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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Lacustrine and Marine Pre-Salt Clastic and Carbonates
Of Brazil and West Africa: Drivers for Reservoir
Quality, Environoments Of Deposition and Analogs
1Department of Geology and Geophysics,
The University of Sydney and DI International
2Department of Geology, University of
Alberta, Edmonton
3Roc Oil Company Limited
4DI International
Recent discoveries, past exploration and ongoing exploration
in pre-salt, lacustrine and marine reservoirs
in Brazil and Angola have often encountered
mindboggling complexity in both carbonates
and clastics. The key to reaching an enlightened
understanding of this complexity relies more on
creative right brain thinking than left brain
logic and data collecting. Discoveries made
in the last several years have challenged our
depositional models and the dogma that the
carbonates and clastics in the pre-salt of Brazil and Angola are
plagued by poor reservoir continuity and quality. In fact, the
reservoir quality of the pre-salt, microbial limestone reservoirs in
the Tupi-Jupiter cluster of fields displays
outstanding reservoir quality. The reservoir
distribution and reservoir properties are a
function of the depositional environment in
large and small rift lakes as well as broad,
apparently marine transgressive systems in the
early Cretaceous. Carbonate and clastic
reservoir distribution and quality in lakes and
marine sag basins pre-salt are controlled by rift
geometry and orientation, lake or ocean depth and cross-section
profile, wave climate and fetch, drainage patterns in the
hinterland, entry points of clastics, lake salinity, local climate and
cycles of lake level fluctuation. These drivers for understanding
reservoir quality and continuity will be reviewed on first principles
from our knowledge of recent and ancient deposits, as well as
theoretical framing. Field analogs and regional data will be
reviewed for the Santos and Campos basins of Brazil as well as
the Kwanza and Cabinda basins of Angola. Our global analog
set for clastic and carbonate reservoirs is really diverse in the
aforementioned drivers, so that no two lacustrine, pore-salt
basins are really alike. Our analog set for both Recent and ancient
lakes is also not as statistically significant as sets for other reservoirs.
Indeed, the microbial carbonate reservoirs being discovered in
the pre-salt of the Santos Basin are likely marine, not lacustrine
carbonates, deposited like other stromatolites. Computer-driven
global climate models for lakes in the present and past are much
more difficult than those for marine and deltaic depositional
environments and have had limited success. An enlightened and
more successful exploration campaign in these high-potential
reservoirs will result from the understanding of these first principles.
Exploration campaigns driven by only seismic
interpretation and
structural
modeling
may be prone to a lower rate of success if not
tempered by a more sophisticated understanding of the drivers
for these complex and often excellent reservoirs.
Jupiter Discovery in Santos Basin, Brazil with Pre-Salt Sequences.