Storm Deposited Small Scale Rhythms from Mesa Reservoir (Iabe Formation) of Angola Block 0: Characteristics, Depositional Model and Lateral Distribution
Eugene Karabanov2, Adriano M. Arsenio1, and Sunday Shepherd2
1Reservoir Management, Cabinda Gulf Oil Company, Luanda, Angola
2Chevron Energy Technology Company, Houston, TX
Small scale (1-10 feet) upward fining rhythms were observed in the mixed clastic-carbonate Mesa reservoir, the Turonian age member of Iabe Formation. Upward fining deposits have excellent reservoir quality and are major part of all Mesa deposits in most fields in Block 0, Offshore Angola.
Based on sedimentary structures, textures, ichnofacies, rhythmic character of deposits, and lateral continuity we conclude that Mesa rocks were formed on the shelf by non-regular strong storm events, and sediments were supplied by fluvial plumes that bypassed through littoral zone to the shelf during flood events. Clastic parts of the rhythms (silt/sandstones and argillaceous siltstones) are interpreted as tempestites created by storms while calcareous sections (indurated, nodular siltstones and silty carbonates) were formed during fair weather conditions and represent normal/background sedimentation. Fining upward small scale rhythms trends are associated with clastic sediment starvation and not with sea level fluctuations.
The pre-Congo River was a major source of clastic material deposited on shelf of Angola, and Mesa units were formed by transportation of fluvial clastic sediments along shore. Impacted by longshore transport, Mesa reservoirs have an elongated shape with beds thinning along drift from South to North. Local relief and tectonics also affect thickness of clastic beds and Mesa sequences.
Depofacies and lithology are the primary controls on reservoir quality of Mesa reservoir, thus knowledge of characteristics, origin and lateral distribution of rhythms is very important.
AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery