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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 dolostone units in the Cambrian Changlangpu Formation of the Sichuan Basin have demonstrated substantial potential for natural gas exploration. A systematic understanding of the various dolomite (and non-carbonate) phases that build these rocks and the diagenetic pathways of the Canglangpu Formation carbonates still needs to be developed. This study combines the petrographic properties of dolomite (and non-carbonate) phases with their geochemical evidence (carbon, oxygen and 87Sr/86Sr isotope data). Moreover, fluid inclusion evidence and in situ rare-earth element (REE) data are presented. The samples studied were taken from hand specimens collected in the field and plugs from cores to explore the properties and evolutions of dolomite in the Cambrian Canglangpu Formation in the central-northern Sichuan Basin. Six dolomite phases are identified: silty crystalline dolomite (D1), fabric-retentive dolomite (D2), fabric-destructive dolomite (D3), fine crystalline dolomite (D4), medium to coarse crystalline dolomite cement (C1a and C1b) and saddle dolomite cement (C2). Silty crystalline dolomite represents the earliest dolomitization and shows δ18O values comparable to those of Cambrian seawater. Fabric-retentive dolomite, fabric-destructive dolomite and fine crystalline dolomite represent reflux dolomitization during shallow burial. Specifically, Fine crystalline dolomite with a turbid-core and clean-rim may have recrystallized from secondary dolomitization of marine-derived porewater. Fabric-destructive dolomite shows the depleted δ13C values, resulting from the secondary dolomitization involving organics degradation in medium to deep burial environment. The fluid inclusion data demonstrates that the medium to coarse crystalline dolomite cement (C1a) formed in relatively elevated temperature and salinity brines. Intrusion of sulfur- and metal-rich (Zn and Fe) medium-temperature hydrothermal fluids gave rise to the formation of medium to coarse crystalline dolomite cement (C1b) and sphalerite mineralization. Cementation of saddle dolomite was promoted by sulfate-rich reducing hydrothermal fluids. δ13C depletion, extremely low REE content and positive δEu in the saddle dolomite cement suggest the presence of thermal sulfate reduction. • Six dolomite phases were identified, including matrix dolomites and dolomite cements. • Dolomitization occurred from near the surface to deep burial. • In situ elements data reveal the properties of variously dolomitizing fluids in one dolomite phase. • The dolomite crystal zoned as fluid reducibility increased from core to rim section. • Medium temperature hydrothermal dolomitization associated with sphalerite mineralization. [ABSTRACT FROM AUTHOR]

The Leikoupo Formation, which was deposited during the Anisian stage in the central Sichuan Basin, has produced significant amounts of conventional gas but there have been few studies on its unconventional resources. Organic-rich shale in the Leikoupo Formation was discovered for the first time in the wells of CT1 and JY1 in the central Sichuan region, but information on its sedimentary environment and climate evolution is scarce. A series of lithofacies, organic geochemistry and inorganic geochemistry analyses were used to analyse core samples of CT1 and JY1 to estimate their vertical changes. The organic matter characteristics, the sedimentary environment and the factors controlling the Leikoupo Formation shale in the Sichuan basin were revealed. The organic-rich shale of the Lei3-2 sub-member was formed in a semi-restricted lagoon environment. The sedimentary environment was dysoxic-anoxic, with high palaeoproductivity, a warm and humid to semi-arid and semi-humid climate. The vertical sea level decreased for a short period, led to an increase in oxygen content in the mixed shale, clayey shale and calcareous shale that were sequentially deposited from bottom to top. In the vertical direction, the palaeo-productivity and aridity decreased, and there was an increase in the salinity and terrigenous debris input. All of these corresponded well with TOC. According to Mo-TOC and Mo–U patterns, the mixed shale was characterised by the highest values of Mo EF , U EF and Mo EF /U EF , followed by clayey shale and calcareous shale, which indicated mixed shale was deposited in a moderately restricted water condition, which progressively became more restricted during the deposition of clayey shale and calcareous shale with the fall of relative sea level. Biomarker compounds (steranes, terpanes), Tmax and other pyrolysis parameters indicated that the organic matter had reached the stage of high-to over-maturity. The gammacerane to C 30 hopane in the ratio indicated that it was the bottom water in which it was deposited was sratified. • The first discovery of Lei3-2 sub-member shale in the Sichuan Basin. • The shale has good original hydrocarbon generation potential. • The shale was formed in a semi-confined environment. • Lithological changes reflect variations in sedimentary environment and sea level. [ABSTRACT FROM AUTHOR]

