Understanding the hydrocarbon accumulation process in deep-ultradeep oil and gas reservoirs in complex structural areas — The Qixia Formation in the Shuangyushi structure in the Northwestern Sichuan Basin, China

Abstract Deep-ultradeep oil and gas reservoirs in complex structural areas have typically undergone multistage tectonic processes that have a considerable influence on the hydrocarbon composition, isotopic composition, and other geochemical characteristics of petroleum. By assessing the charging period of hydrocarbon accumulation during a particular age, we can determine these changes. We study the characteristics of fluid inclusions developed in the dolomite reservoir in the Middle Permian Qixia Formation in the Shuangyushi structure of the northwestern Sichuan Basin using methods such as inclusion petrography, microbeam fluorescence spectroscopy, and inclusion homogenization temperature. Furthermore, the main peak wavelength of the fluorescence spectrum, the red-green ratio/chromatic parameter, Q650/500, as well as the chromatic parameter corresponding to the emission flux at 535 nm (QF535), and their correlations with the homogenization temperature distribution are used to identify the charging period of hydrocarbons in the Qixia Formation. The results indicate that different types of fluid inclusions are developed in the Qixia Formation. The fluorescence of hydrocarbon inclusions is mainly blue, with a small amount of yellow-green and orange-yellow. The λmax of orange-yellow, yellow-green, and blue fluorescence of the hydrocarbon inclusions ranges between 562 and 579 nm, 497 and 548 nm, and 447 and 497 nm, respectively. The distribution of the homogenization temperatures during the same period ranges from 100°C to 110°C, 120°C to 140°C, and 160°C to 170°C, respectively. By combining the burial and thermal evolution histories of the Qixia Formation, we conclude that the Qixia Formation has experienced three stages of oil and gas charging, with the time of 204, 185, and 158 Ma. This method is of great significance in determining the date and duration of oil and gas accumulation and provides technical support for the recovery of ultra-deep hydrocarbon accumulations in complex structural areas.