INFLUENCE OF THERMAL MATURITY AND ORGANIC MATTER TYPE ON PORE STRUCTURE OF TRANSITIONAL ORGANIC-RICH SHALES

Organic matter (OM) nanopores developed in transitional facies shale, i.e., the Upper Permain Longtan and Dalong Formations in the Yangtze Platform, China, were investigated to reveal the corresponding influences of thermal maturity and OM types. A suite of 16 samples were respectively taken from Longtan (type-III) and Dalong (type-II) Formations from two wells covering a maturity from 1.22%-1.43% and 2.62%-2.97% Ro, to perform integrated analysis including field emission scanning electron microscopy (FE-SEM), gas physisorption (N₂ and CO₂), high-pressure CH₄ adsorption and mercury intrusion capillary pressure (MICP) analysis. To study the migration of hydrocarbons, fluid inclusions hosted in calcite veins in Dalong Formation were studied using laser Raman and fluorescence spectroscopy. FE-SEM images suggest that OM pores in different formations vary dramatically in terms of shape, size, and distribution density. OM pores in Dalong Formation are mainly micropores, sparsely distributed in solid bitumen with spot-like and irregular shape. In contrast, there are abundant pores densely developed in pyrobitumen in Dalong Formation with bubble-like, alveolate and spongy shape, and the diameter of OM pores range from several to hundreds of nanometers. Plenty of blue fluorescent oil inclusions and a small amount of CH4 inclusions mixed with C2H6 can be observed within calcite veins in Dalong Formation whereas only CH4 inclusions can be identified within calcite veins in Dalong Formation, indicating that maturity controlled both of the type of generated hydrocarbons and OM pores development in type II OM. The pore structure of type-II Dalong Formation in dry gas window obviously better than that in early stage of wet gas window. It is therefore believed that thermal maturity can significantly contribute to the formation of OM pores in type II OM. Compared with the overlying Dalong Formation with similar maturity, the pore structure and morphology of OM pores are significantly different in type-III Longtan Formation due to difference in OM type. The primary OM pores in terrestrial kerogen debris in Longtan Formation samples have relatively larger pore diameter ranging from hundreds of nanometers to a few micrometers with almost all rounded shape and a certain amount of kerogen debris are nonporous. The change of the proportion of different kerogen maceral appears to influence the pore structure of samples from Longtan Formation.