Laminated texture of shale and its storage space significance in Western Depression, Liaohe Basin

Almost all organic-rich shale contains stratigraphic structures to varying degrees, reflecting different sedimentary facies, shale organic matter abundance, hydrocarbon generation capacity, reservoir space and gas content. The organic matter (OM) can be divided into three types of occurrence state, namely continuous lamellae, discontinuous lamellae and scattered lamellae, based on the sedimentary characteristics of organic-rich shale in lacustrine rift basins and the heterogeneity of the OM distribution. The black shale stratigraphic structure of the Western Depression of Liaohe Basin was studied using core observation, thin section analysis and scanning electron microscopy, and combined with statistical analysis. The organic-rich shale in the fourth member of Shahejie Formation (Sha-4) was mainly deposited in a semi-deep lake under brackish water conditions, developing carbonate-curved continuous laminae and clay-type flat continuous laminae. The Sha-3 shale, deposited in a freshwater deep-semi-deep lake, is dominantly composed of felsic interlaced laminae and dispersed laminae. The continuous carbonate-mixed lamellar shale can be related to chemical interactions, biochemical interactions caused by algae, or a combination of these two. The organic-rich clay shale is mainly composed of seasonal lamellae formed by climate change. The discontinuous striated layers usually consist of clay lamellae, and silt and clay are seasonally input in the form of advection or inter-layer flow. The shale lamellar morphology largely determines hydrocarbon generation capacity and characteristics, and further may influence reservoir capacity. The continuous lamellae are generally characterized by high carbonate content as well as being prone to dissolution, high TOC abundance, strong hydrocarbon generation ability and abundant organic pores. The organic pore-micro-seam reservoir system would contribute to oil and gas accumulation, and carbonate content is positively correlated with hydrocarbon content. In addition to the development of various inorganic pores, the discontinuous lamellae also develop organic pores, bedding cracks, and mineral cleavage cracks, which lead to the development of shale reservoir space and superior shale gas-bearing conditions. The dispersed lamellae are less prone to dissolution and cracking, so the reservoir space is relatively undeveloped and the gas-bearing conditions are relatively poor.