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 andrew.racey@bg-group.com
#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.