A 3D Model for Carbonate Platform Sedimentation: Response to Sea-Level Change and Seafloor Gradients
Authors: Thomsen Rene O. , Dorobek Steve , Galperin Pavel , Lerche Ian 
Published: October 2008

Abstract
Understanding the impact of various depositional controls on the growth of carbonate platforms may help in predicting the continuity and spatial distribution of petroleum reservoir facies within ancient platform successions. This paper presents preliminary results from a 3D model for simulating the growth of carbonate platforms, focusing especially on the behavior of platform-margin facies tracts. The 3D model, described here for the first time, was used to investigate the impact of changing sea level on the growth of carbonate platforms, using a range of initial seafloor gradients. The initial depositional surface used in the model features an along-strike transition from gently dipping to more steeply dipping profiles. Simulations were performed to investigate different aspects of platform growth in response to a single cycle of sea-level change (200-meter amplitude, 200,000 yr cycle duration). Using low (?1°) initial gradients, the model produces a complex depositional unit that consists of several detached platform-margin 'terraces,' each of which has a relatively lowrelief final profile. Following the full cycle of sea-level change, the final depositional unit contains highly diachronous facies boundaries with significant facies dislocations. In contrast, using the same sea-level oscillation, but a steeper (?16°) initial depositional gradient, the model creates a narrower platform, with a terminal depositional profile that is steeper overall. Internal chronostratigraphic relationships within the final depositional unit are also complex, although facies dislocations are more areally limited than in the lowgradient example.

Introduction
Many of the world's major oil and gas fields are found in carbonate platform strata (e.g., the Middle East and the Permian Basin of West Texas). Within these carbonate platform successions, platform-margin facies commonly constitute some of the best reservoir facies so it is important to understand the factors that control the stratigraphic evolution of these facies tracts.
Platform-margin facies develop within a relatively narrow range of water depths so the combined effects of variable seafloor gradients and changes in sea level are critical for understanding the depositional history of these facies. Although investigation and characterization of natural carbonate platform systems is important for understanding the distribution of lithofacies and the internal stratal architecture within each platform, computer models can provide insight into the relative influence of various depositional processes and environmental conditions on carbonate sedimentation. The main objective of this study was to test a preliminary 3D depositional model for carbonate platforms. Here we report preliminary model results that focused on the relative influence of variable seafloor gradients on platform development.
Fig. 1 General flow diagram of the 3D carbonate depositional model showing the complex interrelationship of direct and indirect processes controlling accumulation of carbonate sediments. (Available in full paper)

General description of the model
Our 3D model is based on a geometrical representation where a reference Cartesian coordinate system is set up such that both the most transgressive strandline position and the most basinward extent of carbonate facies lie within the initial model limits, over the entire duration of the model run.