Print this pageBottom-Current Reworked Sands - A "New" Latent Reservoir Type in Deep-Water Sedimentation, Equatorial Guinea and Nigeria
FAMAKINWA, SIMEON BOLAJI, G. SHANMUGAM, and JOHN NIETO
A "new" latent reservoir, bottom-current reworked (BCR) sands of Pliocene age, has been recognized in the Zafiro Field, Equatorial Guinea. Two major types of sands, amalgamated BCR sands with very good poroperms (20-30%, 1000-3000 mD) and isolated BCR sands with moderate permeability (100-1800 mD) are identified. These deep-water reservoir sands have been commonly referred to as "low-resistivity" pay sands, contourites or thinly laminated sands. Petrophysical modelling, using conventional cores and borehole imaging tools, has shown they are high-resistivity sands.
BCR sands consist of fine-grained, well sorted sands and silts, with interbedded mud layers. They are distinguished by their traction features including horizontal, parallel and ripple cross laminations. In the Zafiro Field, these sands often form a thick sediment blanket over the main slump and debris flow-dominated reservoir interval. Their frequent occurrence above the main reservoir sand, rather than below, suggests they are genetically related. Their origin is believed to be closely tied to the predictable pattern of ocean bottom currents sweeping the Atlantic Ocean. Individual BCR units may be thin but the gross intervals are usually thick and extensive.
Conventional cores are critical to achieving a complete evaluation and exploitation of the BCR sands, which have a net/gross ratio of over 40% and contribute to oil production in the Zafiro Field. A suspected BCR interval to the north in Nigeria, flowed over 2200 BOPD. BCR sands are likely to be a regular feature in deep-water sedimentation in offshore Equatorial Guinea and Nigeria.