Geochronology of volcanically associated hydrocarbon charge in the pre-salt carbonates of the Namibe Basin, Angola

N Rochelle-Bates,S Schröder,N.M.W Roberts,R Dixon,R Swart,K Verwer,E Fiordalisi,U Freitag,C.H Ferreira,B.E Van Dongen,A Halton,I Sharp

doi:10.1130/g48019.1

Abstract

Abstract In volcanic rifted margins, the timing of hydrocarbon charge is difficult to predict, but is important in understanding fluid genesis. We investigated whether igneous activity was linked to hydrocarbon charge in the prolific South Atlantic pre-salt petroleum system. To do this, we applied in situ carbonate U-Pb geochronology, a relatively novel tool for dating hydrocarbon migration, to bituminous veins in pre-salt travertines from the rifted onshore Namibe Basin (Angola). To test if fluid flow was synchronous with known volcanic pulses, we also obtained new 40Ar/39Ar geochronology from a nearby volcanic complex. Bitumen is associated with calcite in a first generation of veins and vugs, and with dolomite in younger veins. The dated calcite veins yielded a pooled U-Pb age of 86.2 ± 2.4 Ma, which overlaps the volcanism 40Ar/39Ar age of 89.9 ± 1.8 Ma. The overlapping dates and the localized bitumen occurrence around the dated volcanic center show a clear genetic relationship between Late Cretaceous igneous activity and hydrocarbon charge. The dolomite was dated at 56.8 ± 4.8 Ma, revealing a previously unknown Paleocene/Eocene fluid-flow phase in the basin.

Highlights

The nature and timing of fluid flow can be difficult to predict in ancient volcanic rifted margins due to their highly perturbed geothermal regimes (e.g., Morgan, 1982)
We investigated whether igneous activity was linked to hydrocarbon charge in the prolific South Atlantic pre-salt petroleum system
The data reveal a genetic relationship between hydrocarbon charge and Late Cretaceous igneous activity in the onshore Namibe Basin, where absolute age data were previously limited

Summary

Introduction

The nature and timing of fluid flow can be difficult to predict in ancient volcanic rifted margins due to their highly (albeit locally) perturbed geothermal regimes (e.g., Morgan, 1982). Despite the ubiquity of carbonate minerals in a wide range of hydrological and diagenetic settings, carbonate U-Pb geochronology has only recently been used to date fluid-flow events (see Roberts et al, 2020), and the technique has rarely been applied to bituminous carbonates. Traditional bulk dissolution methods are hindered by low spatial resolution and the potential presence of organic inclusions as sources of common Pb and open-system U (Parnell and Swainbank, 1990). We show that it greatly improves the utility of U-Pb geochronology in providing the timing of bitumen-related mineral authigenesis

Methods

Results

Discussion

Conclusion