Neoproterozoic source rocks and analogue microbial reservoir characterisation: from outcrop to dynamic reservoir simulation

摘要

This research project presents an integrated study of selected aspects of Neoproterozoic carbonate sedimentology in Namibia from a petroleum point of view. In recent years petroleum companies started to explore Neoproterozoic frontier petroleum plays due to the global steady increase for the demand of oil and gas. However, despite an enormous research effort over the last two decades, little is yet known about Neoproterozoic geology and in particular about Precambrian petroleum geology. From a petroleum exploration point of view, the search for Neoproterozoic hydrocarbon systems is a new frontier in petroleum geology. The focus of the first part of this research project has been on the evaluation of potential source rocks along strike of the southern edge of the Owambo Basin in North Namibia. This includes detailed sedimentological characterisation of four transects of the more than 4 km-thick Otavi carbonate platform. 40 samples were collected in the field and subsequently analysed for total organic carbon (TOC) content. None of these samples could be classified as potential source rock due to the low total organic carbon content ( 0.5% TOC). However, 2 samples from the Halali Waterhole showed elevated TOC values and might have acted as source rocks during the Neoproterozoic. Further investigations of these sequences were, however, hampered due to the unclear stratigraphic position in the succession. A key observation is the absence of organic-enriched sediments in the Keilberg Member, north of Tsumeb, which is regarded as potential source rock interval for the Owambo Basin. This shows that the Keilberg Member is not a basin-wide source rock horizon. These results suggest that organic-enriched sediments were deposited during postglacial flooding in tectonically controlled anoxic sub-basins with restricted water circulation. A potential analogue system for the Otavi carbonate platform is the early Silurian-Ordovician post-glacial source rock system of North Africa. The second part of the thesis presents a multidisciplinary, outcrop-based analogue hydrocarbon reservoir characterisation of a microbial carbonate system in the Nama interpretations. The microbial bioherm evolution has been analysed in detail and a generic model describing bioherm evolution has been proposed. Consequently, a new bioherm nomenclature to describe bioherm geometries has been introduced, including merged, composite and stacked bioherm complexes. Structural characterisation of the microbial carbonate system revealed an important phase of fracture development. Syn-depositional fractures are intrinsic to the microbial carbonates due to body forces of an early lithified, fragile microbial framework. This has an important implication for microbial reservoir characterisation because fractures have to be considered in microbial reservoirs independent form the regional setting. Thin-section analyses were used to determine petrophysical properties of the reservoir and non-reservoir facies. Subsequently, terrestrial laser scanning (lidar) was used to acquire high-resolution digital outcrop data of the canyon system in order to construct a static geocellular analogue reservoir model. Object-based and multiple-point statistics facies modelling were chosen to develop a detailed facies model of the microbial carbonates. These facies models served subsequently as basis for dynamic performance studies. The final step of the reservoir characterisation constitutes a streamline fluid flow forecasting analysis, following a top-down modelling philosophy. Based on the integrated reservoir characterisation the analogue microbial carbonate system of the Omkyk Sequence 2 in the Nama Basin is classified as hybrid 3 type reservoir after the carbonate reservoir classification of Ahr (2008). Reservoir heterogeneity and geometry is constrained by depositional processes in combination with syn-depositional fractures. However, the diagenetic history of the Omkyk Sequence 2 has not been studied. Selective dolomitisation of microbialites indicate that diagenetic alterations have affected the carbonate system. Therefore, future research might refine or overturn this classification. Basin of South Namibia. The incentive for this investigation was to better understand potential microbial carbonate reservoirs in the subsurface of the Owambo Basin and elsewhere. The terminal Neoproterozoic Kuibis ramp carbonates are superbly exposed in the Zebra River canyon system, providing the opportunity to study this system in three dimensions from micro- to macro-scale. Reservoir characterisation includes detailed sedimentological and structural field observations and interpretations. The microbial bioherm evolution has been analysed in detail and a generic model describing bioherm evolution has been proposed. Consequently, a new bioherm nomenclature to describe bioherm geometries has been introduced, including merged, composite and stacked bioherm complexes. Structural characterisation of the microbial carbonate system revealed an important phase of fracture development. Syn-depositional fractures are intrinsic to the microbial carbonates due to body forces of an early lithified, fragile microbial framework. This has an important implication for microbial reservoir characterisation because fractures have to be considered in microbial reservoirs independent form the regional setting. Thin-section analyses were used to determine petrophysical properties of the reservoir and non-reservoir facies. Subsequently, terrestrial laser scanning (lidar) was used to acquire high-resolution digital outcrop data of the canyon system in order to construct a static geocellular analogue reservoir model. Object-based and multiple-point statistics facies modelling were chosen to develop a detailed facies model of the microbial carbonates. These facies models served subsequently as basis for dynamic performance studies. The final step of the reservoir characterisation constitutes a streamline fluid flow forecasting analysis, following a top-down modelling philosophy. Based on the integrated reservoir characterisation the analogue microbial carbonate system of the Omkyk Sequence 2 in the Nama Basin is classified as hybrid 3 type reservoir after the carbonate reservoir classification of Ahr (2008). Reservoir heterogeneity and geometry is constrained by depositional processes in combination with syn-depositional fractures. However, the diagenetic history of the Omkyk Sequence 2 has not been studied. Selective dolomitisation of microbialites indicate that diagenetic alterations have affected the carbonate system. Therefore, future research might refine or overturn this classification.