Heterogeneity and Sedimentary Characteristics of Shale Laminae of Fine-Grained Sediments in Alkaline Lacustrine Strata in the Permian Fengcheng Formation, Mahu Sag, NW China

Abstract

The heterogeneity, sedimentary characteristics, and distribution of fine-grained sediments of millimeter-scale laminae were studied in alkaline lacustrine strata in the lower Permian Fengcheng Formation (P1f2) in the Mahu Sag to provide a sound theoretical basis for shale oil exploration and development. They were investigated by using core and thin-section observation, geochemical and elemental analysis, mineral identification, and pore examination. Five types of laminae were distinguished, with different mineral compositions, pore characteristics, and distributions. These laminae occur in four distinct combinations. The pore systems predominantly consist of intercrystalline pores, intragranular pores, and dissolution pores of feldspar and dolomite and also contain imporous alkaline mineral particles. Influenced by variations in salinity, the influx of volcanic–hydrothermal material, and the participation of both endogenous and exogenous materials, the formation has gone through five stages of sedimentary evolution. Furthermore, felsic laminae (FQL), dolomite laminae (DOL)/reedmergnerite laminae (RL)/shortite laminae (SL), and chert laminae (CL) developed in single-terrestrial-source still water deposition, hybrid-source still water deposition, and single-intrasource deposition, respectively. Vertically, the FQL-DOL combination shows favorable reservoir characteristics and is the most extensively developed lamina combination in the Fengcheng Formation that primarily developed in the period of the late Feng 1 Member (P1f1), the early Feng 2 Member (P1f2), the late Feng 2 Member (P1f2), and the early Feng 3 Member (P1f3). The FQL-RL/SL combination primarily developed in the period of the middle P1f2, and the DOL-CL combination is the counterpart in the period of the early P1f2 and the late P1f2 stages. Considering this in conjunction with the longitudinal distribution of lamina combinations, a model is proposed for the distribution of fine-grained sediments in alkaline lacustrine strata.