Modeling Reservoir Architecture of Isolated Carbonate Platforms

Bassant, Phillip, Paul (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. The depositional profiles described below have characteristic facies belt dimensions, geometries, facies-pro-portions 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 platform­centered 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.