A method to differentiate hydrocarbon source (oil vs methane) in authigenic carbonate rock from seeps
- 1South China Sea Institute of Oceanology, Guanghzou, China (ydsun@scsio.ac.cn)
- 2Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266061, China
- 3Shanghai Engineering Research Center of Hadal Science and Technology, College of Marine Sciences, Shanghai Ocean University, Shanghai 201306, China
- 4Institute for Geology, Center for Earth System Science and Sustainability, Universität Hamburg, 20146 Hamburg, Germany
- 5Coastal Studies Institute, College of the Coastal and Environment, Louisiana State University, Baton Rouge, Louisiana 70803, USA
- 6Innovation Academy of South China Sea Ecology and Environmental Engineering, Chinese Academy of Sciences, Guangzhou 510301, China
- 7University of Chinese Academy of Sciences, Beijing 100049, China
Abstract
Numerous marine hydrocarbon seeps have been discovered in the past three decades, the majority of which are dominated by methane-rich fluids. However, an increasing number of modern oil seeps and a few ancient oil-seep deposits have been recognized in recent years. Oil seepage exerts significant control on the composition of the seep-dwelling fauna and may have impacted the marine carbon cycle through geological time to a greater extent than previously recognized. Yet, distinguishing oil-seep from methane-seep deposits is difficult in cases where δ13Ccarb values are higher than approximately -30‰ due to mixing of different carbon sources. Here, we present a comparative study of authigenic carbonates from oil-dominated (site GC232) and methane-dominated (site GC852) seep environments of the northern Gulf of Mexico, aiming to determine the geochemical characteristics of the two types of seep carbonates. We analyzed (1) Major and trace element compositions of carbonates, (2) total organic carbon (TOC), total nitrogen (TN) and carbon isotope (δ13CTOC) of residue after decalcification, (3) sulfur isotope signatures of chromium reducible sulfur (CRS, δ34SCRS) and residue after CRS extraction (δ34STOS ), as well as (4) sulfur contents (TOS) of residue after CRS extraction. Carbonates from the studied oil seep are dominated by aragonite and exhibit lower δ34SCRS values, suggesting carbonate precipitation close to the sediment surface. In addition, oil-seep carbonates are characterized by higher TOC and TOS contents and higher TOC/TN ratios, as well as less negative δ13CTOC values compared to methane-seep carbonates, probably reflecting a contribution of residual crude oil enclosed in oil-seep carbonates. Very low δ13CTOC values (as low as −68.7‰, VPDB) and low TOC/TN ratios of methane-seep carbonates indicate that the enclosed organic matter is derived mainly from the biomass of methanotrophic biota. This study presents new geochemical data that will allow the discrimination of oil-seep from methane-seep deposits. Although some of the geochemical patterns are likely to be affected by late diagenesis, if applied with caution, such patterns can be used to discern the two end-member types of seepage – oil seeps and methane seeps – in the geological record.
How to cite: Sun, Y., Gong, S., Li, N., Peckmann, J., Jin, M., H. Roberts, H., Chen, D., and Feng, D.: A method to differentiate hydrocarbon source (oil vs methane) in authigenic carbonate rock from seeps, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1038, https://doi.org/10.5194/egusphere-egu2020-1038, 2019
