Effects of weathering and fracturing on the physical properties of different types of volcanic rock: Implications for oil reservoirs of the Zhongguai relief, Junggar Basin, NW China

Abstract

Igneous hydrocarbon reservoirs are rare but significant. The Permian and Carboniferous volcanic rocks of the Zhongguai (ZG) relief in northwestern Junggar Basin contain typical large-scale weathered volcanic reservoirs. Weathering and fracturing are the main controlling factors of the volcanic reservoirs. Based on core observations, casting thin sections, and petrophysical measurements, the pore characteristics of the weathered volcanic rocks and the impacts of weathering and leaching on improvement of physical properties among the different rock types were analysed. From the core observation and image logging, a quantitative evaluation method for fractures was established and genetic types and fracture distributions were examined. Finally, from the diagenetic history, the effects of weathering and fracturing on the physical properties of the different types of volcanic rocks were associated with oil and gas distribution. This study showed that the level of porosity improvement caused by weathering and leaching of the different volcanic rocks was in the order of lava > sed–pyroclastic rock > pyroclastic rock, whereas permeability improvement followed the order of sed–pyroclastic rock > lava > pyroclastic rock. The distribution of fractures is controlled mainly by lithology, rock stratum thickness, degree of weathering and leaching, and tectonism. Differences in petrophysical properties make the volcanic breccia and welded breccia more prone to fractures than the other rock types. Additionally, thinner rock strata have more fractures. Weathered fractures are distributed mainly in the range of 0–60 m from the top of the weathered crust. Moreover, weathering plays an important role in the increase of fractures. Structural fractures in the ZG relief formed mainly in the Middle–Late Permian, corresponding to weathering and leaching. The pores and fractures that formed during this period laid the foundation for the formation of volcanic reservoirs. Thus, the effective lava reservoirs are distributed mainly near the weathered crust, while the effective volcanic breccia and welded breccia reservoirs are not restricted by depth.