Abstract: Predicting Oil Charge Types and Quality in the Deepwater Offshore Lower Congo Basin, Angola

Cole, Gary A.; Z. (Alan) Yu; David Ormerod; and Jonathan Smith - BHP

The ultra-deepwater of the offshore Lower Congo Basin, Angola, contains excellent potential for large accumulations of oil. Evidence for this are two recent discoveries in Block 17 where Girassol and Dalia are rumored to contain several billion barrels of recoverable oil. Scout information suggest that the deeper Girassol well contained fresh oil with moderately high API gravity, whereas the more shallow reservoirs at Dalia contained a heavier oil (22 API gravity) of likely biodegraded crude oil. What is not known (or what is not public knowledge) is the origin of the oils - Are they derived from the marine drift section source rocks, or from the pre-salt synrift lacustrine section, or are they mixed oil types?

The primary source rocks in this region, as inferred from wells drilled in the deepwater and shelf areas of the Lower Congo Basin and from published literature (Burwood et al., 1990; Burwood et al., 1995; Burwood, 1997, Cole at al. 1997), are: 1) the synrift pre-salt lacustrine section that can contain 100s meters of very organic rich, oil prone Type I shales; and 2) the drift or post-salt marine section that can contain from 10s to >100 meters of organic rich Type II oil prone shales.

The objective of this paper is to use a 2D modeling package, namely TemisPack, to try to understand the emplacement of the crude oils across Block 17. The main questions to be answered assuming we have two well developed source rocks as described above are: 1) do both source rocks mature enough to expel significant (commercial) quantities of oils and is the timing right to charge the Tertiary reservoirs?, 2) assuming the salt diapirism resulted in windows (no sealing) between the post- and pre-salt sections, can both source rocks contribute to charging of the Tertiary reservoirs at Girassol and Dalia?; and 3) do the modeled results match the observed data?

A seismic section was selected from a regional grid over Block 17 that traversed both the Girassol and Dalia discoveries. A stratigraphic section was constructed based on the nearest well data (Block 4 and Margarida-1 wells), and also from regional seismic horizon picks. The lithology section consisted of a lacustrine sequence typical of the pre-salt sequence discussed in the literature (Burwood et al., 1992); basically, the pre-salt section overlies the basement (assumed to be granitic due to the higher than expected bottom-hole temperatures observed at Margarida-1) and consists of 3 basic units in this model. The basal unit is a coarse clastic unit typical of the Lucula Formation, the middle unit is the lacustrine source rock which was modeled with 5% TOC and a 600 HI (Hydrogen Index), and the upper unit is another coarse clastic sequence typical of the Chela Formation. The pre-salt section ranges from Neocomian to Lower Aptian in age. The Aptian Loeme salt represents the boundary unit between the pre-salt synrift lacustrine sequence and the overlying post-salt drift section. The drift section consists of (from oldest to youngest): 1) a basal carbonate sequence (Pinda carbonates) Albian in age, 2) Upper Albian to Cenomanian Moita Seca, a secondary source rock unit (2% TOC and 350-400 HI) that can contribute some hydrocarbons into the system, 3) the Turonian to Eocene Iabe and Landana, a primary source rock unit for the Block 17 oils which can attain >100m in thickness with 3-5% TOC and 400->500 HIs, 4) the late Oligocene to Recent Malembo Formation, which contains the reservoir sands at the Block 17 Girassol and Dalia discoveries. For migration purposes, the Loeme salt effectively seals the lacustrine petroleum charge from the post salt section, except for an assumed small weld at 31-34 km along the line. This weld provides a large enough window for some migration of lacustrine oil into the drift sequence, and the migrating petroleum fronts through time provide valuable insight into the charging of the Girassol and Dalia sands.

Based on the modeling results, the 2D model across Girassol and Dalia suggests:

1) that both source rocks do mature and expel sufficient oil volumes to contribute to the charging of the Girassol and Dalia Tertiary turbidite reservoirs. Figure 1 shows the transformation ratio at present day across the modeled line.

2) Temperatures during filling of the reservoir sands suggest a higher risk of biodegradation at Dalia as temperatures during charge were <55C, whereas the temperature was about 65C at Girassol. Therefore, Dalia may be heavier due to biodegradation, whereas Girassol should be a fresher, lighter oil.

3) Petroleum saturations through time (Figures 2 and 3), suggest that Girassol is derived mostly from the post-salt marine section with possible minor contributions from the pre-salt lacustrine source rock. Dalia is a likely mix between the lacustrine source and the post-salt marine.

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