Characterizing and Modeling the Heterogeneity of Fluvial Reservoirs: A Case Study From Gypsy Outcrop Site, Pawnee County, Oklahoma

Renqi Jiang, James M. Forgotson Jr., and Daniel J. O'Meara Jr.

The genetic relationships between lithofacies and depositional environments and between lithofacies and permeability make the heterogeneity of fluvial reservoirs predictable and mappable. Two types of geological models (with/without lithofacies-dominated heterogeneity interpolation) have been simulated to illustrate the impact of geological modelling on predictions of oil recovery and sweep efficiency.

Lithofacies, defined by certain types of constituents and sedimentary textures and structures, are responses to certain depositional flow regimes or local hydraulic conditions. The Gypsy outcrop site in Pawnee County, Oklahoma, which includes well exposed roadcuts with 22 cored boreholes behind the outcrop, provides a realistic three dimensional geological site and database for developing reservoir characterization techniques. Four lithofacies that control permeability are recognized at the Gypsy outcrop site. From bottom to top of a complete channel sequence, these are:

(1) mudclast sandstones (low permeability);

(2) sandstones dominated by cross beds and planar laminated sets (high permeability);

(3) ripple and bioturbated sandstone (low permeability);

(4) mudstone and siltstone (flow barrier).

The lithofacies within each channel were mapped based on the Gypsy outcrop and cored borehole data: These maps provided input to geological modelling software for 3-D visualization. A flow simulator was used to study models consisting of up to 500,000 grid cells.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California