Lamina Variation and Its Relationship with Sedimentary Facies in Alkaline Lacustrine, Permian Fengcheng Formation, Junggar Basin, Northwest China

Abstract

The Permian Fengcheng Formation in the Mahu Sag was deposited in a volcanic-alkaline lacustrine evaporative environment and contains a unique variety of fine-grained sediments. This study examines, at a millimeter-scale, the influence of sedimentary microfacies on variability of lamina quality in fine-grained sediments in the second member of the Fengcheng Formation (P1f2). The methods used include thin-section identification, X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy (SEM), nitrogen adsorption, and nuclear magnetic resonance (NMR). Six types of lamina were identified in two different lithofacies: fan-delta front facies (FDFF) and semideep/deep lacustrine facies (SDDLF). The laminae in FDFF are predominantly feldspar-quartz laminae (FQL), reedmergnerite laminae (RL), shortite laminae (SL), alkaline mineral laminae (AML), and chert laminae (CL). The laminae in SDDLF are predominantly FQL, RL, SL, CL, and dolomite laminae (DOL). Variations in reservoir quality, oil-bearing properties, and the fracability of laminae in different sedimentary facies are determined by the combined effects of lamina density, mineral composition, rock structure, organic matter abundance, and microfractures. Analysis of these factors indicates superior reservoir qualities in FDFF. In SDDLF, the pore structure is limited by high lamina density, chert content, and fine grain size with the NMR porosities of FQL, RL, SL, and CL being 1.32, 0.18, 0.84, and 0.39%, respectively. However, in FDFF, the combination of high organic matter content, feldspar, pyrite, and clay minerals has a superior effect on the organic matter and minerals deposited resulting in better pore structure and more storage space for shale oil. The NMR porosities of FQL, RL, SL, and CL are 2.81, 2.53, 1.80, and 1.12%, respectively. Overall, analysis of lamina variations and their relationships with sedimentary facies indicates that the reservoir in FDFF may offer more favorable targets for “sweet spot” evaluation.