Reservoir Architecture of Deep-Lacustrine Sandstones from the Early Cretaceous Recôncavo Rift Basin, Brazil

Abstract

Synrift, lacustrine sedimentation took place along the entire eastern Brazilian margin in response to the Neocomian breakup of Gondwana. In the Reconcavo 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 Balsamo oil field, near the northeastern border fault margin of Reconcavo 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 Reconcavo basin provided virtually unlimited accommodation space for the aggradational stacking of climatically controlled, deepening-upward successions.