Jason Eleson | GeoIntegra Consulting, LLC
Got Microbialites? A Comparison of Subsalt Carbonate Reservoir Characteristics from the Pricaspian Basin, Kazakhstan and Santos/Campos Basin, Brazil
Abstract:
Recent oil discoveries offshore Brazil have highlighted the importance of carbonate microbialite reservoirs. These reservoirs are capable of producing many thousands of barrels of oil per day, but have only become a focus of thorough research in the last two decades. This study draws on observations from Serpukhovian (Lower Mississippian) marine microbial carbonates of Tengiz field in the Pricaspian Basin of Kazakhstan, and observations from multiple lacustrine "microbialite" discoveries in the Santos and Campos Basins, offshore Brazil.
Tengiz is a mulit-billion barrel oil field discovered in 1979, and has three distinct marine depositional domains: shallow platform, offshore wedge & deep marine apron. The majority of the microbialite facies are found in the wedge and apron domains. The wedge reservoir is the locus of deposition for prograding, upper slope microbialite reservoirs, with intercalations of in situ or allochthonous skeletal material and marine muds. The apron deposits contain deep water allochthonous debris flows, turbidites and, more locally, olistostromes that have slid en masse downslope from shallower settings. Wedge and apron deposits are typified by abundant early marine fibrous calcite cements, resulting in poor matrix reservoir quality. This early diagenesis promoted syndepositional fractures that were later enhanced by sulfur-rich fluids migrating throughout these facies, resulting in high production rates.
In contrast with Tengiz microbialites, the "microbialite" reservoir facies discovered multi-billion barrel offshore Brazil fields were deposited in widespread lacustrine basins. The Aptian/Albian Barra Velha Formation has carbonate reservoirs typified by three main lithologic fabrics, commonly observed meter scale packages with the following vertical succession: 1) basal laminated carbonate muds of poor reservoir quality, 2) middle mm-scale carbonate spherule deposits with associated magnesium silicate clays of variable reservoir quality, and 3) upper in situ and reworked microbialite "shrubs" of high reservoir quality.
Prediction of reservoir quality in these lacustrine reservoirs has proven to be a challenge, and debate persists on the origin of these depositional fabrics. Originally, most researchers believed that productive microbialite reservoirs were created by microbial organisms along the flanks of paleo-structural highs in large, deep ancient lake systems. Lines of evidence include scattered occurrences of stromatolitic boundstones in core and image logs, algal filaments preserved in early silica cements, and shrubby textures from cored sections on paleostructural highs within the lake system. However, other researchers have recently posited that the "microbialite" fabrics are actually inorganic precipitates deposited within highly alkaline, smaller lakes. Lines of evidence include low abundance of stromatolitic boundstones from available core and image logs, high alkalinity required to precipitate Mg-silicate clays (pH=10-10.5), general absence of fossils and bioturbation, and increasing lake restriction indicated by high degrees of oxygen fractionation within the spherulitic and shrubby facies.
Determining the actual origin of these fabrics is made more difficult due to 1) the unwillingness of industry to share proprietary data with each other, 2) the abundance of wells drilled in current structural highs, with virtually no wells in lower structural positions, and 3) lack of detailed outcrop and reservoir analogs with similar fabrics and diagenetic alteration. It is clear that carbonate researchers must exercise caution when using reservoirs or outcrops identified as "microbial" as analogs for the Brazil presalt deposits, and further work is required to reconcile the substantial differences that commonly exist between them.