Abstract: Reservoir Lithofacies in the Lance Formation, Jonah Field, Sublette Co., Wyoming

ROBINSON, JOHN W. and LESLEY W. EVANS

The Lance Formation (Upper Cretaceous) in Jonah Field consists of up to 2500 feet of heterolithic continental strata, dominantly sandstone and mudstone, with minor conglomerate and coal. Based on observations from four cores in the upper, middle and lower Lance, three reservoir lithofacies are present: 1) Medium grained, massive to cross-bedded chert litharenite is the primary reservoir with average porosity and permeability of 10 - 12 % and 0.1 - 0.2 mD. This lithofacies was deposited in active channels as migrating bars under lower flow regime conditions. 2) Medium grained, disturbed-bedded, shale class, chert j litharenite was originally deposited as lithofacies 1, but was subsequently deformed by slumping. The resulting change in grain sorting and inclusion of shale clasts decrease porosity and permeability to 9 - 10% and .01 - .05 mD, respectively. | 3) Fine grained, ripple and parallel laminated chert litharenite at the top of individual channel-fill units has poor porosity and permeability (6 - 8% and less than .01 mD), and contributes a minor amount to initial production.

The relative contribution from each lithofacies is a function, of matrix quality (rock properties) which decreases predictably with depth. Reservoir volume of each lithofacies is a function of zone thickness and width which vary in the Lance from isolated braided and meandering stream channels to stacked, amalgamated braided stream systems. Few visible fractures were present in the cores and do not contribute significantly to production in this wellbore. The change from calcite cement in the upper Lance to silica cement in the middle and lower Lance significantly degrades reservoir quality.

AAPG Search and Discovery Article #90946©1997 AAPG Rocky Mountain Section Meeting, Denver, Colorado