Abstract
Non-marine carbonates are not fully understood from several points of view, including facies and petrophysics (porosity and permeability), as well as their controlling processes The Hard Cap of the Mupe Member in the Purbeck Limestone Group of the Wessex Basin contains thrombolites presenting varying porosity degrees. This paper aims to carry out a multi-scale characterization, focusing on Mupe Member petrophysical properties and providing insights into its depositional environments and diagenetic history. Non-marine carbonates are characteristically heterogeneous, and their components are highly diverse, comprising various types of primary micrites, bioclasts and other particles, as well as different types of cement precipitated at different diagenetic stages. These components display distinct geochemistry and petrophysics signatures. This study integrates microtomography, petrographic, geochemistry, and petrophysics data from Hard Cap Purbeck samples obtained from a block store at the Bowers Quarry in Portland (50°32′49.4ʺ N, 2°26′53.1ʺ W), with the aim of better understanding these challenging reservoirs. Hard Cap porosity is predominantly the result of bioturbation and dissolution. Organisms such as ostracods and gastropods are believed to have played a key role in porosity development, as their activity resulted in sediment mixing and the development of pore spaces. Carbonate mineral dissolution also contributed to porosity development, with the most significant dissolution occurring during periods of increased groundwater flow. A stable isotope analysis indicated that dissolution was likely the result of acidic groundwater, which can dissolve carbonate minerals present in the limestone. The Hard Cap cementation process is the primary cause of its low porosity and permeability. The analysis also revealed that the Hard Cap was deposited in a shallow environment with alternating subaerial exposure and inundation periods. The paleo controls concerning Hard Cap porosity development were found to be largely associated to lake level and climate changes. During high lake level periods, the Hard Cap was submerged and subject to increased sedimentation, reducing porosity development. Conversely, during low lake level periods, the Hard Cap was exposed to increased groundwater flow, promoting dissolution and porosity development. Overall, this study provides important insights into the porosity origin of the Mupe Member Hard Cap and the paleo controls that influenced its development. These findings may be useful in the exploration and development of hydrocarbon reservoirs located in similar geological settings.
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Acknowledgements
Special thanks are due to Drs. Pablo Suarez-Gonzalez and M. Isabel Benito from the Complutense University of Madrid (Spain), for their help in the description and interpretation of petrographic and geochemical data. We also would like to thank Dr. Arnaud Gallois (Royal Holloway University of London) and Dr. Peter Burgess (University of Liverpool, UK) for their guidance and discussions during the field trip in south Dorset. We would also like to thank Bernard Vining and Professor Dan Bosence (Royal Holloway University of London) for all the guidance and support. The authors also thank Baker Hughes of Brazil for donating the blocks used in this research to the State University of Rio de Janeiro (UERJ). We would like to thank the National Observatory Petrophysics Laboratory (LabPetrON) team for performing the routine petrophysics analyses, the UERJ Chemostratigraphy and Organic Geochemistry Laboratory (LGQM-UERJ) team for conducting the geochemistry isotopic analyses, and the Federal University of Rio de Janeiro Nuclear Instrumentation laboratory (UFRJ/ COPPE/ Nuclear Engineering Program) team for carrying out the microCT imaging.
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Baptista, R.C., Bergamaschi, S., Ade, M.V.B. et al. Multi-scale characterization from the Hard Cap, Mupe Member, Lower Purbeck Limestone Group, Wessex Basin, Dorset, UK. J. Sediment. Environ. 8, 363–387 (2023). https://doi.org/10.1007/s43217-023-00138-6
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DOI: https://doi.org/10.1007/s43217-023-00138-6