Abstract:

The continental coal-accumulating Tuha Basin is recognized as a significant petroliferous basin in western China. The Jurassic Shuixigou Group contains multiple suites of high-quality source rock sequences bearing coal seams， as well as high-quality sandstone and coal reservoirs， emerging as a primary target for hydrocarbon exploration and a field for strategic breakthroughs in deep coal-rock gas exploration within the basin. In this study， we investigate the impacts of the basin's tectonic and sedimentary evolution on the formation of source rocks. Based on the response characteristics of trace elements to the sedimentary environment， we analyze the developmental and distribution patterns of various types of source rocks and identify favorable locations for natural gas （coal-rock gas） preservation in coal reservoirs. The results suggest that during the deposition of the Shuixigou Group， the tectonic uplift and subsidence processes governed the formation and evolution of the depocenter of the Tabei Sag， driving the depocenter to migrate gradually from north to south. At the end of the Late Jurassic， the northern part of the Tuha Basin underwent significant tectonic uplift， resulting in the continuous migration of the depocenter toward the basin's hinterland and the development of the terrain featuring highs in the east and lows in the west. Consequently， three present-day subsidence centers， i.e.， the Shengbei， Qiudong， and Xiaocaohu sub-sags， are formed， which tend to rise and grow shallower eastward. The depocenter evolution determines the changes in the depth and salinity of water bodies in the sub-sags of the Taibei Sag. Specifically， the water bodies exhibited an increasing depth and decreasing salinity during the deposition of the Badaowan and Sangonghe formations of the Shuixigou Group. In contrast， the water bodies displayed a gradually decreasing depth and increasing salinity during the deposition of the Sangonghe and Xishanyao formations of the group. Mudstones are primarily distributed in the northern part of the basin， thickening from west to east. Influenced by the salinity of paleo-water bodies， the Badaowan—Xishanyao formations are characterized by vertically improving organic matter both in types and abundance. The Shuixigou Group underwent frequent fluctuations in water bodies during its deposition， with coals forming in sedimentary environments with relatively shallow water bodies. This group exhibited shallow water bodies during the initial water transgression in the early deposition of the Badaowan Formation， along with the late water regression in the late deposition of the Badaowan Formation and the early-middle deposition of the Xishanyao Formation， creating favorable coal-accumulation environments. Therefore， the Shuixigou Group represents the primary interval for coal occurrence in the Tuha Basin. Ten types of roof-floor assemblages of coal seams are identified in coal-bearing areas of the basin. Five favorable roof-floor assemblages for the preservation of coal-rock gas， as well as their locations， are determined， serving as the targets for future coal-rock gas exploration.

Key words: hydrocarbon-generating environment, trace element, roof-floor assemblage of a coal seam, coal-measure source rock, coal-rock gas, Shuixigou Group, Taibei Sag, Tuha Basin