Sedimentary characteristics of Late Carboniferous to Early Permian coal measures and its control on symbiotic gas reservoirs in Shouyang area, Qinshui Basin

In recent years, the Ordos Basin has achieved significant breakthroughs in the exploration and development of coal measures symbiotic gas reservoirs, whereas the Qinshui Basin continues to focus primarily on coalbed gas methane reservoirs. Notably, in the northern Qinshui Basin’s Shouyang block, Late Carboniferous to Early Permian the 3rd coal seam of the Shanxi Formation and the 9th and 15th coal seams of the Taiyuan Formation exhibit a high degree of thermal evolution, providing an excellent hydrocarbon source. The marine-continental transitional facies sedimentary environment promotes the development of various reservoir-cover combinations, demonstrating that the area has the essential geological conditions for the formation of coal measures symbiotic gas reservoirs. Drawing on data from 108 CBM exploration wells and previous studies on the depositional environment, the sedimentary characteristics of the Late Carboniferous-Early Permian coal measures have been meticulously investigated. Using a “point-line-surface” approach, the Taiyuan Formation is divided into three depositional units: the first unit (hereafter referred to as Tai-1) represents delta front deposition, the second unit (Tai-2) represents lagoon and tide flat deposition, and the third unit (Tai-3) again represents delta front deposition. The Shanxi Formation is classified into one depositional unit, namely, delta plain deposition. Based on these studies, eight typical lithofacies associations were identified, and a comprehensive analysis yielded three types with four categories of coal-measure gas reservoir covers. The study finds that in the northern part of the study area, only a single coalbed methane reservoir is developed, whereas the eastern part exhibits a combination of coalbed methane and conventional limestone fractured gas reservoir. In the central region, coalbed methane combined with tight sandstone gas predominates, while the northern-central zone is characterized by a mix of tight sandstone gas, coalbed methane, and shale gas.