TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe 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 both an overpressured light-oil reservoir and a world-class source rock. In this unique geological setting found in the South Oman Salt Basin, reservoir characterisation is intimately associated with source rock evaluation. In the absence of any direct geological analogue, the initial petrophysical model was calibrated against extensive core analyses. Porosity interpretation was then based exclusively on wireline bulk density measurements. The limitations of this approach only became apparent as attempts to reconcile data from various sources failed. While efforts aimed at obtaining representative core material and establishing reliable core analysis procedures, in-situ calibration of the petrophysical model was also actively pursued. NMR logging was primarily introduced to reduce uncertainties affecting density-derived porosities in the presence of variable, unknown organic matter content. The first NMR log run in a silicilyte well could not be fully reconciled with density data. Laboratory NMR confirmed the log measurements, but more questions were raised when significant NMR signal was recorded on dry, cleaned samples at the time when new field data could not be explained. Athel silicilyte is unique, and with no reference to confirm or to invalidate new data and interpretation, consistency and integration are key to improving our understanding of the rock. A thorough review of NMR acquisition procedures and processing algorithms was undertaken with logging contractors; job planning was improved, new logging procedures were implemented, tool malfunctions were identified and remedied, processing methods were upgraded. Laboratory procedures were revisited, and NMR core measurements were complemented with additional, independent core analyses.Whereas the initial emphasis was on "effective" porosity determination, NMR logs and core measurements provided new insight into the Athel silicilyte rock. This includes: (i) strong indications that the rock may be mixed-wet; (ii) encouraging results in the area of permeability estimation; and (iii) indeed higher confidence in the assessment of effective porosity. A TOC (Total Organic Carbon) log can also be derived, which could find further application in source rock evaluation and the characterisation of organic-rich reservoirs. Integration of all available log data using a statistical approach now provides a consistent reservoir description, with limited core calibration requirements. Having successfully integrated NMR data to unravel the organic content of the rock, the improved petrophysical model provides the basis for sound reservoir characterisation, the foundation for better exploration, appraisal and development decisions.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractCrucial issues in formation evaluation are the determination of porosity, permeability, hydrocarbon volumes and net-to-gross ratio. NMR logging provides measurements that are directly related to these parameters. The NMR response of fluids contained in pores is governed by their T 2 and T 1 -relaxation times, diffusion coefficient and whether or not they wet the rock. In the case where fluids possess a sufficiently large contrast in these properties and NMR data has been acquired with suitably chosen acquisition parameters (i.e. wait times and/or interecho times) a separation of water, oil and gas NMR responses can be made. From these separate NMR responses the hydrocarbon volumes, porosity and permeability estimates are subsequently calculated. Key in these applications is the ability to include all the acquired log NMR data into the processing towards the desired end result. Methods exists to derive hydrocarbon volumes from T 2distribution or from echo decay data. However, these are all methods in which the difference between just two acquisitions that only differ in either wait time or interecho time are considered. Over the past years we have developed, tested and employed an alternative processing technique named MacNMR (Multi-acquisition NMR). MacNMR takes any number of log acquisitions (wait time and/or interecho time variations) and simultaneously inverts them using an rigorous forward model to the desired water and hydrocarbon T 2 spectra. In this paper we discuss the concepts of MacNMR and demonstrate its versatility in NMR log processing. An example will illustrate its benefits. MacNMR is implemented across Shell operating units worldwide and has been in use for over two years now.
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