Primary depositional and diagenetic processes exert very important influences on shale gas reservoirs. Rock electrical properties are an important basis for making reservoir prediction using the electromagnetic method (EM). However, there is still a lack of understanding about the impact of the sedimentary diagenesis process on shale electrical properties, this study focuses on the impact of diagenesis on rock properties. In this study, rock electrical properties are studied based on electromagnetic experiments. We systematically studied the lithofacies of different depositional paleoenvironments and diagenetic processes, and the influence of diagenetic evolution on the rock electrical properties was discussed, by means of X-ray diffraction (XRD) analysis, field-emission scanning electron microscopy (FE-SEM), low-temperature nitrogen adsorption (LTNA), and trace element (TE) geochemical analysis. Based on the study of mineral composition, grain assemblages and pore systems, we identified four lithofacies in the Longmaxi Formation: siliceous shale, siliceous-argillaceous mixed shale, silty shale and argillaceous shale. Redox proxies (U/Th, V/Cr, and Ni/Co) indicate that the siliceous shale was deposited in a relatively anoxic and reducing environment, indicating a deep-water shelf depositional paleoenvironment. The siliceous-argillaceous shale, silty shale and argillaceous shale were deposited under a relatively dysoxic-oxic environment, indicating a shallow-water shelf depositional paleoenvironment. The order of resistivity values of the lithofacies within the Longmaxi Formation is 36.78 Ω m (siliceous shale), 66.81 Ω m (siliceous-argillaceous mixed shale), 79.54 Ω m (silty shale), and 107.00 Ω m (argillaceous shale), and the resistivity decreases with an increase in porosity. The siliceous shale has the most abundant authigenic quartz, which filled the primary pores forming a rigid framework during the gas window, inhibited compaction, increased the distribution of organic matter (OM), and enhanced the development of OM pores. The high TOC content and high maturity of siliceous shale at the bottom of Longmaxi Formation make the OM pores more developed. Pyrite, conductive fluid and pore network under the main control of OM pores in shale form a conductive circuit when AC voltage is input, which increases the exchange capacity of cations and leads to the phenomenon of low resistivity. The interconnected OM pore network, both depositionally and diagenetically derived, affects the electrical properties of the Longmaxi shale. This study reveals that electrical properties of shale rocks and its variations can be impacted by the depositional environment and diagenetic processes. This work provides resistivity parameters for the electromagnetic exploration of shale gas under complex terrain conditions and provides a theoretical basis for later interpretation. • Shale lithfacies with different sedimentary environments and diagenetic processes have different electrical properties. • The shale of Longmaxi formation has the characteristics of high TOC content and low resistivity. • OM pores network is the fundamental factor affecting the electrical properties in Longmaxi shale. • It provides parameters of rock electricity for EM exploration of complex terrain and for inversion interpretation. [ABSTRACT FROM AUTHOR]

