Characteristics and Origin of Microfracture in Lower Cretaceous Tight Sandstone from Kuqa Foreland Basin, NW China

The Kuqa basin, as a secondary tectonic unit of Tarim basin, located at the front of the south Tianshan Mountain, is a foreland basin formed in the Late Tertiary. The Lower Cretaceous Bashijiqike tight sandstone in the basin is an ultralowpermeability and low-porosity reservoir. Microfractures, such as intragranular microfractures, grain-edge microfractures, and transgranular microfractures, are abundant in the tight sandstones. Microfractures improve storage and permeability and impact distribution of natural gas. Microfractures have a wide range of sizes, shapes, and patterns, but most of them are opening-mode fractures. Microfractures reflect tectonic, overpressure, and diagenetic origins. Based on fluid inclusions, carbon-oxygen stable isotope analysis of fracture-filling cements (calcite and dolomite) and burial history, tectonic microfractures were determined to be formed before early Miocene and at the middle of Miocene. In the middle to late Himalayan movement, the tension microfractures closed and were filled with carbonate cements. Microfractures related to overpressure, being tension microfractures, are commonly filled with bitumen and were formed at the end of Miocene. The transition of stress state from compression to tension by overpressure is the reason that tension microfractures were formed in the compression setting. Diagenetic fractures were formed at the end of the Late Cretaceous to Paleogene. Under intense compaction, grain-crushing crackle fractures in quartz and cleavage fractures in feldspar formed intragranular microfractures. Some transgranular microfractures and grain-edge microfractures caused by diagenesis are along the bedding plane and parallel to the directional mineral grains.