Lithofacies and pore characterization of the Lower Permian Shanxi and Taiyuan shales in the southern North China Basin

Abstract

Marine-continental transitional shales with varied lithofacies are widely distributed in the Lower Permian Shanxi and Taiyuan Formations in the southern North China Basin (sNCB) where they have been subject to frequently changing depositional conditions. Despite their importance, integrative classifications of the lithofacies of such shales are not normalized primarily due to the complex composition of the formations. This work classifies and defines the pore microstructure of the Shanxi and Taiyuan shales (well Mouye-1) from the Zhongmou exploration area. Classification is performed by optical (polarizing) microscopy, X-ray diffraction, and scanning electronic microscopy (SEM) imaging of Ar-ion milled samples, yielding measurements of the total organic carbon (TOC) content, porosity, and nitrogen adsorption. The TOC content is introduced into traditional ternary plots denoting “clay-carbonate-quartz”. Four primary lithofacies are identified from the combined metrics of optical microscopy and inorganic and organic contents. These four divisions comprise silt bearing mudstones, silty mudstones, muddy siltstones, and silty carbonaceous mudstones. The samples exhibit porosities between 1% and 4.5%, with silty carbonaceous mudstones having the highest TOC content and returning the highest porosity. Pores hosted in both the inorganic matrix and organic substrate are imaged by SEM. The predominant and largest pore types are in the inorganic matrix and include inter-particle mineral pores, inter-crystalline mineral pores and secondary denudation pores caused by smectite illitization. The pore size distributions (PSDs) and specific surface areas are recovered from nitrogen gas adsorption using BJH and BET models that reveal a wide range of pore sizes. The pore volumes are predominately associated with larger macro-/mesopores, whereas the specific surface area is primarily from a contribution of smaller micro-/mesopores. Finally the target zone for fracturing and recovery is optimized using these integrated methods for lithofacies description, pore characterization, and petrophysical and geomechanical analysis. This study provides a selective completion strategy to reduce fracturing-treatment expense and improve well productivity.