The Lower Triassic Jialingjiang Formation, characterised by a carbonate–evaporite succession, is one of the important pay layers in the eastern Sichuan Basin. Several diagenetic events were identified in this study, including meteoric dissolution, dolomitisation, anhydrite/gypsum replacement/cementation, TSR, BSR, and multiple calcite cementation (C1–C3). Dolomitisation and anhydrite/gypsum replacement/cementation are the major diagenetic events affecting reservoir porosity. The dolostone is composed of D1, D2, and D3 dolomite. D1 dolomite is very finely crystalline, dominating in the low-energy, restricted depositional facies. D2 dolomite is finely crystalline, constituting a major portion of the dolostones. D3 dolomite is depositional fabric retentive dolomite, including dolo-wacke-/packstone, dolo-grainstone, and dolo-boundstone. Four types of anhydrites (A1–A4) were identified in the core and thin sections. A1 is stratabound anhydrite. A2 is the anhydrite in dolostones, filling in the interparticle spaces in dolo-grainstone and the intraparticle spaces in dolo-wackestone. A3 is the anhydrite in limestones, which occurs non-fabric-selectively. A4 is the anhydrite filling in the fractures. The diagenetic anhydrites (A2–A4) are interpreted to postdate to major dolomitisation. D1 and D2 dolomites are characterised by δ13C VPDB values of −3.4–+7.3 ‰ and δ18O VPDB values of −4.7−-2.4 ‰. The REE patterns of D1–D3 dolomites are characterised by light rare earth element (LREE) depletion relative to heavy rare earth elements (HREE) ((Pr/Tm) N of 0.54–1.16, 0.81 ± 0.20), middle rare earth elements (MREE) enrichment (BSI values of 1.06–1.38, 1.26 ± 0.10), and negative Ce anomalies (Ce/Ce* ratios of 0.74–0.94, 0.83 ± 0.07). The diagenetic anhydrites (A2–A4) show δ34S V-CDT values of +25.9–+45.7 ‰, similar to those of the stratabound anhydrite (+29.6–+43.0 ‰). The large variations in the δ13C VPDB values of D1 and D2 dolomites are attributed to methanogenesis and BSR. The δ18O VPDB values and REE characteristics of D1–D3 dolomites suggest that the carbonates are dolomitised by the mesohaline fluid. The δ34S V-CDT values of A1–A4 indicate that the diagenetic anhydrites are formed by redistribution in a closed system. The petrographical and geochemical data suggest that the Jialingjiang carbonate evolved in a closed system during burial diagenesis. Primary pores dominate in the Jialingjiang carbonate reservoirs, and the boundstone and grainstone show the best porosities based on the porosityversus lithofacies data. Therefore, we propose that the lithofacies, instead of the diagenetic events, are the key factors of the Jialingjiang carbonate. The resisting compaction of dolomitisation helps preserve the reservoir porosity, and anhydrite is the major diagenetic event destroying the reservoir porosity. • Lithofacies are the key factor of the reservoir qualities. • Petrography and geochemical work suggest the closed diagenetic systems. • Jialingjiang carbonate is dolomitized by mesohaline fluids predating diagenetic anhydrite. • Anhydrite selectively replace calcitic part in Jialingjiang carbonate. [ABSTRACT FROM AUTHOR]

The marine carbonate rock in the Changxing-Feixianguan Formation of Xuanhan-Daxian area, northeastern Sichuan, the Sichuan Basin holds abundant natural gas resource, where the dolomite includes two types, the one with original texture (grain dolomite, reef dolomite) and the one without original texture (idiomorphic to anhedral-crystal dolomite, porphyritic dolomite). The analysis on the isotopes of C, O, Sr and other geochemical features shows that the dolomitizing process happened in fluid rich in Mg2+ and lean in Ca2+ and brought in Fe2+, Mn2+ and took out Sr2+; with the rise of recrystallization, the Fe2+, Mn2+ contents tend to increase while δ18O PDB and Srcontent tend to decrease. According to the variation of 87Sr/86Sr and other geochemical features, it is considered that the dolomitizing fluid participating in the reservoir diagenesisis the high salinity sea water later than the end of Feixianguan stage, and the dolomitization takes place mainly through the multiple thermal convective cycle effect. The micritic dolomite is of evaporative dolomitization origin with the characteristics of high Sr2+, Na+ and low Mn2+ which reflects the open environment of the surface exposed in the quasi syngenetic period, while the reef dolomite is of high Mg/Ca dolomitizing fluid origin with the characteristics of lowSr2+, Na+ and Mn2+, whose diagenetic environment changed from near surface to shallow burial environment; crystalline dolomite is formed by metasomatism of low Mg/Ca dolomitizing fluid at deep burial depth and high temperature, with the increase of burial depth,in the high temperature and high pressure deep sealing system, its Fe2+ and Mn2+ elements increased while Sr2+ decreased. Through reconstruction of dynamic diagenesis-pore evolution, it is concluded that the low porosity of shallow beach grain dolomite is resulted from early compaction and medium-term