Formation mechanism of high-quality reservoirs in intermediate volcanic rocks: A case study of Longfengshan area in Songliao Basin

The basin margin slope area of Changling fault depression in Songliao Basin is dominated by intermediate-basic volcanic rocks of Huoshiling Formation, and there is still a lack of in-depth understanding of the development pattern and formation mechanism of high-quality reservoirs in inter-mediate-basic volcanic rocks in this area. Taking the intermediate volcanic rocks in Longfengshan area of Changling fault depression as an example, a comprehensive study was undertaken via core obser-vations, thin section porosity analysis, rock physical property tests, scanning electron microscopy and X-ray diffraction analysis, combined with well log and seismic data. As a result, formation mechanism of a high quality reservoir was made clear based on dissolution modification effects of the volcanic rocks, and the development model of the high quality reservoir under the dual control of lithology and fluid was established. The research shows that the volcanic rocks in this area consist of six lithologies: andesitic hydrothermal breccia, andesite, andesitic volcanic breccia, andesitic tuff, dacitic tuff and sedimentary tuff, which are controlled by structure, eruptive source, eruptive action and palaeotopography. The proximal facies of the volcanic edifice were located on the basin margin, distributed along faults mainly consisting of overflow facies intermixed with eruptive facies, and gradually transformed into thick eruptive facies (middle-source facies) and volcanic sedimentary facies (distal facies). The reservoir space of volcanic rocks was dominated by dissolution pores, which were formed by weathering and dissolution by organic acidic fluids. The thermal debris flow subphase in the middle-source facies of the volcanic edifice had a large amount of easily dissolved volcanic ash, which was close to the source rock in the centre of the basin and was a favourable area for the development of dissolution pores. This study is providing a guide for oil and gas exploration in the intermediate-basic volcanic rocks in the slope of Changling fault depression in Songliao Basin.