Characteristics of fractured and porous volcanic reservoirs and the major controlling factors of their physical properties;A case study from the Carboniferous volcanic rocks in Ludong - Wucaiwan area, Junggar Basin
Volcanic reservoirs have characteristics of fracture-pore dual-porosity, so the relevant studies will focus on the identification of their lithology and the major controlling factors of their physical properties. Taking the Carboniferous volcanic rocks in the Ludong - Wucaiwan area of the Junggar Basin as an example, we analyje the lithology, poroperm characteristics and diagenesis of the volcanic reservoirs through observation of cores, thin sections, regular logging and FMI imaging. The following conclusions are obtained: (1) The neutral-basic lavas are mainly distributed in the eastern Wucaiwan sag, while the acidic lavas and volcantclastic rocks mainly occur in the western and eastern parts of Dinan salient, and volcanic rocks with different lithologies have different logging responses. (2) The pores of the volcanic reservoirs can be grouped into 3 categories and 9 types, and there are 4 types of pore combinations, including primary pore type, secondary dissolution pore (cavity) type, secondary dissolution pore-fracture hybrid type, and fracture type. (3) The intermediate-basic lavas are dominated by dissolved fractures and weathered fractures, and the physical properties are the best in acid lavas with the combinations of matrix dissolution pores - dissolved fractures and/or weather fractures. (4) Factors controlling volcanic reservoir quality mainly include lithology, tectonic landform and epigenesis. According to relative values of porosity and permeability, volcanic reservoirs can be divided into 4 types. Volcanic reservoirs close to volcanic vents have relatively good quality and fractures may be highly developed on structural highs, thus, oil and gas production are high, and the volcanic reservoirs in this central basin are dominated by dissolution pores and contain mainly oil and less gas. Secondary dissolution and metasomatism-filling are the major factors influencing volcanic reservoir quality in the area, and the former contributes a lot to the reservoir quality.
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