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Microbialite reservoirs are of great importance in oil and gas exploration. However, there is still a lack of comprehensive studies on the formation mechanisms of thrombolitic reservoirs, a specific type of microbialite. This research focuses on the oldest thrombolitic dolomite reservoir located within the Precambrian Dengying Formation in the central Sichuan Basin, southwestern China. A multi-disciplinary approach was employed to characterize different thrombolite facies and elucidate the formation mechanism of thrombolitic dolomite reservoirs and their controlling factors, involving core observation, thin-section analysis, cathodoluminescence, scanning electron microscope (SEM) microscopy, elemental analysis using LA-ICP-MS, and carbon and oxygen stable isotope analysis. Based on variations in texture, four types of thrombolite were identified: 1) distinct clotted thrombolite, 2) diffuse and regular clotted thrombolite, 3) diffuse and irregular clotted thrombolite, and 4) composite clotted thrombolite. Notably, the diffuse clotted thrombolitic dolomite is the prevalent lithology in the reservoir. Through modification by meteoric water, organic acid fluids, and hydrothermal fluids, a reservoir with dominant porosity in the form of primary growth-framework pores, dissolution pores, and vugs was formed. This resulted in the development of two high-quality reservoir intervals within the Second Member and at the top of the Fourth Member of the Dengying Formation. The growth-framework porosity of the thrombolites, epigenetic karstification, and tectonic fracturing were mainly conducive to reservoir development. However, various types of cementation have reduced porosity and connectivity within the reservoir. Overall, this study is a valuable example of the methodology required to understand meso- and microstructures of deep-buried thrombotic dolomite reservoirs, including their heterogeneities and diagenesis, as the original structures influence diagenesis. [ABSTRACT FROM AUTHOR]

In recent years, studying dolomite diagenesis and controlling factors of reservoir development from microscopic perspective has become a hot subject in deep carbonate gas exploration. In this study, we have carried out a fine classification of different types of dolomite reservoirs in the Fei-3 Member of the Lower Triassic Feixianguan Formation in the Puguang area, and the distribution law and main controlling factors of dolomite reservoirs have been clarified. The results show that the dolomite reservoirs of the Fei-3 Member in the Puguang area include three rock types: residual structure dolomite, microbial dolomite, and crystalline dolomite, and the main reservoir space types are intergranular pores, solution pores/caves, and inter-crystalline pores, respectively, and the solution fractures are mainly developed in the residual structure and microbial dolomites. Most of the dolomite reservoirs are distributed in the Fei 31 sub-member, and the residual structure dolomite is mainly distributed in the relative paleomorphic highs of the Fei 31 sub-member; microbial dolomite is mainly developed in the platform environment with shallow water and intermittent exposure, and is controlled by the growth conditions of microorganisms; the crystalline dolomite is mostly distributed in the relatively shallow water and relatively limited dolomite flat environment. On the plane, the dolomite reservoirs are distributed in the form of clumps. The distribution of residual structure dolomite reservoir is controlled by paleotopography and early exposure corrosion. Microbial dolomite is formed in the microbial mat microfacies, and the distribution range of microbial mat controls the development of microbial dolomites. The diagenetic types of the dolomites in the target layer include dolomitization, dissolution, rupture, and material filling. Various kinds of pores formed by syngenetic dissolution provide channels for later buried dissolution, and organic acids formed by organic matter maturation under deep burial conditions play a crucial role in dissolution of dolomite and promote the formation of dissolution pores. In the burial stage, hydrocarbons enter the dolomite, which can effectively prevent the growth of late authigenic minerals, thus promoting the preservation of pores. The structural fracture system formed by late tectonic movement can communicate with different pore groups and improve the reservoir quality.

The genesis of deep-to-ultra-deep dolomite reservoirs in the Tarim Basin is crucial for exploration and development. The Cambrian subsalt dolomite reservoirs in the Tarim Basin are widely distributed, marking significant prospects for ultra-deep reservoir exploration. Based on big data methodologies, this study collects and analyzes porosity and permeability data of carbonate reservoirs in the western Tarim Basin, specifically targeting the Cambrian deep-oil and gas-reservoir research. Through an examination of the sedimentary evolution and distribution of carbonate–evaporite sequences, and considering sedimentary facies, stratigraphic sediment thickness, fault zone distribution, and source-reservoir assemblages as primary reference factors, the study explores the macro-distribution patterns of porosity and permeability, categorizing three favorable reservoir zones. The controlling factors for the development of Cambrian carbonate reservoirs on the western part of the Tarim Basin are analyzed from the perspectives of sedimentary and diagenetic periods. Factors such as tectonic activity, depositional environment, microbial activity, and pressure dissolution are analyzed to understand the main causes of differences in porosity and permeability distribution. Comprehensive analysis reveals that the porosity and permeability of the Series2 carbonate reservoirs are notably high, with extensive distribution areas, particularly in the Bachu–Tazhong and Keping regions. The geological pattern of "Three Paleo-uplifts and Two Depressions" facilitated the formation of inner-ramp and intra-platform shoals, creating conducive conditions for the emergence of high-porosity reservoirs. The characteristics of reservoir development are predominantly influenced by diagenetic and tectonic activities. The Miaolingian is chiefly affected by diagenesis, featuring high permeability but lower porosity and smaller distribution range; dolomitization, dissolution, and filling processes under a dry and hot paleoclimate significantly contribute to the formation and preservation of reservoir spaces. In the Furongian, the Keping and Bachu areas display elevated porosity and permeability levels, along with substantial sedimentary thickness. The conservation and development of porosity within thick dolomite sequences are mainly governed by high-energy-particulate shallow-shoal sedimentary facies and various dissolution actions during diagenesis, potentially indicating larger reserves. [ABSTRACT FROM AUTHOR]

