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Exploration of New Horizons in Mature Area: A New Understanding of Oil/Gas Accumulation in Low Porosity and Permeability Area


The central Jungar Basin was abundant with oil/gas from Carboniferous to Cretaceous. However, Triassic was characterized by low porosity and permeability, which was listed as “Forbidden Zone” for exploration in the past. The study on accumulation evolution of three periods (accumulation period, oil/gas adjustment period-present) in Triassic Baikouquan Formation has been carry out in this paper, the evolutionary relationship between porosity of different facies and depth has been fitted by geological statistics, and analysis of threshold property for oil/gas charging in different periods has been conducted according to the mineral composition and diagenetic evolution analysis. Based on above study, it can be confirmed that delta front is dominated by medium pores in Early Cretaceous (the crucial accumulation period), where oil/gas can be charged in large amount; when secondary porosity increased along with the buried depth which can pay the porosity reduction by compaction, oil/gas can be charged continuously in the delta front. However, reservoirs in the delta plain get tight, leading the porosity lower than the threshold property where oil/gas can be charged in most area, which can form the seal in the updipping direction. All this can form favorable conditions for wide range of accumulation in the up-hydrocarbon migration surface and nose-like structure of the hydrocarbon migration system. The area of newly increased favorable exploration area is about 1500 Km2, which was the important replacement area for the oil/gas exploration in central Jungar Basin. The study results also may be helpful for multi-horizons potential exploration in mature exploration area and evaluation in low porosity and permeability area.