Abstract: Petroleum Geology of the Lombo East and Essungo Oil Fields, Block 2, Offshore Congo Basin, Angola

Koning, Tako - Texaco Angola Inc.; Odette Gracas De Deus - Sonangol; Elder S. T. Pierobon - Braspetro

All the oil fields in Block 2, offshore Congo Basin, Angola, including the Lombo East field, produce from Albian age Pinda carbonates. The single exception is the Essungo field which produces from Tertiary sandstones.

The Pinda formation consists primarily of shallow marine carbonates deposited on a westward sloping carbonate ramp. The Pinda carbonates include oncoid-ooid grainstones, packstones and wackestones. The Pinda carbonates were pervasively dolomitized during multiple dolomitization events.

The principal structural elements in Block 2 which affect the Pinda strata include syn-rift basin architecture, post-rift listric detachment and associated faulting as well as transverse deformation and offset. Northwards oriented listric and antithetic faulting has resulted in a variety of oil-trapping structures including simple rollovers and horst blocks. Salt rafting and down-to-the-basin gravity sliding of Pinda "rafts" resulted in a number of fields consisting of blocks of Pinda carbonate with separate oil-water contacts.

The Lombo East field is located in 35 meters of water 30 kilometers offshore. Lower Cretaceous fluviatile sandstones and conglomerates onlap basement and are overlain by thin Aptian anhydrites. Sandy Aptian-Albian dolomites occur above the anhydrite beds which become progressively less sandy higher in the sequence. Near the top of the dolomite is a high energy carbonate shoal facies wherein the hydrocarbons are trapped.

Regional subsidence and localized structuring formed the Lombo East carbonate raft. The reservoir consists of fine to coarse crystalline vugular dolomite with 15 to 20% porosity. The oil gravity is 40 degrees API. The field occurs within a 1600 acre closure with vertical relief of about 250 meters and an oil column of 130 meters. The reservoir is overlain and sealed by thick impermeable Albian limestones and silty, tight dolomites. The Lombo East field was discovered in 1984 and has produced to date about 80 million barrels of oil.

The Essungo field produces from Tertiary (Oligocene and Eocene) sandstones. Essungo was discovered in 1975 in 25 meters of water and has produced almost 30 million barrels of oil to date. The field occurs within a subtle structural closure and oil is trapped both stratigraphically and structurally. Sand deposition is interpreted to have occurred within a deep water environment. The reservoirs consist of fine-grained conglomeratic sands which are in general poorly sorted. A variety of separate oil-water contacts occur within the multiple reservoir sands. Average porosities within the field range from 16 to 22% and the oil gravity ranges from 32 to 39 degrees API.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil