Influence of the Emeishan basalt eruption on organic matter pore structure and chemical characteristics : A case study of shale from Longmaxi shale in southern Sichuan Basin
Authors: Liang Zhikai , Jiang Zhenxue , Muhammad Arif , Tang Xianglu , Xue Zixin , Wang Meng 
Published: March 2024

The Emeishan Large Igneous Province (ELIP) at the end of the Middle Permian was a very important geothermal event, which formed a giant intrusive and eruptive magmatic system, and had a great impact on the maturity of Organic matter (OM), hydrocarbon production process, storage capacity and gas content. In order to study the influence of the ELIP on OM characteristics and shale gas development. In this study, the pore structure and kerogen macromolecule structure of OM were characterised using various techniques in several typical wells of Longmaxi Formation shale in the southwestern part of the Sichuan Basin, including laser Raman, X-ray diffraction, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), low-pressure N2 and CO2 adsorption, in order to clarify the controlling effect of ELIP on the maturity and carbonation of Longmaxi Formation shale, to reveal the connection between ELIP and the kerogen chemical and pore structure, and to establish a model for the maturity and carbonation of the OM under the influence of volcanic activities model. The results show that the middle zone of the near-mantle column reaches the stage of high-over maturity, and the resistivity is generally lower than about 10 &#937;&#183;m, with a negative correlation with TOC. ELIP promotes higher geothermal heat flows, resulting in regular changes of the kerogen molecular structur. In the Middle zone of ELIP, the aliphatic content and oxygenated functional groups decrease, the relative content of sp2 carbon and the degree of OM carbonation increases. The number of stacked layers of aromatic basic structural units in the macromolecular structure of casein increases, the stripe length increases significantly, and the arrangement order gradually increases with consistent orientation. The microporous volume and surface area of the OM in the middle zone of ELIP are smaller than those in the outer zone, which may be attributed to the further condensation of the aromatic compounds caused by the higher paleogeodetic heat flux values, which leads to the increase of &#960;-bonds between carbon atoms, the gradual decrease of layer spacing, and the gradual change of the OM into graphite-like crystals with higher Young's modulus and lower Poisson's ratio. The high degree of OM carbonation reduces the hydrocarbon generating capacity, and in the case of gas dissipation in the area of strong fracture activity, the overall pore fluid pressure is low, which destroys the pore preservation conditions, and the microporosity collapses and closes. Overall, the OM close to the mantle column has a higher degree of graphitization, and the microporous volume and surface area are drastically reduced, while the preservation conditions are destroyed by the fracture activity, resulting in a significant decrease in both the storage capacity of the OM. The present study improves the exploration theory of high overmature organic-rich marine shale, and provides geological basis for the selection of marine shale gas-rich areas in southern China.