COUGHLIN, R. M., W. O. BEMENT, and W. V. MALONEY, Pecten International Company, Houston, TX

ABSTRACT: Petroleum Geology of the Deltaic Sequence, Rio Del Rey Basin, Offshore Cameroon

The Rio Del Rey Basin of offshore Cameroon comprises the easternmost portion of the Niger delta complex. In Cameroon, the deltaic package is late Miocene to Pliocene and consists of multiple cycles that prograded from north to south across the basin. In this delta flank setting, the overall package is thinner and stratigraphic correlation simpler than in the depocenter to the west, so the distribution of reservoirs and seals is well defined. Productive reservoirs are shallow, typically less than 2000 m, and mostly hydropressured to slightly overpressured. Reservoir properties generally are excellent and seismic bright-spot technology has played a major role in exploration and development efforts.

Rio Del Rey is characterized by three main structural styles from north to south: (1) detachment-based growth faults, (2) highly faulted, mobile shale cored domes and ridges, and (3) toe thrusts and folds. Virtually all individual traps are fault dependent and lack of sufficient internal seals in sand-rich wave-dominated deltaic sequences can limit the objective window. Most fields are downthrown fault traps in which hanging-wall reservoirs are effectively sealed against older prodelta and marine shales. All large oil fields are located in the dome and ridge province and are potential analogs for similar structures now being explored on the upper slope off Nigeria.

The best quality source rocks identified thus far are Paleocene to Eocene marine shales. Oil vs. gas distribution in the basin is a function of variations in the kerogen type, organic richness, and maturity of these source rocks. A significant quantity of the gas found to date in Rio Del Rey is biogenic in origin. The Cameroon charge model may aid in better understanding hydrocarbon distribution in offshore Nigeria, where the Miocene deltaic section is much thicker and potential Paleocene-Eocene source rocks likely are unpenetrated.

The shallow deltaic play in Cameroon is relatively mature. Future exploration likely will focus on identifying deeper, nonbright-spot-supported opportunities including potential stratigraphic traps.

AAPG Search and Discovery Article #90990©1993 AAPG International Conference and Exhibition, The Hague, Netherlands, October 17-20, 1993.