The evaluation of the pore structure in dolomite, particularly with regard to pore heterogeneity, geometry, and connectivity, is crucial for oil and gas field production and reservoir prediction. The subsalt dolomite reservoir in the Ordovician strata of the Ordos Basin has shown promising exploration results and is anticipated to have a high hydrocarbon potential. However, there has been limited research on the pore structure and primary controlling factors of the Ordovician Majiagou reservoir in the south-central Ordos Basin. Therefore, we conducted a comprehensive analysis of the pore structure and fractal characteristics using routine petrophysical measurements, thin-section analysis, and high-pressure mercury injection (HPMI) data. We also discussed the relationship between fractal dimension, reservoir physical properties, and pore structure, along with exploring the origin of potentially prolific reservoirs. Our observations from the thin section identified four main pore types: intercrystalline pores, intercrystalline dissolved pores, dissolved pores, and micro-fractures. The data from HPMI revealed that the average pore-throat radii range from 0.009 μιτι to 0.015 μιτι with porosity ranging from 0.4% to 5.26%, and permeability ranging from 0.011 mD to 0.059 mD. They were further categorized into three reservoir types: dissolved pore type, intra-crystalline (dissolved) pore type, and micro-porous type. The fractal dimension was calculated based on HPMI data, and the reservoir's fractal characteristics were divided into two segments. The dissolved pore type was identified as the potentially prolific reservoir due to its larger pore size and volume, moderate permeability, and homogeneity on pore structure. Additionally, the fractal dimension is negatively correlated with porosity and permeability and positively correlated with sorting coefficient and skewness, suggesting that fractal dimensions are valuable for evaluating reservoir quality and quantitatively characterizing pore networks. [ABSTRACT FROM AUTHOR]

The complex seismic responses of heterogeneous reservoirs can be related to the fabric structure, pore/microcrack shape, mineral composition and fluid distribution of the rock in situ. The pore structure refers to the geometric shape, size, spatial distribution and interconnectedness of pores, microcracks and throats. It is closely related to the storage space of reservoirs and the spatial distribution of oil/gas. Understanding the pore structure is crucial for the development of processes to increase oil/gas production capacity. Six dolomite samples from the Gaoshiti-Moxi Longwangmiao Formation are sorted out for measurements, and the ultrasonic and seismic attenuation are determined by using the spectral ratio method and the enhanced frequency shift method, respectively. When predicting the pore structure, we assume that the aspect ratio and volume fraction of pores and microcracks correspond to a normal distribution. On this basis, a model with the Voigt–Reuss–Hill average (VRH), differential effective medium (DEM) theory and infinituple-porosity media (IPM) theory is proposed. The acoustic wave responses in terms of reservoir porosity and standard deviation of normal distribution are analyzed, and multiscale 3D rock physics templates (RPT) are created. The calibrations of the templates are performed with the ultrasonic and seismic data, and then the templates are applied to the field data. The results show that the estimated porosity and pore structure (corresponding to the mean aspect ratio and standard deviation of a normal distribution, respectively) are in substantial agreement with the log data and the actual gas production results. [ABSTRACT FROM AUTHOR]

Microbialite reservoirs are of great importance in oil and gas exploration. However, there is still a lack of comprehensive studies on the formation mechanisms of thrombolitic reservoirs, a specific type of microbialite. This research focuses on the oldest thrombolitic dolomite reservoir located within the Precambrian Dengying Formation in the central Sichuan Basin, southwestern China. A multi-disciplinary approach was employed to characterize different thrombolite facies and elucidate the formation mechanism of thrombolitic dolomite reservoirs and their controlling factors, involving core observation, thin-section analysis, cathodoluminescence, scanning electron microscope (SEM) microscopy, elemental analysis using LA-ICP-MS, and carbon and oxygen stable isotope analysis. Based on variations in texture, four types of thrombolite were identified: 1) distinct clotted thrombolite, 2) diffuse and regular clotted thrombolite, 3) diffuse and irregular clotted thrombolite, and 4) composite clotted thrombolite. Notably, the diffuse clotted thrombolitic dolomite is the prevalent lithology in the reservoir. Through modification by meteoric water, organic acid fluids, and hydrothermal fluids, a reservoir with dominant porosity in the form of primary growth-framework pores, dissolution pores, and vugs was formed. This resulted in the development of two high-quality reservoir intervals within the Second Member and at the top of the Fourth Member of the Dengying Formation. The growth-framework porosity of the thrombolites, epigenetic karstification, and tectonic fracturing were mainly conducive to reservoir development. However, various types of cementation have reduced porosity and connectivity within the reservoir. Overall, this study is a valuable example of the methodology required to understand meso- and microstructures of deep-buried thrombolitic dolomite reservoirs, including their heterogeneities and diagenesis, as the original structures influence diagenesis.

