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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]

Channels are considered effective conduits that deliver terrigenous sediments into related depositional basins, and they are the targets of petroleum exploration and production. Three-dimensional (3D) seismic data, well data, and attribute maps were used to study five types of channels observed in the Shahejie Formation of the Zhanhua Depression, namely erosional channels, confined channels, semi-confined channels, distributary channels, and transitional channels. The channels originated from hyperpycnal flows related to bed load, suspended load, and lofting processes from proximal to distal regions, which were primarily accumulated on medium and low terrains. The hyperpycnal channel systems were primarily identified as three stages with morphological changes in the Es3l and Es3m intervals on a seismic section. Stage 1 was developed during the late period of the Es3l interval, and stages 2 and 3 were developed during the early and middle periods, respectively, of the Es3m interval. Stage 1 is the initial/young stage formed during a phase of low accommodation. Channels of stage 1 are predominantly entrenched with moderately aggregated width and thick channelized splays. Stage 2 is the transitional stage formed during a phase of increasing accommodation. Channels of stage 2 are predominantly meandering with narrowly aggregated width and dispersed frontal splays. Stage 3 is a waned stage, formed during the events of increasing lake levels without comparable increase in sediment flux. This stage involves slight incisions, broad U-shaped morphology channels, and the prevalence of overbank sediments. The depositional architectures, processes, and the development of hyperpycnal channel systems correspond with the lake level and climate conditions of the proposed stages and were controlled by tectonic activity and source supply. Understanding the distribution and development of lacustrine hyperpycnal channels is important to improve and enrich theories of lacustrine sediment gravity flow. • Five types of channels were identified in the Shahejie Formation of the Zhanhua Depression. • Hyperpycnal channel systems, consisted by five types of channels, were categorized as three stages. • Tectonic activity, sediment supply, climate conditions, and lake level were demonstrated as the factors controlling the evolution of the lacustrine hyperpycnal channel system. [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]

Siliceous mudstones and chert-shale couplets are primary source rocks for many large paleo-oil reservoirs over the world. It is well established that photosynthetic organisms can be exceptionally well preserved in silica precipitates, but the fate of the trapped organic matter (OM) during burial remains poorly studied. Here, dense unicellular microbial fossils have been recently discovered in the cherts of the Late Paleozoic alkaline saline lake deposit in NW Junggar Basin, NW China. We use petrographic observations and organic geochemical analysis to study the origin of unicellular microbes and how silicified organic matter generate and expel hydrocarbons. A 300-m-long oil shale core shows overall low levels of porosity, permeability and total organic carbon contents (generally <0.5 wt%), but very high extracts (on average 0.26 wt%), suggesting that the rich extracts were mostly indigenous and the original organic matter had a high hydrocarbon conversion. Analysis of biomarkers and silicified fossils both suggest that the bio-precursor for the organic matter is a special green alga. Reconstruction of silica diagenesis of cherts indicates that both the trapped lipids and hydrocarbons in silicified microbial cells and extracellular polymeric substances had been effectively expelled out during the transformations of Opal-A to microcrystalline quartz and during OM thermal maturation. Exceptional OM preservation and effective oil expelling ways render siliceous source rocks as excellent unconventional hydrocarbon reservoirs. • Dense unicellular microbe fossils are found in Early Permian lacustrine cherts. • Microbe preserved by cherts is a special green alga. • Two stages of bitumen expulsions were associated with silica diagenesis. • Organic biosignatures in cherts have already been largely lost before metamorphism. [ABSTRACT FROM AUTHOR]

