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Abstract: Neocomian - Upper Aptian Pre-Salt Sequence of Southern Kwanza Basin: A Regional View

Uncini, Giuseppe - Agip Angola; Brandao Mario - Sonangol; and Giovannelli Antonio - Agip

To date the Kwanza Basin lacks a defined stratigraphic framework, despite several works conducted by different authors. The establishment of a reliable geological model is also complicated by scant paleontological data and the ensuing local absence of certain time lines. Palynomorphs and ostracods are in practice the only fossil groups that supply detailed results.

The understanding of lithostratigraphy should be made easier by the definition of a regional scale geological model, rather than by the classification of individual lithotypes recognized by wells. Therefore, the goal of this paper is to highlight similarities in the pre-salt succession, between Kwanza and Lower Congo (Cabinda) Basins; the area of study is the offshore southern Kwanza Basin, where five wells, drilled in the early ?80s, penetrated the pre-Aptian sequence.

During the Neocomian - Barremian rifting stage, lacustrine basins, located in meridian trending structural troughs (grabens and half grabens), developed in the Kwanza area. The related sedimentary successions consist of siliciclastic (shales, siltstones, sandstones and conglomerates) and carbonate deposits (bivalve coquinas).

In the offshore southern Kwanza Basin, the oldest sediments penetrated by the wells (Mucua 1 and possibly Maboque 1) are Neocomian - Early Barremian in age (palynozone CV-CIV, Jardine et al, 1974, Doyle et al, 1977, Grosdidier et al, 1995), while the uppermost part of this sedimentary stage reaches the CVI palynozone (Late Barremian). From a regional point of view, they are equivalent to the Bucomazi-Erva and Toca Formations of Lower Congo Basin; similar facies are also known in Brazil (Campos Basin, Lagoa Feia Fm.; Abrahao & Warme, 1990).

The Bucomazi Eq. facies are represented by alternating quartzose and feldspathic arenites with abundant fragments of magmatic and metamorphic rocks, siltstones, shales and minor occurrence of chalky limestones. This sequence contains organic rich facies with good source characteristics (T.O.C. up to 1.14%, fair to very good Petroleum Potential and type II kerogen).

Skeletal carbonates of ?Toca Equivalent Fm.? have been cored in three wells (Denden 1, Abacaxi 1 and Maboque 1) and consist of shoal facies developed along the margins of the lacustrine basins (Lomando, 1996, 1997). They are presumably associated with two different stratigraphic-structural settings:

1) low-angle ramps on the antithetic margins of half grabens.

2) sunk steps of structural highs related to synthetic faults with respect to the master faults bordering the highs (perched platforms).

In Lower Aptian (CVII-CVIII palynozones), separation of the African and South American continental plates and the resulting isostatic uplift of the newly formed margins caused the peneplanation of the existing paleomorphologies and the outstanding break-up unconformity.

The subsequent transgressive sequence (Grey Cuvo Fm., equivalent to the Chela Fm. of the Lower Congo Basin) is Late Aptian (CIX palynozone) in age. It is represented by fluvial and lacustrine continental deposits grading upwards to intertidal evaporitic sediments (dolomites and anhydrites).

This sedimentary cycle ends with anhydrite and massive halite, which characterize the Loeme Formation, testifying to the existence of regionally extensive sabkha environment at the end of the Aptian.

Potential reservoir facies were found in the ?Toca Equivalent Formation?. Calcirudites and dolorudites, sometimes containing dead oil, locally show good vuggy-mouldic porosity, but scarce pore interconnection (poor permeability). A complex diagenetic evolution affected these skeletal and bioclastic deposits, and together with fracturing plays a key role in determining the ?Toca Equivalent Formation? reservoir performance.

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