The evaluation of the pore structure in dolomite, particularly with regard to pore heterogeneity, geometry, and connectivity, is crucial for oil and gas field production and reservoir prediction. The subsalt dolomite reservoir in the Ordovician strata of the Ordos Basin has shown promising exploration results and is anticipated to have a high hydrocarbon potential. However, there has been limited research on the pore structure and primary controlling factors of the Ordovician Majiagou reservoir in the south-central Ordos Basin. Therefore, we conducted a comprehensive analysis of the pore structure and fractal characteristics using routine petrophysical measurements, thin-section analysis, and high-pressure mercury injection (HPMI) data. We also discussed the relationship between fractal dimension, reservoir physical properties, and pore structure, along with exploring the origin of potentially prolific reservoirs. Our observations from the thin section identified four main pore types: intercrystalline pores, intercrystalline dissolved pores, dissolved pores, and micro-fractures. The data from HPMI revealed that the average pore–throat radii range from 0.009 μm to 0.015 μm with porosity ranging from 0.4% to 5.26%, and permeability ranging from 0.011 mD to 0.059 mD. They were further categorized into three reservoir types: dissolved pore type, intra-crystalline (dissolved) pore type, and micro-porous type. The fractal dimension was calculated based on HPMI data, and the reservoir’s fractal characteristics were divided into two segments. The dissolved pore type was identified as the potentially prolific reservoir due to its larger pore size and volume, moderate permeability, and homogeneity on pore structure. Additionally, the fractal dimension is negatively correlated with porosity and permeability and positively correlated with sorting coefficient and skewness, suggesting that fractal dimensions are valuable for evaluating reservoir quality and quantitatively characterizing pore networks.

The Ediacaran–Ordovician strata within three major marine basins (Tarim, Sichuan, and Ordos) in China are analyzed. Based on previous studies focusing on the characteristics of the Neoproterozoic–Cambrian strata within the three major basins (East Siberian, Oman, and Officer in Australian) overseas, the carbonate–evaporite assemblages in the target interval are divided into three types: intercalated carbonate and gypsum salt, interbedded carbonate and gypsum salt, and coexisted carbonate, gypsum salt and clastic rock. Moreover, the concept and definition of the carbonate–evaporite assemblage are clarified. The results indicate that the oil and gas in the carbonate–evaporite assemblage are originated from two types of source rocks: shale and argillaceous carbonate, and confirmed the capability of gypsum salt in the saline environment to drive the source rock hydrocarbon generation. The dolomite reservoirs are classified in two types: gypsum-bearing dolomite flat, and grain shoal & microbial mound. This study clarifies that the penecontemporaneous or epigenic leaching of atmospheric fresh water mainly controlled the large-scale development of reservoirs. Afterwards, burial dissolution transformed and reworked the reservoirs. The hydrocarbon accumulation in carbonate-evaporite assemblage can be categorized into eight sub-models under three models (sub-evaporite hydrocarbon accumulation, supra-evaporite hydrocarbon accumulation, and inter-evaporite hydrocarbon accumulation). As a result, the Cambrian strata in the Tazhong Uplift North Slope, Maigaiti Slope and Mazatag Front Uplift Zone of the Tarim Basin, the Cambrian strata in the eastern-southern area of the Sichuan Basin, and the inter-evaporite Ma-4 Member of Ordovician in the Ordos Basin, China, are defined as favorable targets for future exploration.

The Ediacaran-Ordovician strata within three major marine basins (Tarim, Sichuan, and Ordos) in China are analyzed. Based on previous studies focusing on the characteristics of the Neoproterozoic-Cambrian strata within the three major basins (East Siberian, Oman, and Officer in Australian) overseas, the carbonate-evaporite assemblages in the target interval are divided into three types: intercalated carbonate and gypsum salt, interbedded carbonate and gypsum salt, and coexisted carbonate, gypsum salt and clastic rock. Moreover, the concept and definition of the carbonate-evaporite assemblage are clarified. The results indicate that the oil and gas in the carbonate-evaporite assemblage are originated from two types of source rocks: shale and argillaceous carbonate, and confirmed the capability of gypsum salt in the saline environment to drive the source rock hydrocarbon generation. The dolomite reservoirs are classified in two types: gypsum-bearing dolomite flat, and grain shoal & microbial mound. This study clarifies that the penecontemporaneous or epigenic leaching of atmospheric fresh water mainly controlled the large-scale development of reservoirs. Afterwards, burial dissolution transformed and reworked the reservoirs. The hydrocarbon accumulation in carbonate-evaporite assemblage can be categorized into eight sub-models under three models (sub-evaporite hydrocarbon accumulation, supra-evaporite hydrocarbon accumulation, and inter-evaporite hydrocarbon accumulation). As a result, the Cambrian strata in the Tazhong Uplift North Slope, Maigaiti Slope and Mazatag Front Uplift Zone of the Tarim Basin, the Cambrian strata in the eastern-southern area of the Sichuan Basin, and the inter-evaporite Ma-4 Member of Ordovician in the Ordos Basin, China, are defined as favorable targets for future exploration. [ABSTRACT FROM AUTHOR]

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Sucrosic dolomite, an important hydrocarbon reservoir, has long been the focus of carbonate sedimentological and reservoir geological studies. This study investigated a kind of heterogeneous sucrosic dolomite in the Lower Permian Qixia Formation of NW Sichuan Basin, which has recently been the location of giant natural gas discoveries. The heterogeneous sucrosic dolomite is characterized by coexistence of porous euhedral dolomite and tight anhedral dolomite, and it is mainly distributed in the platform-marginal shoal facies with a quasi-layered structure. Further geochemical analysis, including C, O, and Sr isotopes as well as rare earth elements, reveals that the euhedral dolomite and anhedral dolomite have similar geochemical properties to the matrix limestone representing coeval seawater, and they were mainly generated from dolomitization by the closed marine-related fluid (left-leaning REE and δPr < 1) in the shallow burial. The difference in crystal morphology, porosity, and permeability between the euhedral dolomite and anhedral dolomite is mainly related to the compositional and textural heterogeneities of the host rocks. Due to the dissolution of meteoric water (relatively flat REE and low Y/Ho) in the early diagenetic stage caused by high frequent exposures, quasi-layered vugs and caves were formed in the grainstones. In the process of shallow burial dolomitization, the loose-filled carbonate sands formed the porous euhedral dolomite due to sufficient space, while the matrix limestone formed the tight anhedral dolomite due to relatively poor porosity and permeability. Accordingly, the paleogeomorphic highland controlled platform-marginal shoal superimposed by meteoric water dissolution in the early diagenetic stage is the main factor for the formation of Qixia Formation reservoirs, while dolomitization is mainly manifested as the inheritance and adjustment of pre-existing pores in the host rock. Therefore, the exploration direction for dolomite reservoirs in the Qixia Formation in the Sichuan Basin should be shifted to the favorable sedimentary facies-controlled reservoir model, which can also be referential for other cases under similar geological setting. [ABSTRACT FROM AUTHOR]

