成岩过程对五峰-龙马溪组页岩地震岩石物理特征的影响

五峰-龙马溪组页岩是目前国内页岩气勘探的首选层位，而其地震岩石物理特征是利用地震方法进行"甜点"预测的重要基础之一，但对五峰-龙马溪组页岩地震弹性特征变化规律的研究并未考虑沉积、成岩过程的影响，致使相应的规律性认识缺乏地质意义.在对五峰-龙马溪组页岩样品系统声学测量基础上，分析了页岩样品地震弹性性质的变化规律.利用X射线衍射分析、扫描电镜（SEM）、阴极发光（CL）与能谱分析确定了五峰-龙马溪组页岩在不同沉积环境下的成岩过程，并讨论了成岩过程与地震弹性性质变化规律的因果关系.研究结果表明，页岩中有机质（TOC）受高热演化程度的影响，其密度通常高于1.4 g·cm^-3，并接近于有机碳密度上限1.6 g·cm^-3（石墨密度）.五峰-龙马溪组页岩地震弹性性质变化规律整体受沉积环境控制，沉积环境的差异形成不同的成岩过程，致使地震弹性特征也表现出不同的变化规律.表现在五峰-龙马溪页岩样品动态岩石物理特征主要受岩石结构控制（支撑颗粒弹性性质），而孔隙度、TOC含量以及孔隙形状则为对地震弹性特征影响的次一级因素.五峰-龙马溪组页岩上段为浅水陆棚相，机械压实与化学压实（硅质胶结）为先后两个过程，造成样品表现出高的速度-孔隙度变化率、高速度比（泊松比）、高各向异性以及低TOC含量的特征.五峰-龙马溪组页岩下段为深水陆棚相，机械压实过程中同时伴有生物成因的硅质胶结，造成岩石样品表现出较高TOC含量与孔隙度、各向异性较弱以及较小的速度-孔隙度变化率.研究结果可为五峰-龙马溪页气储层的测井解释和地震"甜点"预测提供依据.

The influence of the diagenetic process on seismic rock physical properties of Wufeng and Longmaxi Formation shale

Wufeng-Longmaxi Formation shale is the prior horizon for shale gas exploration in China. Its seismic rock physical properties are the important basis for "sweet spot" prediction by the seismic method. While previous research on this issue does not consider the influence of the diagenetic process, the resultant physical parameters lack geological implication. In this paper, seismic rock physics properties of the shale gas samples from Wufeng-Longmaxi Formation are studied based on acoustic experiments. Using X-diffraction analysis, scanning electron microscope (SEM) combined with cathode luminescence (CL) and energy spectrum analysis, depositional and diagenetic processes of Wufeng-Longmaxi Formation shale in different sedimentary environments are determined, and the influence of diagenetic evolution on seismic rock physical properties is discussed. The results show that bulk density of the shale gas samples is over 1.4 g·cm^-3, close to the upper limit value of 1.6 g·cm^-3 for organic matter, due to the high thermal evolution. Seismic rock physics properties and their variations are controlled by the sedimentary environment, and different sedimentary environments created varied diagenetic processes, which resulted in different seismic rock physics properties. Rock texture (elastic properties of load-bearing grains) was the primary factor to determine the seismic rock physics properties for the formation, and porosity and the content of organic matter (TOC) were only the second factor. The upper part of the Wufeng-Longmaxi Formation shale is interpreted as a shallow shelf, in which mechanical compaction and chemical compaction (silica cementation) are two sequential processes, which resulted in much larger change of velocity with porosity, high Poisson's ratio and strong anisotropy of the shale. The lower part of the Wufeng-Longmaxi Formation shale is interpreted as a deep shelf, where the mechanical compaction was accompanied by biogenic quartz cementation, which resulted in relatively smaller change of velocity with porosity, low Poisson's ratio and week anisotropy of the shale. Our results can provide a basis for well-logging interpretation and "sweet spot" discrimination using the seismic method for Wufeng-Longmaxi Formation shale.