浅层页岩气储层孔隙分形特征及控制因素——以云南昭通地区龙马溪组为例

Fractal Characteristics and Controlling Factors of Pores in Shallow Shale Gas Reservoirs:A Case Study of Longmaxi Formation in Zhaotong Area,Yunnan Province

为定量表征云南昭通国家级页岩气示范区龙马溪组页岩孔隙结构复杂程度及对页岩气影响,通过对24块龙马溪组页岩进行TOC质量分数、全岩矿物组分、低温氮气吸附测试,从页岩孔隙结构及矿物组分两个方面分析讨论二者对页岩孔隙分形维数的影响。结果显示:昭通地区龙马溪一段不同小层之间孔隙发育,非均质性强,有机质质量分数在1小层最高,纵向上具有降低—升高—降低的“三段式”变化,页岩孔隙分形维数具有分段特点,其中分形维数D_(2)介于2.6994~2.9124之间,平均为2.8265,分形维数D_(1)介于2.6174~2.7194之间,平均为2.6734,孔隙结构复杂,分形维数在1小层最高,向上逐渐减小。页岩分形维数与孔隙结构参数及地质因素相关性分形显示:TOC质量分数及石英体积分数控制微孔的发育,提供主要的比表面积,与分形维数具有良好正相关关系,有利于页岩气的赋存,总含气量及吸附气比例高;黏土矿物及白云石体积分数主要控制介孔及宏孔的发育,提供主要的孔体积,与分形维数呈负相关。结合生烃、储集及开发因素可知,1小层及3小层因其TOC质量分数高、分形维数大、脆性强的特点,有利于页岩气滞留,成为页岩气开发的有利层位。

In order to quantitatively characterize the complexity of shale pore structures of Longmaxi Formation in Zhaotong national shale gas demonstration zone,Yunnan Province and its influence on shale gas,24 pieces of shales collected from Longmaxi Formation were tested for TOC mass fraction,whole rock mineral composition and low-temperature nitrogen adsorption.And their influence on fractal dimension of shale pores was analyzed and discussed from two aspects of shale pore structures and mineral composition.The results show that there is a strong heterogeneity in pore development among different sub-layers in Member 1 of Longmaxi Formation in Zhaotong area,with the highest content of organic matter in the Layer 1,with a“three-stage”variation of decreasing-increasing-decreasing in the longitudinal direction.Shale pores are segmented,with fractal dimension D_(2)ranging from 2.6994 to 2.9124,with an average of 2.8265 and fractal dimension D_(1)ranging from 2.6174 to 2.7194.The pore structures are complex and the fractal dimension is highest in the Layer 1 and gradually decreases upwards.The correlation between fractal dimension of shale with pore structure parameters and geological factors shows that TOC mass fraction and quartz volume fraction control the development of micropores,provide the main specific surface area,and have a good positive correlation with fractal dimension,which is conducive to the occurrence of shale gas,with higher total gas content and adsorbed gas ratio;The volume fraction of clay minerals and dolomite mainly controls the development of mesopores and macropores,provides the main pore volume,and has a negative correlation with fractal dimension.The combination of hydrocarbon generation,reservoir and development factors suggests that Layer 1 and Layer 3 are beneficial to shale gas retention due to their high TOC content,large fractal dimension and strong brittleness,making them become favorable horizons for shale gas development.