging, thin-section authentication, and geochemistry tests, we studied for the first time the growth characteristics, regularities of distribution, genesis, and geological significance of dedolomitization in rock with typical fabric. From this study, we arrived at three conclusions: (1) the microstructure type divisions of dedolomite are as follows: A. a large micrite core and clear calcite rim, B. dolomite crystal, C. gypsum crystal, and D. rhombohedral pores; (2) rocks had low porosity and high density after dedolomitization in Eastern Sichuan, which is detrimental to reservoir development; and (3) micrite-microlite crystal dolomite and micrite-microlite crystal gypsum dolomite are the protoliths before dedolomitization. Sabkha is the primary sedimentary environment, in which the sabkha brine was kept within the layer and would become the main fluid of dolomitization in the high-quality dolomite reservoir from Eastern Sichuan. Analyzing the instances of dedolomitization has important geological significance in some ways, such as recovering the protolith, recovering the ancient environment, and discussing the relationships between diagenetic fluid properties and the reservoir's development. [ABSTRACT FROM AUTHOR]

Natural gas was found in Upper Permian Changxing dolomite reservoirs of the NE Sichuan Basin, China. Researching the mechanisms for dolomitization was essential for petroleum exploration. Outcrop samples from Upper Permian Changhsingian Panlongdong and Yanggudong sections were studied using statistical work, cathodoluminescence (CL), electron microprobe, major element analysis and isotope studies (δO and δC), in an attempt to determine the migration pathway and origin of the dolomitizing fluids. The dolomite content, average dolomite size, δO values and Mg/Ca ratios were all decreased downwardly in the two sections, indicating that the Changxing dolomitizing fluid migrated downwardly from the top of Changxing Formation (sediments). The Mg, Na, Fe, Mn and Sr elements were uniformly distributed in dolomite crystals which proved that the dolomites were formed in the same fluid without time interval. The element content in Changxing dolomites was similar to that of the Feixiangan sakbha dolomites and the δO values of Changxing dolomite were much higher than those of the Changxing limestone, which suggested that the dolomitizing fluid was originated from evaporative seawater. In conclusion, we concluded that the coeval Changhsingian evaporative seawater caused dolomitization. [ABSTRACT FROM AUTHOR]

Origin of dolostone remained a controversial subject, although numerous dolomitization models had been proposed to date. Because of the dolomitization's potential to be hydrocarbon reservoirs, one debatable issue was the role of dolomitization in porosity construction or destruction. Based upon case studies of dolostone reservoirs in various geological settings such as evaporative tidal flat (Ordos Basin, NW China), evaporative platform (Sichuan Basin, SW China), and burial and hydrothermal diagenesis (Tarim Basin, NW China), here we systematically discuss the origin of porosity in dolostone reservoirs. Contrary to traditional concepts, which regarded dolomitization as a significant mechanism for porosity creation, we found two dominant factors controlling reservoir development in dolostones, i.e., porosity inherited from precursor carbonates and porosity resulted from post-dolomitization dissolution. Actually, dolomitization rarely had a notable effect on porosity creation but rather in many cases destroyed pre-existing porosity such as saddle dolostone precipitation in vugs and fractures. Porosity in dolostones associated with evaporative tidal flat or evaporative platform was generally created by subaerial dissolution of evaporites and/or undolomitized components. Porosity in burial dolostones was inherited mostly from precursor carbonates, which could be enlarged due to subsequent dissolution. Intercrystalline porosity in hydrothermal dolostones was either formed during dolo- mitization or inherited from precursor carbonates, whereas dissolution-enlarged intercrystalline pores and/or vugs were usually interpreted to be the result of hydrothermal alteration. These understandings on dolostone porosity shed light on reservoir prediction. Dolostone reservoirs associated with evaporative tidal flat were laterally distributed as banded or quasi-stratified shapes in evaporite-bearing dolostones, and vertically presented as multi-interval patterns on tops of shallowing-upward cycles. Dolostone reservoirs associated with evaporative platform commonly occurred along epiplatforms or beneath evaporite beds, and vertically presented as multi-interval patterns in dolostones and/or evaporite-bearing dolostones of reef/shoal facies. Constrained by primary sedimentary facies, burial dolostone reservoirs were distributed in dolomitized, porous sediments of reef/shoal facies, and occurred vertically as multi-interval patterns in crystalline dolostones on tops of shallowing-upward cycles. Hydrothermal dolomitization was obviously controlled by conduits (e.g., faults, unconformities), along which lenticular reservoirs could develop. [ABSTRACT FROM AUTHOR]