Based on drill core and thin section observation, major geochemical element analysis and conventional well log analysis, this study summarizes the characteristics and thickness of weathering crust of Pennsylvanian volcanic rocks (Carboniferous) in Shixi area, Junggar Basin. The weathering crust is identified and divided into three types according to the petrological characteristics and well log interpretations in Shixi area, and the isopach of weathering crust is mapped. The results show that: (1) With the increase of depth, the weathering weakens, and the rocks become less fractured with decreased porosity; (2) the weathering crust of the Upper Carboniferous volcanic rocks can be divided into strongly weathered and mildly weathered layers in Shixi area; (3) the weathering crust is relatively thicker in Dinan uplift and Shixi uplift. This study provides research basis for further evaluation of Upper Carboniferous volcanic reservoir, and will benefit for well location deployment and potential oilfield development in the Shixi area.

The identification of volcanic lithofacies is the basis for establishing volcanic institutions and making high-quality reservoir predictions. As the formation of Carboniferous volcanic rocks in Shixi area of Junggar Basin is strongly related to the cyclonic period of volcanic eruption and the location of volcanic channel development, the lithological variety changes rapidly and the lithofacies are complex. Moreover, the volcanic rocks had undergone strong weathering modification at later stage, the identification of volcanic facies becomes more difficult. This paper uses core observation, thin section identification, logging data and seismic attributes to classify and identify the lithology and lithofacies, and to summarize the methods of lithology classification and lithofacies identification of volcanic rocks in the study area. The lithology is mainly andesite and dacite, followed by volcanic breccia and tuff. Basalt and rhyolite develop locally in a few localities. The lithofacies of Carboniferous Period in Shixi area mainly consists of intermediate-acid effusive facies, followed by explosive facies, with basic effusive facies being rare and thinly developed, according to the well data. The volcanic conduit facies is developed only in a few places. The lithofacies are mainly identified based on the characteristics of the logging curves and the combination of external morphology and internal structure of the seismic reflections. The research method of this paper provides a case for the study of Carboniferous volcanic rocks and the dissection of volcanic edifices in Shixi Area.

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 intermediate-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 observations, 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.

In order to study the microscopic pore characteristics of andesite reservoir and the effect of mineral content on the andesite pore, this study takes the andesite of the Huoshiling Formation in Longfengshan of Changling fault depression, Songliao Basin as a subject. The andesite reservoir space was discerned through the observation of cores and casting thin sections. Besides, the pore size distribution of andesites and their mineral contents were quantitatively characterized by high-pressure mercury injection, nitrogen adsorption and XRD, respectively. The results show that: (1) There are various types of reservoir space in andesites, including vesicles, amygdule, intergranular pores, matrix dissolution pores and dissolution pores of amygdala, and three types of fractures including dissolution, structural and explosion fractures. (2) The pore size distribution of andesite is complex. The main pore size of andesite is mid-pore (10-20 nm) with some large-pores (>50 nm). Mid-pore and large-pore provide the main specific surface area, which are the main space for gas storage. (3) The andesite reservoir space in the study area is mainly controlled by dissolution, as supported by the relationship between the change of mineral content and porosity evolution. The porosity of andesites decreases with the increase of quartz and chlorite content, but increases with the increase of soluble mineral, e.g., feldspar content.