轮南地区压力场演化及其成藏意义

Evolution of geopressure field in Lunnan Area and its significance on hydrocarbon accumulation

选择轮南主体区剖面七条和临近凹陷剖面两条,利用压力数值模拟手段,对轮南地区的压力场演化进行了恢复和模拟。结合实测压力资料和钻井资料,发现部分地区石炭系和上奥陶统中存在弱超压、寒武系在凹陷内存在超压。通过分析,石炭系超压在轮南主体区主要是水热增压作用和泥岩转换作用造成的,在凹陷内则可能主要由于欠压实引起。上奥陶统内部的超压和石炭系在形成时间、特征和发育机理上相似,不过轮南地区上奥陶统覆盖区域较少。寒武系超压和三个成藏期相对应,在凹陷内形成明显的三期超压现象,以喜山期超压幅度最大。整体上看,石炭系和上奥陶统弱超压,对成藏动力方面贡献不大,但是超压的形成增强了该层的封闭性,对喜山期天然气成藏有重要影响。寒武系超压为油气初次运移提供了动力,但是超压幅度有限,推测其在油气二次运移过程中贡献不大。

Using numerical simulation, seven profiles crossing Lunnan Area and two profiles along the sag （Caohu sag and Manjiaer sag） were selected to restitute and simulate the pressure field evolution. The result of the simulation, measured pressure data and drilling data showed there was weak overpressure in Carboniferous strata and upper Ordovician strata and prominent overpressure existed in the Cambrian strata. Analyzing the result of the simulation and measured data, in the main area of Lunnan Area, the overpressure in Carboniferous strata was formed by aquathermal pressuring and shale mineral transformation at the late Himalayan epoch, but in the sag, the overpressure was formed by under compaction at Late Hercynian. The overpressure in Carboniferous strata and upper Ordovician strata had the same characteristics such as forming time, development mechanism and overpressure features. The overpressure in Cambrian strata had three growth times in the sag, late Caledonian to early Hercynian, later Hercynian and Himalayan. They match up to three accumulation times in Lunnan Area and the strongest overpressure happened in Himalayan movement. In conclusion, weak overpressure in Carboniferous strata and upper Ordovician strata contributed a little to hydrocarbon accumulation dynamics but enhanced sealing ability of formations, so it had a great impact upon Himalayan gas accumulation. The overpressure in Cambrianprovided dynamics for the primary migration of oil and gas.