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[Objective] The dispute in the paleogeographic pattern of the carbonate platform and distribution of reservoir facies during the deposition of the Dengying Formation in the middle-upper area, hinders the expansion of the exploration domain from the Mianyang-Changning intracratonic sag to other areas. [Methods] Detailed lithofacies and facies analysis was conducted on more than 30 sections (wells), [Results] further confirming that the mound-shoal is marked by massive peloidal/ooidal dolograinstone and columnar, domal dolostromatolite, which were deposited around the platform (or inner ramp) margin of the Middle-Upper Yangtze Platform gradually shifting outward into tempestite of the middle-outer ramp or slump dolobreccia, argillaceous dolomite and chert of slope-basin facies. Owing to the mound-shoal barrier, its back was dominated by tidal flat and lagoon. Of these, the tidal flat facies were marked by microbial dololaminite, domal dolostromatolite, dolothrombolite with minor peloidal dolograinstone, and dolomudstone, which are commonly arranged into meter-centimeter scale cycles. The lagoon facies were composed mainly of dolomudstone and peloidal dolowackestone. In general, the mound-shoal facies is present in the lower Dengying Formation along the platform to slope transition (or inner ramp to middle ramp transition) . In the Upper Yangtze Platform, it occurs in the Deng 1 (e.g., wells Pengtan 1, Ziyang 1, and sections Qingping, Beidoushan) or Deng 2 members at the Baijiaba, Yulin, and Meiziwan sections, or in both the Deng 1 and Deng 2 members at the Songlin-Yankong areas. On Middle Yangtze Platform, it occurs in the Hamajing (e. g., sections Sancha, Bailuya, and Tianping) or Baimatuo members of the upper Dengying Formation at the Miaohe section. Specifically, the mound- shoal facies shows repeated transgressions and regressions and occurs in different stratigraphic position of the Dengying Formation at many sections (Baiguoping, Xiaofenghe, Dengyingxia, Wushan, Xuejiadian, and Dengjiaya) . In contrast, the tidal flat and lagoon facies are widely present on the whole Middle-Upper Yangtze Platform, commonly associated with the grape-like dolomite at the lower-middle part of the Deng 2 member (or correlated stratigraphy) . The spatiotemporal distributions of these facies show a progradational trend of the Dengying Formation on the Middle- Upper Yangtze Platform, which consists of 2.5 depositional sequences with three progradations (shallowing) and two retrogradations (deepening) . The regressive system tract of Sequence 1 is formed by the lower Dengying Formation (the Deng 1 to middle Deng 2 or Hamajing members), characterized by the upward evolution of lagoon or moundshoal facies into tidal flat facies. On the Upper Yangtze Platform, Sequence 2 occurs in the upper Deng 2 member, marked by the regression of mound-shoal facies and the presence of more lagoonal dolomudstone, followed by the reoccurrence of tidal flat facies. On the Middle Yangtze Platform, Sequence 2 is present in the Shibantan member, represented by the deepening into middle ramp limestone or lagoonal dolomudstone and then shallowing into moundshoal or tidal flat facies. Sequence 3 on the Middle Yangtze Platform shows a similar deepening (transgression) and shallowing (regression) cycle. On the Upper Yangtze Platform, the transgressive systems tract of Sequence 3 is located in the siliciclastic rock of the Deng 3 member and the following transgressive systems tract is marked by the transition of lagoonal dolomudstone to tidal flat dolomicrobialite. [Conclusions] Owing to the denudation of the Dengying Formation at the platform margin-slope area and Upper Yangtze Platform beyond the current plate boundary, the platform marginal mound-shoal facies are locally present in the Deng 4 member. In this case, certain areas (e.g., the Songlin-Yankong area and periphery of the Middle Yangtze Platform) with platform marginal mound-shoal facies in the lower-middle Dengying Formation (Deng 1 to 2 or Hamajing to Shibantan members) are important exploration targets. Moreover, the lower Deng 2 and upper Deng 4 members (or coeval strata) formed during progradations composed of microbial dolomite throughout the Middle-Upper Yangtze Platform (or inner ramp) interior are also important exploration targets. Because the microbial dolomite was widely developed throughout the Middle-Upper Yangtze Platform interior, decoding the formation mechanism and distribution pattern of high-quality reservoir is the key to finding new hydrocarbon provinces. [ABSTRACT FROM AUTHOR]

Alkaline lakes (pH > 9) are among the few modern sedimentary environments that are hydrochemically favourable for low‐temperature dolomite formation. While Mg‐clays and Mg‐evaporites also form more easily in alkaline environments, few studies have focused on how the kinetically inhibited dolomite wins the competition for Mg2+. Here, a basin‐wide survey of the distribution, paragenesis and stable C, O and Mg isotopes of main Mg‐rich minerals in the Late Palaeozoic saline alkaline lake deposit of the north‐west Junggar Basin, north‐west China, is conducted to study the influence of the formation and diagenesis of eitelite, northupite and Mg‐clays on dolomite formation. Large, isolated dolomite crystals (20 to 70 μm in diameter), show positive δ13C values (ranging from +1 to +7‰) and a restricted distribution in the mudstones of the lake‐transitional zone. These crystals have been interpreted as organogenic dolomite driven by methanogenesis via fermentation of organic substrates. The δ18O values of dolomitic mudstones (from −7.4 to +3.4‰), calcitic mudstones (from −15.1 to −3.3‰) and bedded Na‐carbonate evaporites (from +0.08 to +3.7‰), together with their Mg isotopic compositions, suggest that dolomite was not enriched in the most concentrated environments or during stages with most Mg sources, but in the organic‐rich deposits containing few other authigenic Mg‐rich minerals. Dolomite is at a competitive disadvantage for Mg2+ ions compared to Mg‐evaporite and Mg‐clay minerals due to its slow crystallization rates and the deficiency of micritic calcium carbonate precursors. However, it can nucleate and progressively grow into large crystals (>20 μm) if bacterial methanogenesis could effectively lower porewater pH (<8.5) and induce the dissolution of generated eitelite, northupite or Mg‐clays. These findings suggest that high salinity and/or high alkalinity are not always favourable conditions for dolomite formation and highlight the active role of pH fluctuations in inducing low‐temperature dolomite formation. [ABSTRACT FROM AUTHOR]

