黔东南岑巩地区下寒武统黑色页岩孔隙结构特征

利用矿物组分、有机地球化学、氮气吸附、物性及扫描电镜等资料,对黔东南岑巩地区下寒武统牛蹄塘组和变马冲组页岩孔隙结构特征进行了系统研究。牛蹄塘组与变马冲组贫有机质页岩以平板和狭缝形黏土粒间孔为主,平均孔径普遍大于5 nm;牛蹄塘组富有机质页岩以狭缝形和少量墨水瓶状孔为主,平均孔径普遍小于3～4 nm,比表面积为贫有机质页岩的2～3倍。页岩总孔容与比表面积、黏土含量与平均孔径均为正相关关系,而前二者与后二者均为负相关关系。有利保存条件下页岩具有相对"高孔低渗"特征,峰值孔径较大,有机质孔发育程度较高,孔渗相关性强。不利保存条件下页岩储层致密,孔隙度与峰值孔径较低,裂缝过度发育造成相对"低孔高渗"特征。有机碳含量对孔隙结构具有重要控制作用,与孔容、比表面及孔隙度总体正相关,与平均孔径负相关;过高有机碳含量层段页岩塑性增强,孔径较低,压实作用和不利保存条件等因素使部分狭窄孔喉发生萎缩、坍塌与闭合,导致孔隙度、孔容、比表面和脆性与有机碳含量出现负相关关系。

Pore structure characteristics of the Lower Cambrian black shale in the Cengong block,southeastern Guizhou area

Based on the mineralogy, organic geochemistry, nitrogen adsorption, physical properties and FE-SEM observation, the pore structure characteristics of the Niutitang and Bianmachong marine shales of the Lower Cambrian in the southeastern Guizhou area were systematically analyzed. The dominant pore types of the organic-lean shale with a high clay mineral content in the Niutitang and Bianmachong formations are plate-like and slit-like inter particle pores with average pore sizes commonly greater than 5 nm. However, the organic-rich shale in the Niutitang Formation mainly contains slit-like and ink-bottle-like pores with average pore sizes less than 3-4 nm, and the specific surface area is 2-3 times of that of the organic-lean shale. In addition, the total pore volume and specific surface area have a positive correlation, and there is also a correlation between clay mineral content and average pore size, but the correlations between total pore volume/specific surface area and clay mineral content/average pore size are negative. Under favorable preservation conditions, the shale has a relatively high development level of organic matter pores, greater values of porosity, pore size and peak diameter with a positive correlation between porosity and permeability, which are represented as “high porosity and low permeability”. Under unfavorable preservation conditions, the reservoir parameters have lower values except the permeability, which leads to the overdevelopment of fractures and has the feature of “low porosity and high permeability”. Besides, the TOC content has a significant control on pore structure and generally has a positive correlation with total pore volume, specific surface area and porosity and a negative correlation with average pore size. Nevertheless, in the intervals with an excessive TOC content, the decrease and increase of pore size and ductility with increasing TOC coupled with compaction and/or unfavorable preservation will result in the atrophy, collapse and close of narrow pores and throats which leads to the negative correlations between TOC and reservoir parameters.