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[Display omitted] • The bitumen in wall rocks and the Danzhai paleo-oil reservoir both originate from the Lower Cambrian source rock. • The paleo-oil reservoir and the ore sulfur both derive from seawater sulfate in the strata. The metallogenic metals are mainly sourced from the underlying strata and basement. • Bitumen Re-Os and sphalerite Rb-Sr isotopes yield isochron ages of 424±160 Ma and 398±4.7 Ma, respectively. • Hydrocarbons supply a large amount of reduced sulfur for mineralization through TSR reaction. The Niujiaotang MVT Pb-Zn ore field is located at the southwestern end of the West Hunan-East Guizhou (WHEG) metallogenic belt, which overlaps spatially with the Majiang paleo-oil reservoir. The Pb-Zn mineralization is closely associated with oil-gas accumulation processes. However, the genetic link between the paleo-oil reservoir and the Pb-Zn deposits remains unclear. In this paper, we present new findings of bitumen (paleo-oil reservoir) and its features in the deposits, and apply various isotopic geochemical methods and 2D seismic data to elucidate the source rock and coupling relationship of the paleo-oil reservoir and the Pb-Zn deposits. This study reveals that the ore has a simple mineral composition, mainly comprising sphalerite, pyrite, and dolomite. Solid bitumen occurs widely in the ore and the host dolostone, and coexists with minerals from the mineralization stage. Laser Raman spectroscopy shows that the bitumen in the wall rocks of the Chengong deposit and the Danzhai paleo-oil reservoir are both overmature, and their carbon isotope compositions (δ13C PDB = –29.17 ‰ – –31.20 ‰, average –30.10 ‰, n = 6) and initial 187Os/188Os ratio (1.08±0.19) suggest that they have the Lower Cambrian source rock similar to the Majiang paleo-oil reservoir. The δ34S value of pyrite in the bitumen from the wall rocks in the Chengong deposit ranges from +28.15 ‰ to +44.12 ‰ (average +33.28 ‰, n = 18), which is consistent with the sulfur isotope composition of the ore sulfide reported previously, indicating that they both derived from sulfate in the Middle-Upper Cambrian strata. Bitumen Re-Os and sphalerite Rb-Sr isotopic dating indicate that oil reservoir accumulation in the ore-hosting strata preceded Pb-Zn mineralization, occurring at 424±160 Ma (MSWD = 1.3) and 398±4.7 Ma (MSWD = 0.66), respectively. The initial 87Sr/86Sr ratio of sphalerite is 0.709983±0.000027, which, after comparison and in combination with previous studies, suggests that the metallogenic metals mainly originated from the underlying strata and basement. 2D seismic data show that the Zaolou fault (F 2) that controls the mineralization is a deep fault that extends to the basin basement, and acts as a migration pathway for hydrocarbon fluids and ore-forming materials. We propose that liquid hydrocarbons played a significant role in Pb-Zn mineralization, acting as reductants to reduce sulfate through the thermochemical sulfate reduction (TSR) reaction and provide abundant reduced sulfur (H2S/HS−) for Pb-Zn mineralization. Finally, we present a comprehensive model of hydrocarbon accumulation and mineralization in the Niujiaotang ore field. [ABSTRACT FROM AUTHOR]

Numerous studies have been conducted on the isotopic geochemical characteristics of shale gas. However, a systematic study of the relationship between the isotopic geochemical characteristics and gas content has not been addressed to date. In this study, we investigated the geochemical and isotopic compositions of shale gas samples, and the gas content of the Lower Silurian Longmaxi marine shale in the Huangjinba area, southern Sichuan Basin, China. In addition, we analyzed the quantitative relationship between the geochemical parameters and the gas content. The Longmaxi shale gas is a dry gas with a low wetness (0.12%–0.67%), dominated by methane (89.49%–99.00%) with a small amount of ethane (0.11%–0.61%). The average values of δ13C methane , δ13C ethane and δ13C propane are −29.8‰, −35.0‰, and −33.2‰, characterized by a partial isotopic reversal (δ13C methane > δ13C propane > δ13C ethane) with a maturity level (Ro ≈ 2.30%–2.51%). The total gas content of the Longmaxi shale varies widely from 0.42 m3/t to 3.88 m3/t. Correlation analyses revealed that the ethane isotope (δ13C ethane) and the magnitude of isotopic reversal (δ13C methane - δ13C ethane) exhibit strong correlations with the total gas content and the free gas content. This study demonstrates that carbon isotopic reversal of shale gas largely depends on the secondary cracking, and thermal maturity and residual liquid hydrocarbons content are important factors affecting the magnitude of carbon isotopic reversal. Based on the quantitative study conducted in this paper, we provide a new proxy for predicting the gas content of high maturity shale using an isotopic method. • Carbon isotopic reversal of shale gas largely depends on the secondary cracking. • Sufficient residual liquid hydrocarbon is necessary for the carbon isotopic reversal. • Wetness of 1.4% in high maturity shale gas indicates the peak of secondary cracking. • δ13C methane –δ13C ethane can be used to predict the gas content of high maturity shale. [ABSTRACT FROM AUTHOR]

• The methane in Weiyuan area comes from different hydrocarbon generation materials. • Nitrogen in Xiuwu basin is derived from atmosphere and deep crust-upper mantle. • The sealing capacity and the anticline are the reason for the high methane content. • The detachment layers, faults and volcanism are the reasons for the high nitrogen. Marine shale gas exploration in southern China has successes and failures. Under the condition of great hydrocarbon generation material basis, shale gas wells drilled from some shale gas blocks are rich in methane, while the wells in other shale gas blocks with high nitrogen and low hydrocarbon gas, which indicates that they have different accumulation mechanisms. Therefore, the study of gas composition in shale will help us to figure out the mechanism of shale gas accumulation and loss. In this paper, the Lower Cambrian shale from Wei-201 well in Upper Yangtze Weiyuan Block and Jiangye-1 well from Lower Yangtze Xiuwu Basin are selected as research object, and shale samples are used for tests and experiments including analysis of gas composition and nitrogen isotope, test of porosity and TOC content, overburden permeability test, permeability test before and after methane adsorption under different osmotic pressure, permeability test parallel and vertical to the bedding surface, FIB-SEM (Focus Ion Beam Scanning Electron Microscope) and FIB-HIM (Focused Ion Beam Helium Ion Microscope). Finally, the reasons for the difference in the gas components of the Lower Cambrian shale gas in Weiyuan Block and Xiuwu Basin are studied by means of seismic interpretation, core description and outcrop observation besides the tests and experiments. The results show that the gas components of the Lower Cambrian shale in Weiyuan Block, the Upper Yangtze, mainly consist of methane, derived from liquid hydrocarbon cracking. The sealing capacity of roof and floor, the great self-sealing of shale and the flat anticline structure contribute to the high methane content in shale gas. The Lower Cambrian shale gas in Xiuwu Basin, the Lower Yangtze, is mostly nitrogen, which is derived both from atmosphere and deep crust-upper mantle. The detachment layer at the bottom of the Lower Cambrian, the widely developed deep faults and the Jurassic volcanic activity are the reasons for the high nitrogen and low hydrocarbon of shale gas. Based on the above analysis, the patterns are summarized for shale gas accumulation in the simple anticline background and reservoir destruction in the complex syncline background. [ABSTRACT FROM AUTHOR]