Initial porosity reduction phase of ultra-deep tight sandstone reservoirs:A case study of the Rub' Al Khali Basin in the Middle East

The Ordovician S Formation in the Rub' Al Khali Basin in the Middle East is a typical ultra-deep tight sandstone reservoir. Siliceous cementation dominated by the secondary overgrowth of quartz is the main cause for reservoir porosity reduction in the study area. The period of the pyrobitumen charge is taken as the beginning of reservoir tightening, while the secondary overgrowth of quartz, which sharply reduced porosity, is taken as the end of reservoir tightening. We combined mineral cathodoluminescence, fluid inclusion temperature, authigenic illite isotopic dating with tectonic subsidence, burial and thermal evolution history analyses, and concluded that the Ordovician reservoirs were cemented from the Late Triassic to Early Jurassic with a tightening rate of 30.6 m/Ma. The reservoir continued thermal evolution after tightening. Oil partially cracked to gas, some of which condensed in traps, and some of which escaped upwards or migrated through micro-pores to become diffused mixtures of oil and gas.