Abstract:

In recent years, the Ordos Basin has achieved significant breakthroughs in coal-measure symbiotic gas reservoirs. However, the exploration and development focus in the Qinshui Basin, which is part of the same structural unit, remains on coalbed methane. The late Carboniferous to early Permian sedimentary environment in the Shouyang block of the northern Qinshui Basin is a transitional marine-land environment. Notably, Coal-3 in the Shanxi Formation and Coal-9 and Coal-15 in the Taiyuan Formation, which exhibit a high degree of thermal evolution, are considered excellent source rocks. The marine-continental transitional facies is conducive to the formation of various reservoir-cap combinations, indicating that the area possesses the basic geological conditions necessary for the development of coal-system gas symbiotic reservoirs. Based on the previous studies and data from 108 CBM exploration wells, the sedimentary characteristics of the late Carboniferous to early Permian coal measures have been thoroughly examined. By the research method of “point-line-surface”, the Taiyuan Formation is divided into three subfacies. The first section is the front of the delta deposition, the second section is the lagoon-chaoping deposition, and the third section is the front of the delta deposition. The Shanxi Formation is categorized into one subfacies, that is the delta plain deposition. From these studies, eight typical lithofacies combinations have been summarized. A comprehensive analysis has yielded three types and four categories of coal gas storage covers. This paper outlines these findings and identifies four types of coal-based gas symbiotic gas reservoir combinations. In the study area, it is observed that only a single coalbed methane reservoir is present in the northern part. In contrast, the eastern part has developed coalbed methane plus conventional limestone fissure gas, the middle part mainly features coal seam plus tight sandstone gas, and the central-north is characterized by a combination of tight sandstone gas, coalbed methane, and shale gas.

Key words: late Carboniferous to early Permian, coal measure formation, sedimentary characteristic, lithofacies associations, symbiotic association