ABSTRACT

The log evaluation of an expelling source rock is notoriously challenging because the kerogen and the generated oil coexist in the source rock. The Athel silicilyte is a unique geological setting found in the South Oman Salt Basin where the issue of source rock evaluation is directly related to that of reservoir characterisation. With no secondary migration path available, this sealed intra-salt source rock matured into a large reservoir, with micro-crystalline porosities up to 30 pu and oil saturations in excess of 80 su. In the absence of known analogues, well Al Noor-2 was fully cored, and extensive core analyses provided the basis for reservoir description. A simple density-porosity model was calibrated against the core data. The foundation of this model was recently challenged, when the analysis of cores taken in shallower Athel prospects revealed that the organic matter comprised in the matrix is highly soluble in organic solvents. There was significant concern that the Al Noor-2 cores may have been affected in a similar manner, albeit to a lesser degree. Petrophysics and geochemistry teamed up to resolve the challenge. A detailed analytical programme confirmed the initial observation, and established that the shallower, less mature Athel prospects contain viscous bitumens rather than kerogen. The organic matter comprised in the deep Athel silicilyte is mature, and is Soluble in organic solvents: it is a kerogen. TOC (Total Organic Carbon) measurements on cuttings showed that the organic matter content of Athel silicilyte varies with depth in a given well. The resulting grain density changes should be accounted for in the interpretation of the density log. Traditional wireline-based TOC evaluation methods developed for the evaluation of source rocks do not readily apply, and have limited accuracy. Furthermore, knowledge of TOC alone is insufficient to determine the grain density reliably, due to the occurrence of additional minerals, including pyrite. A more general petrophysical model, honouring all available data simultaneously, is being calibrated to provide the detail required for reservoir characterisation. Measurement procedures, processing methods, and evaluation techniques have had to be revised. The integration of disciplines and the introduction of new measurements provided the key to bracket the organic matter fraction and to reduce evaluation uncertainties to an acceptable level. Preliminary results obtained from NMR (Nuclear Magnetic Resonance) in the laboratory and in the field are promising and indicate the potential of the technique to unravel the unusual porosity of the Athel silicilyte, with possible further application to the characterisation of organic-rich reservoirs and source-rocks.

This content is only available via PDF.
You can access this article if you purchase or spend a download.