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Abstract: Constraints for Plate Reconstruction using Gravity Data: Implications for Source and Reservoir Distribution

Dickson, William G. and Robert E. Fryklund - Union Texas Petroleum; Chris G. Green - Getech

Gravity data were compiled from numerous sources, merged and processed (see companion paper). The resulting Bouguer Horizonal Derivative (HD) grids of the circum-Atlantic region were attached to plate outlines in a plate reconstruction software package. Six near-restorations were produced for times between 55Ma and 165Ma and plotted at 1/10,000,000 scale. No attempt was made to close the syn-rift half grabens, nor were later volcanic features excised. The grids were simply rotated and translated back in time with the intervening data ?consumed.?

The maps clarify the configuration of pre-rift Brazil and West Africa. Sharply-imaged transforms connect corresponding points on opposite sides of the Atlantic. The contrast between the cool, quiet oceanic and the hot, dissected continental terranes with greater detail than typical bathymetric and magnetics-based maps highlights imperfections in plate outlines and matches. The transforms fade to background in the oldest oceanic crust showing density uniformity (or annealing) associated with the crust of the Cretaceous magnetic quiet period.

Transforms east of the mid-Atlantic Ridge fit the published ne-sw trends tracking across the prominent, sub-parallel Cameroon Line of volcanoes. The Cameroon Line, like the Walvis Ridge, does not follow a transform, in accord with a ?Hot Spot? origin, although some of the ?leaky? transforms from the Chain Fracture Zone on north do exhibit strings of seamounts related to north-south separation besides pure transform motion.

Reconstruction emphasizes the zipper-like continental separation between the Chain Fracture Zone and the Walvis Ridge, matching wide basins on one side with opposing narrow basins on the other. The particularly well defined West African plate limit helped locate the corresponding limit of the offshore Campos and Espirito Santo Basins. Locally, this extends eastward the recognized limit of continental crust and hence the area with possible syn-rift lacustrine source rocks.

Several factors improve risking of hydrocarbon prospectivity including clearer definition of highlands as syn-rift sediment provenances. Where the basins were narrow, initial deposition of the drift-age delta systems could potentially have reached the opposite side of the rift. A calculation using a drift rate of 10 cm/yr indicates that basin separation would not have equaled the maximum syn-rift basin width of 250 km until about 80Ma! Hence early drift source and reservoir provenances may in places have been from opposite sides of the rift than at present.

Examples in the Camamu, Campos, Congo and Gabon Basins delineate half-graben compartments which likely restricted circulation, promoting the development of lacustrine source rocks. Definition of these half-graben compartments is crucial in extrapolating drilling results. The accommodation zones linking half-grabens show avenues for paleo rivers and deltas, particularly those lineaments corresponding to known oceanic transforms. Locally, well control offshore Gabon indicates sand /shale ratio preferences and variations in hydrocarbon gravities associated with these accommodation zones, suggesting their continuing influence on sedimentation and fluid flow.

Transform influences also apply to sediments deposited on oceanic crust where potential source rocks would all be marine. Given lower heat flows typical of oceanic regimes, and starting from the cooler temperatures of bathyal to abyssal environments, source rocks require much thicker overburden than the syn-rift section to generate hydrocarbons. Higher heat flows (and structural deformation) associated with transforms may improve local prospectivity.

Overprinting of post-rift prograding delta systems on the syn-rift section in the Amazon, Potiguar, Niger and Congo fans is characteristic of multi-phase basins. This masks the syn-rift section and produces misfits in reconstructions unless resolved with multiple displays and data sets (per our companion paper).

Future work requires better control of plate outlines and improved means to tie arbitrary data such as well locations and field outlines to plates. Improvements to gravity data will reduce artifacts and increase resolution perhaps to prospect levels.

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