The petroleum geology of the Early Eocene El Garia Formation, Hasdrubal field, offshore Tunisia

The Hasdrubal field (offshore Tunisia) comprises art Early Eocene shallow-m arine nummulitic limestone reservoir (the El Garia Formation) sourced by de ep-marine mudstones and limestones of the generally age-equivalent Bou Dabb ous Formation. The field is located on a NNW-SSE trending horst between a s eries of en-echelon normal to oblique faults, and is clip-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 Cherahil Formati on form the main seal. The El Garia Formation reservoirs significant volumes of hydrocarbons in Tu nisia and Libya. A detailed micropalaeontological and nannofossil study has been undertaken of the Fl Garia Formation and the immediately over- and un derlying formations which together form the Metlaoui Group, using subsurfac e data from the Hasdrubal field. This has permitted a detailed chronostrati graphic 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 reservo ir 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 dri lled along the El Garia nummulite "bank" trend have failed to encounter the nummulite reservoir facies, and why, even where this facies was encountere d, the limestones were frequently tight and/or contained limited hydrocarbo ns. It is also suggested that proximity to source is a critical factor; wit h the development of dissolution porosity by acidic pore waters migrating i n 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.