三塘湖盆地灰质页岩孔隙发育特征及对页岩油赋存的影响

Large quantities of calcareous and organic-rich shales are developed in semi-deep to deep lake facies during numerous terrestrial basins in China. Due to their excellent hydrocarbon generation and reservoir capacity, these calcareous shales become the target of exploration and exploitation for shale oil with terrestrial origin. There are various types of pores with pore width ranging from nanometers to micrometers. Porosity and permeability are not accurate enough to describe the pores in calcareous shales. Appropriate pore study should supply fundamental information about pore types, sizes, connection and their effectiveness to store shale oil. In the meantime, pore arrangements and their impacts on storage of shale oil changes during the diagenetic process. Both of the above are the key and difficult part in reservoir assessment for calcareous shales. In this project, the samples of calcareous shales will be collected from the second section with various depths in the Lucaogou group. N2 adsorption and mercury intrusion are conducted to describe pore size distribution and pore volume, while Field Emission Scanning Electron Microscopy is combined to identify the relationship between pore width and pore types. Then, the contribution of pores with various types to whole pore volume can be reached and evaluated. Subsequently, the correlation between pore volume at certain pore width ranges and S1 is carried out to evaluate the reservoir capacity of pores with certain types and sizes. Finally, samples with different maturities are analyzed to investigate the evolution of pores and its influence on shale oil storage. The findings will demonstrate the evolving trend for pores in calcareous shales within the oil generation window, strengthens the understanding of accumulative mechanism for shale oil and supply primary data and theoretical reference for shale oil exploration and exploitation.

中国众多陆相盆地的半深湖-深湖亚相大都发育富有机质灰质页岩，因其良好的生烃和储集能力，成为页岩油勘探开发的重点。灰质页岩中孔隙类型多、孔隙多处于纳米-微米尺度，常规孔隙度、渗透率在其储层描述中不够精细而被慎用。灰质页岩中孔隙研究需体现孔隙类型、大小、连通性的信息，进而评估不同孔隙的储烃有效性；同时，随着成岩演化的进行，孔隙的组成和储烃有效性均发生动态变化，这是灰质页岩储层评价的重点和难点。因此，本项目以埋深跨度大的芦草沟二段灰质页岩为研究对象，在高分辨扫描电镜技术确定孔隙大小和孔隙类型的对应关系基础上，利用气体吸附和压汞联用描绘孔隙大小特征，建立不同类型孔隙对总孔的贡献差异；再通过某段孔宽的孔体积与含油性关系，来评估不同孔隙储烃的有效性；最终将研究对象扩大至整个生油窗范畴，来探讨不同孔隙的演化规律及其对页岩油赋存的影响。研究结果可以加深对页岩油储集机理的认识，为页岩油勘探开发提供理论依据。

灰质页岩生油窗内孔隙演化受有机质成熟、排烃作用和不稳定矿物（如白云岩、方解石、长石等）溶蚀的影响。easy%Ro>1.0%后，有机质大量转化为烃类分子并大量排烃，TOC迅速减小，排出烃大量增加；有机质收缩和排烃过程引起少量微孔、大量介孔和宏孔、（沿层里面方向）排烃缝的发育；生成的烃类优先充填页岩自身孔隙，导致微孔、介孔和宏孔体积迅速减小；抽提处理后，可溶沥青堵塞的孔隙暴露出来，表现为抽提后孔隙体积的大幅增加；生烃过程生成的有机酸优先溶蚀碳酸盐中方解石、白云石、白云质方解石、长石等，出现矿物溶蚀孔，溶蚀孔联结形成溶蚀带；有机质收缩、排烃、溶蚀等形成的孔和缝整体表现为总孔隙度的迅速增加；孔隙增加与有机质降解计算的孔隙度增加基本吻合；灰质页岩纳米孔隙的发育为已生成的烃类提供了赋存空间，因此整体上表现出含油性（S1）迅速增加；孔隙体积与游离烃相互关系表明，>20nm孔隙是页岩油有效孔隙。因此，灰质页岩有机质成熟阶段和不稳定矿物组成等因素均影响了其纳米孔隙的发育和含油性特征，初步预测easyRo=1.0%（vRo大约0.95%）后页岩油储层经济可采性高。