Oil Origin, Charging History and Crucial Controls in the Carboniferous of western Junggar Basin, China: Formation Mechanisms for Igneous Rock Reservoirs

Abstract The Carboniferous igneous rocks in the western margin of Junggar Basin, western China developed large-scale oil reservoirs. Hongshan-Zhongguai Bulge, the location of Jinlong Oilfield where oil and gas exploration in igneous rocks has achieved efficient breakthroughs in recent years, was studied as an example, because a large number of igneous rocks and crude oil samples are available in this area. The Carboniferous igneous rock reservoirs are typical fault-controlled block reservoirs, and are distributed in a stepped-like manner. Based on analysis of biomarkers, inclusions and reservoir cores under microscope, the oil sources, filling history and major controls for the igneous rock reservoirs that associated with accumulation mechanisms were studied. The biomarker geochemistry reveals that the oil in the igneous rocks was derived from the Permian source rocks including formations of Jiamuhe, Fengcheng and Lower Wuerhe (P2w), and the P2w is the primary source rock. Three important periods of oil charging in the igneous rock reservoirs are suggested: Late Triassic to Early Jurassic, Early Cretaceous, and Late Cretaceous to Early Tertiary. Volcanic breccia of eruptive facies is most favorable for igneous rock reservoir, followed by andesite of effusive facies. Strike-slip faults, overthrust faults and secondary normal faults formed into a fault network, interwoven with the unconformities in the top of the Carboniferous system and within the Permian strata, forming a vertical and horizontal three-dimensional petroleum migration pathway. Hydrocarbons migrated to igneous rock reservoirs from the Permian source rocks along strike-slip faults. The faults communicate the hydrocarbon source rocks with the upper weathering crust reservoirs and control the petroleum distribution. Due to weathering and leaching below the unconformity at the top of the igneous rock reservoirs, oil and gas are most abundant in the upper 150m. In the fault belt the rocks are cemented by minerals of carbonate, clay, zeolite, quartz, and serpentine, and these types of cementation along faults were recognized as important factors for laterally sealing oil and gas in igneous rock reservoirs.