Evidence for Seasonal, Abiotic Precipitation and Accumulation of Carbonate Sediment in the Pre-salt “Sag” Successions of Offshore Brazil

Lacustrine carbonate facies comprise the highly productive reservoirs of the pre-salt “sag” successions, offshore Brazil. Depositional processes remain somewhat enigmatic, with the predominant model being that microbial organisms played an important role in sediment production and accumulation. Preliminary observations of cores and sidewall cores, however, indicate that largely abiotic precipitation of different carbonate phases formed most of the important depositional reservoir facies at some locations.

Primary aragonite, calcite, and dolomite were precipitated in various water depths, both on the lake floor and within previously deposited sediment. The resulting grain types and sediment fabrics are highly variable, ranging from ooids, spherules, composite grains, and morphologically complex, precipitated framestone fabrics (“shrubs”). Microfabric analysis indicates that individual laminations formed within lake-floor crusts and framestone facies at sub-millimeter to perhaps as much as 1-cm thicknesses via some quasi-periodic processes.
(Sub-)Millimeter-thick crusts of pure, micritic carbonate in deeper water facies commonly are draped by silt-rich and pyritic layers, which indicate times when carbonate accumulation was followed by deposition of very fine-grained siliciclastic sediment. In shallower water framestone facies, similar, but slightly thicker crusts of precipitated micritic carbonate cover detrital carbonate sediment that filled between the frame, forming regularly spaced laminations of carbonate grainstone/packstone and precipitated micrite that bridge between approximately 3 to 5 mm-thick growth bands in the frame.

Similar scales and types of carbonate layering are observed in other facies and suggest that carbonate precipitation was dominantly abiotic, quasi-periodic, and likely seasonal in nature in some depositional environments. Pure carbonate layers may indicate times of high seasonal input of highly alkaline water to the lake systems (more wet?), which were followed by times when wind-blown silt was supplied to far-offshore lacustrine environments (more arid?). These various types of regularly layered, sediment fabrics have important implications for reservoir quality and estimates of longer term accumulation rates in the various Early Cretaceous lacustrine environments of offshore Brazil.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California