cementation, yet the reservoir quality has been apparently improved by the later hydrocarbon dissolution; compared with shallow beach grain dolomite,the reef dolomite has smaller initial porosity, but stronger compressive capacity. The early compaction and medium-term cementation have less impact on the reef dolomite, and the later dissolution strength is slightly weaker than that of the granular dolomite, thus the change range of porosity is smaller than that of the grain dolomite; the micrite-silt dolomite mainly suffered porosity reduction caused by compaction, and the later burial dissolution and recrystallization are key to the quality improvement of this kind of dolomite reservoir. • There are two types of dolomite in the study area: the original fabric and without original fabric. • The high salinity marine source fluid of late period is involved in the dolomitization process of dolomite reservoir. • Dissolution is the key to the formation of high quality reservoir. • Compaction and cementation are the key factors to reduce the porosity of dolomite reservoir. [ABSTRACT FROM AUTHOR]

The Lower Cambrian Longwangmiao Formation is a significant deep carbonate (dolomite) reservoir that has great hydrocarbon potential in the Sichuan Basin, South China. However, the origin of the Longwangmiao dolomite and the mechanism of dolomitization remain a debate that needs better understanding due to its crucial importance in determining reservoir quality. In the current study, a multi-technique approach of petrography, geochemistry (C- and O-isotopes, major and trace elements including rare earth elements), and microthermometry has been applied to reconstruct the mechanisms of dolomitization. Petrographic examinations revealed three types of matrix dolomite: (1) near-micritic (dolomicrite), fabric retentive texture (Md1, 10–50 μm), (2) finely to medium crystalline dolomite, subhedral to euhedral crystals (Md2, 60–120 μm), and (3) medium to coarsely crystalline dolomite, subhedral to nonplanar crystals (Md3, 100–300 μm). The similarity of PAAS-normalized REE patterns of the investigated dolomites to that of their precursor lime mudstone and their δ13C (C1 = 0.3 ± 0.3‰, Md1 = −1.3 ± 0.6‰, Md2 = −0.6 ± 1.6‰, and Md3 = 0.3 ± 1.5‰) and δ18O (C1 = −9.4 ± 0.2‰, Md1 = −7.8 ± 0.8‰, Md2 = −8.7 ± 0.5‰, and Md3 = −8.9 ± 0.6‰) values suggest that the parent dolomitizing fluids of Md1, Md2, and Md3 dolomites were likely modified seawater. The significant enrichment of Na (1266 ± 1316 ppm) and K (4649 ± 3199 ppm), tight structure and presence of moldic pores in Md1 and its slightly elevated δ18O values suggest that it formed in a near-surface (i.e., mesosaline to penesaline) environment from slightly evaporated seawater, which is consistent with its Sr/Ca molar ratios (0.0075−0.0090). The Md2 dolomite is crosscut by stylolites and has a coarser grain size, higher Mn (295 ± 172 ppm), Nd SN /Yb SN (1.33 ± 0.06) and U (0.49 ± 0.11 ppm), and lower Ce/Ce* (0.83 ± 0.02) values than Md1. This suggests a shallow to mid-burial dolomitization with possible downwards seepage-reflux of Mg2+-rich evaporated brines. The Md3 dolomite has the largest crystal size and mosaic contacts, lowest Ce/Ce* (0.81 ± 0.02), and highest U (0.64 ± 0.15 ppm) and Mn (487 ± 240 ppm) values, which imply a deeper dolomitization environment than Md2. Thus, it precipitated likely in a mid-to deep burial setting at a higher temperature (T h = 120−135 °C), from the residual Mg2+-rich fluids stored in the strata pores. Considering the regional geology, we suggest the sedimentary facies and paleogeomorphology controlled the spatial distribution of early dolomites, while sealevel fluctuations influenced the initial time and scale of dolomitization. • Origin of dolomites reflected by stable isotopic, elemental, and fluid inclusion constraints. • Dolomitization of Md1 to Md2 and Md3 took place from early to late. • Controls of depositional environment and sea-level fluctuation on dolomitization. • Dolomitizing model for the Longwangmiao carbonates in the eastern Sichuan basin. [ABSTRACT FROM AUTHOR]

