Controls of interlayers on the development and distribution of natural fractures in lacustrine shale reservoirs: A case study of the Da'anzhai member in the Fuling area in the eastern Sichuan Basin

Lacustrine shale reservoirs with interbedded organic-rich shales and thinly layered tight shell limestones are mainly developed in the Da'anzhai Member of the Lower Jurassic Ziliujing Formation in the Fuling area. These reservoirs generally have low porosities and permeabilities. Natural fractures provide effective spaces for these reservoirs and significantly improve fluid flow capability. However, little research has been conducted on the genesis and distribution of fractures in the Da'anzhai Shale Member. This study aims to understand the features, distribution, and factors that influence natural fractures of these shale reservoirs in the study area. Based on field outcrop observations, core descriptions, and thin section analysis, four types of natural fractures are developed in the study area: tensile fractures, shear fractures, interlayer bedding fractures, and diagenetic shrinkage fractures. Most low-angle and horizontal natural fractures have lengths of 5 cm-10 cm, and the effective fracture density is 0.90 pieces/m. The lithology (Y), thickness of the rock layer (H), regional tectonic stress (F), and fault scale (S) have a significant impact on the development of natural fractures. Most natural fractures develop when there is a large number of laminations in thin rock layers. The large regional tectonic stress in the Late Himalayan, which had a large fracture density index, leads to the formation of natural fractures in numerical simulations. Large faults would have co-developed with more fractures than small faults due to the high energy of large faults. The relationship between the effective permeability and the four parameters (i.e., Y, H, F, and S) was established. Based on this relationship, it is concluded that H is the most critical factor influencing the effective permeability of natural fractures in the study area. As a result, the middle to upper section in the 2nd subsection of the Da'anzhai Member, in which the lithofacies is thinly laminated shales with shells, can be considered to contain favourable shale gas layers.