Carbonate sedimentation and diagenesis in rift settings

Eocene Thebes Formation, Hammam Fauran Fault Block, Western Sinai, Egypt

Rift basins remain key targets for exploration and production, especially with the development of plays in the South Atlantic. Nevertheless, there remains uncertainty as to the processes governing carbonate platform growth within this environment, particularly the transition from terrestrial to marine sedimentation under conditions of fluctuating clastic sediment supply. Furthermore, the interplay of fluctuations in relative sea level, tectonism and geothermal convection of fluids along faults has a critical control on diagenetic modification, and hence porosity distribution.

Project 1: Timing of diagenetic modification of pre-rift carbonate sediments within the Sinai rift, Gulf of Suez

This project constitutes the HFF-DOL consortium, which aims to determine the processes governing fault-fracture controlled dolomitisation on the Hammam Fauran Fault Block, Western Sinai. It is a multi-scale, multi-disciplinary study that integrates field-based sedimentology and structural geology with diagenetic and geochemical characterisation, petrophysical analysis and reservoir modelling. The results highlight the evolution of fluid flux during rift initiation and early syn-rift tectonism.

Palaeosol capping an upward-shallowing platform top succession, Great Orme Limestone, Lower Carboniferous, North Wales

 

Project 2: Characterisation of the Lower Carboniferous carbonate platforms of northern England.

A range of studies over the last five years have focused upon the relationship between platform-bounding basement faults, dolomitisation and mineralisation.  In these studies, detailed field data has permitted reconstruction of the history of diagenetic modification through combined sedimentological analysis, kinematic reconstruction, geochemical fingerprinting and fluid flow modelling (Hollis, 2011; Juerges, 2012; Frazer et al., 2013).  The results highlight the inter-relationship of structural deformation during post-rift thermal subsidence, expulsion of metal-enriched brines from juxtaposed hanging wall basins and platform-top diagenesis.  New projects are also focused on the inter-relationship of carbonate platform growth and clastic sediment supply

Project 3: Tectonostratigraphic evolution of Cretaceous syn-rift deposits in Angola

Lower Cretaceous rift graben are filled with pre-salt clastics and carbonates. The Aptian salt marks the end of rifting and is followed by post-rift sediments.

A third project started in autumn 2014 and extends this theme into the South Atlantic. Cretaceous rift deposits are exposed along coastal southern Angola (Benguela and Namibe basins), and form calibration points for current exploration activities offshore. The onshore geology is poorly understood, but initial studies indicate presence of carbonate buildups, and influence of fluid flow on carbonate precipitation and diagenesis. These outcrops provide further constraints on the regional tectonostratigraphic framework: ages and stratigraphic relationships of various rock units, depositional environments, basin configuration, paleogeography, and key controls on facies distribution. The project will tie together on- and offshore stratigraphy, establish improved tectonostratigraphic relations and determine the implications of pre-salt observations for regional rift evolution. It will then establish key controls on facies distribution, such as volcanics, rift structure, clastic input, paleogeography.

Phase 1 has mapped onshore geology using satellite imagery to start building a tectonostratigraphic framework.

Stromatolitic carbonates in the pre-salt of the Namibe Basin.
The post-rift Albian is dominated by clastics in the Namibe Basin, but tends to be carbonate-dominated further north.

Project 4: Controls on stratigraphic development and reservoir distribution of shelf margin carbonates: Jurassic Atlantic margin - western Morocco

Stratigraphic overview of the Middle-Upper Jurassic near Agadir, illustrating the excellent exposure conditions. The Jurassic consists of interbedded marine limestones and continental-marine siliciclastics arranged in several transgressive-regressive cycles.

 

Oxfordian reef exposed near Cap Rhir, north of Agadir. These dolomitized coral reefs form reservoirs in the subsurface at Cap Juby field.

This study has started in 2015 and examines the extensive Jurassic carbonates along the Atlantic margin of western Morocco (Agadir, Essaouira and Tarfaya basins). Although well exposed, the Jurassic has received little regional study, and there is no coherent stratigraphic scheme for the succession. The study therefore focusses on refining our understanding of the stratigraphic architecture, age relationships, paleogeography and facies trends in the basin. A large part of the work will be outcrop-based, logging extensive sections, and conducting a regional stratigraphic-sedimentological study. Results will be integrated with subsurface data, including seismic and well data. There is also scope for detailed examination of geobody architecture using digital mapping techniques such as differential global positioning system (DGPS), digital photogrammetry (using multiple digital images to reconstruct three dimensional information) and LIDAR. The dense dataset will allow the mapping of facies, geological object distribution and architecture.

Morocco, together with the conjugate margin in Nova Scotia, is an area of active oil and gas exploration, and the results from this study will have an important input to understanding the petroleum system and development of the passive margin.

▲ Up to the top