The Archaeocyathid Extinction Carbon Isotope Excursion (AECE) and Mingxinsi Carbon Isotope Excursion (MICE) are major events in the Early Cambrian sedimentary records but controversially heterogeneous in their magnitudes, as well as local and global correlations. This paper presents a high-resolution δ13C carb profile from the Shiliu section in the western Yangtze Platform (China) that spans the lower part of the Lower Cambrian. The investigated Lower Tianheban interval (∼110 m) is dominated by marine carbonates (lime mudstones) alternating occasionally with thinly-bedded black shale and mudstone interbeds. The investigated micritic carbonates were examined through a multi-technique protocol, combining petrographic and geochemical assessments, to evaluate the degree of sample preservation. The δ13C carb values (−2.4‰–1.8‰) show an insignificant correlation with the δ18O carb values (−12.6‰ to −8.7‰, R2 = 0.012), the Mn/Sr ratios (R2 = 0.001), and Al concentrations (R2 = 0.041). This supports the preservation of predominantly at least near-primary δ13C carb signatures, which can be utilized to reconstruct a reliable, high-resolution δ13C carb profile for chemostratigraphic correlations in the area and beyond. The profile displays a lower positive excursion of ∼3‰ overlain immediately by a negative excursion of ∼2.3‰. The archaeocyath zonation scheme suggests that the lower excursion is correlated with the global MICE event and the upper excursion with the AECE event. The results provide an alternative option for interpreting the carbon isotope excursions in the Lower Cambrian within the Yangtze Platform as well as other equivalent sections at a global scale. • δ13C carb composition of the Lower Cambrian carbonates in northwestern Yangtze Platform. • Distinct δ13C carb excursions correlated with the MICE and AECE events. • Alternative interpretation of the MICE and AECE events in the Yangtze Platform. • A useful C-isotope profile for chemostratigraphic correlations in Yangtze Platform and beyond. [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 Barremian and Aptian correspond to periods of marked changes in the atmosphere-ocean system, and offer valuable information about global tectonic events. Here, we present integrated high-resolution carbonate carbon-isotope (δ13C carb) and geochemical data that precede the Early Aptian oceanic anoxic event (OAE 1 a) in the Qiangtang Basin (Eastern Tethys). We identified a characteristic and well-correlatable Barremian-Lower Aptian carbon-isotope pattern that can be categorized into 10 Segments. The most notable features of the δ13C carb record are multiple negative carbon-isotope excursions in the Barremian-Aptian, and each is possibly linked to environmental changes. Two carbon isotopic plateaus are observed in the Upper Barremian, which are generally related to increased primary productivity, and to the concomitant development of dys-to anaerobic conditions. Our data indicate the development of anoxic conditions during shale deposition, which occurred under the conditions of a semi-enclosed lagoon environment. Such conditions, were likely localized, and not the primary mechanism responsible for the deposition of organic-rich sediments within the Shenglihe succession. The deposition of organic-rich shales was very likely due to global/regional warming. Rising temperatures and hydrological cycle strengthening during shale deposition are believed to have increased primary productivity, and thus resulted in the expansion of the local oxygen minimum zone within the water column. • Barremian was a time of marked changes in the atmosphere-ocean system. • δ13C record is subdivided into 10 δ13C segments prior to the OAE 1a. • Positive carbon isotope excursion is prominent in the Upper Barremian. • Global warming would raise surface productivity. [ABSTRACT FROM AUTHOR]

Intrabasinal slope belts, which were previously thought to lack sandy sediments, have been largely understudied due to their distance from their sediment provenance. Thus, our study sought to conduct a comprehensive geological study of the facies architecture and sediment infilling processes in the intrabasinal slope belts of the Zhanhua Depression during the Paleogene using seismic data, drill cores and well logs. Three distinctive depositional systems were identified: near-shore subaqueous fan, shallow/deep lakes, and braided-river delta. The facies architecture and sediment infilling processes were primarily characterized by different stages of diversiform tectonic geomorphologies in cyclothem motifs of rift lacustrine basins. The geomorphological characteristics of the Shahejie Formation exhibited multiple alternating uplifts and sags, resulting in multiple intra- and extra-basinal sediment provenances, while those of the Dongying Formation displayed a single depocenter with a wide and gentle geomorphology derived from several extra-basinal sediment provenances. Coarse-grained sediments formed near-shore subaqueous fans, primarily accumulated on the slopes around the Kenli Uplift during the member 3 of the Shehejie Formation interval; medium- and fine-grained sediments deposited in the proximal and distal environment of the delta front and sandy sediments formed thick-bedded sandy beach bars on subaqueous uplifts during member 2 of the Shehejie Formation interval; ultrafine-grained, fine-grained, and carbonate sediments deposited as thin-bedded carbonate banks and sandy beach bars in a shallow/deep lake during member 1 of the Shehejie Formation interval; and fine-to medium-grained sediments abundantly accumulated in the proximal environment of the delta front by passing the uplifts and infilling the slopes during the Dongying Formation interval. This integrated study demonstrates the effectiveness of a process-based analytical approach and depositional models in predicting facies architecture and sediment infilling processes. Furthermore, it provides guidelines for oil and gas exploration in intrabasinal slope belts of rift basins. Image 1 • 3D seismic data, core data, and well-logs were analysed to identify depositional systems. • Strata thickness and geomorphology was restored from 3D seismic data. • Facies architecture and sediment infill process of intra-basinal slopes are studied. [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]