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Ancient dolomite reservoirs play an increasingly important role in deep oil and gas exploration. The mechanism of formation and preservation of dolomite reservoirs is complex, which is always the key issue. With the discovery of deep oil and gas in the Ediacaran dolomites of the world, the upper Ediacaran Qigebrak Formation in the Tabei area has begun to attract attention, but its reservoir space difference and formation mechanism have yet to be clarified. Based on ultra-deep drilling cores and field outcrops in the Tabei area, the lithofacies, reservoir space, and formation mechanism are systematically analyzed by macro to micro, and qualitative to quantitative petrology: (1) The types of dolomite can be divided into five major categories, including microbial dolomite, granular dolomite, residual granular dolomite, crystalline dolomite and karst breccias. (2) The main types of reservoir space are microbial-framework pores, microbial-mold pores, and non-fabric selective dissolution pores. Spongiomicrobialite, karst breccias, and fine-grained dolomite are the dominant reservoir rock types. (3) High-frequency sedimentary cycles and meteoric dissolution are the key factors of reservoir formation. Two sets of large-scale reservoirs are present: the first set is mainly controlled by the supergene karst of the Keping movement, and the second set is mainly controlled by high-frequency sedimentary cycles in the penecontemporaneous period. The reservoirs formed at the shallow burial stage and were preserved until the deep burial stage. (4) The quality of a deep reservoir depends on the geological events that affect the processes of pore reduction and increase. Cementation, compaction and pressure solution are the main destructive diagenetic processes; however, the reservoir space can still be effectively preserved under the influence of constructive diagenetic processes, such as meteoric dissolution and early dolomitization. This research has important theoretical and practical significance for revealing the formation mechanism of upper Ediacaran deep dolomite reservoirs in the Tarim Basin. [ABSTRACT FROM AUTHOR]

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In recent years, great breakthroughs have been achieved for the exploration in the Permian Qixia Formation of the Shuangyushi structure in the north and the Pingluoba structure in the south of the Western Depression of the Sichuan Basin (Chuanxi Depression), while the exploration of the same formation in the middle segment of the Chuanxi Depression has not been carried out. In order to accelerate the ultra-deep marine natural gas exploration of the Qixia Formation, based on a large number of outcrops around the middle segment of the Chuanxi Depression, combined with seismic and drilling data and previous research results, this study systematically combed the oil and gas geological conditions of the Qixia Formation and comprehensively analyzed its exploration potential. Results show that the Qixia Formation in the study area has comparable sedimentary background as the north and south segments of the Chuanxi Depression, and is a platform margin sedimentary facies with superior reservoir conditions. Moreover, the Qixia Formation in the study area is connected with the hydrocarbon generation center of the Cambrian Qiongzhusi Formation by source faults. The tight limestone and shale of Permian and the gypsolyte of Triassic can be potential excellent cap rocks. Therefore, the source-reservoir-cap assemblage of the Qixia Formation in this area is preferential for large-scale hydrocarbon accumulation.

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In recent years, with the fine exploration of carbonate reservoirs in the Qixia Formation, central Sichuan Basin, researchers have made important oil and gas discoveries in the dolomite reservoirs. However, the characteristics, genesis and controlling factors of dolomite reservoirs in the Qixia Formation in this area are still unclear. In this paper, the petrology, pore structures, physical properties and geochemical characteristics of the dolomite reservoirs in the Permian Qixia Formation in the central Sichuan Basin have been systematically studied based on a large number of cores, thin sections, physical property and geochemical tests. Furthermore, the genesis and main controlling factors of dolomite reservoirs are clarified. The study shows that the dolomites have the characteristics of small single-layer thickness and multi-layer development, and they are usually interbeded with the limestones. The reservoir types are mainly fine crystal and fine medium crystal dolomite, and the reservoir spaces include intercrystalline pores and dissolution pores (or caves). Moreover, the reservoir physical properties are characterized by medium porosity and medium to high permeability. Quasi-syngenetic dolomitization is the main origin of dolomite in the target layer, and the dolomite was slightly modified by hydrothermal solution at the end of the Maokou Formation period. The development of dolomite reservoirs in the Qixia Formation in the study area is affected by palaeogeomorphology, sedimentation and diagenesis. The granular shoal facies developed in the high parts of the paleogeomorphology provides the material basis for the formation of dolomite reservoirs. The high frequency sequence interface controls the development of dolomite reservoirs in the highstand systems tract. In addition, the quasi-syngenetic dissolution promots the development of the early secondary pores and provids an effective channel for the migration of the diagenetic fluids in the later stage. In the direction of orthogonal horizontal deposition, the dolomites have the characteristics of thin monolayer thickness, multiple sequences and are interbedded with limestone. The dolomite shoals in the study area are distributed as clumps in the plane, which has great potential for hydrocarbon exploration.

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Based on the latest drilling core, thin section, 3D seismic, well logging data as well as exploration results, the sequence stratigraphy and sedimentary microfacies of the Middle—Lower Ordovician carbonates in Gucheng area, and their controlling effects on the development of reservoir were examined by the theories and methods of fine carbonate sedimentology. The results show that the Middle—Lower Ordovician in Gucheng area is a set of typical carbonate ramp deposits, which can be divided into 10 microfacies in 4 subfacies as follows: back ramp, inner shallow ramp, outer shallow ramp, and deep ramp. The back ramp subfacies consists of muddy-dolomitic flat and dolomitic lagoon microfacies; it is dominated by lamellar micrite dolomite tight in lithology. The inner shallow ramp subfacies includes dolomitic shoal and dolomitic flat in shoal top and dolomitic flat between shoals microfacies; it is mainly composed of crystal dolomite with metasomatic residual structure, as well as abundant karst vugs and intercrystalline pores. The crystal dolomite has an average porosity of 4.36%. The outer ramp subfacies includes medium—high and low energy grain shoal and inter-shoal microfacies; it is dominated by sandy limestone, oolitic limestone, and micrite limestone with few pores. The deep ramp subfacies is dominated by low-energy argillaceous deposits, with local presences of storm shoal microfacies. The Lower—Middle Ordovician has six third-order sequences from bottom to top, among which sequence (SQ3) represents the third member of Yingying Formation. The main reservoir has three fourth-order high-frequency sequences. Apparently, the favorable reservoir in the study area is jointly controlled by sedimentary microfacies and high frequency sequence in the carbonate ramp. The former controls the primary pore structure, and the latter controls the intensities of karstification exposure and dolomitization in the penecontemporaneous period. The dolomitic shoals and top dolomitic flats of different stages, superimposed and connected into favorable reservoirs of considerable scale like “platform margin”, are favorable exploration facies in the carbonate ramp.

Aiming at the scientific problem that only part of dolomite acts as dolomite reservoir, this paper takes the multiple dolomite-bearing formations in the Tarim and Ordos basins, NW China and Sichuan Basin, SW China as the study object, by means of mineral petrological analysis and geochemical methods including carbonate clumped isotope, U-Pb isotopic dating, etc., to rebuild the dolomitization pathway and evaluate its effects on reservoir formation. On the basis of detailed rock thin section observation, five dolomitic structural components are identified, including original fabric-retained dolomite (microbial and/or micrite structure), buried metasomatic dolomite I (subhedral–euhedral fine, medium and coarse crystalline structure), buried metasomatic dolomite II (allotriomorphic–subhedral fine, medium and coarse crystalline structure), buried precipitation dolomite and coarse crystalline saddle dolomite. Among them, the first three exist in the form of rocks, the latter two occur as dolomite minerals filling in pores and fractures. The corresponding petrological and geochemical identification templates for them are established. Based on the identification of the five dolomitic structural components, six dolomitization pathways for three types of reservoirs (preserved dolomite, reworked dolomite and limestone buried dolomitization) are distinguished. The initial porosity of the original rock before dolomitization and the dolomitization pathway are the main factors controlling the development of dolomite reservoirs. The preserved dolomite and reworked dolomite types have the most favorable dolomitization pathway for reservoir formation, and are large scale and controlled by sedimentary facies in development and distribution, making them the first choices for oil and gas exploration in deep carbonate formations.

Prediction of petrophysical properties of deep dolomite reservoir using elastic parameter data is challenging and of great uncertainty. Changes in the petrophysical properties generally induce perturbations in elastic properties. Rock-physics model, which plays a role as a bridge between petrophysical properties and elastic properties, determines the accuracy of inversion for petrophysical properties using elastic properties. Different pore structures lead to variations of rock-physics relationships, and in dolomite reservoir, the influence of pore structure on elastic properties is larger than that of petrophysical properties. We first propose a statistical rock-physics model, in which we consider the effect of pore structure on the nonlinear rock-physical relationship between petrophysical properties and elastic properties of dolomite reservoirs. Then, we propose a Bayesian inversion approach of using elastic properties to predict petrophysical properties and use weight factors to address the difference in accuracy of the input elastic properties in the Bayesian inversion framework. Examples illustrate the proposed approach may produce petrophysical properties of high accuracy for deep dolomite reservoirs. [ABSTRACT FROM AUTHOR]

