下扬子地区二叠系海相页岩孔隙特征新认识及页岩气勘探启示

New cognition on pore structure characteristics of Permian marine shale in the Lower Yangtze Region and its implications for shale gas exploration

下扬子地区中上二叠统大隆组和孤峰组页岩具有厚度大、有机碳含量高、分布范围广、有机质热演化程度适中等特点,有可能成为页岩气勘探下一步的重要领域。为了给该区页岩气勘探开发提供目标与方向,选取下扬子宣泾地区大隆组、孤峰组两套页岩为研究对象,运用扫描电镜、高压压汞、CO2及N2吸附等手段,定量表征了不同尺度的页岩孔隙的发育特征;利用FHH(Frenkel-Halsey-Hill)模型计算了样品孔隙分形维数,进而结合TOC、矿物组分讨论了孔隙结构与分形维数的关系。研究结果表明:①大隆组页岩以富泥硅质、富泥/硅混合质为优势岩相,具有较低的比表面积、孔容以及较高的平均孔径,孔隙发育主要受黏土矿物含量的控制,孤峰组以硅质页岩为优势岩相,具有较高的比表面积、孔容以及较低的平均孔径,孔隙发育受有机质及脆性矿物含量的控制;②分形维数影响因素可以归根于微孔发育的控制因素,大隆组页岩分形维数D_(1)介于2.4515~2.5513/2.5227、D_(2)介于2.5817~2.6578/2.6246,孤峰组页岩分形维数D_(1)介于2.5817~2.6578/2.6246、D_(2)介于2.7227~2.871/2.813,孤峰组页岩表现出更为复杂的孔隙结构特征;③分形维数D_(1)对比表面积、孔隙发育及矿物组分更具敏感性,D_(2)对表征平均孔径效果更好。结论认为,具有高分形维数D_(1)、D_(2)的大隆组以及低D_(1)、高D_(2)的孤峰组页岩段可以作为下扬子地区二叠系海相页岩气勘探的有利对象,富烃凹陷内构造变形较弱的局部超压区(带)将是该区页岩气勘探的有利方向。

Shale of the Middle-Upper Permian Dalong Formation and Gufeng Formation in the Lower Yangtze region is characterized by large thickness,high total organic carbon(TOC),wide distribution and moderate organic thermal evolution degree,so it may be the next important field of shale gas exploration.In order to point out the target and direction of shale gas exploration and development in this region,this paper selects the Dalong Formation and Gufeng Formation shale in the Xuanjing area of Lower Yangtze as the research object to quantitatively describe the development characteristics of shale pores of different scales by means of scanning electron microscopy(SEM),high pressure mercury injection,CO2 and N2 adsorption.Then,the fractal dimension of sample pores is calculated by using the FHH(Frenkel-Halsey-Hill)model.Finally,combined with TOC and mineral compositions,the relationship between pore structure and fractal dimension is discussed.And the following research results are obtained.First,dominant lithofacies of Dalong Formation are argillaceous-rich siliceous shale and argillaceous-rich/siliceous mixed shale,which has smaller specific surface area and pore volume,larger average pore diameter.The pore development is mainly controlled by clay mineral content.Dominant lithofacies of Gufeng Formation is siliceous shale,which has a larger specific surface area and pore volume,and smaller average pore diameter.The pore development is controlled by organic matter and brittle mineral content.Second,the influencing factors of fractal dimension can be attributed to the controlling factors of micropore development.The fractal dimension D_(1) of Dalong Formation ranges from 2.4515 to 2.5513(average 2.5227),and D_(2) ranges from 2.5817 to 2.6578(average 2.6246).The fractal dimension D_(1) of Gufeng Formation ranges from 2.5817 to 2.6578(average 2.6246),and D_(2) ranges from 2.7227 to 2.871(average 2.813).Gufeng Formation shale presents the characteristics of more complex pore structure.Third,the fractal dimension D_(1) is more sensitive to specific surface area,pore development and mineral composition,while D_(2) is more effective in characterizing the average pore diameter.In conclusion,Dalong Formation shale with high D_(1) and D_(2) and Gufeng Formation shale with low D_(1) and high D_(2) can be taken as the favorable exploration targets of Permian marine shale gas in the Lower Yangtze region,and the regional overpressure zones(belts)with weaker structural deformation in hydrocarbon rich sags will be the favorable shale gas exploration targets in this area.