Characteristics and formation mechanisms of large volcanic rock oil reservoirs: A case study of the Carboniferous rocks in the Kebai fault zone of Junggar Basin, China

Abstract

Volcanic hydrocarbon reservoirs are rare and may be overlooked. The Carboniferous volcanic rocks of the Kebai fault zone in the western Junggar Basin contain hydrocarbon (HC) reservoirs in volcanic rock with proven oil reserves of 9.76 × 108 bbl that have a complex filling history. We have investigated the lithology and properties of these volcanic rock HC reservoirs as well as diagenesis and control of faults and fractures in oil reservoirs. The lithology of these Carboniferous volcanic rocks is primarily andesite and tuff. Also present were volcanic breccia and metamorphic rock in addition to rhyolite, felsite, diabase, and granite in the volcanic lava. On the basis of microscopic examination, five types of pores and fractures were observed: (1) fracture–dissolved phenocrystal pore, (2) fracture–intergranular pore, (3) fracture–gas pore, (4) fracture–dissolved intragranular pore, and (5) fracture–dissolved matrix pore. The fractures in these rocks are a significant factor in connecting the pores. Diagenetic processes that control reservoir quality include compaction, filling of pores and fractures, cementation, metasomatism, and grain dissolution. The volcanic reservoirs show a variety of lithologies, and oil has been discovered in all types of Carboniferous rocks. The controlling factors for oil distribution in these Carboniferous volcanic rocks are faulting, fracture development, and degree of weathering when they were subaerially exposed in the Permian. The area in which these faults and fractures developed is the primary area of oil enrichment with high yields. The objectives of this study were to (1) describe the characteristics of different types of volcanic rocks and reservoirs found in this basin and (2) characterize the diagenetic history of these rocks and document how diagenesis controls porosity and permeability.