The Lower Silurian Longmaxi Formation in the south of Sichuan is a key player in the exploration and development of shale gas in China. Due to a highly complex topographic area, electromagnetic methods (EM) become important exploration means in this area. Many studies have been conducted on the shale mineral composition and electrical properties of shale, however, the correlation between sedimentary environments and the electrical properties of shale remain poorly understood. The electrical properties and sedimentary environment of the organic-rich shale of the Longmaxi Formation have been studied by means of X-ray diffraction, organic geochemistry, scanning electron microscopy and complex resistivity measurements. The discovered high quartz content of the Longmaxi Formation shale results in low resistivity. Deep-water shelf biogenic quartz contributes lower resistivity more than that of shallow-water terrigenous quartz. The deep-water anoxic and organic sedimentary environment led to major enrichment of pyrite, leading to a high polarization effect in shale. We present the correlation between the lithofacies types and electrical properties of Longmaxi Formation. The mixed siliceous shale lithofacies is the most favorable among the three lithofacies, which is characterized by high total organic carbon (TOC) content, high brittleness mineral content, high polarization and low resistivity ("three high and one low"). This feature is an effective identification of shale gas reservoirs by electromagnetic prospecting. Our study can provide constraints on electrical parameters of rocks for electromagnetic "sweet spot" exploration of shale gas, and so this has important geological significance to shale gas exploration and development.

Silurian shale gas has been commercially extracted Zhaotong shale gas demonstration area,in order to realize the sustainable development of this place and search for new potential target strata of shale gas, well A targeted at the carboniferous Jiusi formation shale gas exploration was deployed and implemented to explore the gas content of Jiusi formation shale gas. Based on the systematic testing of organic carbon, major content, trace amount and rare earth elements in well A, combined with petrology, biological relic fossil characteristics and chromatography-mass spectrometry, the geochemical characteristics of elements in Jiusi formation in well A were analyzed, and the REDOX conditions in Jiusi sedimentary stage were identified, revealing the sedimentary environment for the development of black shale. The research results show that the petrological characteristics, major, trace and rare earth elements of Jiusi formation in A well of Zhaotong shale gas demonstration area are quite different from those of Wufeng formation and Longmaxi formation. The organic carbon content of Jiusi group in well A drilling is relatively high, with an average value of 1.13%, and the TOC content is not strongly correlated with depth. The content of Al2O3 in the main elements of Jiusi formation is high, mainly calcareous shale and clayey shale. The REDOX indexes such as trace elements and rare earth elements indicate that the depositional stage of the Jiusi formation was a relatively oxygen-rich environment, which was mutually verified with the content of TOC. Chondrites combination fossil and shale chromatogram mass spectrum characteristics in the gray mudstone of the Jiusi formation also reveal that the Jiusi formation developed in the paleosedimentary environment of the transition facies of sea and land, and its paleoproductivity was lower than that of the Wufeng formation and Longmaxi formation, and the abundance of organic matter in the Jiusi formation was relatively low. Field gas content test proves that Jiusi formation in Zhaotong shale gas demonstration area has good potential for shale gas exploration.

The Carboniferous-Permian coal measures in China contain abundant natural gas resources. Shale, coal and tight sandstone reservoirs are developed in coal measures, and the quantitative characterization of the pore structures of different types of reservoirs can provide scientific guidance for the sweet spot prediction of tight reservoirs. In this study, taking the Shan 2 Member coal measure of the Shanxi Formation in the eastern Ordos Basin as an example, the pore structures of shale, coal rock and tight sandstone were systematically studied based on organic geochemistry, scanning electron microscopy, high-pressure mercury injection, and low-temperature N2 and CO2 adsorption experiments. The results show that the microscopic pore structures of different types of reservoirs in the Shan 2 Member coal measures are quite different. Shale and tight sandstone mainly develop clay mineral pores at mesopore scale, followed by intragranular and dissolution pores developed in quartz and feldspar minerals, while organic pores are rarely developed. A large number of macro-scale clay mineral pores and micro-fractures are developed in tight sandstone, meanwhile the pore connectivity of tight sandstone is better than that of shale. A large number of micro to nano-scale organic pores are developed in coal, and the specific surface area of micropores in coal is much larger than that of mesopores in shale and tight sandstone. Sandstone, shale, and coal are frequently interbedded in coal measure strata. Tight sandstone provide the main storage space for free gas, and pores in shale and coal absorb a large amount of natural gas. Sandstone-shale-coal assemblages and sandstone-coal assemblages are the key targets for the exploration of hydrocarbons in the Shanxi Formation coal measures in the study area.

By performing scanning electron microscopy, microscopic observations, whole-rock X-ray diffraction analysis, organic geochemistry analysis, and elemental analysis on drill core specimens and thin sections, in this study, we classified the shale types of the Wufeng Formation-Member 1 of the Longmaxi Formation in western Hubei, southern China, and explored the development characteristics and formation environments of the different shale types. The results show that (1) the shales of the Wufeng Formation-Member 1 of the Longmaxi Formation are composed of three types of shale: siliceous shale, mixed clay-siliceous shale, and clay shale. The siliceous shale is a type of shale unique to deep-water environments; clay shale is the main type of shale formed in shallow-water environments; and mixed clay-siliceous shale falls between the two. (2) The changes in shale type are characterized by multiple depositional cycles in the vertical direction with strong heterogeneity and an obvious tripartite character, and the siliceous shales gradually thicken as they laterally extend northwestward, with their last depositional cycle gradually ending at a later time. (3) The Late Ordovician-Early Silurian paleoenvironment can be divided into six evolutionary stages (A, B, C, D, E, and F) from early to late. In particular, the sea level was relatively lower in stages A and F when the bottom water was mainly oxygen rich with higher terrigenous inputs and a lower paleoproductivity, which led to the formation of clay shales poor in organic matter but rich in terrigenous quartz clasts. The sea level was higher in stages B, C, and D when the bottom water was anoxic with lower terrigenous inputs and a higher paleoproductivity, which led to the formation of siliceous shales rich in organic matter and biogenic silica. The total organic carbon (TOC) contents of siliceous shales decrease in the order of stage C > stage D > stage B, which is mainly attributed to the different degrees of water restriction in the three stages and the consequently different paleoproductivities. Stage E corresponds to the mixed clay-siliceous shales, the depositional environment of which is between those of the siliceous shales and the clay shales, thereby resulting in the mineral composition and TOC content of the mixed clay-siliceous shales being between those of the other two shale types.

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