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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
Abstract
AAPG Bulletin, V.
1Manuscript received March 13, 1998;
revised manuscript received January 25, 1999; final acceptance March 16,
1999.
2Petrobras, Av. Chile 65, sala 1704,
20035-900 Rio de Janeiro, RJ, Brazil; e-mail: [email protected]
ABSTRACT
Synrift, lacustrine sedimentation took place
along the entire eastern Brazilian margin in response to the Neocomian
breakup of Gondwana. In the Recôncavo basin, a deep lake was developed
and filled mainly by a thick (>2000 m) succession of dark-colored, organic-rich
mudstones and sandstones, and by subordinate, oncolite- and Ostracod-rich
carbonates. In the Fazenda Bálsamo oil field, near the northeastern
border fault margin of Recôncavo basin, deep-lacustrine reservoirs
comprise a succession up to 424 m thick composed mainly of laminated (lower
section) and massive (upper section), medium- to fine-grained sandstones.
Their porosities and permeabilities typically range from 14 to 23%, and
100 to 500 md, respectively. The sandy reservoirs and interbedded source-rock
mudstones comprise eight transgressive successions (mostly 20-120 m thick)
bounded by basin-wide, mudstone marker beds. The average duration (about
83,000 yr) of each succession suggests that climatically driven lake level
fluctuations may have modulated the cyclic, deep-lacustrine sedimentation
in the study area; however, the interaction of changes in lake level with
tectonic activity defined the dominant type of reservoir facies and the
geometry and position of sand bodies. Laminated sandstones form sandstone
bodies that are 600-1200 m wide, 1.5-4.5 km long, and up to 46 m thick,
which fill northeast-oriented, fault-bounded troughs. These rocks probably
were deposited by long-duration, sand-rich density underflows preferentially
developed during rising lake level or highstands. These rocks may reflect
an increasingly humid climate and more powerful and sediment-loaded influent
streams. The more intense fault activity during the deposition of the lower
reservoir section would have axially focused density underflows coming
from the northeastern end of the basin. Massive sandstones comprise turbidite
channel-fills that are 100-800 m wide, more than 2 km long, and up to 38
m thick. Fault activity diminished during the deposition of the upper reservoir
section, allowing the cutting and filling of northwest-oriented channels
by turbidity currents derived from the nearby border fault margin, preferentially
during falling lake level or lowstands. Despite the local erosion associated
with some turbidite channels, no unconformity or widespread erosion surface
can be recognized within the studied succession. The relatively rapid and
continuous tectonic subsidence along the eastern border fault margin of
the Recôncavo basin provided virtually unlimited accommodation space
for the aggradational stacking of climatically controlled, deepening-upward
successions.
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