Abstract:

Paragenesis, coexploration and comining of the natural gas in coal measures and its overlying strata is an important way for making the full use of natural gas resources and for improving the economic benefit of coal measures gas (CMG) development. In the central and southern Qinshui Basin, the continuouslydeposited Upper Carboniferous to Triassic strata constitute favorable hydrocarbon sourcereservoircaprock assemblage, the thermal evolution degree of the coal organic matters is high, the hydrocarbon generation and expulsion are intense, and CMG shows are very strong, which provides the abundant gas source and reservoiring prospects for CMG accumulation. Tectonic heat event during the middle Yanshan Movement led to the two strong secondary hydrocarbon generation and expulsion from the PermoCarboniferous coal seams, providing a rich gas source for the shale and tight sandstone reservoirs in the coal measures and overlying strata. Natural gas in coal seams is expelled by the diffusion, caprocks breakthrough and seepage, and the zones with the maximum cumulative hydrocarbon expelling intensity occur along the compound syncline axis. Active and moderately active zones of the hydrocarbon expulsion are distributed in middle and northern composite syncline axis, where the level of the natural gas emission is relatively low and is favorable for free gas accumulation. Overall, in the study area, the free gas system develops with the bottom source (coal measures of the Taiyuan and Shanxi formations), the bottom to middle reservoir (Taiyuan to Lower Shihezi formations), and the upper cap(the Upper Permian to Triassic), the various kinds of the traps such as the structural, lithologic and water power ones occur, and the free gas accumulation is controlled by two critical geological periods, including the structural differentiation and abnormal thermal event during the middle Yanshan Movement and the continuous basin return since the Cenozoic.

Key words: Qinshui Basin, coal measures, free natural gas, sourcereservoircaprock assemblage, hydrocarbon expelling intensity, reservoiring prospect