Date of Award:

5-2003

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Geosciences

Department name when degree awarded

Geology

Committee Chair(s)

Bradley D. Ritts

Committee

Bradley D. Ritts

Committee

Robert Oaks

Committee

Peter Kolesar

Abstract

The abundance of good reservoir and source rocks offshore Cabinda, Angola, makes the area an attractive and successful hydrocarbon province. Block 0, offshore Cabinda, lies in the Lower Congo basin along the western coast of Africa. The stratigraphy of Block 0 consists of two major oil-rich sequences: the rift sequence (primarily lacustrine) and the post-rift sequence (primarily marine). These are separated by a thick section of evaporites, and thus are referred to as the pre-salt sequence and post-salt sequence, respectively.

The Chela Formation, mid-Aptian in age, was deposited before the salt. It consists of sandstones and conglomerates locally interbedded with carbonates. It is the youngest unit of the pre-salt sequence. The top of the Chela Formation is gradational into the salt, whereas the base of the Chela unconformably overlies older pre-salt units. The regional pre-Chela unconformity corresponds to the "break up unconformity" developed as a direct response of Gondwanaland rifting that resulted in the opening of the South Atlantic Ocean and culminated with the separation of the South American and African continents. In this context, the Chela Formation represents an early post-rift transgressive unit that spans the transition from continental to marine conditions. The thickness of the Chela Formation in Block O is variable. This unit thickens westward from O to 305 m. Environments of deposition within the Chela Formation range from coastal non-marine to shallow-marine environments. The Chela Formation onshore Cabinda is a fining-upward sequence with coarse sandstone and conglomerate in the base grading to finer sandstone interbedded with mudstone. Offshore, the unit is represented by a sequence of very-fine to fine-grained sandstone, grading to siltstone, and locally interbedded with dolomitic carbonates. Sediment in the Chela Formation has a continental-block provenance, evidence that Chela detritus was supplied from an external granitic source on the southern African continent. Although no commercial accumulations of hydrocarbons have been found offshore in the Chela sandstones to date, the unit represents a potential reservoir because of hydrocarbons log porosities between 15 and 30 porosity units, the presence of hydrocarbon shows onshore, and an excellent overlying seal of thick salt.

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