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Based on drilling cores, well logging and seismic data, source rocks and reservoirs are evaluated; and the natural gas genesis is identified through the analysis of natural gas isotopes, components and fluid inclusions, to study the gas accumulation conditions of the gypsum salt rock related strata of the Ordovician lower assemblage in Ordos Basin. (1) The natural gas from Ordovician lower assemblage is high thermal evolution dry gas from marine source rock, characterized by relatively light δ13C value of methane and heavy δ13C value of ethane. The natural gas is identified as gas cracking from crude oil according to component analysis. Thermochemical sulfate reduction (TSR) reaction has happened between the hydrocarbon fluid and sulfate as sulfur crystals are found in the cores, hydrogen sulfide is found in the natural gas, and hydrocarbon and hydrogen sulfide fluid inclusions are widespread in secondary minerals. (2) Around the gypsum-salt lows, argillaceous rocks are extensive in the Ordovician lower assemblage, reaching a cumulative thickness of 20–80 m. The effective source rocks include argillaceous rock rich in organic laminae, algal clump and algal dolomite. Analysis shows that the source rocks have a dominant TOC of 0.1%–0.5%, 0.31% on average and 3.24% at maximum. The source rocks have an average TOC of 0.58% after recovered through organic acid salt method, indicating the source rocks have high hydrocarbon supply potential. (3) In the sedimentary period, the palaeo-uplift controlled the distribution of reservoirs. The inherited secondary palaeo-uplift in Wushenqi–Jingbian east of the central palaeo-uplift and the low uplift formed by thick salt rocks near Shenmu–Zizhou area controlled the distribution of penecontemporaneous grain shoal dolomite reservoirs. The salinization sedimentary environment of gypsum salt rock can promote the development of reservoir. There are three types of dolomite reservoirs, the one with intercrystalline pore, with dissolution pore, and with fracture; intercrystalline and dissolution pores are main reservoir spaces. (4) There are two types of cap rocks, namely tight carbonate rock and gypsum-salt rock, constituting two types of source-reservoir-cap assemblages respectively. The general accumulation model is characterized by marine source rock supplying hydrocarbon, beach facies limy dolomite reservoir, small fractures acting as migration pathways, and structural-lithologic traps as accumulation zones. (5) The third and fourth members of Majiagou Formation are major target layers in the lower assemblage. The Wushengqi–Jingbian secondary paleo-uplift area and Shenmu–Zizhou low uplift are dolomite and limestone transition zone, there develops tight limestone to the east of the uplift zone, which is conducive to the formation of gas reservoir sealed by lithology in the updip. Two risk exploration wells drilled recently have encouraging results, indicating that the two uplift zones are important prospects.

Great breakthroughs have been recently made in exploring oil and gas in the Middle Permian Qixia Formation, the southwestern Sichuan Basin, China. These discoveries in dolomitic reservoirs revealed abundant hydrocarbon reserves, agreeing on traditional predictions associated with dolomite prospect, which also left the controversy about the nature of dolomitization. Therefore, it is particularly important to research dolomite reservoirs characteristics and their genesis. In this study, comprehensive analyses, including pore-casted thin-section observation, scanning electron microscopy, physical property analysis, mercury injection measurement, fluid inclusion test, and isotopic measurement, are carried out to characterize Qixia Formation dolomites that are sampled from five outcrop sections and nine wells, followed by an investigation into controlling factors of these characteristics and establishment of pore evolution models. Granular dolomites, especially fine-grained and coarser dolomites, are demonstrated to be the best reservoir rocks, with their reservoir capacity positively related to dolomite grain sizes. There are mainly four types of pore-throat structures, namely large pores with wide throats, medium pores with medium throats, small pores with narrow throats, and fracture pore-throat structure. Dolomite reservoirs in the study area have experienced complex diagenetic changes, including the meteoric water dissolution in the penecontemporaneous stage, dolomitization in the shallow-burial stage, and hydrothermal- and formation water-related dissolution during the middle-deep burial stage. High-quality reservoirs are mainly distributed in the shoal facies deposits, especially where both dolomitization and multi-stage dissolution have occurred. [ABSTRACT FROM AUTHOR]

Dolostones of tidal flat facies are the major reservoir rock type of the fourth member of Leikoupo Formation, Longmen Mountain front, western Sichuan Basin. The reservoir controls are dominated by phase belt, dolomitization and denudation. Core observation, cast thin section identification, laser carbon and oxygen isotopes, cathode luminescence, electron probe and inclusion thermometry were used to study the denudation effect. This reservoir went through three periods of denudation, including penecontemporaneous period, epidiagenetic period and buried period, which separately formed intragranular and moldic pores, gypsum caves, and fault-line cavities. The studies of karstification and gypsum dissolution in the paleo-epigenetic period showed that, the distinguishing signs of meteoric water impacts in episode I of the Indosinian Movement include the irregular network of weathering cracks, karst and gypsum breccia, de-anointed calcite maintaining gypsum board shape or right-angle edge shape, widely developed paste mold holes and paste-dissolved holes. These products are featured by low carbon and oxygen stable isotope values, a wide range of strontium isotope values, low content of Sr-Mn-Fe-K-Na trace elements, low-temperature single-phase water-soluble inclusions. The dissolution of gypsum in episode I of the Indosinian Movement is the key for reservoir generation.

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The dolomite reservoirs in the fourth member of Leikoupo formation of the western Sichuan gas field have good prospects for natural gas exploration and development. However, the conventional and one-dimensional nuclear magnetic resonance (1D NMR) logging data are not sufficient for a definitive classification of the reservoir fluid type, likely because of heterogeneity of the reservoirs. Through geological characteristics analysis, log response analysis and two-dimensional (2D) NMR experiment, the environmental factors and technical parameters leading to the differences between the laboratory results and actual logging data were determined. According to the analysis of T2-T1 and T1/T2 (R) differences, the distribution range of drilling fluid, bound fluid, movable water and natural gas signals in the 2D NMR spectra was delineated to build a gas-water 2D NMR identification chart. The results obtained using the identification chart were verified with the actual drilling test results. Complementary to conventional and 1D NMR logging, the 2D NMR identification method was proposed, and can be used to determine the fluid properties in the dolomite reservoirs effectively.

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Copyright of Natural Gas Geoscience is the property of Natural Gas Geoscience 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 Geoscience is the property of Natural Gas Geoscience 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.)

