Characteristics of Volcanic Reservoirs and Hydrocarbon Accumulation of Carboniferous System in Junggar Basin, China

The Carboniferous volcanic reservoirs in Junggar Basin contain rich hydrocarbon resources, implying a great exploration potential, so that they have become a key replacement target for “three-dimensional exploration”. The study on the Carboniferous volcanic reservoirs and their hydrocarbon accumulation elements is significant for clarifying the orientation for exploration. In this paper, based on 37 reserves reports and 3 200 reservoir test data, the Carboniferous volcanic reservoirs in Junggar Basin were discussed from the prospective of lithology and lithofacies, physical properties, reservoir types, main controls on hydrocarbon accumulation, and hydrocarbon accumulation patterns. It is found that the Carboniferous in the basin is mostly in the multi-island ocean-volcanic island arc structural-sedimentary environment, so it is geologically eligible for forming in-situ volcanic reservoirs. The volcanic rocks are: (1) mostly distributed along deep and large faults, with the lithology and lithofacies controlled by volcanic architectures; (2) dominantly lava, followed by volcaniclastic lava and volcaniclastic rock; (3) distributed in the periphery of hydrocarbon-generating sag and within the source rocks horizontally, and concentrated in the weathering crust at the top longitudinally, possibly leading to reworked weathering crust reservoir; and (4) liable to form inner reservoirs. The volcanic reservoirs can be concluded into four hydrocarbon accumulation patterns, i.e., self-generating & self-storing in paleo-uplift and vertical migration, self-generating & self-storing in paleo-uplift and lateral migration, young-generating & old-storing in fault zone and vertical migration, and young-generating & old-storing in paleo-uplift and lateral migration. Future exploration will focus on the effective source rock development and hydrocarbon supply zones and the self-generating & self-storing and young-generating & old-storing patterns. The exploration prospects are determined to be the Ludong-Wucaiwan-Baijiahai slope belt and the southern slope belt of the Shaqi uplift (self-generating & self-storing pattern) in eastern Junggar, and the fault and nasal arch zone at the northwestern margin and the nasal arch zone (deep and large structure) in the Luxi area (young-generating & old-storing pattern) in western Junggar.