鄂尔多斯盆地东北部上古生界煤系气成藏机制探讨

Discussion on Upper Paleozoic Coal Measures Gas Reservoiring Mechanism in Northeastern Ordos Basin

鄂尔多斯盆地东北部上古生界发育多套含煤碎屑岩系,为上古生界气藏主要烃源岩。结合煤系气概念及其内涵,将上古生界天然气藏以下石盒子组区域分布的＂桃花泥岩＂为界划分为下部（C2b-P2x）煤系气藏以及上部（P2sh-P3q）非煤系气藏,并在沉降史模拟与构造热年代学分析的基础上,结合上古生界气藏压力分布特征,探讨了其成藏机制,认为上古生界天然气成藏可划分为4个阶段,其中煤系气成藏主要集中于145-115 Ma的加载增压原生成藏阶段以及115-65 Ma的压力调整再平衡成藏阶段;早白垩世末期以来南北生烃能力差异、煤系气横向运移的生气增压能量亏损、煤系气作为非煤系气源以及后期天然气逸散共同造成了煤系气藏纵向和横向上的压力分异;上古生界煤系气勘查应主要集中于研究区南部的米脂地区。

In the northeastern Ordos Basin has developed multiple sets of upper Paleozoic coal-bearing clastic rock series thus the main gas reservoir source rock. Combined with coal measures gas concept and its conotation, divided the upper Paleozoic natural gas reservoir into lower part（C2b-P2x） coal measures gas reservoir and upper part（P2sh-P3q） non coal measures gas reservoir, taking the regionally distributed Taohua mudstone in Xiashihezi Formation as the boundary. Based on subsidence history simulation and tectonic thermal chronological analysis, combined with upper Paleozoic gas reservoir pressure distribution features, discussed its reservoiring mechanism. It is considered that the upper Paleozoic natural gas reservoring can be divided into 4 stages, in which, coal measures gas reservoring mainly concentrated in loading pressurized primary reservoiring stage（145-115 Ma） and pressure adjusting rebalanced reservoring stage（115-65 Ma）. Since the end of early Cretaceous, hydrocarbon generation potential is different between south and north, coal measures gas lateral migration gas generation presurizing energy loss, coal measures gas played as non coal measures gas source and succeeding natural gas dissipation togather made vertical and lateral pressure differentiation. The prospecting of upper Plaeozoic coal measures gas should be mainly concentrated in Mizhi district in the southern study area.