Analyzing Reservoir Architecture of Isolated Carbonate Platforms*
Phillip Bassant1 and Paul M. (Mitch) Harris1
Search and Discovery Article #40295 (2008)
Posted August 7, 2008
*Adapted from oral presentation at AAPG Annual Convention, Calgary, Alberta, June 16-19, 2005. See companion article, “Modeling Reservoir Architecture of Isolated Carbonate Platforms,” Search and Discovery Article #40294 (2008).
Forward stratigraphic modeling of a conceptual isolated carbonate platform produces four distinct depositional profiles, determined essentially by water depth, with characteristic facies belt dimensions and lateral relationships. Profile A (shallowest) shows a grainstone shoal margin on the high-energy edge of the platform, 250-500 m wide, with a raised rim and shallow platform interior dominated by packstones. Profile B also shows a high-energy grainstone rim, 500-1000 m wide with no significant margin relief, and a platform interior dominated by packstones. Profile C occurs in a deeper bathymetric setting; high-energy conditions flood the platform, and platform-centered grainstone shoals develop with widths of 2000 – 5000 m. Profile D (deepest profile) has deeper water packstones developed across the platform top, with no grainstone development.
In an aggrading platform with only monotonous sea-level rise and no sea-level cyclicity, only profile B develops. This is the stable-state for platform-growth in this model. During sea-level stillstands, profile A will eventually develop. During a deepening sequence, profiles B, C, and D develop in rapid succession prior to final drowning. Profiles C and D can be considered transient or unstable states, as their productivity rates are too low to keep up with sea-level rise, and thus are rare during times of monotonous sea-level rise. However, when sea-level cycles are introduced unstable profiles C and D may dominate the platform. Grainstones (profile C) or packstones (profile D) can dominate platform-top deposition throughout the cycle, with abrupt shallowing to the raised grainstone rim (profile A) occurring at maximum sea-level fall.
The depositional profiles described above have characteristic facies belt dimensions, geometries, facies-proportions and stratigraphic occurrences. These simulations help to predict facies belt geometries and constrain facies belt dimensions for isolated platform reservoirs found in the Caspian Basin.
1. N-G increases with increased accommodation rate (up to drowning threshold).
2. The simulator produces a limited number of depositional profiles (solutions) showing variations in reservoir distribution.
3. Even a simple simulation resembles reality.
4. Bathymetry alone will not uniquely define the depositional profile for a given system: multiple possibilities exist (partially dependent on rate).
5. Interpreted SB & MFS positions relative to accommodation cycle changes with cycle amplitude.
Weber, L.J., B.P. Francis, P.M. Harris, and M. Clark, 2003, Stratigraphy, lithofacies, and reservoir distribution, Tengiz field, Kazakhstan: Permo-Carboniferous Carbonate Platforms and Reefs, SEPM Special Publication 78, p. 351-394.