Diagenetic Control on Reservoir Quality Within a Single Depositional Unit: Example from Frontier Formation, Powder River Basin, Wyoming

J. M. Beggs, W. L. Bredar

A sandstone unit of the Wall Creek Member of the Frontier Formation was studied in core from two adjacent wells, one an oil producer and the other a dry hole. The wells are 4,500 ft apart; the sandstone is 65 ft lower structurally in the dry hole. Porosity in the pay zone ranges from 13.3 to 16.1% and permeability from 13 to 36 md, whereas in the equivalent interval in the dry hole porosity ranges between 5.5 and 6.7% and permeability between 0.04 and 0.11 md. The interval in both wells is comprised of fine to coarse-grained sandstone, moderately well to well sorted, and characterized by cross-bedding. The sandstone is interpreted to have been deposited in an upper shoreface regime. There are no significant differences between the sandstones in the two wells in terms of d trital mineralogy, texture, or depositional environment. The difference in reservoir quality is due to a greater volume of authigenic quartz cement in the downdip well.

Diagenetic features common to both wells are chlorite grain coatings, plagioclase alteration (including dissolution, albitization, and partial replacement by ferroan dolomite), syntaxial quartz overgrowths (as prisms in the producer but pervasive in the dry hole), and precipitation of intergranular ferroan dolomite. The ferroan dolomite occurs in discrete patches and as pervasive cement in a 2-3 ft thick band at a similar stratigraphic position in each well. There is a general inverse relationship between abundance of chlorite grain coatings and secondary quartz which, based on textural evidence, is at least partly due to chlorite replacement. The abundance of quartz overgrowths in the dry hole is believed to be due to preferential cementation in the downdip parts of the sand unit aft r oil emplacement. The ferroan dolomite-cemented band may mark a former position of an oil-water contact. The carbonate patches are not believed to be relicts of a dissolution event since they are commonly bounded by straight crystal faces within pores.

Because the main control on the existence of good porosity in this reservoir is the inhibition of pervasive quartz cementation, a predictive diagenetic model is very important to further successful exploitation.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.