湖相碳酸盐岩致密储层有机质赋存状态与孔隙演化微观机理

Organic matter occurrence and microscopic mechanism of pore formation in the lacustrine tight carbonate reservoirs

孔隙发育机理及预测是湖相碳酸盐岩致密储层研究中的一大难题。以渤海湾盆地束鹿凹陷沙三下亚段致密碳酸盐岩为对象，开展热压模拟实验模拟岩石埋深演化过程。通过对实验岩石产物进行地球化学、离子抛光扫描电镜及低温氮气吸附实验分析，研究了有机质赋存状态及成熟作用对其储集孔隙的影响，揭示了致密储层有机质孔隙热演化机制。结果表明：碳酸盐岩中有机质主要与泥质或灰泥质基质以络合-复合的形式存在，成熟作用对有机质孔隙形成与演化影响巨大。随有机质成熟度（ R0 ）从0.5%增加到1.5%，有机质丰度（TOC）减少一半以上（从2.07%降低到0.85%），孔隙总比表面积和孔隙体积增加3～4倍。在低成熟-生油窗阶段（R0 =0.5%～1.0%），有机质孔隙不很发育；随成熟度升高，有机质逐渐发育裂缝和孔隙，尤其进入湿气窗-干气窗阶段（R0≥1.5%），孔隙比表面积和总孔隙体积快速增加，孔隙大量生成。有机质的分解是孔隙总体积和比表面增加的主要原因。新生成孔隙主要为有机孔，尤其以微孔-介孔对成熟度变化最为敏感，直径小于5nm的孔隙对新增总孔隙体积和比表面积贡献最大。孔隙总体积和比表面的增加主要依赖于新增孔隙的形成，而不是原有孔隙的扩大。

Pore formation mechanism and prediction is one of the difficult scientific problems for study of lacustrine tight carbonate reservoirs. This paper aims to study the thermal evolution of pores during burial through hydrous pyrolysis ex-periments,in which the samples were collected from the lower part of the 3 rd Member of Shahejie Formation in the Shulu sag,Bohai Bay Basin. The geochemical analysis,focus ion bean milling Scanning Electronic Microscope（ FIB-SEM） and lower temperature nitrogen adsorption were performed to examine the impacts of organic matter occurrence and thermal maturation on pore development and to reveal thermal evolution mechanism of organic pores in the tight reservoirs. The experimental result shows that the organic matter mainly occurs as combination of organic matter-argillaceous or marl matrix in the lacustrine carbonates,and the maturation has significant impact on the organic pore formation and evolution. With thermal maturity increasing from 0. 5% to 1. 5%（R0）,the total organic carbon（TOC）lowers more than half from 2. 07% down to 0. 85%,and porous volume and total specific surface area increase by 3 to 4 times. The organic pores is poor developed during the low thermal maturity-oil window（R0=0. 5%-1. 0%）,but increases dramatically during high thermal maturity-gas window stage （ R0≥1. 5%） due to the development of pores and fractures. The decomposition of organic matter should be the primary explanation for the increase of pore volume and specific surface area. The newly generated pores are dominated by organic pores,with the micro-and meso-organic pores being most sensitive to maturity.The pores with diameter less than 5nm make the biggest contribution to the additional total pore volume and specific surface area. The increase of total pore volume and specific surface area mainly comes from the formation of new pores instead of the enlargement of the existing pores.