Travertines formed of crystalline crust have been widely reported, but there has not been focus on their geochemical characteristics. We therefore carefully conducted a series of geochemical investigations and U-Th dating on a travertine mound mainly composed of crystalline crust from Sobcha (southwest China) to determine their geochemical features and geological implications. The Sobcha travertines dominantly consist of granular crystals and fan crystals and show δ13C from 3.4‰ to 4.9‰ V-PDB, δ18O from −26.7‰ to −23.7‰ V-PDB, and 87Sr/86Sr from 0.712458 to 0.712951. When normalized to PASS, the Sobcha travertines exhibit MREE enrichment relative to HREE and LREE, HREE enrichment relative to LREE, and positive Eu anomalies. The δ13C signatures and mother CO2 evaluation of the Sobcha travertines show that the Sobcha travertines were thermogene travertines largely receiving mother CO2 from (upper) mantle (i.e., magmatic CO2) or a mixture of soil-derived CO2 and CO2 related to carbonate decarbonation. The 87Sr/86Sr of the Sobcha travertines is out of the 87Sr/86Sr ranges of local deposits exposed at Sobcha and surrounding areas but is well matched with the mean 87Sr/86Sr of Nadi Kangri volcanic rocks which cropped out to the northeast of the studied travertines (over 20 km away). This might indicate the important role of the Nadi Kangri volcanic rocks in suppling Sr to the studied travertines, but more studies are required. The LREE depletion compared to MREE and HREE in the Sobcha travertines was interpreted to be caused by the difference in geochemical mobility between LREEs and HREEs during water–rock interaction at depth, while the MREE enrichment compared to HREE was considered to be most likely inherited from reservoir/aquifer rocks. The positive Eu anomalies of the Sobcha travertines may result from very high reservoir temperatures and/or preferential dissolution of Eu-rich minerals/rocks (especially plagioclase). The Sobcha travertine mounds displays no or very slight vertical variations in δ13C, 87Sr/86Sr, and REE patterns, indicating the compositional stability of mother CO2 and paleo-fluids. However, a significant vertical increase in δ18O was observed and was explained as the result of gradual water temperature decrease related to climate cooling, self-closure of the vents, or mound vertical growth. The findings in this study might help us better understand the deposition of crystalline crust in Ca2+-deficient hot spring systems. [ABSTRACT FROM AUTHOR]

The identification markers of paleokarst have been described in detail and include a combination of surface geology, drilling data and the isotopic characteristics of carbon, oxygen and strontium. The relationships between the vertical zoning, plane distribution and karstification degree of paleokarst and the reservoir development were further analyzed. The study shows that karstic breccias are the most direct sign of paleokarst features. Karsts of different genetic types and the intensity of dissolution have different stable isotopic characteristics from the surrounding cements, and their formation and evolution are significantly influenced by fluid sources, fluid nature and isotopic fractionation. The identification markers of small scale pinholes are quite different from that of large scale holes. The development and evolution of palaeokarsts are mainly controlled by structure, palaeoclimate and lithology. The best lithology for the development of reservoirs is dolomite in the C2hl2 formation. The best zones for the development of the reservoir is located in the upper and lower dissolution intervals of the C2hl2 formation and the best position for the development of reservoir is located in the karst highlands, remnant hills and sloping fields in the karst slope. Key words: eastern Sichuan, Huanglong Formation, palaeokarst reservoir, geochemistry, karst topography, karst cycle

Travertine mounds are common spring‐constructed landforms with highly variable geometries, but factors controlling their development have not yet been fully understood. To investigate processes linked with their lateral and vertical growth, this study investigated travertine mounds at Heinitang (China) and compared them with typical mounds around the world. Calcium concentration variation modelling and height evaluation of travertine mounds were also performed. The lateral extent of travertine mounds was found to be mainly controlled by Ca2+ and HCO3‐ concentrations and water discharge: relatively small metre‐scale mounds, like those from Heinitang, are largely the products of Ca2+‐deficiency (<150 mg L−1, commonly <100 mg L−1) or HCO3‐‐deficiency (<500 mg L−1) springs with low discharges (<0.1 L s−1), whereas springs with higher Ca2+ and HCO3‐ concentrations and discharges give rise to the formation of wider mounds. Additional factors affecting mound lateral development include infiltration, evaporation, micro‐organisms, water cooling, CO2 degassing and flow paths. Height of travertine mounds cannot be simply considered to represent the piezometric surface, because the vertical growth of travertine mounds is also influenced by the water table change triggered by tectonic pulses and climate fluctuations, and the closing of conduits induced by self‐sealing. The extinction of travertine mounds at Heinitang is, for example, probably caused by the water table lowering triggered by earthquakes or climate drying after 4 kyr bp. These findings offer new inputs for the better understanding of factors controlling travertine mound development and might assist in the reconstruction of fluid properties, hydrodynamics, tectonics and climate fluctuations in fossil geothermal systems with travertine mounds. [ABSTRACT FROM AUTHOR]