EFFECT OF ORGANIC MATURITY ON SHALE GAS GENESIS AND SHALE RESERVOIR DEVELOPMENT: A CASE STUDY ON MARINE SHALE IN THE UPPER YANGTZE REGION, SOUTH CHINA

Compared with the North American shale formations, the marine shale in southem china has undergone multiple stages of complex structural and thermal evolution. According to previous exploration experience, excessive thermal evolution brings certain risks to shale gas exploration and development. Therefore, it is a significant problem to be solved to study the impact of maturity upon the genesis of shale gas and development features of organic pores.

This paper takes the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation in the Sichuan Basin and Lower Cambrian Niutitang Formation in Outer Margin of the Sichuan Basin. A series of geochemical research methods, such as TOC analysis, gas component analysis, gas isotope analysis, were adopted to study the impact of different thermal evolution stages of organic matter upon the genesis of shale gas. Meanwhile, the nanoscale micro-imaging technique, such as FIB-SEM and FIB-HIM, was used to analyze the development of organic matter pores. The experiments results are applied on studying the effect of organic maturity on shale gas genesis and reservoir development characteristics.

As shown from the results, when R_o=0.5%~1.2%, the kerogen generates liquid hydrocarbon by thermal evolution and develop kerogen pores. The liquid hydrocarbon is largely filled into the inorganic mineral matrix pores. when R_o=1.2%~3.5%, the marine shale gas is dominated by the methane and other hydrocarbon gases, since the mixture of cracking gas from liquid hydrocarbons and kerogen-cracking gas cause the carbon isotope reversal. Besides, the pyrobitumen pores with large quantity, high diameter, strong connectivity and storage capacity were primarily developed. When R_o>3.5%, the organic matter is at the graphitization stage. Most of the hydrocarbon gas such as methane dissipation at this stage, while the shale gas is mainly composed by nitrogen. The nitrogen is originated from the atmosphere and the thermal evolution process when the organic matter is at high-over maturity stage, and the organic matter pores (pyrobitumen and kerogen pores) with small quantity and diameter, bad connectivity and storage capacity are developed. Finally, the main component of shale gas, the genesis of shale gas and pattern of organic matter pores under different thermal evolution stages of organic matter are summarized, which provides technical support for the exploration and development of shale gas.