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PSModeling Reservoir Architecture of Isolated Carbonate Platforms*
By
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).
1 Chevron Energy Technology Company, San Ramon, California, USA ([email protected]; [email protected])
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.
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Parameters were chosen to resemble approximately a Carboniferous grain-dominated platform with microbial boundstone slopes like Tengiz.
Input parameters:
Implications for Sequence Stratigraphic Interpretations 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
In high amplitude accommodation cycles, SB and MFS are in
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. |