A. Racey+*, H. W. Bailey#, D. Beckett*, L. T. Gallagher#, M. J. Hampton# and J. McQuilken*

*BG Group, 100 Thames Valley Park Drive, Reading, Berkshire, RG6 1PT.

+ author for correspondence: email

#Network Stratigraphic Consulting, Unit 60, The Enterprise Centre, Cranborne Road, Potters Bar, Hertfordshire, EN6 3DQ.

The Hasdrubal field (offshore Tunisia) comprises an Early Eocene shallow-marine nummulitic limestone reservoir (the El Garia Formation) sourced by deep-marine mudstones and limestones of the generally age-equivalent Bou Dabbous Formation. The field is located on a NNW-SSE trending horst between a series of en-échelon normal to oblique faults, and is dip-closed except to the north where a stratigraphic pinch-out into the Bou Dabbous Formation is inferred. Middle Eocene shales and dense limestones of the Souar Formation form the main seal.

The El Garia Formation reservoirs significant volumes of hydrocarbons in Tunisia and Libya. A detailed micropalaeontological and nannofossil study has been undertaken of the El Garia Formation and the immediately over- and underlying formations which together form the Metlaoui Group, using subsurface data from the Hasdrubal field. This has permitted a detailed chronostratigraphic and sequence stratigraphic framework to be developed, including the recognition of three flooding events, which can partly be calibrated with second-order sequences, thus permitting the correlation of discrete reservoir units across the field. A further six microfaunal events are recognized between the Chouabine Formation and the "Compact Micrite Member" within the Metlaoui Group.

Previous depositional models for the El Garia Formation are discussed and a new model is proposed. The model partly explains why a number of wells drilled along the El Garia nummulite "bank" trend have failed to encounter the nummulite reservoir facies, and why, even where this facies was encountered, the limestones were frequently tight and/or contained limited hydrocarbons. It is also suggested that proximity to source is a critical factor, with the development of dissolution porosity by acidic pore waters migrating in advance of hydrocarbons. This is critical for enhancing reservoir quality and thus promoting the capacity to reservoir hydrocarbons, as indicated by the location of existing discoveries.

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