Genetic mechanism of high-quality reservoirs in Permian tight fan delta conglomerates at the northwestern margin of the Junggar Basin, northwestern China

Fan delta conglomerate reservoirs in the Permian Jiamuhe Formation in the Zhongguai area at the northwestern margin of the Junggar Basin, northwestern China, are reservoirs for large accumulations of natural tight gas. The tight conglomerates and sandstones are mainly litharenites with a large amount of texturally and compositionally immature volcanic clastic materials. Core porosities demonstrate the development of anomalously high porosities at depths of 3200-4200 m (10,500-13,800 ft) and 4500-4900 m (14,800-16,100 ft). The Permian reservoirs experienced initial rapid subsidence, uplift, and further subsidence, and diagenetic reactions occurred in these reservoirs involving compaction; precipitation of chlorite clays, calcites, zeolites, iron oxides, kaolinite, and quartz cements; and dissolution of unstable minerals including analcite, laumontite, feldspars, and rock fragments. Low-porosity reservoirs, with extensive compaction and (or) cementation of calcite and heulandite-Ca, consist of only few visible secondary pores formed by dissolution of feldspars and rock fragments. Reservoirs with anomalously high porosity, however, experienced relatively weak compaction and contain significant amounts of secondary pores formed by dissolution of mainly laumontites, some feldspars, and rock fragments. Comprehensive studies of sedimentary features and diagenesis of the reservoirs indicate that these anomalously high porosities originate from chronological coupling of four important geological processes. (1) Sedimentary facies and detrital compositions controlled distribution of various zeolites in reservoirs, with abundant laumonites developed in reservoirs in the fan delta front subfacies. (2) Early precipitation of the laumontites inhibited compaction during deep burial; however, they provided unstable minerals for secondary porosity development. (3) The Permian reservoirs experienced subaerial exposure erosion during the uplift stage, and meteoric freshwater leached laumontite cements and aluminosilicate grains to form secondary pores in reservoirs beneath the unconformity. (4) Hydrocarbon emplacement at relative shallow depth during a second subsidence period preserved the secondary pores in reservoirs by retarding late carbonate cementation.