Characterization and genetic significance of mold pores in tight sandstone: A case from the lower part of the Yanchang Formation in the Wuqi–Ansai area

The lower part of the Yanchang Formation in the Wuqi–Ansai area consists of three layers which are generally characterized by low (5–10%) to extra-low (<5%) porosity and extra-low (10–100 md) to ultra-low (<10md) permeability. Based on microscopic observation of casting thin sections, mercury injection tests, micro-CT scanning, inclusion temperature measurement, and fluorescence tests, it was determined that the pore types of the reservoirs mainly included secondary pores between pores, intraparticle pores in feldspar, intercrystallite pores in clay, mold pores, and laumonite dissolution pores. With decreasing porosity and permeability of the reservoirs, the proportions of intraparticle pores in feldspar and mold pores increased; however, the proportion of interparticle pores decreased. The pores of the reservoirs, which were dominated by mold pores, mostly showed coarse skewness and good sorting with few pore throats and low coordination numbers. These pores were spatially isolated and rarely connected by throats. Quartz microcrystals were observed to be closely attached to the pore walls and growing along them. Diagenetic analysis indicated that the mold pores formed after the sandstones were intensely compacted and before the reservoirs were densified. As the hydrocarbon source rock matured and discharged acidic fluids, the grains and matrix in the sandstone reservoirs gradually dissolved to form pores, and some of the feldspar particles were even completely eroded into mold pores. In the following densification process of cements filling pores, the mold pores located on hydrocarbon migration pathways were filled by small amounts of oil and gas; thus, they could be retained after densification. Pores of this type were morphologically isolated because of the cementation of most throats. After the intense and selective corrosion of the reservoirs, some macropores were filled by small amounts of oil and gas during the later densification of reservoirs. This led to the survival of mold pores and has made the mold pores a symbol of the process in which the reservoirs were densified after hydrocarbon accumulation.