Reservoir characteristics and genesis of the Ordovician M54−1a in the central and eastern parts of the Ordos Basin, China

Abstract

M54−1a is a new natural gas exploration target of the Ordovician in the Ordos Basin and is expected to have large-scale reserves and a production increasing area after M51 and M52 weathering crust gas reservoir. Based on the data of the drilling core, thin section, cathodoluminescence, physical properties, and geochemical characteristics, a study has been conducted about the genesis and distribution of the M54−1a reservoir in the central and eastern parts of the Ordos Basin. The study results showed the following: (1) The M54−1a reservoir is composed of fine-silt-crystalline dolomite containing anhydrite nodules in lithology. The reservoir storage space is mainly the moldic pores of anhydrite nodules with a small amount of anhydrite crystal moldic pores and micro-fractures. The average porosity is 5.54%, and the average permeability is 1.53 × 10−3μm2. (2) The reservoir pores have experienced anhydrite precipitation, atmospheric freshwater dissolution, mesocrystalline calcite filling, quartz filling, medium-coarse crystal dolomite filling, megacryst calcite filling, and cataclasis. (3) Differential dissolution and differential filling influenced by paleogeomorphology controlled the planar distribution of the reservoir. The results of the comprehensive analysis indicate that the superimposed area between the high part of sedimentary paleogeomorphology and the karst slope at the west of Yulin-Jingbian-Yanan as well as the Hongdunjie-Qingyangcha area are high-quality reservoir distribution areas with strong dissolution and low filling degree. The study results provide the theoretical basis for the next exploration and well deployment.