Precambrian Analogs for Pre-Salt Lacustrine Carbonates
Bishop, James W.; Wasson, Matthew S.; Murphy-Bishop, Megan A.; Sumner, Dawn Y.
The large pre-salt discoveries of the South Atlantic have led to a surge of interest in lacustrine carbonates. Several aspects distinguish the pre-salt reservoirs: (1) size, (2) lacustrine rift setting, (3) microbial and/or heavily precipitated fabrics, and (4) unusual geochemistry. However, to date they have defied most analogs. Crucially, modern and ancient lakes rarely have carbonate deposits of comparable size to allow comparisons of platform architecture. Here, we examine the utility of Precambrian marine analogs, focusing on Neoarchean platforms (Transvaal, Hamersley) and paying special attention to rules that may guide facies distribution.
(1) Size: The Neoarchean platforms span 100s of km and are up to several km thick. Outcrop windows can be several km wide and 10s to 100s of m thick. Thus, platforms, their buildups and facies belts attain the scale of pre-salt reservoirs.
(2) Setting: The Neoarchean platforms are long-lived and formed in rift to passive margin settings. A demonstrably marine origin is difficult to prove for units in the Transvaal and Hamersley, though currently most are believed to be marine.
(3) Fabrics: Precambrian and pre-salt lacustrine platforms both have fabrics rich in microbialites and/or precipitates. Precambrian platforms contain a number of precipitated fabrics unusual to the Phanerozoic marine record, including large aragonite botryoids, giant ooids, molar tooth structures, and thick seafloor cement crusts. Many are intimately related to microbial structures, such as in fenestrate microbialites, tube structures, and a variety of stromatolites. The distribution of many of these cement-rich facies is predictable across the platforms.
(4) Geochemistry: Unusual Precambrian facies speak to an unusual seawater chemistry (e.g., Grotzinger, 1989). Cement-rich fabrics required high crystal growth rates relative to sedimentation, likely achieved by high saturation coupled with inhibited nucleation. The Archean ocean likely had a paucity of [SO42-] and was alkaline, at times with [HCO3-] exceeding [Ca2+]. Such conditions are more typical of lakes, both modern and ancient. Unusual minerals are also common to pre-salt and Precambrian platforms (e.g., early diagenetic talc and smectitic clays in unusual coated grains).
Given these similarities, we believe aspects of Precambrian carbonate platforms are useful as analogs for some pre-salt lacustrine reservoirs.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013