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The Formation Mechanism and Distribution of Volcanic Hydrocarbon Reservoirs in China

Abstract

Volcanic hydrocarbon reservoirs are distributed widely in sedimentary basins in China. Volcanic rocks have changed from “forbidden area” to “target area” in exploration. There are mainly intermediate rocks and acidic rocks in east China which primarily developed in intracontinental rift settings during Jurassic to Cretaceous and Paleogene to Neogene, and intermediate rocks and basic rocks in west China which developed in intracontinental rift and island-arc environment duiring Carboniferous to Permian. The volcanism in sedimentary basins in China can promote the formation of high-quality source rocks and the biogenic hydrocarbon generation process. Most of volcanic hydrocarbon reservoirs discovered in China are biogenic origin and the near-source volcanic rocks are most favorable for hydrocarbon accumulation. Besides, volcanism also plays an important role in the generation and accumulation of the abiogenic gases including hydrocarbon gases and CO2. The volcanic reservoirs are chiefly composed of rhyolite, dacite, andesite, basalt and pyroclastic rocks in China. It can be classified as five lithofacies of which explosive and effusive facies are favorable facies in exploration. It is indicated that secondary reservoir space which includes dissolved pores, structural fractures and weathered fractures makes a dominant contribution to the improvement of volcanic reservoir capacity. The property of volcanic reservoirs is affected by several factors, such as lithology and lithofacies, eruption environment, tectonics and diagenesis. The migration of hydrocarbon predominantly relies on faults and crusts of weathering in the volcanic hydrocarbon system in China. The features of faults control the distance and period of vertical migration while the area of crusts of weathering controls the distance of lateral migration and the scale of hydrocarbon accumulation. Under the control of all factors, the volcanic hydrocarbon accumulation chiefly develops in certain positions, including the top of volcanic cycles, unconformity surfaces, and areas near deep-seated faults. The volcanic hydrocarbon reservoirs in east China are lithologic and sturctural-lithologic reservoirs and those in west China belong to stratigraphic reservoirs.