Genesis and Accumulation of Paleo-Oil Reservoir in Dabei Area, Kuqa Depression, Northwest China: Implications for Tight-Gas Accumulation

Paleo-oil reservoir is of great importance to understand hydrocarbon enrichment mechanism and hydrocarbon exploration potential, but is yet poorly investigated in Kuqa Depression. The occurrence of the paleo-oil reservoir in Dabei area was proved by quantitative grain fluorescence (QGF) and fluid inclusion petrography. Development history of the paleo-oil reservoir was reconstructed through: (1) oil-source correlation; (2) time coupling of source rock maturation, porosity evolution and migration pathways. The impact of paleo-oil reservoir on tight-gas accumulation was consequently discussed. Results suggest that considerable oil was accumulated in the K₁bs reservoir with paleo oil-water contact in Dabei 2 Well and Dabei 201 Well at 5 800 and 6 040 m, respectively. Crude oil was primarily sourced from Triassic source rocks with Jurassic source rocks of secondary importance, which was at oil generation window (0.7%–1.1% Ro) during 9–6 and 7.5–5 Ma, respectively. The occurrence of K₁bs tight reservoir (porosity < 12%) was about 25 Ma, while faults and associated fractures at Kelasue structural belt were developed approximately from 8 to 3.5 Ma. Therefore, the tight oil accumulation was formed during 8–5 Ma. The paleo-oil reservoir in Dabei 1 gas field was destroyed by the evaporation fractionation in later stage.