The Cambrian Jiulaodong Formation of the Wei-201 well block in the Sichuan Basin was investigated for shale gas potential. In the subsurface, the thermally mature formation attained a stable thickness of 234 m encompassing an area of approximately [Formula: see text] and representing a potential gas resource. The total gas content measurements from canistered samples was more than the estimated total gas storage capacity of the free gas, absorbed gas, and gas dissolved in water and in oil. The canister gas content ranged between 0.971 and [Formula: see text] and averaged [Formula: see text]. The average estimated gas in place was 2.5 billion cubic meters for the formation in the Weiyuan area. Reflectance measurements for thermal maturity range between 2.60% and 3.06% and average 2.84%. The results of our total organic carbon content (TOC) content analysis conducted on the core shale samples indicate that the TOC content of the formation ranges from 0.87% to 3.57% and averages 2.2%. The mineral composition of marine mudstone formation of the Jiulaodong shale is relatively consistent. Brittle mineral content increases with organic carbon content and is approximately 32%–43%, of which quartz content is 29%–40% with a very low amount of clay mineral as the mixed layer. The amount of illite-smectite ranges from 0% to 1% and the brittleness index range from 37% to 62% and average 57.1%. The Cambrian Jiulaodong Formation ha very good petroleum-source rock potential due to its average TOC content of greater than 2%, average canister gas content of [Formula: see text], good type I kerogen, high maturity with average 2.84% of source rocks that are characterized by a fairly high abundance of organic matter increasing from top to bottom and a large thickness of 234 m. Natural fractures, cracks, and pores developed in the Jiulaodong Formation also provide space for shale gas storage, and its average brittleness index is greater than 57%, which is good for fracability.

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Hydrothermal dolomite paleokarst reservoir is a type of porous carbonate reservoir, which has a secondary porosity and can store a large amount of oil and gas underground. The reservoir is formed by magnesium-rich hydrothermal fluids during the karstification and later stages of the transformation. Due to the strong heterogeneity and thin thickness of hydrothermal dolomite paleokarst reservoirs, it is a real challenge to characterize the spatial distribution of the reservoirs. In this paper, we studied the hydrothermal dolomite paleokarst reservoir in the Wolonghe gasfield of the eastern Sichuan Basin. First, based on detailed observations of core samples, the characteristics and storage space types of the dolomite reservoir were described. Secondly, the petrophysical parameters of the paleokarst reservoirs were analyzed, and then the indicator factor for the dolomite reservoirs was established. Thirdly, using the time–depth conversion method, the geological characteristics near boreholes were connected with a three-dimensional (3D) seismic dataset. Several petrophysical parameters were predicted by prestack synchronous inversion technology, including the P-wave velocity, S-wave velocity, P-wave impedance, and the hydrothermal dolomite paleokarst reservoir indicator factor. Finally, the hydrothermal dolomite paleokarst reservoirs were quantitatively predicted, and their distribution model was built. The 3D geophysical characterization approach improves our understanding of hydrothermal dolomite paleokarst reservoirs, and can also be applied to other similar heterogeneous reservoirs.

The marine shale of the Upper Ordovician Wufeng Formation–Lower Silurian Longmaxi Formation is the main source rock and the target of shale gas exploration in the southern Sichuan Basin. The maturity of organic matter (OM) is a vital indicator for source rock evaluation. Due to the lack of vitrinite, the organic matter maturity of the Wufeng–Longmaxi Formations in the southern Sichuan Basin is difficult to accurately evaluate. In total, 33 core samples of the Wufeng–Longmaxi Formations in the southern Sichuan Basin were selected to observe the optical characteristics of solid bitumen and graptolites and measure their random reflectance. Simultaneously, Raman spectroscopic parameters of kerogen were also used to quantitatively analyze the change in maturity. By using Raman spectroscopic parameters as mediators, conversion equations between graptolite random reflectance (GRor) and equivalent vitrinite reflectance (EqVRo) were established. Taking the calculation results of EqVRo as constraints, the tectono-thermal evolution history of Wufeng–Longmaxi Shale in the southern Sichuan Basin is constructed through basin modelling. The results show that the maturity of Wufeng–Longmaxi Shale in the western Changning, Luzhou-western Chongqing, eastern Changning and Weiyuan areas decreases successively. The EqVRo falls in the ranges of 3.61%~3.91%, 2.92%~3.57%, 3.08%~3.25%, 2.41%~3.12%, and the average EqVRo is 3.73%, 3.30%, 3.18% and 2.80%, respectively. Thermal evolution in western Changning was controlled by the thermal effect of the Emeishan mantle plume and paleo-burial depth, while the thermal evolution of other areas was mainly controlled by paleo-burial depth. This study provides a reliable parameter for the evaluation of thermal maturity and makes a more accurate calibration of the maturity of the Wufeng–Longmaxi Formations in the southern Sichuan Basin; it also expounds the factors for the differences in thermal evolution in different parts of the area.

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