Orderly coexistence and accumulation models of conventional and unconventional hydrocarbons in Lower Permian Fengcheng Formation, Mahu sag, Junggar Basin

By using the latest geological, seismic, drilling and logging data, this article studies the basic conditions for the formation of the total petroleum system and the orderly coexisting characteristics and accumulation models of conventional & unconventional reservoirs in the Lower Permian Fengcheng Formation in the Junggar Basin. Controlled by thermal evolution, hydrocarbon generation and expulsion process of the high-quality source rocks in alkaline lake as well as the characteristics of multi-type reservoirs (conglomerate, sandstone, dolomite and shale), conventional structure-lithologic reservoirs and tight oil and shale oil reservoirs controlled by source-reservoir structure have been formed. On the plane, mature conventional reservoirs, medium-high mature tight oil, and medium-high mature shale oil reservoirs coexist orderly from the slope area around Mahu sag to the sag. Based on the orderly coexisting characteristics of conventional and unconventional reservoirs in the Fengcheng Formation, it is clear that oil and gas in the Fengcheng Formation accumulate continuously over a large area in three accumulation models: integrated source-reservoir, source-reservoir in close contact, and separated source-reservoir model. The three accumulation models differ in relationship between source-reservoir structure, reservoir lithology and spatial distribution, hydrocarbon migration, oil and gas type. It is pointed out that the conventional & unconventional oil and gas should be explored and developed as a whole to achieve an overall breakthrough of the total petroleum system. This study is expected to enrich the geological theory of oil and gas enrichment in continental basins and to provide an analogy for exploration and research in other hydrocarbon-rich sags.