车镇凹陷车西北带下古生界煤成气成藏差异及模式分析

Differences and control modes of coal gas accumulations in Lower Paleozoic in northwest of Chezhen Sag,Jiyang Depression,Bohai Bay Basin

济阳坳陷煤成气勘探已历经30余年,在孤北、惠民、临清、车西等地区均获得突破,但未形成规模储量,有必要针对煤成气的成藏主控因素进一步深化认识。以地震、录井、测井及生产测试资料为基础,利用碳同位素等分析化验手段,探讨济阳坳陷车镇凹陷车西洼陷北部陡坡带(车西北带)下古生界天然气成因类型,分析研究区下古生界煤成气藏的成藏差异及主控因素。研究结果表明,车西北带下古生界天然气类型为煤成气,主要来自上古生界煤系地层,具有良好的区域成藏地质条件。煤系地层埋深普遍超过4100 m,已进入二次生烃阶段,且埋深和厚度决定了生烃量的差异。邻近断层的断距对煤成气成藏具有至关重要的控制作用,其不仅决定圈-源对接程度,也决定下古生界储层发育带的宽度及圈闭的有效性。当邻近受控断层的断距大于150 m时,断层上升盘下古生界圈闭与下降盘上古生界烃源岩对接,上古生界的煤成气沿断层就近运移至下古生界被断层改造的碳酸盐岩储层内,在有利圈闭内即可近源成藏。

After more than 30 years of exploration for coal gas in Jiyang Depression,breakthroughs have been made in Gubei,Huimin,Linqing,Chexi,and other areas,but large-scale reserves have not been formed.Therefore,it is necessary to further explore the main factors controlling the accumulations of coal gas.Based on the seismic survey,mud logging,well logging,and production test data,the genetic types of the Lower Paleozoic natural gas in the northern steep slope zone in Chexi Subsag,Chezhen Sag,Jiyang Depression(the northwest of Chezhen Sag).In addition,the differences and main controlling factors of accumulation for the Lower Paleozoic coal gas reservoirs in the study area were analyzed.The results demonstrate that the Lower Paleozoic natural gas in the northwest of Chezhen Sag is coal gas,which mainly comes from the coal-bearing strata of the Upper Paleozoic,leading to favorable geological conditions for accumulation.The coal-bearing strata have a buried depth generally over 4100 m and have entered the stage of secondary hydrocarbon generation,and the differences in hydrocarbon generation quantity are determined by their buried depth and thickness.The fault displacement of adjacent faults plays a crucial role in coal gas accumulation:It dominates both trap-source connection and the width of the Lower Paleozoic reservoir,thereby directly determining the effectiveness of the trap.When the fault displacement of adjacent controlled faults is more than 150 m,the Lower Paleozoic trap in the hanging wall can be connected with the Upper Paleozoic source rock in the footwall.The coal gas from the Upper Paleozoic migrates along the fault into the Lower Paleozoic carbonate rock reservoir transformed by the fault,and then accumulates in the favorable trap near the source.