Approaches to rifting and heatflow modeling of hydrocarbon systems

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					  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


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%).

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Description: Approaches to rifting and heatflow modeling of hydrocarbon systems