Formation Mechanism and Controlling Factors of Organic Pores in Organic-Rich Shale

Abstract

Organic pore formed in the process of organic thermal evolution, is the trace of shale gas generation, diffusion and accumulation left behind, and is also the refection of gas generation and storage capacity for shale gas in shale reservoirs. The previous studies on organic pore system were mainly focused on certain sedimentary facies shale. In this study, marine, continental, and marine-continental transitional shales are selected in typical areas in China to make a comprehensive analysis on the formation mechanism, developmental characteristics and controlling factors of organic pores. The main testing methods employed in this study are the characterizing methods for shale pore system and the related geochemical tests.

Results showed that the organic pores are essentially determined by the alterability of the inner structure of organic macerals during thermal evolution and the porous regularity is controlled by gas-expansion-force. Organic pores of the over mature marine shales from Lower Silurian Longmaxi Formation in the range of 2.2-3.2%VRr, are widely distributed and characterized by big round or oval-shaped pores with pore diameter and surface porosity in the range of 10-200nm and 15-40%, respectively. Additionally, organic pores of the low mature continental shales from Eocene-Oligocene Shahejie Formation in the range of 0.5-1.2%VRr, are poorly developed and featured with small isolated pores with pore diameter and surface porosity in the range of 5-50nm and 0-8%, respectively. While organic pores of the high-over mature transitional shales from Lower Permian Shanxi Formation in the range of 1.4-2.7%VRr, are rarely formed and characterized by small scattered pores with pore diameter and surface porosity in the range of 200-8000nm and 0-2%, respectively.

Except for the geochemical properties of organic matter, organic pores are strongly influenced by the mechanical properties and the occurrence pattern of organic macerals. The over-compaction of the organic matter in the over-mature stage will cause serious deformation of organic pores.

This study can be helpful to better understand the hydrocarbon generation mechanism and the gas storage capacity of organic matters in shale reservoirs, and also can provide a selection index for favorable zone of shale gas.

Keywords: Organic-rich shale, organic pores, formation mechanism, developmental characteristics, controlling factors

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018