The Middle−Upper Xixiangchi Formation of the Sichuan Basin consists of mixed carbonates and siliciclastics. The carbonates are dominated by dolomitized lime-mudstones and dolo–grainstones of shallow warm-water environments. Petrographic examination of carbonates reveals two types of dolomite: (i) fabric retentive dolomicrite (D1, micritic to near-micritic, 4–50 μm) and (ii) fabric-destructive dolomite (eu-to subhedral crystals with cloudy cores and clear rims, 50–300 μm). The mean ∑REE values of D1 (15.3 ± 6.3 ppm) and D2 (14.6 ± 7.1 ppm) and the nearly identical mean PAAS-normalized patterns that coincide at some points, suggest an origin from similar parent fluids that were not dramatically changed by water-rock interaction through basinal sedimentary rocks. The estimated Sr/Ca molar ratios of D1 (0.0043–0.0055) parent fluids and their estimated δ18O values imply an origin from a mixture of meteoric and seawaters, which is also consistent with the petrographic evidence. The partially coinciding normalized REE pattern of D2 with that of the earlier D1 suggests that the latter was formed at a mid-burial setting rather than deep burial, which is consistent with higher homogenization temperatures of primary two-phased fluid inclusions retained in D2 (T h = 118.1 ± 5.3 °C). Therefore, D1 is suggested to have formed (1) during a penecontemporaneous period that resulted from a slight evaporative pumping effect within a near-surface diagenetic environment, and/or (2) during post-penecontemporaneous or penecontemporaneous seepage-reflux dolomitization, induced by sea-level fluctuations, during early stages of diagenesis at near-surface conditions. D2 formed likely through the recrystallization of D1 or a continuation of reflux dolomitization of the same precursor at mid-burial rather than deep burial settings. • Origins of D1 and D2 dolomites are reflected by their geochemical compositions. • Dolomitizing fluids were derived from slightly evaporated seawater and basinal brines. • Similar parent fluids of D1 and D2 are reflected by their nearly identical PAAS-normalized REEs patterns. • Recrystallization of D1 likely accounts for comparable δ18O values with D2. [ABSTRACT FROM AUTHOR]

Complex lithology association along vertical direction results in thin layers with different interbed patterns of the Lower Triassic Jialingjiang Formation (T 1 j), Sichuan Basin. The prediction of thin tight reservoirs in the Jialingjiang Formation is important for further oil and gas exploration. However, seismic data in the eastern Sichuan Basin has poor lateral continuity of seismic events because of strong interference noises, which makes it difficult to pick horizons. Moreover, low dominant frequency (<25 Hz), low seismic resolution, and weak response characteristics of strong transverse heterogeneity on logging parameters make it challenging for the thin reservoirs prediction. To tackle these issues, in this study, we propose a target-oriented workflow by integrating seismic data processing with interpretation to delineate the thin reservoirs and corresponding hydrocarbon production capacity. In the processing workflow, we first remove the noise interferences using the structure-oriented filtering (SOF), which can improve the effective information of seismic events and also preserve the features of fault structures. Second, we propose a novel method based on spectral shaping and sparse inversion to enhance the seismic resolution. Tests on filed data demonstrate that the effective information and resolution of processed seismic data are significantly enhanced. Then, in the seismic interpretation, fluid mobility and spectral decomposition are first used to delineate the spatial distribution of favorable reservoirs. Furthermore, multi-attribute integrated interpretation involving impedance analysis, reservoir thickness estimation and porosity inversion is used to evaluate the storage capacity and productivity of the thin reservoirs. The estimated impedance, reservoir thickness and porosity show good agreement with well information, demonstrating the feasibility of the proposed method for accurately predicting thin reservoirs in this area. Multi-attribute integrated interpretation revealed that the favorable thin reservoirs are mainly developed from the northeast to the southwest in the study area, which can provide effective guidelines for future production well drilling and distribution delineations of a favorable reservoir in the Eastern Sichuan Basin. • Structure-oriented filtering (SOF) can remove noises and enhance the effective continuities and structural features. • A new resolution enhancement (RE) method improves the seismic resolution significantly and reduces the artifacts. • The target-oriented processing workflow can markedly improve the quality of the seismic data. • Multi-attribute analyses indicating the favorable thin reservoirs is developed for the target zone. [ABSTRACT FROM AUTHOR]