Genesis and reservoir significance of multi-scale natural fractures in Kuqa foreland thrust belt, Tarim Basin, NW China

Abstract

Natural fractures in the deep Cretaceous Bashijiqike Formation sandstone reservoirs of Kuqa foreland thrust belt, NW China, are classified according to fracture aperture based on the data of outcrops, cores, thin sections and imaging logging, using industrial CT scanning, laser scanning confocal microscope (LSCM), cathodoluminescence (CL), electron probe, and scanning electron microscopy (SEM). The types, characteristics, genesis ages and formation sequence as well as reservoir significance of such natural fractures are examined. Four categories of fractures are classified. Category I (aperture>100 μm) is macro structural fractures, which cut single sand body to form the dominant migration pathway, helping to increase the reservoir permeability. Category II (aperture=10–100 μm) is associated micro structural fractures, which cut matrix grains to connect large matrix pores and improve the seepage performance. Category III (aperture=1–10 μm) is micro digenetic fractures at grain edges, which connect medium and small pores to improve the pore network connectivity and the gas migration and charging efficiency. Category IV (aperture<1 μm) is nano-scale matrix fissures, which connect intragranular micro pores to expand the reservoir space, thereby increasing the reserves scale. Category I and Category II fractures were developed in three stages (early, middle and late). The early- and middle-stage fractures, predominantly half-filled–filled fractures, were formed before early Pliocene when extensive oil and gas charging had not occurred. The late-stage opened fractures were formed after the Late Pliocene, they were at the same time as or later slightly than extensive oil and gas charge. The fracture network has low contribution to porosity, but it can improve the permeability by 2–3 orders of magnitudes in the parallel direction of fractures. Key words: deep reservoir, tight sandstone, natural fracture, genetic type, formation sequence, Kuqa foreland basin