Wold, Christopher N.¹, Jay E. Leonard¹, Brett Edwards², Robert Coskey¹, P. Jeffrey Brown³, Marshall Titus¹ 
(1) Platte River Associates, Inc, Boulder, CO
(2) Custer Resources, Evergreen, CO 
(3) Exploration Analysis, Incorporated, Grapevine, TX

ABSTRACT: Reality Checks for Basin Centered Gas Play Risk Assessments

To date, most of the industry evaluated plays based on the probability assessment system for plays published by White and others in the 1980’s and 1990’s. Play shared probability was based on estimated adequacy of charge/migration timing, play independent probability was based on an “average” prospect, and volumes were estimated from an analog field size distribution. Reality checks are critical to correct implementation of play risk analysis. Methods based on petroleum systems modelling techniques are described. 
A stochastic compaction, thermal, kinetic model is used to estimate gas charge from one or more source rocks. Uncertainty of the quality and quantity of organic matter in the source rock are input to the simulator. Other important components include the extent of the pod of active source rock and amount of hydrocarbon retained by the source rock or lost during secondary migration. A fill to spill (P1) migration simulation is used with minimum and maximum scenarios for the extent and richness of source rock to estimate a range of play fetch area sizes. Secondary migration losses are estimated from: 1) total volume of sub economic traps, 2) bypass of the play along faults, 3) inadequate top seal along a lateral migration fairway, and 4) pore saturation in the migration fairway estimated from secondary migration scenarios. The calculated cumulative probability volume distribution of migrated and entrapped hydrocarbon is a reality check on play reserves estimated from the older approach. Examples from the Jonah Area, Green River basin, Wyoming will be presented.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.