Aurélien Virgone

Sedimentology and Sequence Stratigraphy of the Late Precambrian Carbonates of the Mbuji-Mayi Supergroup in the Sankuru-Mbuji-Mayi-Lomami-Lovoy Basin (Democratic Republic of the Congo)

The late Mesoproterozoic–middle Neoproterozoic carbonate succession (1155 Ma–800 Ma) of the Mbuji-Mayi Supergroup (Democratic Republic of Congo) represents a classic late Precambrian carbonate sequence whose architecture is poorly known. Here we present new data and synthesis of microfacies analysis, sequence stratigraphy, Fischer plots coupled with C and O isotopes, to evaluate the paleoecology and sea level variations of the carbonate series of the Mbuji-Mayi Supergroup, and to establish hierarchical approach stratigraphic framework from which to resolve the evolution of the Sankuru-Mbuji-Mayi-Lomami-Lovoy Basin. Our microfacies and sequence stratigraphy analyses show that the carbonate succession consists of strata accumulated on a ramp, during cyclic sedimentation across the inner ramp. Here plurimetric ‘thin’ peritidal cycles (±4 m-thick on average) record a relative maximum sea level of ca. 4 m, with fluctuations in the range around 1–4 m. This shallow-water depth and the abundance of cyanobacteria suggest that water column was oxygenated. By contrast the subtidal cyclic facies at the outer/middle ramp, preserve ‘thick’ subtidal sequences characterized by an average thickness of ±17 m. Accurate relative sea level fluctuations are difficult to assess in this ‘deeper’ environment since the facies could have been deposited in a wide range of shallow water that did not completely fill the accomodation space or available space. A probable magnitude for sea-level fluctuations here is around 10–20 m. These data are the first to place a quantitative constraint on the late Mesoproterozoic to middle Neoproterozoic carbonate deposits that have lively covered much of the Congo Shield at the end of the Precambrian, and is therefore an important type section for Central Africa.

Multiscale approach to (micro)porosity quantification in continental spring carbonate facies: Case study from the Cakmak quarry (Denizli, Turkey)

Carbonate spring deposits gained renewed interest as potential contributors to subsurface reservoirs and as continental archives of environmental changes. In contrast to their fabrics, petrophysical characteristics - and especially the importance of microporosity (< 1 mu m) - are less understood. This study presents the combination of advanced petrophysical and imaging techniques to investigate the pore network characteristics of three, common and widespread spring carbonate facies, as exposed in the Pleistocene Cakmak quarry (Denizli, Turkey): the extended Pond, the dipping crystalline Proximal Slope Facies and the draping Apron and Channel Facies deposits formed by encrustation of biological substrate. Integrating mercury injection capillary pressure, bulk and diffusion Nuclear Magnetic Resonance (NMR), NMR profiling and Brunauer-Emmett-Teller (BET) measurements with microscopy and micro-computer tomography (mu-CT), shows that NMR T-2 distributions systematically display a single group of micro-sized pore bodies, making up between 6 and 33% of the pore space (average NMR T-2 cut-off value: 62 ms). Micropore bodies are systematically located within cloudy crystal cores of granular and dendritic crystal textures in all facies. The investigated properties therefore do not reveal differences in micropore size or shape with respect to more or less biology-associated facies. The pore network of the travertine facies is distinctive in terms of (i) the percentage of microporosity, (ii) the connectivity of micropores with meso- to macropores, and (ii) the degree of heterogeneity at micro- and macroscale. Results show that an approach involving different NMR experiments provided the most complete view on the 3-D pore network especially when microporosity and connectivity are of interest.

Continental Carbonates Reservoirs: The Importance of Analogues to Understand Presalt Discoveries

Recent discoveries offshore Brazil have induced a renewal of interest in the study of recent and ancient continental carbonate systems which developed in a wide range of depositional settings, reflecting aerial to subaqueous environments. Recent and ancient continental carbonate analogs provide some keys to depict the sedimentologic/sequential pattern observed at the core scale and help in the understanding of the impact of climate change, fluid flow and water chemistry on the carbonate factory. It is noteworthy that the widespread microbial development in continental carbonate systems occurs in stratigraphic intervals typified by specific climatic and geodynamic conditions, and sometimes coincides with similar development in the marine realm. Stromatolites are more developped in high water level condition. But comparative studies between intracratonic (Recent Great Salt Lake; Eocene Green River lacustrine systems) and rift lacustrine systems demonstrates that they are more extensive on a flat substrate. The control exerted by the topography may increase during abrupt alternations of arid and humid periods, influencing the water chemistry and, accordingly, leading to the development of anoxic and/or evaporitic conditions. The key issue is therefore to understand the development of carbonate in lacustrine condition, how the sedimentary bodies and features can be preserved, and how their good reservoir properties can be maintained. High subsidence rate will influence the preservation potential of the relevant carbonate bodies, while the geothermal gradient, water chemistry or volcanic activity will impact the reservoir properties. In addition, meteoric or thermogenic travertine deposits, are an additional carbonate product that must be considered in the evaluation of continental carbonate reservoir systems.

The Khuff Formation in the Middle East: New Insight into Regional Stratigraphy and Palaeoenvironmental Reconstruction using Bio-assemblages and Facies Analysis

A multidisciplinary synthesis of outcrop and subsurface data of the carbonates and evaporites of the Late Permian Khuff Formation was carried out in order to constrain the spatial and stratigraphic distribution of the depositional facies.