Mechanisms of Petroleum Accumulation in the Liaodong Bay, Bohai Bay Basin, China: Origin and Occurrence of Crude Oils Presented by Rucai Zhang

Abstract

Liaodong Bay is located in the northeast of Bohai Bay Basin of China and covers an area of 12500 km2. The origin of the eighteen major oil fields in the Liaodong Bay, Bohai Bay basin was studied based on the results of Rock-Eval pyrolysis on more than 1000 samples and biomarker analysis on 139 source rock samples and 157 oil samples. There are three organic-rich source rock intervals in the Liaodong Bay: the third and the first member of the Eocene Shahejie Formation (Es3, Es1) and the third member of the Oligocene Dongying Formation (Ed3). Es3 and Es1 source rocks are both characterized by low C19/C23 tricyclic terpane (C19/C23 TT) and low C24 tetracyclic terpane/C26 tricyclic terpane (C24 TeT/C26 TT). However, Es3 source rock displays high 4-methyl steranes index(4-MSI) but low gammacerane. Es1 source rock displays relatively low 4-MSI but high gammacerane. These indicate they both deposited in anoxic environment with minor terrigenous organic matter input. Ed3 source rock is characterized mainly by high C19/C23 TT, high C24 TeT/C26 TT, but low gammacerane and low 4-MSI, which indicates it deposited in freshwater, suboxic environment with substantial terrigenous organic matter input. The three source rock intervals have different biomarker assemblages due to environmental and ecological changes resulting from shifts in tectonic subsidence and climate. Three oil families derived from Es3, Es1 and Ed3, respectively, and three types of mixed oils have been identified. Almost 70% of large oil fields in the Liaodong Bay display considerable heterogeneities in biomarker compositions and originated from more than one source rock interval, which suggests that mixing of oils derived from multiple source rock intervals or multiple generative kitchens. Es3- and Es1-derived oils experienced relatively long-distance lateral migration and accumulated in traps in the Liaoxi sag or on the Liaoxi uplift. Es3-derived oils had migrated short distances and accumulated in the Liaozhong sag. Ed3-derived oils had migrated short distances and accumulated in the Liaozhong sag. Such a petroleum distribution pattern has important implications for future exploration. There is considerable exploration potential for Es3- and Es1-derived oils on the basin slopes, and traps close to or within the generative kitchens have better chance to contain oils generated from the Es3 or Ed3 Formations.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017