Abstract

The Penglai 19-3 (PL19-3) oil field, the largest offshore oil field in China, was found in shallow reservoirs (700–1700 m, 2297–5577 ft) within the most active fault zone in east China. The PL19-3 anticline was not finally formed until about 2.0 Ma and is cut by densely distributed faults. Source rock and crude oil samples from the PL19-3 field were analyzed to determine the origin and formation mechanisms of this large oil field. Three organic-rich, oil-prone source rock intervals exist in the Bozhong subbasin, each of which has a distinct biomarker assemblage. Oil samples from different wells have different biomarker associations, and three source-related oil classes were identified within the PL19-3 field based on biomarker compositions and multivariate analysis of the data. The PL19-3 field displays considerable compositional heterogeneity. The compositional heterogeneity within the field and comparison between oil samples from the PL19-3 field and those from nearby structures suggest three field-filling directions, which is consistent with the results of migration pathway modeling. The PL19-3 field was charged in the north by oil generated from Dongying Formation source rocks in the eastern Bozhong depression and Bodong depression, in the southeast by oil generated from Shahejie Formation source rocks in the Miaoxi depression, and in the northwest by oil generated from Shahejie Formation source rocks in the central Bozhong depression. Oil charge from multiple source rock intervals and multiple generative kitchens and focusing of oil originating from a large area of the Bozhong depression into the same trap resulted in rapid oil accumulation in the PL19-3 structure and the formation of this large oil field in a very young trap within an active fault zone.

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