Abstract: Integrated Reservoir Description and Analysis of the Lance Formation at Jonah Field, Sublette County, Wyoming

John W. Robinson, Denise L. Delozier, Ron Flinch

Log, core, and production data from the 16 wells in Jonah field have been used to characterize sandstone reservoirs in the Lance Formation (Cretaceous) in the northern Green River basin. The Lance Fm. is composed of 2500 feet of heterolithic fluvial strata that were deposited on a broad alluvial plain. Sandstones were deposited in east-flowing channels 10-20 feet deep and 150-4000 feet wide; some amalgamated sandstone intervals are >100 feet thick and over a mile wide. Fluvial architecture varies from isolated meandering river deposits to amalgamated braided river deposits. Sandstones are dominantly composed of detrital chert and quartz grains.

The Lance Fm. has been divided into several informal pay intervals that have different reservoir character and performance. Wardell interval sandstones produce gas in eight wells and are poor reservoirs due to fine grain size, high clay and cement content, and greater depth. Yellow Point interval sandstones have shown average performance in five wells. The Jonah interval produces in 10 wells and is the most prolific pay zone with up to 150 net feet of sandstone having core porosity of 8 to 12% and permeability of .01-0.9 mD. Upper and middle Lance sandstones have better than average performance from five wells. All pay intervals require greater than 8% porosity and less than 35% water saturation. Pre-frac pressure build-up analysis indicates insitu permeabilities of 3-20 microdarcys a d suggests that fractures are a significant contributor to deliverability.

Estimated reserves of 0.4 - 4.0 BCFG/well are based on decline curve analysis. Liquid yields vary from 6 to 86 BO/MMCFG and increase with depth. Pressure gradients range from .55 to .59 psi/ft. Reservoir overpressure is a result of continuous migration of hydrocarbons into available pore space via microfracture seepage.

AAPG Search and Discovery Article #90952©1996 AAPG Rocky Mountain Section Meeting, Billings, Montana