The influence of fracture development on quality and distribution of volcanic reservoirs: a case study from the carboniferous volcanic reservoirs in the Xiquan area, eastern Junggar Basin

The distribution pattern of fractures in the Carboniferous volcanic rocks controls the oil and gas distribution in the Xiquan area located in the eastern Junggar Basin. This paper describes the types, characteristics, distribution of fractures in the volcanic rocks, and the evolutionary history of fractures, as well as the influence of fractures on the reservoir distribution in this area, based on analyzing cores, casting thin sections, imaging logging data, the measurement of the physical properties and integrated with the production testing. The lithology of these Carboniferous volcanic rocks is primarily andesite and volcanic breccia. Basalt, dacite, tuff, breccia lava, tuffaceous sandstone, and tuffite were also present. The fracture types are mainly shrinkage fractures, weathered fractures, dissolution fractures, and structural fractures, the majority of which are structural fractures and weathered fractures. The distribution of fractures has obvious zonation characteristics in the vertical direction. Fractures are mainly distributed at four intervals under the top surface of the Carboniferous unconformity: 0–50 m, 65–115 m, 125–155 m, and 170–210 m. There are mostly weathered fractures and dissolution fractures in the 0–50-m interval. The physical properties and oil production of andesite are the highest, and weathering fractures and dissolution fractures play a major role in controlling the secondary pores of volcanic lava and the formation of favorable reservoirs in this interval. The structural fractures were developed in the intervals of 65–115 m, 125–155 m, and 170–210 m. The physical properties and oil production of volcanic breccia are the highest, and therefore, structural fractures mainly control the formation of favorable reservoir in volcanic breccia in these three intervals. The evolution of fractures in the Carboniferous volcanic rocks experienced four phases: the condensation and shrinkage fracture stage, which developed the primary pores in the middle-late Carboniferous period; the weathered fracture and dissolution fracture stage, which greatly increased the porosity in the late Carboniferous period to the early-middle Permian period; the structural fracture and weathered fracture stage, which greatly increased the porosity and permeability of the reservoir from the end of the Triassic to the early Cretaceous period; and the filled fracture and dissolution fracture stage, which can also greatly increase the porosity of the volcanic reservoir and occurred wholly within the late Cretaceous period until the present.