PSModeling Reservoir Architecture of Isolated Carbonate Platforms*
Phillip Bassant1 and Paul M. (Mitch) Harris1
Search and Discovery Article #40294 (2008)
Posted August 7, 2008
*Adapted from poster presentation at AAPG International Conference & Exhibition, Paris, France, September 11-14, 2005. See companion article, “Analyzing Reservoir Architecture of Isolated Carbonate Platforms,” Search and Discovery Article #40295 (2008).
Forward stratigraphic modeling of a conceptual isolated carbonate platform produces four distinct depositional profiles determined essentially by water depth. The depositional profiles described below 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 like those found in the Caspian Basin.
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.
Parameters were chosen to resemble approximately a Carboniferous grain-dominated platform with microbial boundstone slopes like Tengiz.
Bathymetry alone will not uniquely define the depositional profile for a given system: multiple depositional profiles exist (partly dependent on rate) for the same bathymetry.
In low amplitude accommodation cycles, Sequence Boundaries (SB) are 180 degrees out of phase with accommodation cycle (SB occurs at sea-level highs). Maximum Flooding Surface (MFS) is 90 degrees out of phase with accommodation cycles.
In high amplitude accommodation cycles, SB and MFS are in phase with accommodation lows and highs, respectively.
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.