Analyzing Reservoir Architecture of Isolated Carbonate Platforms

Phillip Bassant and Paul M. (Mitch) Harris
ChevronTexaco Energy Technology Company, San Ramon, CA

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 still-stands 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.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005