Paleogene lacustrine dolomicrite of Unit II of the lower Xingouzui Formation in the southern part of the Qianjiang Depression, China forms “tight carbonate” hydrocarbon reservoirs that can be divided into the following four dolomicrite types: dolomicrite, muddy dolomicrite, sandy dolomicrite, and gypsiferous dolomicrite. These four dolomicrite types occur in the following three distinct combinations, which are referred to as subfacies: (1) interbedded dolomicrite (or sandy dolomicrite) and dark-gray siliciclastic mudstone, interpreted as open lacustrine deposits, (2) grayish-dark siliciclastic mudstone with thinly-bedded muddy dolomicrite or gypsiferous dolomicrite, interpreted as restricted lacustrine deposits, and (3) grayish-green mudstone with thinlybedded brown-yellow siltstone, interpreted as lacustrine shoreline deposits. The spatial distribution of these subfacies was controlled by the paleo-provenance system, paleo-microgeomorphology, and lake level fluctuations. In the open lacustrine subfacies, dolomicrite or sandy dolomicrite are often vertically interbedded with dark argillite rocks, forming a suitable petroleum source rockreservoir association. The dolomicrite and sandy dolomicrite reservoirs possess many secondary crystalline pores and few dissolution pores, with mesopore porosities in the range of 15%–25%. The dark mud shale interval has total organic carbon (TOC) values exceeding 4% (averaging 1.29%). The organic matter is mainly type II2, with vitrinite reflectance values averaging 0.57, which indicates potential for the generation of immature oil. The oil-source correlation results indicated that the n-alkanes of the crude oil and source rock samples from Unit II of the lower Xingouzui Formation exhibit single peak values, with a main peak (post-peak) in C22 and a Pr/Ph ratio $#x003C; 1 in a C2920R $#x003E; C2720R $#x003E; C2820R distribution. These features indicate that source-reservoir interbedding is characteristic of selfsourced reservoirs. The interbedded dolomicrite (or sandy dolomicrite) and dark-gray siliciclastic mudstone, i.e., subfacies (1), is a favorable target for hydrocarbons in the study area.

The Middle Ordovician subsalt Majiagou Formation in the Ordos Basin comprises pervasively dolomitized shallow marine limestone and is a major reservoir rich in natural gas resources. Four types of dolomite matrix and cement were identified based on petrographic textures: (very) finely crystalline, non-planar to planar-s matrix dolomite (Md1); finely to medium crystalline, planar-s to planar-e matrix dolomite (Md2); microbialites comprising dolomite microcrystals (Md3); and finely to coarsely crystalline dolomite cement (Cd). The Md1 and Md2 dolomites were controlled by alternating lagoon-shoal facies and have δ13C values (− 1.89 to + 1.45‰ VPDB for Md1, − 1.35 to + 0.42‰ VPDB for Md2) that fall within or are slightly higher than the coeval seawater, suggesting the dolomitizing fluid of evaporated seawater. Md2 dolomite was then subjected to penecontemporaneous karstification by meteoric water and burial recrystallization by sealed brines during diagenesis, as indicated by its relatively lower δ18O values (− 8.89 to − 5.73‰ VPDB) and higher 87Sr/86Sr ratios (0.708920–0.710199). Md3 dolomite comprises thrombolite and stromatolite and is interpreted to form by a combination of initial microbial mediation and later replacive dolomitization related to evaporated seawater. Cd dolomite was associated with early-formed karst system in the Md2 host dolomite. The lowest δ18O values (− 11.78 to − 10.18‰ VPDB) and 87Sr/86Sr ratios (0.708688–0.708725) and fluid inclusion data (Th: 123–175 °C) indicate involvement of hydrothermal fluid from which the Cd dolomite precipitated during deep burial. These results reveal the multi-stage dolomitization history of the Majiagou Formation and provide new constraints on fluid origins and dolomites evolution during deep burial in old superimposed basins, such as the Ordos Basin and elsewhere. [ABSTRACT FROM AUTHOR]

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Based on the unconventional thought of “source rock oil and gas” theory and “exploration into source kitchen”, the exploration process of continental shale oil “six determinations” thinking(determining the background, defining source rocks, fixing reservoirs, ascertaining plays, confirming resources and finalizing sweet spots)was innovatively established, its geological characteristics and significant differences with the conventional exploration process were analyzed, and the oil exploration process of the lower first member of Shahejie Formation(E2-3s1)in Qikou sag of Bohai Bay Basin was disscussed as an example. Background analysis shows that semi-deep lake to deep lake and shore-shallow lake to lake bay facies areas are widely distributed, and multiple slope structural belts are developed in the environment of Paleogene lake basin expansion and salinization in Qikou sag, so that shale oil formation and accumulating background are favorable. Source rock evaluation indicates that dark mudstone, oil shale and organic-rich shale are developed in the lower first member of Shahejie Formation with large thickness and wide distribution, and the main body is in mature oil generation stage, so the hydrocarbon generation conditions are good. Reservoir evaluation believes that the development of reservoirs in the lower first member of Shahejie Formation is controlled by both internal and external sources, forming a large area of silty-fine sandstone with multi-stage gravity flow sedimentation and mud-microcrystalline dolomite reservoirs(the thickness is 12.7-70.0 m), overlapping development and contiguous distribution, showing low porosity and ultra-low permeability. According to the shale oil play evaluation, the lower first member of Shahejie Formation characterized by “source-reservoir one body” with four types of configurations between source and reservoir. The superimposed areas of sources and reservoirs are mainly distributed in the middle-low slope area and middle slope area of the gentle depression. Vertically, the turbidite sheet sandstone reservoir and dolomite reservoir are overlapped and horizontally connected with the conventional sandstone reservoirs in high slope and fracture zone. The resource evaluation reveals that the total median value of the two types of shale oil geological resources is 5.86×108 t, the remaining resources account for more than 60%, the buried depth is moderate, and the exploration potential is large. According to the prediction of sweet spots, several exploration wells have drilled industrial oil flow, and shale oil enrichment is controlled by structure, microfacies and diagenesis. There are two types of sweet spot layers consisted of tight sandstone and dolomite, and four sweet spot sections. Qibei low slope, Qinan slope, Banqiao slope and Qibei middle slope in Qikou sag are favorable sweet spot areas for recent exploration. Three suggestions for “exploration into source kitchen” are put forward after further consideration deeply. Firstly, the research on the hydrocarbon-generating contribution and distribution of carbonate source rocks should be paid more attention to in order to find out the center and strata of main source rocks. Secondly, from the perspective of the extension and development of “source-reservoir one body” feature and “source control” theory in unconventional field, some “unconventional thinking” should be opened up such as exploring the source rock(area)to tap all potential, studying all kinds of reservoirs to search for all kinds of sweet spots, carrying out comprehensive exploration combined with the research of conventional and unconventional hydrocarbon reservoir. Thirdly, it is necessary to continue to probe and focus on the methods and technologies of identification and evaluation for shale oil sweet spot layers and sections, so as to solve the technical problems in the evaluation of selected areas. [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.)

