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2018 AAPG International Conference and Exhibition

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Characteristics of High-Quality Source Rock in Brink Sags, the Southern Bohai Sea

Abstract

Brink sags are distributed in the margins of sedimentary basins with high denudation degree. The study area is located in the Southern Bohai Sea and contains four typical brink sags which are Laizhouwan, Qingdong, Huanghekou and Miaoxi sag. These sags are controlled by both extentional faults and strike-slip faults. Five sets of lacustrine source rocks are developed in study area, including E3d3, E3s1, E3s2, E2s3, E2s4. The quality of source rocks and petroleum exploration are thought to be bleak in brink sags. However, several large oil fields have been found in the Southern Bohai Sea in recent years. In this study, a series of experiments are performed to evaluate the source rocks in brink sags objectively. The results and conclusions are as follows: 1) E2s3M+L and E2s4U are the high-quality source rocks in study area. The two sets of source rocks are similar in thermal evolution degree (Low-middle mature stage) and abundance of organic matter (The average of TOC is about 2%; the average of S1+S2 is about 10mg/g). The types of kerogen are sapropelic-humic in E2s3M+L and sapropelic in E2s4U. 2) E2s3M+L and E2s4U are featured by inhomogeneous salinization. The carbonate content (13~28.5%) and GI (0.02-0.10) of E2s3M+L correspond to the low salinity. High GI (0.20~0.47) stand for the high salinity only occurs in KL16 structure. However, strong inhomogeneous salinization was found in E2s4U. The carbonate content (36.8~41.6%) and GI (0.06 to 0.46) show low, moderate and high salinity in different regions. 3) Low-middle values of Pr/Ph, 4-MSI and (nC21+nC22)/(nC28+nC29) in E2s3M+L show the weak reduction-oxidation environment with large terrestrial input. The E2s4U with higher value of the parameters above indicate the strong reduction-weak oxidation environment with large input of floating algae and benthic algae. 4) Based on the correlation of oil-source rock, it can be confirmed that E2s3M+L, E2s4U are the chief source rocks of the study area. Oil in this area were mainly E2s3M+L-derived. However, oil from the following regions, such as the east region in Huanghekou sag, the gentle slope belt in southern Laizhouwan sag and the middle region in Miaoxi sag, were mixed souces of E2s3M+L and E2s4U. According to the results, it is safe to say that high-quality source rocks are well developed in brink sags. The favorable condition of the source rock enable the brink sags to provide suffient hydrocarbon sources to form large hydrocarbon reserviors.