Seismic facies-based reservoir characterisation of a deepwater Miocene canyon complex, offshore Mauritania.
David Benjamin Spence
Honours Degree, 2005
University of Adelaide
Offshore Mauritania has been the target of deepwater hydrocarbon exploration since 1999. This region is dominated by deepwater canyon systems that still occur in the modern day. The bulk of the plays in the region are mid-slope canyon reservoirs with structural control from salt diapirism in the salt sub-basin, offshore from west of the capital, Nouakchott.
The region of interest is located within canyon fill up-dip from the Tiof Oil and Gas Field some 100km west of Nouakchott. The Tiof Field is hosted by a Miocene mid-slope canyon complex adjacent a large salt diapir. The study location is up-dip from the salt diapir, which separates it from the Tiof Field.
The aim of this project was to establish from seismic data the likelihood of reservoir sands occurring in a region considered to have limited hydrocarbon potential, for the purpose of gas disposal. Hydrocarbon accumulations in the region are well supported by various seismic direct hydrocarbon indicators (DHIs), reducing the risk of exploration. However well control has shown that there is often no acoustic impedance contrast between shales and brine sands. This means seismic amplitude-based studies are unreliable when the target reservoirs are water saturated.
Data available for the study included a composite 3D seismic survey and data from the Tiof wells.
One approach taken was to determine depositional environment from reflection configuration rather than using amplitude analysis. Complimenting this approach was the extraction of various seismic attributes and the use of facies analysis.
The seismic interpretation of the deepwater canyon found that it had a branching morphology up-dip consisting of a significantly larger branch and a smaller branch. The development of the diapir as a syn-depositional event has had an influence on the depositional processes occurring in the canyon.
Facies analysis determined that turbidite sands were more likely to have been deposited midway up the canyon slope rather than close to the diapir. The canyon was found to be a debrite dominated environment with turbidite sands associated with debris flows. The lithology of the debris flows was undetermined. Broad scale analysis showed little evidence of channelisation except in one location.
The evidence of channelisation was a horseshoe shaped feature that had what are interpreted to be direct hydrocarbon indicators associated with it. This feature was imaged in plan view and it was shown to sweep as it aggraded. Frequency domain displays were used to image the extent of the reservoir sands and they were found to truncate abruptly down dip.
This channel has been identified as the most likely location for reservoir quality sands suitable for gas disposal. It suggests that sands are present in the up-dip region. Attribute analysis did not yield any specific attribute combination suitable for lithology determination, but textural analysis associated with various attributes was important in its recognition.
The up-dip Tiof region may be suitable for disposal purposes but limitations may exist in terms of volume and reservoir connectivity. The use of the region for disposal purposes must be preceded by a more comprehensive study of the target area, taking into account seal integrity and reservoir volume. This study suggests that the region is not barren of hydrocarbon prospects and that there is potential for it to be exploited in the production and development of the Tiof Field.