Seismic geomorphology and reservoir characterization of isolated tertiary reefs in the Eastern Sirt Basin, Libya

Abstract

The study area is situated on Ajdabiya trough, in the eastern part of Sirt basin, north-central Libya. The main play is Upper Sabil Carbonate, within large pinnacle reef traps. About twenty pinnacle reefs have been identified in the Ajdabiya trough within the Paleocene carbonates; some of these are oil bearing. The reservoirs are charged from the underlying mature Upper Cretaceous shales via faults and fractures. The aims of this study are to improve our understanding of the depositional and geomorphological processes, the reservoir characterization and the paleogeographic environment of the Paleocene reefs in the Sirt basin. In addition, it aims to provide a better understanding of the evolution of Sirt basin, which remains controversial. The present study is based on three-dimensional post-stack seismic and well logs. Well data have been used to indicate the lithological and petrophysical variations within the reefs. The seismic geomorphology has been used to assess how karst features have affected reservoir characterization. It is intended to improve the existing Intisar ‘A’ geological model using the properties estimated from the seismic inversion to enhance the interpolation and modelling, and to predict the properties of the Intisar ‘B’ and ‘C’ reservoirs, where minimum well control data is available. The study also provides a description of the Paleocene carbonate shelf and the opportunity to clarify and develop an understanding of the depositional processes, based on the combined interpretation of seismic attributes. Karstification exhibits both positive and negative impacts on the quality of carbonate reservoirs. Carbonate dissolution can increase porosity and permeability, whereas seal quality may be reduced because of the collapse and deformation of strata overlying the karst features. Intisar ‘B’ and ‘C’ reefs contain only residual traces of oil, while Intisar ‘A’ has hydrocarbon fill factors as high as 85%. The seismic attributes indicate buried karst-collapse features within the Intisar ‘B’ and ‘C’ reefs, with an impact on the reservoir quality and the production of oil within these fields. These karstcollapse features provide pathways for upward movement of hydrocarbons into the Eocene reservoirs. This may be the main contributing factor to the escape of hydrocarbons from these two reef fields. The shale section above the reservoir is a weak cap rock and allows for upward hydrocarbon migration. The relationship of the karstcollapse features with an abnormally large upward movement of water explains the high water cuts in these two reef fields. The inverted impedance clearly highlighted variations in facies and porosity in the Intisar reefs which are not apparent in the conventional seismic data. Five vertical facies units have been characterized in these reefs. The porosity of the south-eastern part of Intisar ‘A’ reef was significantly improved by faulting/fracturing and dissolution. The talus deposits are located on the north-eastern flank of Intisar ‘A’ reef. Well A6 on the southeast flank of the Intisar ‘A’ reef was the only remaining significant producer. It is most likely that it is draining oil from the talus zone. The Intisar pinnacle reefs developed within the low energy outer-shelf environment in the protected southern embayment of the Ajdabiya trough. The distribution of the Intisar reefs is probably controlled by the Lower Sabil Carbonate paleo-highs’ topography. The Intisar ‘E’ and ‘L’ reefs may have developed along in the inner shelf margin where the broken discontinuous barrier reef complex has initiated a semi-lagoonal open marine area; this is the ideal condition for the continued growth of Intisar ‘E’ and ‘N’ pinnacle reefs in the inner shelf area. There are no evaporite sediments within the area surrounding or overlying these reefs, so the history of the Intisar reefs fits the Darwin transgressive type theory very well. The Intisar reefs have relatively the same size, shape and history as the Intisar ‘A’ reef. A practical geological model has been proposed that describes the evolution of these reefs.