Petroleum geology of Carboniferous volcanic weathered crust in northern Xinjiang, China

Abstract Previous studies on volcanic rock reservoirs were focused on lithology, lithofacies and fractures. Primary processes may lead to high porosity and permeability, and secondary processes tend to decrease primary porosity. However, this is not the case in northern Xinjiang, where Carboniferous volcanic rocks covered an area of 378×103 km2 dominated by intermediate-basic rocks of low porosity. At the end of the Carboniferous, these rocks were uplifted, underwent denudation and weathering, and then formed a weathered crust. All types of volcanic rocks might form favorable reservoirs after long-term weathering. The reservoir properties of the crust were controlled by weathering degree and fault development. Four types of reservoir porosity developed: dissolutional pore, pore-fracture, fracture and fracture-cave. Dissolutional pores and fractures are the main reservoir spaces. Under the influence of fractures and an enhanced fracture dissolution environment, the volcanic reservoirs have improved physical properties in deeper burial settings. The depth range of effective reservoirs under the unconformity, either close to, or far from fracture zones, is between 1100 m and 550 m, with maximum porosities of 32% and 24%, respectively. Hydrocarbons have accumulated close to the effective source rock zone due to strong heterogeneities of the volcanic reservoir. The overlying Carboniferous mudstone formed effective caprock. The current structural highs and slope zone coincided well with the paleogeomorphology and form traps for hydrocarbons. Faults and fractures control hydrocarbon enrichment. This is contrary to the view that the Carboniferous is simply impermeable basement in this region, and not capable of hydrocarbon generation.