Origin and accumulation of Paleogene oil in Qingdong sag, Jiyang depression, eastern China

This study reports on the geochemical examination of 18 core samples and 22 crude oil samples from the Qingdong and Dongying sags. Both conventional and heavy oils coexist in the Qingdong Sag, with the heavy oil resulting from biodegradation. Based on the oil-source rock correlation, the conventional oil mainly originates from the Es-3(1) source rock in the central and northern sub-sags, with the biodegraded oil in the western slope having been derived from the Es-4(u) source rock in the central and northern sub-sags. The biodegraded oil in the transition belt is mostly derived from the Dongying Sag (Es-4(u)). The biodegraded oil in the Qingdong-5 area originated from the Es-4(u) member near the Qingdong-5 area in the southern sub-sags. Two stages of oil charging were identified in the Qingdong Sag based on the basin modelling. The first stage occurred during the deposition of the Dongying Formation (Ed) in the Paleogene, and the second stage occurred during the deposition of the Guantao Formation from the Neogene to Quaternary (Ng-Q). The conventional oil in the central and northern parts of the study area mainly accumulated during the second stage and migrated both laterally through the sand-bodies and vertically along the steep faults. The heavy oil in the transition belt mostly accumulated during the second stage, migrating a long distance along the en-echelon step-fault zone, and suffered severe biodegradation. Both stages of oil charging were responsible for the heavy oil in the Qingdong-5 area and the western slope. The former migrated a short distance before being trapped, while the latter moved long distance along the fault strike. The oil charged during the first stage was subject to alteration, eventually being mixed with oil from the second charging event.