Effect of Organic Matter Type and Maturity on Organic Matter Pore Formation of Transitional Facies Shales: A Case Study on Upper Permian Longtan and Dalong Shales in Middle Yangtze Region, China

Abstract

Organic matter (OM) nanopores developed in transitional facies shales, i.e., the Upper Permian Longtan and Dalong Formations in the Yangtze Platform, China, were investigated to determine the corre- sponding influence of thermal maturity and OM types within the geological conditions. A suite of 16 core samples were taken from Type-III Longtan shales and Type-II Dalong shales from two wells covering a ma- turity (Ro, vitrinite reflectance) ranging from 1.22% to 1.43% and 2.62% to 2.97%, respectively. Integrated analysis of the shale samples was carried out, including field-emission scanning electron microscopy (FESEM), low-pressure N₂ and CO₂ adsorption, high-pressure CH₄ adsorption, and mercury intrusion capillary pressure (MICP) analysis. The fluid inclusions of liquid and gas hydrocarbons trapped in calcite vein samples in Dalong shales of two wells were studied using laser Raman and fluorescence spectroscopy. FE-SEM images indicated that OM pores in different formations varied substantially in terms of shape, size, and distribution density. OM pores in Type-II Dalong shales of Well XY1 were mainly micropore, sparsely distributed in the gas-prone kerogen with a spot-like and irregular shape, while bitumen rarely developed observable pores. In contrast, the morphology of OM pores in Type-III Longtan shales were significantly different, which was due to differences in the OM type. The primary OM pores in some terrestrial woody debris in Longtan shales had a relatively larger pore diameter, ranging from hundreds of nanometers to a few micrometers and were al- most all rounded in shape, which might be one of the factors contributing to larger pore volume and gas adsorption capacity than Dalong shales of Well XY1. Comparing Dalong shales of Well XY1 with relatively lower thermal maturity, there were abundant spongy-like pores, densely developed in the pyrobitumen in Type-II Dalong shales of Well EY1, with an irregular shape and diameter ranging from several to hundreds of nanometers. Many blue fluorescent oil inclusions and a small number of CH4 inclusions mixed with C2H6 could be observed within calcite veins in Dalong shales of Well XY1, whereas only CH4 inclusions could be identified within calcite veins in Dalong shales of Well EJ1. Therefore, thermal maturity not only controlled the type of hydrocarbons generated, but also makes a significant contribution to the formation of OM pores, resulting in larger pore volumes and adsorption capacity of Type-II shale samples in the dry gas window.

Keywords

Middle Yangtze area / Permian / transitional facies shales / pore structure / thermal maturity / organic matter type / organic matter pore / petroleum geology

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