Bedding-Parallel Fractures in Deep Lacustrine Shales and their Significance for Hydrocarbon Accumulation

Deep lacustrine shales have extremely low porosity and permeability. It is difficult to form an efficient seepage system with only the matrix pores in the shale. Bedding fractures play an important role in the enrichment and production of shale oil. Bedding-parallel fractures are small in scale but highly developed in lacustrine shales. Macroscopically, bedding-parallel fractures usually parallel or are approximately parallel to the bedding plane, with short extension lengths in the lateral direction and an intermittent distribution. The dip angle of bedding-parallel fractures is consistent with the stratigraphic occurrence, mainly concentrated in 0°∼10°. The aperture of bedding-parallel fractures is very small and follows a power-law distribution, with more than 70% of bedding-parallel fractures smaller than 10 μm. Bedding-parallel fractures develop a large number of microscopic pores and fractures at a microscopic scale, which can provide effective storage space for shale oil. Bedding-parallel fractures are also the main seepage channels of lacustrine shale reservoirs, which can increase the reservoir permeability by two orders of magnitude and significantly improve the seepage conditions of the reservoir.