Influence Of Granite Intrusion On The Mineralogy And Pore Structure Of Shales

Shale gas can either adsorb or freely exist in the organic rich clay-shale and its interlayers, and shales serve as sources, seals, and reservoirs. With the development of horizontal well drilling and hydraulically fractured technology, it is possible to develop low-porosity, low-permeability shale reservior effectively. The characteristics of shale pore stucture remain the key matter in the exploration of shale gas. And preservation conditions of shale gas, especially tectonic movement and magmatism, are generally critical factors of evaluating shales. This paper summarizes the recent techniques and classification methods used in pore structures of shales, and presents examples on the Paleozoic shales in the Lower Yangtze region. In order to investigate the influence of granite intrusion on shale gas preservation in mineralogy, organic geochemistry and pore structure, we selected the Luocun section in South China, where the early Cambrian shales (i.e., the Hetang Formation) were intruded by an early Cretaceous granite dike.Image analysis and physical measurements are the common techniques to characterize shale gas system. Image analysis (e.g. Scanning Electron Microscopes, Transmission Electron Microscopes, and Atomic Force Microscopes) can obtain the morphologic information and distrubition with image processing software. However, Physical measurements (e.g. Gas adsorption, Mercury porosimetry methods and Nuclear Magnetic Resonance methods) can aquire shale pore size, distrubition, porosity and the specific surface area in a statistical sense. Based on the techniques, classification methods of shale pore strcture can be divied into three categories:one classification method based on image annlysis (interparticle pores, intraparticle pores, intraparticle organic matter pores and microcracks), one based on physical measurements (macropore, mesopore and micropore), and one conbined these two methods. Pores of the Paleozoic shales (Hetang Formation, Hule Formation and Ningguo Formation) in the Lower Yangtze region are interparticle pores, intraparticle pores and organic matter pores. The range of pore size is 0.084～36.5μm with the average 1.04μm, and the mesopores are more than 50%。Contact metamorphism will lead to significant changes in mineralogy, organic geochemistry and microstructure (e.g., microcracks, pore size and shape) of shales. To reveal thermal and mechanical effects of granite intrusion on shales, we systematically investigated geochemical and microstructural features of the Hetang Formation along the Luocun section, as well as an original sample of the Hetang Formation from the Mufushan area for comparison. Compared with silty mudstones from the Mufushan section without any intrusion, the siliceous shales in the Luocun section have been silicified and contain some banded black carbonaceous residues, with less clay minerals when closer to the dike. The equivalent vitrinite reflectance is 2.6% at a distance of 50 m to the dike and raises to 3.7% next to the dike, suggesting that the granite dike has enhanced the thermal maturity of organic matter. The variation of vitrinite reflectance along the Luocun section reveals that the aureole width is ～37% of the dike width. However, there is no evident correlation between total organic carbon content of shales and their distance to the dike. Pores in shales from the Luocun section include interparticle pores, intraparticle pores, and a few large (>50 nm) intraparticle organic matter pores. With decreasing distance to the dike, the specific surface area of shale samples reduces from 5.22 to 0.16 m2/g due to decreasing pore volume of mesopores and micropores. However, thermal expansion of shales and thermal reaction products of organic matter have produced macropores and microcracks in shales, which will enhance permeability of shales and result in fast escape of shale gas during intrusion of granites. Therefore heat and siliceous fluids from granites will modify composition and pore structure of shales and hinder preservation of shale gas within the contact aureole.