Pore Structure Characteristics and Controlling Factors of Different Lithofacies in Lacustrine Shale: A Case Study of Fengcheng Formation in Mahu Sag, Junggar Basin

Abstract

The practice of shale oil exploration and development all over the world has shown that reservoir quality is a key factor restricting oil and gas production, the study of ultra-deep (4 500 m) reservoirs in petroliferous basins is a hot spot for oil and gas exploration in the world today. In this paper, taking the Fengcheng Formation (P₁f) in Mahu sag of Junggar Basin as an example, through experiments such as geochemistry, whole rock analysis, FE-SEM, N₂ adsorption, and CO₂ adsorption, the lithofacies are separated, and the pore structure properties of the various lithofacies are compared. Discuss the primary influencing elements that lead to pore structure differences. The results reveal that the shale of the P₁f can be separated into four lithofacies (namely felsic shale, mixed shale, lime shale and dolomitic shale), according to the mineral composition. The shale pores of P₁f develop intergranular pores, intragranular pores, organic pores and micro-fractures. Among them, intergranular pores, intragranular pores and micro-fractures are the most developed, and the pore size is larger, which is a favorable storage space. Different lithofacies have different pore development characteristics. With the change of lithofacies (mixed shale-dolomitic shale-lime shale-felsic shale), the size of the developed pores changes from small to large, and the pore structure changes from “mainly microporous” to “mainly macropores”. The felsic shale has developed laminae, mainly mesopores and macropores, with the largest pore diameter and the highest content of brittle minerals, which is the dominant lithofacies in the shale oil reservoir of the P₁f. TOC, felsic and clay minerals all affect the pore development of shale in the P₁f, but the influence of organic matter and inorganic minerals on pore volume varies with pore size. The purpose of this article is to characterize the pore structures and regulators of various lithofacies in lacustrine shale and to provide new ideas for ultra-deep shale oil exploration.

Keywords

Mahu sag / Fengcheng Formation / ultra-deep shale / shale lithofacies / pore structure / controlling factors

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