Paleoenvironmental characteristics and evidence of subsag migration within the Laizhouwan Sag in the Bohai Sea

• Published in: Journal of petroleum science & engineering 2022-03, Vol.210, p.109961, Article 109961
• Main Authors: Wang, Feilong, Tang, Guomin, Wang, Deying, Pan, Wenjing, Yang, Hong, Awan, Rizwan Sarwar
• Format: Article
• Language: English
• Subjects: Bohai sea; Laizhouwan sag; Paleoenvironment; Paleoproductivity; Palynofacies; Sag-migration
• ISSN: 0920-4105; 1873-4715
• DOI: 10.1016/j.petrol.2021.109961

The Laizhouwan Sag comprises many sets of source rocks, and their crude oil properties are complex and changeable. This research evaluates the quality of source rocks using mudstone samples of the Shahejie Formation from typical wells from northern and southern subsags by incorporating a detailed organic-inorganic geochemical, palynology and petrological analysis. Moreover, it analyzes the difference between the paleoenvironment and paleoproductivity. It also discusses the migration characteristics of the sag in combination with seismic data to reveal the fundamental reason for the complex oil source characteristics of the Laizhouwan sag. The results show there are two sets of effective source rocks in the northern subsag of the Laizhouwan Sag, (i) the third member of the Shahejie Formation(E2s3) (ii) the fourth member of the Shahejie Formation (E2s4). Additionally, the E2s4 is a high-quality, effective source rock in the southern subsag of Laizhouwan Sag. (2) During the sedimentary period of the E2s4 in Laizhouwan Sag, the water bodies of the southern and northern subsags were separated. The organic matter (OM) in the E2s4 of northern subsag mainly comes from lower algae, mainly amorphous, and deposited in a weak reducing environment of brackish water. The OM in the E2s4 of southern subsags mainly comes from terrigenous higher plants, mainly composed of terrigenous amorphous assemblages, and it was deposited in a strongly reducing environment of brackish water to saline water. (3) Moreover, the water bodies of the southern and northern subsags gradually changed to unified water bodies during the sedimentary period of the E2s3. During this period, OM comes from lower algae, mainly from amorphous material, and the water bodies were transformed to a weak oxidation-reduction environment from freshwater-brackish water. The subsidence center of the Laizhouwan sag continued to move southward during the sedimentary period of E2s4-E2s3. Gradually the southern subsag became a sedimentary center containing higher paleoproductivity and eutrophic lake settings. The differences in organic source, sedimentary environment and paleoproductivity of source rocks in different subsags lead to the complexity of crude oil generation features. •Different subsags have different paleo-environment, revealing the reason of complex oil source characteristics.•From E2s4 to E2s3, the subsidence center continued to move south, the southern subsag became the center of sedimentation.•Ther