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Banded travertines are important parts of fissure ridge systems, but studies on geochemical characterization of banded travertines are limited. This study investigated the lithofacies and stable carbon and oxygen isotopic features of banded travertines from Xiagei (southwestern China) to examine their formation mechanisms. Petrographic analyses of the banded travertines revealed two lithotypes: thick-laminated palisade crystalline crust and thin-laminated composite crystalline crust. δ13C and δ18O of the Xiagei banded travertines range from 2.82‰ to 4.50‰ V-PDB, and from −25.86‰ to −20.90‰ V-PDB. Parent CO2 evaluation shows that the Xiagei banded travertines mainly received CO2 from the decarbonation of marine carbonates, but the contributions of magmatic CO2 and the dissolution of marine carbonates are also unneglectable. Significantly, the magmatic-derived CO2 might indicate that the delamination of the lithosphere along with the asthenosphere upwelling could be taking place in the eastern Tibetan plateau. Paleotemperature calculation shows that the Xiagei travertines were precipitated from moderate- to high-temperature hot springs (44.3 to 86.8 °C). Interestingly, the thick-laminated palisade crystalline crust and thin-laminated composite crystalline crust display calculated paleotemperature between 66.6 and 86.8 °C and between 56.6 and 77.7 °C, respectively, reflecting the great role of water temperature in controlling the lithofacies of banded travertines. A comparison between the banded travertines at Xiagei and other areas also shows temperature is a non-negligible factor controlling banded travertine precipitation. However, this does not mean that water temperature is the decisive controlling factor and more studies on banded travertines are still indispensable to disclose the potential factors controlling the factors/processes affecting banded travertine lithofacies. This study provides a good example for understanding the relationship between lithofacies and stable isotopic geochemical characteristics of travertine deposits. [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]

This study describes and interprets the mineral and facies assemblages of lacustrine shales that occurred in an ancient volcanic-related alkaline playa-lake, considering the importance of diagenetic pathways (calcification, dolomitization, borosilicate replacement, and silicification). Three laminated lithofacies (LF) have been differentiated in the Early Permian Fengcheng Formation, NW Junggar Basin: an alginite-poor, dolomite-lean, and argillaceous-rich lithofacies deposited during high lake level stages (LF-A), an alginite-moderate, dolomite-moderate, and evaporite-present lithofacies deposited during low lake level stages (LF-B), and an alginite-dense, dolomite-rich and reedmergnerite (NaBSi 3 O 8)-rich lithofacies deposited during periods of intense hydrothermal input (LF-C). Depositional environments influenced subsequent diagenetic pathways of the three laminated lithofacies. In the shallow-water LF-B sediments, extensive evaporite crystallization, dissolution and subsequent calcification or dolomitization occurred during eodiagenesis. The deep-water LF-A sediments underwent slight eodiagenetic modifications, but developed abundant calcite-filling sub-horizontal fractures during moderate inversion and uplift in mesodiagenetic regime. With increasing depth of burial, the previous vugs-filling calcite and dolomite were preserved well or only slightly replaced by reedmergnerite in the LF-A and LF-B sediments, but intensely replaced by reedmergnerite in the LF-C sediments during mesodiagenesis. The reedmergnerite enrichment processes in the LF-C sediments were closely related to organic matter production and thermal evolution. Algae blooms induced by spring input functioned as an important media in sinking hydrothermal boron from the not very concentrated waters. Later degradation and thermal evolution of alginite released boron and organic acids into interstitial waters, which caused carbonate dissolution and promoted reedmergnerite formation. The formation of reedmergnerite during mesodiagenesis restrained silicification process of carbonate minerals in ancient volcanic-related alkaline lake deposits. • Tuff-rich lithofacies is algae-poor and dolomite-rich lithofacies is algae-rich. • Algae, primary dolomite, and diagenetic reedmergnerite are genetically related. • Primary environments determine subsequent diagenetic pathways of dolomitic shales. • Mesodiagenetic formation of reedmergnerite restrained late silicification process. [ABSTRACT FROM AUTHOR]