陆相含煤岩系米氏旋回及其与煤储层特性的耦合机理

The changes of paleoclimate, and high resolution sequence stratigraphic cycles drived by the Milankovitch astronomical cycles can be identified in coal seams of the nonmarine basins. The primary coalbed methane(CBM) reservoir parameters, such as morphology and thickness of the coal seams, porosity and permeability of coal reservoirs, gas-bearing characteristics, along with superimposed relation of the generating, reservoiring and capping rocks are highly correlated to peat-swamp type, palaeohydrologic condition and paleobotany which are also corresponding to the Milankovitch astronomical cycles. We take the multiple superposed CBM systems of the Jurassic of the Muli Basin in the Qilian mountain area as a sample, rebuilt the Milankovitch astronomical cycles of the Jurassic of the Muli Basin using fast Fourier transform and continuous wavelet analysis of coal facies parameters, and analysis of carbon isotope, XRD and ICP-MS. The primary CBM reservoir parameters are studied by scanning electron microscope, mercury porosimetry and gas adsorption. Finely quantitative comparison between the Milankovitch cycles and the primary CBM reservoir parameters enables us to work out the periodicity, responding mechanism of these CBM reservoir parameters. Finally, the accumulation dynamic mechanisms of the nonmarine multiple superposed CBM reservoirs are summarized.

煤层作为一种特殊的沉积有机岩，完整地记录了米兰科维奇天文周期控制的古气候变化以及它所驱动的高频地层旋回。陆相低煤阶煤层气主要成藏参数，如煤储层形态分布、孔裂隙特征及“生+储+盖”组合，与米氏天文周期响应下的成煤泥炭沼泽植物类型、古水文条件及沼泽形成演化高度相关。为了揭示陆相盆地煤层气成藏与米氏旋回的响应机制，以祁连山木里煤田侏罗系多煤层含气体系为例，采用基于煤相参数快速傅里叶变换和连续小波分析、X射线衍射、电感耦合等离子质谱和碳同位素分析，结合扫描电子显微镜和压汞/液氮等煤层气储层特性分析，开展精细的定量比较研究，分析陆相多煤层含气系统成藏主控因素在米氏旋回内的周期性变动规律、响应机制及各因素之间的联系，研究陆相多煤层含气体系的成藏特征，探索米氏旋回与陆相多煤层含气系统成藏的耦合关系。

陆相含煤盆地成煤期碎屑沉积基本中断，这一时期形成的含煤地层中完整地记录了米兰科维奇天文周期控制的古气候信息，而古气候及其引起的沼泽基准面变化、成煤植物更替和碎屑物质供给等特征制约着煤层气储层物性及其非均质性。本研究采用高分辨率地层同位素及测井数据进行了频谱分析，识别出了低频、中频和高频区的3组显著频率范围，分别为0.2、0.7、1.2周期/米。采用密度梯度离心分离、激光拉曼实验等技术对不同显微组分的大分子超微结构进行了定量研究，揭示了不同煤级煤中显微组分间的超微结构特征及演化规律；采用偏光显微镜、体视镜和场发射扫描电镜对低煤阶煤中垂直层理方向和平行层理方向裂隙进行了定量研究，揭示了储层渗透性非均质性发育特征及应力受控机制，为现场煤层气开发过程中选取渗透性最优方位提供了科学依据；采用电感耦合等离子质谱、X射线荧光衍射和偏光显微镜对泥质岩和煤层进行了岩矿组成、微量元素和常量元素分析，揭示了柴达木盆地侏罗系古气候演化及聚煤特征；采用野外实测剖面、室内岩矿鉴定和沉积学分析开展了柴达木盆地侏罗纪含煤岩系沉积体系及盆地动力分析，提出了overfilled型至balance-filled型的盆地演化模式及聚煤规律。本项目的研究，对于进一步探寻煤储层孔裂隙非均质性、煤系生储盖赋存规律及受控于盆地演化和古气候的相应机制具有重要意义。