Abstract: Balanced
Filled Lakes Worldwide:
Insights for Optimum Source Character and Distribution in Brazilian Continental
Margin Basins
Bohacs, Kevin M., and Neal, Jack E. - Exxon Production Research Co.
Balanced
-filled lake systems contain
the most prolific lacustrine source rocks and beneficent facies juxtapositions
for hydrocarbon accumulation, based on observations of lacustrine strata
of many different ages and basins (e.g. Quaternary of east Africa, Tertiary
of USA & Asia, Cretaceous of China & Africa). Published data from
Early Cretaceous lakes of the Brazilian continental margin indicate that,
although each area contains various lake-basin types, their dominant source
facies are in
Balanced
-filled lake systems. Organic-rich rocks are best
developed in this lake-basin type due to an optimal interaction of productivity,
preservation, and concentration.
Balanced
-filled lakes are one of three lake-basin
types (Overfilled,
Balanced
-filled, and Underfilled) recognized from recurring
lithofacies associations and stratal stacking patterns. Lake type derives
from interpreted interplay between potential accommodation and sediment+water
supply over sequence-set time scales. These factors are approximately equal
over time in a
Balanced
-filled lake system. The concept of lake-basin type
is useful for sorting out the complexities of lacustrine deposition to
derive a predictive framework. Each lake type has distinctive facies and
stacking patterns at the sequence and para-sequence scale that reveal the
link of sediment flux to changing lake level. Understanding this link leads
to predictability in the sequence stratigraphy and distribution of hydrocarbon
system play elements in each lake type.
Many limnogeologists have noted the common occurrence
of lake basins in the rock record with depositional environments that evolve
from fluvial, through deep, then shallowing lake, back to fluvial. This
"lake-sandwich" evolution is usually attributed to tectonics or climate.
We observe lake type to evolve as a function of both tectonics and climate,
with climate especially controlling the type of lake during maximum subsidence
rate. Basins with dry climates commonly evolve from fluvial (Overfilled)
to deep lake (Balanced
-filled) to evaporitic lake (
Balanced
-filled to Underfilled).
The Campos Basin fits this evolution as paleoclimate was dry during the
rift phase. Composite well logs through the Campos Basin clearly show this
evolution and its impact on source facies (Fig. 1). The organically enriched
Late Jiquiá interval represents the
Balanced
-Fill phase. From seismic
data, the rift fill evolves from a chaotic, low amplitude, areally restricted
seismic facies to a higher amplitude, continuous to semi-continuous, parallel-bedded
and more widespread package (Fig. 2). This transition marks the evolution
of lake types from Overfilled to
Balanced
-filled and Underfilled. Basins
with more humid climates may evolve only to
Balanced
-filled at peak subsidence
(e. g., Recôncavo Basin).
Balanced
-filled lakes can be shallow or
deep with thick or thin sequences but they share similar geochemical, biofacies,
sedimentological, and sequence-stratigraphic attributes. Sequence boundaries
are formed by a mix of erosion and desiccation- erosion may be best developed
during transgression. Lake-water chemistry varies systematically between
fresh and saline/alkaline. Most organic-rich rocks are deposited in the
profundal zone, with subordinate amounts in the lake plain behind mixed
biogenic- clastic shorelines. Organic matter is dominantly algal Type I,
typically with TOC < 30% and HI < 750 mgHC/g C. Organic
facies are relatively constant laterally, changing only relatively close
to shore.
Comparisons of geochemical, geological, and geophysical
data from Balanced
-filled lakes worldwide with Brazilian basins provides
a greater understanding of source rocks in the lacustrine systems responsible
for most of the giant oil accumulations in offshore Brazil. This integrated
geological-geochemical model allows prediction of not only hydrocarbon
quality, but also source character and distribution, to serve as input
for analysis of migration and trap fill.
AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil