松辽盆地白垩系嫩江组煤系页岩孔隙结构及分形特征

Pore structure and fractal characteristics of coal-bearing Cretaceous Nenjiang shales from Songliao Basin, Northeast China

页岩微观孔隙系统是气体赋存场所,孔隙结构精细刻画对于储层研究而言至关重要。页岩储层孔隙具有较强的复杂性和非均质性,而且煤系页岩孔隙发育特征研究显著滞后于海相页岩储层,亟需开展煤系页岩孔隙非均质性特征及其对含气性影响的深入研究。以松辽盆地松原地区白垩系嫩江组煤系页岩岩心为研究对象,通过开展全岩有机碳(TOC)和岩矿测定、孔隙度分析、低温N2吸附—脱附和CH4等温吸附实验,探讨了煤系页岩基质组分对孔隙结构和非均质性特征的影响,孔隙结构特征与分形特征的关系,以及孔隙结构和非均质性对甲烷吸附特征的影响。结果表明:①嫩江组页岩有机质丰度变化不显著(TOC平均含量为2.38%),多发育墨水瓶状孔;富含黏土矿物(平均含量为55.6%),多发育狭缝状孔隙;页岩孔隙表面具有明显的分形特性,分形维数D1和D2均值分别为2.54和2.74,内部结构较表面结构更为复杂。②有机碳的富集通过影响微小孔孔隙发育,增加比表面积使孔隙分形维数增大;黏土矿物发育致使介孔和宏孔数量增多,增加了孔隙的分形维数。③微小孔隙发育较大比表面积,增加了孔隙结构的复杂性和非均质性,促使分形特征显著,拓展了吸附点位,提升了吸附能力。相关研究将为煤系页岩储层综合评价、成藏理论研究提供科学理论依据。

Shale oil and gas resources mainly exist in the pore and fracture system.Quantitative characterization of pore development characteristics and gas-bearing characteristics are the key to shale reservoir evaluation.The pore development of shale reservoir has strong complexity and heterogeneity,and the research on pore development characteristics of coal measure shale lags behind that of marine shale reservoir,thus it is urgent to investigate pore heterogeneity characteristics of coal-bearing shale and its influence on gas bearing property.Therefore,the coal-bearing Cretaceous Nenjiang shales from Songyuan area of Songliao Basin were selected as the research object in this study.Through the total organic carbon(TOC),X-ray diffraction experiment,porosity analysis,nitrogen adsorption desorption experiment and methane isothermal adsorption experiments,the characteristics of pore structure,heterogeneity and gas bearing property of coal-bearing shale were analyzed.The influence of rock and mineral components on the pore structure and heterogeneity characteristics,the relationship between pore structure characteristics and fractal characteristics,and the effects of pore structure and heterogeneity on methane adsorption capacity were also discussed.The results show that:(1)The organic matter abundance of the shale in the Nenjiang Formation does not change significantly(the average TOC content is 2.38%),and ink bottle-shaped pores are mostly developed;it is rich in clay minerals(average content 55.6%),and the slit-shaped pores are mostly developed.The pore surface of shale has obvious fractal characteristics,and the average fractal dimension D1 and D2 are 2.54 and 2.74,and the internal structure is more complex than the surface structure.(2)The enrichment of organic carbon increases the specific surface area by affecting the development of micropores and pores,and increases the fractal dimension of pores;the development of clay minerals increases the number of mesopores and macropores,increasing the fractal dimension of pores.(3)Small pores develop larger specific surface area,which increases the complexity and heterogeneity of the pore structure,promotes the remarkable fractal characteristics,expands the adsorption sites,and improves the adsorption capacity.This work will provide scientific theoretical basis for comprehensive evaluation of coal-bearing shale reservoir and shale gas reservoir forming theory research.