Abstract
The Namibe Basin developed in the Early Cretaceous as part of the South Atlantic rift system between Africa and South America. Magmatic activity occurred during the syn-rift (Valanginian-Hauterivian) and post-rift (Coniacian-Early Campanian) phases, with the latter triggering fluid circulation events that had significant impacts on reservoir quality and hydrocarbon prospectivity. The continental carbonates of the Mariquita Member (Mb.) developed during the Late Cretaceous magmatic pulse and record the associated fluid circulation. Thus, study of these carbonates provides insights into paleo-fluid sources and circulation pathways, which can ultimately help to constrain diagenetic phases, their effects on reservoir quality, and hydrocarbon re-mobilisation.
This study analyses the tectonostratigraphic setting, facies and diagenesis of the Mariquita Mb. carbonates by integrating field, petrographic and geochemical data. The carbonates are stratigraphically bound by Upper Cretaceous magmatic rocks, and constitute a series of fault-associated spring mound systems that pass laterally into lacustrine sediments. The main diagenetic events include pervasive dolomitisation, silicification and multi-stage fracturing. Fluid temperatures during dolomitisation were between ∼32 and 56 °C, while silicification occurred between ∼70 and > 260 °C. Fluids likely followed fault-controlled convective circulation pathways. The fluid compositions comprised infiltrated/modified meteoric and marine waters, as suggested by negative carbon isotopes, enriched oxygen and strontium isotopes, and REE data. Fluid modification occurred via interaction with subsurface rock units, such as basement, volcanic/siliciclastic aquifers and evaporites. However, higher temperatures during silicification could suggest late contributions from magmatic fluids too. Bitumen within porosity possibly reflects regional hydrocarbon mobilisation processes driven by magmatic fluids.
| Original language | English |
|---|---|
| Article number | 105351 |
| Journal | Marine and Petroleum Geology |
| Volume | 134 |
| DOIs | |
| Publication status | Published - Dec 2021 |
Bibliographical note
Funding Information:This publication contains work conducted during a PhD study undertaken as part of the Natural Environment Research Council (NERC) Centre for Doctoral Training (CDT) in Oil & Gas, funded by NERC (grant number NE/M00578X/1 ) and the University of Manchester . BP and Equinor provided additional funding, samples and logistical support, and are further thanked for permission to publish. Jon Fellowes, John Waters, Heath Bagshaw, Paul Lythgoe and Abby Ragazzon-Smith provided technical support for the analyses carried out within the Williamson Research Centre at the University of Manchester, while Steve Crowley carried out stable isotopes analyses at the University of Liverpool. The authors would also like to acknowledge the technical and human support provided by SG Iker of UPV/EHU and European funding (ERDF and ESF) for strontium isotope analyses. Luz Gomis-Cartesio (Equinor) provided assistance during fieldwork, while Cathy Hollis (University of Manchester), Luc Bulot (Aix-Marseille University/University of Manchester) and Enrico Capezzuoli (University of Florence) provided useful comments during the manuscript preparation. Finally, the authors would like to thank the Associate Editor Marco Brandano (Sapienza University of Rome), Alex Bump (The University of Texas at Austin) and an anonymous reviewer for providing useful reviews that greatly improved the manuscript.
Publisher Copyright:
© 2021