Effective fractures and their contribution to the reservoirs in deep tight sandstones in the Kuqa Depression, Tarim Basin, China

Abstract

The Mesozoic Cretaceous reservoirs with a burial depth of more than 4500 m (14,764 ft) in the Kuqa Depression of the Tarim Basin are rich in natural gas resources, categorized under the typical deep tight sandstone reservoirs. Based on the analysis of cores, borehole image logs, thin sections, and laboratory testing, this study comprehensively evaluated the effective fractures in deep tight sandstone reservoirs and illuminated the contribution of effective fractures to the properties of these reservoirs. Tectonic fractures are an important component in these reservoirs. More than 70% of these fractures are not filled by any minerals, and their apertures vary greatly, measured from a few to hundreds of microns. The influencing factors of the effective fractures in these reservoirs include tectonism, diagenesis, and in-situ stresses. The fractures that are formed at earlier tectonic movements are more susceptible to be filled with minerals. The diagenetic facies of strong cementation exhibit a higher abundance of mineral-filled fractures, while dissolution can cause fracture apertures to become larger and makes them more effective. The unfilled fractures, which are parallel to the maximum horizontal stress component, show larger apertures and therefore are more effective. Laboratory tests indicate that effective fractures contribute less to the reservoir storage space, with an average porosity contribution rate of 3.95%. However, the permeability of samples with microscopic effective fractures was found 1-2 orders of magnitude higher than those without such fractures, since these fractures are main pathways that can connect scattered pores to improve the effectiveness of storage space. Moreover, the macroscopic effective fractures with a smaller angle from the maximum horizontal stress component decide the dominant reservoir fluid flow direction. Consequently, effective fractures exhibit a leading contribution to the reservoir permeability.