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Predicting Net/Gross and Pay Thickness in Icehouse Carbonate Reservoirs through Forward Modeling: Implications for Evaluating Reservoir Potential in Different Basin Types

Pollitt, David A.1; Burgess, Peter M.2; Wright, Paul V.3
1 School of Earth and Ocean Sciences, Cardiff University, Cardiff, United Kingdom.
2 Department of Earth Sciences, Royal Holloway University of London, Egham, United Kingdom.
3 BG Group, Reading, United Kingdom.

Numerical simulations, field tested against outcrops across a range of basin settings, show that subsidence is the ultimate arbiter on development, preservation, thickness and composition of reservoir-prone Upper Carboniferous carbonate cyclothems. For a known subsidence regime these simulations predict both cycle thickness and reservoir thickness, allowing estimations of reservoir net pay thickness as well as net/gross.

Upper Carboniferous platform carbonates are hydrocarbon reservoirs in many basins, especially in the midcontinent USA and PreCaspian region (Tengiz, Karachaganak and Kashagan). In many reservoirs of this age the best reservoir quality is found as primary inter-particle porosity in grainstones, controlled by primary depositional factors that may therefore relate to cyclothem development.

To evaluate the relative importance of eustatic and tectonic controls on the stacking patterns of cyclical icehouse platform carbonates two contrasting sections of Pennsylvanian age were studied and compared against numerical simulations: the Gobbler Formation of southern New Mexico, and the Honaker Trail Formation of south-east Utah. These represent sections which are broadly contemporaneous, but are from disparate tectonic settings.

Numerical forward modeling can provide insights into the parameter combinations that are likely to generate the greatest thicknesses of grainstone lithofacies and therefore the best reservoir properties. The one-dimensional numerical model presented here incorporates published parameters thought to control carbonate deposition. Systematically varying these parameters over thousands of model runs allows mapping of the parameter-space and identification of areas of preferential reservoir development.

Modeling results suggest that whilst regular oscillations of relative sea level are required for cycle genesis, overall control of vertical proportions of reservoir facies, pay thickness and net/gross is ultimately determined by tectonic subsidence of the basin. Furthermore, modeling suggests subsidence rates comparable to moderately to rapidly subsiding extensional basins and moderately subsiding foreland basins will generate the greatest thicknesses of reservoir-prone grainstone. These results represent a simple but very useful predictive model for use in exploration for icehouse carbonate reservoirs.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009