Abstract A number of condensate reservoirs with high concentrations of H2S have been discovered in the deep dolomite reservoirs of the lower Ordovician Yingshan Formation (O1y) in the Tazhong Uplift, where the formation water has a high pH value. In the O1y reservoir, the concentrations of Mg2+ and SO4 2− in the formation water are higher than those in the upper Ordovician formation. The concentration of H2S in the condensate reservoirs and the concentration of Mg2+ in the formation water correlate well in the O1y reservoirs of the Tazhong Uplift, which indicates a presumed thermochemical sulfate reduction (TSR) origin of H2S according to the oxidation theory of contact ion-pairs (CIPs). Besides, the pH values of the formation water are positively correlated with the concentration of H2S in the condensate reservoirs, which may indicate that high pH might be another factor to promote and maintain TSR. Oil–source correlation of biomarkers in the sulfuretted condensates indicates the Cambrian source rocks could be the origin of condensates. The formation water in the condensate reservoirs of O1y is similar to that in the Cambrian; therefore, the TSR of sulfate-CIPs likely occurred in the Cambrian. High H2S-bearing condensates are mainly located near the No. 1 Fault and NE-SW strike-slip faults, which are the major migration pathway of deep fluids in the Tazhong Uplift. The redox between sulfate-CIPs and hydrocarbons is the generation mechanism of H2S in the deep dolomite condensate reservoirs of the Tazhong Uplift. This finding should be helpful to predict the fluid properties of deep dolomite reservoirs.

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The Cambrian dolomite reservoir in Tarim Basin is characterized by old age, deep burial and strong diagenesis, whose primary porosity is almost completely destroyed，and the development of effective fracture is the key to the quality of reservoir. Research on the effectiveness and filling characteristics of the fractures of Cambrian dolomite reservoir in the northern Tarim Basin is based on the core and thin-sections, formation microscanner image (FMI) logging, scanning electron microscope (SEM), cathode luminescence microscope (CLM), electron microprobe and fluid inclusion methods. The results indicate that the fracture is growing in the northern Tarim Basin, in which the fractures in the southern Tianshan area and Tahe area are different in occurrence, linear density, shapes and the filling characteristics. In the southern Tianshan area, the linear density and effectiveness of fractures are lower than those in Tahe area, and the filling minerals are dominated by idiomorphic dolomite and quartz, followed by calcite, gypsum and pyrite. Dolomite-filling is divided into three stages and the corresponding temperature ranges are 70 ℃-90 ℃, 90 ℃-110 ℃ and more than 110 ℃, which are caused by diagenetic fluid in strata; the quartz-filling is divided into two phases, the first is caused by the clay minerals alteration, and the second may be the hydrothermal genesis. Tahe area, in which the linear density and effectiveness of fractures are well and the dissolution develops, is a favorable exploration zone; the filling minerals are made of hydrothermal minerals, which are dominated by calcite and followed by gypsum, saddle dolomite, clay mineral, asphalt, etc., and the barite occurs in the form of metasomatic dolomite. Calcite-filling is four times in Tahe area and the corresponding temperature ranges are 70 ℃-100 ℃, 100 ℃-130 ℃, 130 ℃-180 ℃ and more than 180 ℃, among the first three periods are the genesis of diagenetic fluid in different buried depth, and the last one is caused by high temperature hydrothermal fluid from the lower migration. [ABSTRACT FROM AUTHOR]

Copyright of Acta Sedimentologica Sinica is the property of Acta Sedimentologica Sinica Editorial Office 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.)

Recent natural gas discoveries indicate that non-karstification-dominated reservoirs exist in the intracratonic Ordos Basin. This study examines the sedimentological and geochemical characteristics needed to clarify the depositional model and diagenetic evolution process of this newly discovered reservoir type. The depositional environment of the dolomite reservoir can be characterized as a tidal flat that grew from the Central Paleo-uplift to the eastern depression by cyclic progradation on an epeiric platform. A tidal flat sequence can extend laterally as a progradational wedge in each cycle of sea level fluctuation. The sheet-shaped peritidal shoal facies associations patched on the wedge represent potential dolomite reservoirs and can be recognized by the presence of doloarenite that has been altered into a vaguely relict grained-texture by diagenesis. Although continuing destructive diagenesis has led to reservoir densification, burial dolomitization and burial dissolution with facies selectivity have tended to occur in peritidal shoal facies associations, thus improving the quality of the dolomite reservoirs. These models provide new insights for targeting deep dolomite hydrocarbon reservoirs in intracratonic basins. [ABSTRACT FROM AUTHOR]