Storage space, pore structure, and primary control of igneous rock reservoirs in Chepaizi Bulge, Junggar Basin, western China: Significance for oil accumulation

The Chepaizi Bulge of the Junggar Basin contains large-scale fault-controlled petroleum reservoirs in the Carboniferous igneous rock basement. Breccia, tuffaceous breccia, andesite, basalt, and tuff are the major lithology of the igneous rocks, and the major hydrocarbon accumulation was found in the volcanic breccia, andesite, and basalt reservoirs. The types of igneous rocks were predicted by their different logging responses, and were differentiated in the plot of acoustic versus gamma ray loggings when the core is not available. Secondary dissolution pores and fractures (primary tectonic fractures) were the major reservoir spaces. Fractures mainly occurred within 250 m below the top surface of the Carboniferous system. The fractures formed in the early, middle and late stages with different causes were identified. The reservoirs were dominated by mesopores with excessively low permeability, and the volcanic breccia reservoirs exhibited higher porosity (2.0%–16.31%, averages at 7.95%) and permeability (0.01 md–3.13 md, average at 0.11 md) than the basalt–andesite reservoirs (averages at 5.8% and 0.07md) on the whole. Volcanic breccia samples show larger average throat radius, average capillary radius and maximum connected throat radius compared with the basalt samples. The major oil accumulation is mainly distributed along faults, because there is more reservoir space and superior oil migration channels as a result of the induced fractures. Diagenetic cements including minerals of carbonate, clay, zeolite, serpentine, and quartz, were commonly observed in the fault zone, and these types of diagenetic cementation are crucial to the fault sealing, and are of great significance in the formation of large igneous rock reservoirs.