煤系叠置含气系统关键层发育的沉积机理

In recent years, the simultaneous exploration and exploitation of coal series gases, including coalbed methane (CBM), tight sandstone gas and shale gas, is the frontiers of unconventional gas geology, which need geologist take efforts to break through in China. However, geological characteristics of coal series originates from the superposed coexistence of multiple sets of fluid pressure systems and could not been resolved with conventional measures. The accurate prediction of critical layers (impermeable and airtight layer) which has the function to divide the superimposition gas system has become the core issue to solve the problem. Based on this background, the late Paleozoic coal series is chosen to study which possess typical geological conditions and some effect in previous simultaneous extraction on the eastern margin of Ordos basin, and the log response and blocking performance evaluation of critical layers as the constraint conditions to research the spatial and temporal distribution of critical layers in the high-resolution sequence stratigraphic framework and the spatiotemporal coupling of critical layers with gas containing of gas system and fluid pressure system. Then, it is possible to interpret the sedimentary control mechanism of critical layers in coal series superimposed gas system, establish the sedimentary control mode of superimposed gas system, and deepen the objective knowledge of superimposed gas system in coal series and enrichment regularity of unconventional gas, and to provide a scientific basis for the simultaneous exploration and exploitation of coal series gases that widely distributed and richly reserved in China.

以煤层气、致密砂岩气、页岩气为核心的煤系气共探共采是我国非常规天然气地质界近年来着力突破的前缘领域。然而，煤系客观地质条件往往造成多套流体压力系统叠置共生，共采可行性难题突出，对叠置含气系统具有分划性作用的关键层（隔水阻气层）的准确预测成为解决这一难题的核心科学问题。基于这一背景，选择地质条件典型且前期共采取得一定效果的鄂尔多斯盆地东缘晚古生代煤系为研究对象，以关键层的测井响应及封堵性能评价为约束条件，研究高分辨率层序地层格架内关键层时空分布规律以及关键层与含气系统含气性/流体压力系统的时空耦合关系，阐释叠置含气系统关键层发育的沉积控制机理，建立叠置含气系统沉积控制模式，深化对煤系叠置含气系统及其非常规天然气富集规律的客观认识，并为我国分布广泛且资源量巨大的煤系气共探共采提供科学依据。

鄂尔多斯盆地东缘煤层气、致密砂岩气及页岩气“三气”共存，该区上古生界煤系储层垂向上存在着多个独立的含气系统。基于该区临兴区块上石炭统本溪组—下二叠统山西组钻井岩心观察及钻井、测井资料分析结果，详细刻画了鄂尔多斯东缘上古生界煤系含气系统关键层的时空分布规律，阐明了与之对应的地质记录，揭示了层序地层格架内的特征界面与含气系统成藏边界的耦合机制，查明了高频层序格架内的关键层与含气系统的时空耦合关系。阐释多层叠置含气系统关键层发育的沉积机理。研究结果表明，该区煤系叠置含气系统的发育受控于层序格架内具低孔隙度、低渗透率和高突破压力的含菱铁矿层（关键层）的空间展布，关键层的发育受控于区域海侵事件层的空间分布，主要发育于最大海泛面附近并构成叠置含气系统隔水阻气的边界；二级层序内最大海泛面附近广泛发育的关键层，构成了全区稳定发育的含气系统边界，三级层序最大海泛面附近发育的关键层常因后期（水下）分流河道冲刷切割而保存不全，促使相邻层序的煤系储层组合成统一的含气系统，为该区煤层气与致密砂岩气的共采创造了有利的地质条件。