2019 AAPG Annual Convention and Exhibition:

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Unusual Carbonate Facies and Precipitates: Is the Precambrian the Key to the Cretaceous Pre-Salt Carbonates? - PART 1

Abstract

Many analogs have been proposed for the unusual Brazilian Pre-Salt Petroleum System from lacustrine environments to high-temperature travertines. The lateral and vertical extent of the lacustrine carbonates is unparalleled in the geological record, making the identification of useful analogs challenging. A better understanding of how this system developed would have profound implications for exploration efforts away from the main trend of discovered fields. A significant core-based data set for Pre-salt lacustrine carbonates that has been acquired that allows for the identification of unusual facies types and pore systems. Several aspects of the pre-salt Barra Velha Formation make it difficult to find a direct modern analogue. Except for ostracods, the succession is largely devoid of body fossils and there are no signs of trace fossils or infaunal bioturbation; it is essentially a barren sequence. The formation has a substantial thickness, in places over 500 m. The abundance of laminated carbonate facies and a suite of distinctive and usual carbonate textures, transitioning into thick, marine-fed evaporites, pose challenges in resolving depositional environments. Precambrian carbonate facies types and dimensions may have relevance to understanding reservoir facies formed in restricted lacustrine settings where calcified invertebrates are not a source of carbonate, such as the Pre-salt Barra Velha Formation. We will highlight key Precambrian fabrics and their lateral extent including isopachous and microdigitate stromatolites, tufa deposits, and various laminated facies. The interpretation of these facies assemblages indicates that as water chemistry became more restricted during the carbonate-to evaporite transitions, chemical processes dominate over biological processes. In the Nama Group of Namibia facies transitions can be demonstrated through lateral tracing of beds and include up-dip grainstones grading downdip into spatially extensive tracts of microbial laminites and finely laminated carbonate mudstones. Internal facies in Ediacaran-Cambrian Ara Group carbonate reservoirs in the South Oman Salt Basin show strong facies differentiation according to spatial and temporal variations in accommodation space. Microbially influenced facies form laterally extensive sheets that, in some cases, may have covered vast tracts of the shallow subtidal depositional surface.