Coupling relationship between natural gas charging and deep sandstone reservoir formation: A case from the Kuqa Depression, Tarim Basin

Abstract

The formation mechanism of deep effective reservoirs is elaborated based on the analysis of the conditions for forming natural gas accumulations in deep Kuqa Depression. Natural gas charging is closely related to the formation of effective sandstone reservoirs in the deep zone: reservoirs which capture oil and gas in the early period are of better quality during the long burial period; reservoirs which do not capture oil and gas in the early period is compact after deep burial because of compaction. Oil and gas charging before the reservoir is deeply buried significantly inhibits mechanical compaction and fracture closure as well as densification, so the effective pores and fractures will be preserved. The formation conditions of large gas fields in Kuqa Depression are as follows: sufficient coal gas source; great gas generation potential of high-evolution Jurassic source rocks; secondary pores and micro-fissures developed in deep sandstone reservoirs; wide distribution of gypsum-salt cap rock with large thickness; good association of source-reservoir-cap in space; well developed hydrocarbon migration pathways of dominantly faults; high expulsion efficiency; big migration and accumulation coefficient and deep or ultra-deep traps developed below salt structures. Key words: deep zone, low porosity and low permeability reservoir, coal-formed gas, fault transportation system, oil and gas charging, Kuqa Depression