Approaches to rifting and heatflow modeling of hydrocarbon systems along the continental margin basins of Angola, Congo, and Brazil P.R.Smith, I.Norton, C.Tapscott, D.A.Hartman, R.Cunningham, D.Advocate and W.B. Maze (ExxonMobil Exploration Co., P.O. Box 4778, Houston, Texas 77210-4778, U.S.A., ph. 281-423-7029, fax 281- 423-5202, email firstname.lastname@example.org) Abstract Recent license and exploration activities offshore Congo Republic, Angola and Brazil have focused on deepwater areas along the continental slope. With the high cost of seismic data and exploration drilling in deepwater, lowering play and prospect risk by using basin modeling has become critical for deepwater exploration economics. Rifting and heatflow modeling was used to identify and delineate potential hydrocarbon systems quickly with limited data in the South Atlantic salt basins, offshore Angola, Congo and Brazil. The procedures taken to define regions of lower charge risk include: - Down-hole temperature data, basement structure and composition, and regional tectonic elements, heat probes, and gas hydrate heatflow estimates were used to constrain present-day heatflow distribution. - Subsidence history analysis constrained by maturity data and gravity mapping were used t o define paleo-heat-flow distribution, - 1D and 3D thermal models were used to develop Level of Maturity maps of present-day and paleo-maturity for key source beds, - Thermal models were also used to develop yield and yield timing maps to delineate areas of recent (late Neogene) charge from individual sources. These recent charge areas were coupled with areas of high-quality deepwater sandstone reservoir, migration pathways and traps t o identify areas of low play and prospect risk. A relatively wide continental margin underlain by rifted and attenuated continental crust is interpreted along the South Atlantic salt basins. Thick synrift sediments were deposited in grabens that locally contain rich, lacustrine source rocks. Thick, widespread salt was deposited during marine incursion and subsequent restriction of the initial basins in the Aptian. The salt was deposited on a relatively gentle surface unlike the previous higher relief, rifted terrain. The local slope of the presalt surface, underlying rigid basement highs and compactable thick synrift sediments played subsequent roles in structural deformation as the continental margin was loaded by younger sediments. An Albian carbonate platform and margin developed overlying the salt along the eastern shoreline of the South Atlantic. Rapid, late and post-Albian seafloor spreading resulted in subsidence and oceanward tilting of the continental margin. The tilting initiated regional, gravity-driven extension, with associated downdip contraction, downdip increase in salt thickness or vertical salt structures. Intraslope basins formed by the extension, salt withdrawal and compression, trap Cretaceous and early Tertiary coarse grain clastics and promote the preservation of organic matter. These margins provide a classic example of the interplay between initial rift geometry, involving basement-involved deformation, and subsequent subsidence patterns which controlled patterns of detached deformation. A second stage of gravity-driven extension with associated contraction and salt deformation occurred during the Tertiary uplift of western Africa. This was coupled with extensive middle and late Tertiary loading by depositional systems associated with the ancestral Congo, Campos and other local river systems. Sea level fluctuations and climate changes in the late Tertiary also influenced the distribution and amount of deposition. Large amounts of middle and late Tertiary sediments are focused within the intra-slope basins resulting in numerous reservoir units and adequate sediment load for recent hydrocarbon generation. The Aptian salt distribution and deformation style controlled the major Tertiary structural trends. The combination of vertical and horizontal deformation processes resulted in a relatively complex distribution of Tertiary structures and structural styles. Extensional areas are linked to loading and salt withdrawal on the shelf and upper slope. Contractional trends develop in the lower slope and rise and are often reflected in the water bottom topography. Belts of extensional structures, vertical salt structures, and contractional structures can occur from shallow to deep water. The distribution of structures and structural styles are generally not homogeneous within the belts due to the multiple episodes of salt deformation and the complex interplay between structure and sedimentation. Abundant structuring and the stacked nature of both reservoir and source units within an active hydrocarbon generation area provide for an effective hydrocarbon system (historical success ratio >30%).