--> Abstract: Hydrocarbon Reservoirs in Oolitic-oncolitic to Siliciclastic Ramp Series (Pinda Group, Albian), Offshore, Angola, by H. Eichenseer, F. Walgenwotz, P. Biondi, and J. P. Leduc; #90942 (1997).

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Abstract: Hydrocarbon Reservoirs in Oolitic-oncolitic to Siliciclastic Ramp Series (Pinda Group, Albian), Offshore, Angola

EICHENSEER H., F. WALGENWITZ, P. BIONDI and J.P. LEDUC

The mixed ramp sequences of the Albian Pinda Group contain large amounts of hydrocarbons in northern Angola. Important discoveries have been made recently in Block 3 (offshore Angola, operated by ELF) with proven reserves, exceeding one billion bbl. of recoverable oil.

The oil-prone ramp series of the Albian Pinda Group reach a thickness of more than 600m. This overall transgressive succession developed during the earliest spreading stage of the South Atlantic in Albian time. Its major regional seals consist of outer-neritic shales, which were widespread during the Cretaceous peak-transgression. Source rock evolution started during the Aptian in lacustrine to lagoonal settings and continued throughout Albian time in suboxic basinal areas. Typical HC-traps are halokinetic "turtle-back" structures, which formed during Late Cretaceous raft tectonics.

Major reservoirs are found in the wave-dominated sandbelt facies of proximal ramp settings. Stratigraphically, the most important reservoir layers are positioned at or near the boundaries of high-frequency sequences. Early to late diagenetic dolomitization, dissolution and fracturing selectively modified the original reservoir facies.

Typical reservoir facies are:

- Shoreface sandstones with primary interparticle porosities
- Sandy peloidal to oolitic dolomites with intercrystal porosities
- Oolitic-oncolitic dolomites with intercrystal porosities and
dissolution vugs

Well cemented, mid-ramp dolomites, intensely fractured. Final burial dissolution along fractures makes of these dolomites good reservoirs.

Locally, late diagenetic anhydrite precipitation (remobilized from Late Aptian evaporitic series) reduced the effective porosities. This phenomenon is mainly restricted to the outer flanks of the "turtle-back" structures.

AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria