Mixed carbonate-siliciclastic reservoir characterization and hydrocarbon accumulation process of the Ganchaigou area in the western Qaidam Basin, Tibet Plateau

The reservoir characteristics and palaeo-fluid evolution process of Ganchaigou area in west Qaidam basin was analyzed through microscopic observation, fluid inclusions, quantitative grain fluorescence and tectonic evolution history. The reservoir rock in the Ganchaigou is dominated by carbonate rock and the pore space. The Ganchaigou area has experienced two stages of hydrocarbon charge and the destruction of late shallow reservoirs: the first stage was the deposition period of the Shangganchaigou Formation (approximately 27 Ma), which experienced low-mature and immature crude oil charge, with a Ro range from 0.35 to 0.72%. It has the characteristics of continuous accumulation of shale oil under self-sealing and self-storage conditions. The corresponding fluid inclusions are mainly yellow–brown and yellow in fluorescence color. The development of saline lacustrine source rocks, the early rapid burial of source rocks and tight carbonate-siliciclastic reservoir made the accumulation of early crude oil possible. The second stage is the late deposition period of the Xiayoushashan Formation (approximately 15 Ma ago), which experienced mature crude oil charge, with a Ro range from 0.51 to1.11%. Affected by the Himalayan orogenic movement, the shallow and deep traps formed a vertical stacking relationship, leading to a deep and shallow multilayered hydrocarbon accumulation model. After the deposition of the Shangyoushashan Formation, orogenic activities in the Eastern Kunlun Range and Altyn Tage Range intensified, a large amount of denudation in the shallow strata occurred, and the integrity of shallow local traps was destroyed, resulting in the loss of crude oil from the shallow traps and a large amount of oil sands forming in outcrop. The traps developed in deep strata still have great exploration potential in this area.