Geological and Geochemical Characteristics of Black Shale in Yanxi Formation of Middle Ordovician in Central Hunan

Abstract

The Middle Ordovician Yanxi Formation (O2y) in the central area of Hunan province with a set of organic-rich marine shales is having great potential for prospecting of shale gas. Through lots of fieldworks and laboratory tests, this paper has concluded the geological characteristics of Yanxi Formation in this area by comprehensively investigation and analysis on the distribution and geochemical characteristics of the organic- rich marine shales and the features of reservoir. In the study area, it is found that the black shales of Yanxi formation are characterized by widely distribution, great thickness (ranging from 40 to 60m) and high level of organic carbon (ranging from 0.87% to 6.20%, generally higher than2%), the thermal maturity is at the mature to over mature stage. All of these characteristics of the black shares are benefit for generating gas. Mineral composition of black shales include quartz, feldspar, calcite, dolomite, clay minerals and small amounts of pyrite etc., the brittle minerals percentage is ranging from 47% to 87%. The pore types include intergranular pore, intracrystalline pore, mould pore, dissolved pore and organic pore etc, the aperture size of pores varies. Above all, O2y in Hunan province is a new horizon with great potential for shale gas, especially the area along with Xingning-Qingtong-Ningyuan has been defined as an exploration target with priority.