Characteristics of Carboniferous volcanic reservoirs in the Chun-feng Oilfield of the Junggar basin, China

Abstract

As a major petroliferous layer, Carboniferous volcanic reservoirs have developed widely in the Chun-feng Oilfield in the Junggar basin. Volcanic reservoir rocks have been the principal target for exploration and development in the study area, having relatively high porosity and permeability which is favorable for hydrocarbon accumulation. According to observation of thin sections of core samples and logging lithology, the volcanic reservoir rocks of the Chun-feng Oilfield can be divided into 5 types according to their attitudes and composition. These are volcanic fulgurite rocks, clastic volcanic rocks, volcanic clastic rocks, sedimentary volcanic rocks, and volcanic sedimentary rocks. Lithofacies has also been studied according to their lithological distribution and associations. According to observation of thin sections and SEM images of core samples, the pore system of the volcanic reservoir is mainly composed of primary pores, secondary pores, and fractures. Statistical analysis of measured data shows that volcanic reservoirs have poor reservoir properties, with generally low porosity and permeability. Reformed by late diagenesis and tectogenesis, a weathered crust structure has developed in the Carboniferous volcanic reservoirs in the study area, which is composed of five zones. These are soil zone, hydrolytic zone, leaching zone, disintegrating zone, and original zone. Of these, the leaching zone has developed the best reservoirs, with relatively high porosity, followed by the disintegration zone. With low porosity, the reservoirs of the soil and hydrolytic zones act generally as cover layers. There are three main factors which determine the physical properties of volcanic reservoirs. The principal factor is that lithofacies control the primary pore types, which have an influence on subsequent reformation. Faults tend to promote the development of secondary porosity, and paleogeography restricts the distribution of favorable volcanic reservoirs in the area.