Microscopic pore structure of Ahe tight sand gas reservoirs of the Low Jurassic in Kuqa Depression and its controls on tight gas enrichment

Kuqa Depression has been an important region for exploring deep tight sand gas reservoirs in the Tarim Basin during the past few years. The study aims to discuss the micro-structure features of the Ahe tight sandstone reservoirs in the Dibei tight sand gas pool in the Kuqa Depression via reservoir property and inclusion tests, casting thin section observation, MIP(mercury intrusion porosimetry), as well as x-ray micro-CT scanning. What' s more, combined with logging interpretation and fluid inclusion data, the controlling effect of microscopic pore structure on tight sand gas enrichment is revealed. The results show that the tight sand reservoirs in the Lower Jurassic Ahe Formation is characterized by a majority of dissolved pores(including intragranular dissolved pores in feldspar, lithic debris, and dissolved pores in cements)and micro-fractures, together with limited residual intergranular pores. The micro-structure of the Ahe sand reservoir pores can be classified into three types. Specifically, type one mainly developed in coarse-grained sandstones is characterized by capillary bundle-shaped pore system, with poor pore throat sorting, large pore throat radius but low pore-throat ratio, and relatively high permeability; type two mainly occurring in the coarse-to-fine grained sandstones is characterized ink-bottle-shaped pore system with relatively poor pore throat sorting, pore throat radius smaller than that of type one mentioned above but large pore-throat ratio; type three mainly appearing in fine-to-silty sandstone also has ink-bottle-shaped pore throat system with relatively good pore throat sorting and pore throat radius smaller than that of type two and low permeability. Type one pore structure can serve as the pathway for tight gas migration, type two is favorable for tight sand gas enrichment, while type three reservoir has no gas charging. © 2020, OIL & GAS GEOLOGY Editorial Board. All right reserved.