Caracterização multi-escala da Camada Hard cap, Do Membro Mupe, Grupo Purbeck, Bacia De Wessex, Dorset, Reino Unido: origem e paleocontroles da porosidade

Abstract

Non-marine carbonates are not fully understood from various perspectives, including facies and petrophysics (porosity and permeability), as well as their processes and controls. Their components are highly diverse, comprising various types of micrites as well as different types of cement precipitated at different diagenetic stages. These components exhibit distinct geochemical and petrophysical signatures. Multiscale characterization of samples from the Hard Cap layer obtained from a quarry in Portland provided insights into depositional environments and diagenetic history. Petrographic analysis, X-ray diffraction, cathodoluminescence, scanning electron microscopy-energy dispersive spectroscopy, electron microprobe analysis, stable carbon-oxygen isotopes, X-ray microtomography, and petrophysical analyses were performed with the aim of better understanding these challenging reservoirs. The porosity of the Hard Cap layer is predominantly the result of bioturbation and dissolution. Organisms such as ostracods and gastropods are believed to have played a key role in the development of porosity as their activity resulted in sediment mixing and the creation of porous spaces. Mineral dissolution contributed to porosity development, with the most significant dissolution occurring during periods of increased groundwater flow. Cementation was the main cause of low porosity and permeability. The analysis revealed that the Hard Cap was deposited in a shallow environment with alternating subaerial exposure and flooding periods. Paleo-controls related to the development of Hard Cap porosity are closely associated with lake level and environmental changes. During high-level periods, the Hard Cap was submerged and subject to increased sedimentation, reducing porosity development. On the other hand, during low-level periods, the Hard Cap was exposed to increased groundwater flow, promoting dissolution and porosity development. The multiscale characterization of the Hard Cap layer allowed inferences about the depositional conditions that affected variations in carbonate facies and depositional cycles. From these findings, the hypothesis emerges that the deposition of Hard Cap layers may have been influenced by a marginal marine environment subject to significant geochemical control.