Characteristics of deep coal reservoirs and structural fractures controlling in Middle-Lower Jurassic of the Kuqa Depression in the Tarim Basin, China

Coal represents a crucial energy form within the carbon cycle of the Earth's lithosphere and serves as a significant reservoir for hydrocarbon energy sources, such as natural gas. As a plastic stratum, coal rock governs the formation of structural fractures under intense stress conditions in deep strata, thereby influencing the accumulation of oil and gas. In recent years, China National Petroleum Corporation (CNPC) has extended its coalbed methane exploration from depths shallower than 1,000 meters to those exceeding 2,000 meters, achieving large-scale production in the Junggar Basin and the Ordos Basin. This endeavor has become a pivotal business segment for the company in enhancing its reserves and production capacity.

The coal reservoirs in the Middle-Lower Jurassic of the Kuqa Depression in the Tarim Basin are buried at depths ranging from 0 to 8,000 meters. The cumulative thickness of coal seams is 40 to 260 meters, with a single layer thickness of 1 to 19.5 meters. The coal reservoirs buried at depths of 1,500 to 5,000 meters are relatively continuously distributed, covering an area of 5,730 km². Based on experimental analysis such as CT scanning, scanning electron microscopy, nuclear magnetic resonance, and nitrogen adsorption, outcrop and drilling samples have been conducted. It is reported that the coal reservoirs in this area have developed cleats, which are distributed in a linear and networked manner. There are 8 to 15 surface cleats per 10 cm and 13 to 24 end cleats per 10 cm. The reservoir spaces are connected, with microfractures being developed, indicating good reservoir conditions. The microfractures are mainly vertical and interlayer fractures, with good connectivity. The surface porosity ranges from 2.32% to 12.32%, and the connected porosity is 5.12%.Scanning electron microscopy reveals the development of cellular pores, primary pores, and hydrocarbon-generating pores. The matrix porosity is 10.4% to 15.5%, with the development of micro-pores (less than 10 nm) and mesopores (10 to 1,000 nm), which is conducive to the occurrence of adsorbed gas and free gas. Micropores (less than 2 nm) account for 63.4%, serving as the main adsorption space, while macropores with diameters ranging from 100 to 3,000 μm are the main occurrence space for free gas. Under the later strong thrust nappe action, the concentrated development area of coal is a regional differential structural deformation and strike-slip transformation zone, which controls the generation of fractures and the development intensity of structural fractures, and is a potential sweet-spot for reservoir development.