Formation mechanisms and distribution of weathered volcanic reservoirs: A case study of the carboniferous volcanic rocks in Northwest Junggar Basin, China

In view of the complex reservoir heterogeneities within weathered volcanic rocks, taking the Carboniferous volcanic weathered profile reservoir of the northwest Junggar Basin as an example, this study aims to characterize weathered volcanic rocks by the core, thin slice, log interpretation, and experimental test. The Carboniferous volcanic rocks are mainly composed of volcanic lava, volcanic breccia, and tuff, etc, which formed in a volcanic arc environment. As a result of the weathering and denudation, the weathered profile can be divided into four layers: weathered clay layer, intensive weathered zone, weak weathered zone, and unweathered compacted zone. The preserved void space mainly includes primary porosity (both vesicular and fractures), secondary pores, and fractures enhanced by weathering and structural processes. Among them, the volcanic facies control the first-order properties of the reservoir. The degree of weathering-leaching controls the thickness of the weathered part of the reservoir. The faults and fractures control the seepage/permeability capacity of weathered volcanic reservoir. On the plane, the weathered volcanic reservoirs mainly distributed along the fault zone, tectonic zone, and favorable lithofacies zone. On the longitudinal direction, they are always within the weathered profile toward the top of a certain depth range (20-240 m).