Remotely constraining the temporal evolution of offshore oil systems

Alexander J Corrick,David Selby,Andrew S Ross,Philip A Hall,Christine Trefry,David M Mckirdy,Se Gong

doi:10.1038/s41598-018-37884-x

Abstract

An understanding of the temporal evolution of a petroleum system is fundamental to interpreting where hydrocarbons may be trapped in the subsurface. However, traditional exploration methods provide few absolute constraints on the timing of petroleum generation. Here we show that 187Re/187Os geochronology may be applied to natural crude oil seepage to determine when petroleum generation occurred in offshore sedimentary basins. Using asphaltites collected from the South Australian coastline, our determined Re-Os age (68 ± 15 million years ago) is consistent with their derivation from a Late Cretaceous source rock in the nearby Bight Basin, an interpretation similarly favoured by source-specific biomarker constraints. Furthermore, the calculated initial 187Os/188Os composition of the asphaltites, a value inherited from the source rock at the time of oil generation, suggests that the source rock represents the later stage of Oceanic Anoxic Event 2. Our results demonstrate a new approach to identifying the origin of crude oils encountered in coastal environments by providing direct constraints on the timing of petroleum generation and potential source rock intervals in poorly characterised offshore sedimentary basins prior to exploratory drilling.

Highlights

Natural seepage is a major contributor of oil to the marine environment, estimated at ~600,000 tonnes per year[1]
Previous applications of Re-Os dating using oils and bitumens have been restricted to samples collected in-situ, rather than examples released from the subsurface into the marine environment through natural seepage
Through the application of Re-Os radiometric dating to heavy asphaltic crude oil released from natural seafloor seepage we have demonstrated that the technique may be applied to constrain the timing of hydrocarbon generation and potential source intervals within offshore sedimentary basins

Summary

Introduction

Natural seepage is a major contributor of oil to the marine environment, estimated at ~600,000 tonnes per year[1]. A combination of highly reproducible source-specific biomarkers and other hydrocarbons constrains the origin of these asphaltites to a single oil family expelled early in the oil generation window from a Mesozoic marine shale deposited under anoxic/sulfidic conditions[2,17]. Their biomarker composition is unique, and yet to be linked to any produced oil worldwide[19]. Our findings demonstrate that the Re-Os isotope systematics of hydrocarbon seepage can provide important information constraining the evolution of offshore oil systems prior to costly exploratory drilling

Methods

Results

Discussion

Conclusion