--> --> The lamina of shale and its influence to the migration and accumulation of natural gas in the Silurian Longmaxi Formation in the Southern Sichuan Basin

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The lamina of shale and its influence to the migration and accumulation of natural gas in the Silurian Longmaxi Formation in the Southern Sichuan Basin

Abstract

The shale in the Silurian Longmaxi Formation in the Southern Sichuan Basin, which is highly matured and laterally stable in distribution, primarily contains four lithofacies including black siliceous shale, silty shale, mixed siliceous and calcareous shale, and argillaceous shale upwardly. Among them, the black siliceous shale at the bottom is the primary pay layer, which shows high content of methane and large brittleness. Compared with other lithofacies, the laminas are well developed in this interval by the observation of field outcrop samples, drilling cores, ordinary thin sections, and FMI logging images. Therefore, it is considered that the laminas may influence the migration and accumulation of shale gas to some extent. Macroscopically, the migration and accumulation are respectively related to permeability and porosity of shale, which can be interpreted by connected porosity system allowing for the transportation of methane and adequate storage space for the occurrence of natural gas microscopically, notwithstanding their diametrically different occurrence states in varying porosity system. The shale with different types of lamina may differ in the porosity system which can influence the migration and accumulation of methane. First, the shale samples (field outcrops and drilling cores) in the Silurian Longmaxi Formation were selected as the research object to conduct fine descriptions of mineralogical and petrological characteristics, the assemblage between organic matter and minerals, and occurrence of organic matter, thereby classifying laminas in the shale. Second, the shale samples with different types of laminae will be optimized for physical properties analysis and NMR (Nuclear Magmatic Resonance) test to monitor the T2 signal sequential imbibition of brine and oil, which can be used to evaluate the pore connectivity and physical properties of shales with different types of laminae. For the porosity system in shale, it is a living organism of organic-hosted pore occurred in organic matters including dominant kerogen and a minor of pyrobitumen and solid bitumen, and of inorganic pore dominated by intercrystalline pore in clay mineral. The dual fluid NMR method can give a quantitative evaluation on the different porosity systems in shale according to wettability variance of different porosity systems. Based on the experimental results, the analysis of the intrinsic relationship between pore type, morphology, interface effect of medium, and pore connectivity for different lamina types will be conducted in order to reveal the differences in physical properties and pore connectivity between shales with different types of lamina, and comparing with the results of porosity system from different shales, thereby elucidating its influence and significance to the migration and accumulation of shale gas. This research is to promote the exploration and development of shale gas which has been increasingly drawn the attention of the public and to increase the supply of natural gas resources which gives new stable substitute for conventional oil and gas resources, thereby playing an important role in protecting the environment through energy conservation and emission reduction.