Quantitative characterization of fracture-pore distribution and effects on production capacity of weathered volcanic crust reservoirs: Insights from volcanic gas reservoirs of the Dixi area, Junggar Basin, Western China

Abstract

Igneous hydrocarbon reservoirs worldwide are of significant importance to the petroleum industry for oil and gas exploration. The Carboniferous weathered volcanic crust reservoirs of the Dinan salient in the northern Junggar Basin are major exploration targets of PetroChina. The present study analyzes the geological, well logging, and production data to formulate development and distribution methods for quantitative characterization of weathered volcanic crust fractures and examine their fracture-pore distributions. The effects of the fracture-pore distribution on the productivity of various lithologies of weathered volcanic crust reservoirs were investigated. Various volcanic lithofacies demonstrate the lithological differences in their physical properties, fracture development, and distributions due to the conspicuous variations in weathering resistance among different volcanic rock types. The present study revealed the level of physical properties of volcanic rocks, which are ranked as follows: volcanic breccia > mafic lava > tuff > felsic intrusive rock. However, the degree of fracture development ranked differently from most developed to least developed as follows: felsic intrusive rock > mafic lava > volcanic breccia > tuff. The fracture distribution was primarily controlled by lithology, rock stratum thickness, degree of weathering and leaching, along with tectonism. Compared to other rock types, felsic intrusive rock and mafic lava were more prone to fracturing owing to different petrophysical properties. Additionally, fractures typically developed in single lithological bodies with a thickness of less than 15 m and were characterized by the “single-section” developing mode. For lithological bodies of thickness greater than 15 m, fracture development was more limited, primarily concentrated at the top and bottom of the lithological body and characterized by the “bi-section” or “tri-section” developing modes. These differences can be quantitatively characterized by the fracture development thickness, fracture layer density, and fracture development thickness ratio. Moreover, the influence of the degree of fracture development on the productivity of mafic lava and felsic intrusive gas reservoirs was higher than that of the porosity; however, volcanic breccia and tuff exhibited the opposite effect. The production capacity of weathered volcanic crust reservoirs is affected by the following factors in the order of degree of fracture development > porosity > lithology > thickness of a single lithological body > distance from the top surface of the weathered crust. These quantitative characterization methods can provide valuable technical support for optimizing favorable exploration targets.