Silurian Reef Reservoir Heterogeneity in the Illinois Basin with a Focus on Germantown and Wapella East Fields. Reservoir Quality Variability as a Result of Differences in Depositional Environments, Diagenesis and Erosional Truncation at the End of the Silurian

Abstract

As part of an ongoing investigation of the Silurian Reef reservoir systems in the Illinois Basin, we focus on Germantown and Wapella East Field in the Illinois Basin. The Wapella East Field was discovered at a depth of 1,112 feet on December 2, 1962. The discovery well was the T. P. Kiley #1 well with an initial production of 154 BOPD. Commercial oil production has been from Silurian reef-associated carbonates. The Wapella East has produced more than 4.1 MMBO. It has an areal extent of approximately 550 acres. The maximum observed gross pay interval is ~73 feet, and the thickest observed net pay (greater than 10% porosity) is ~49 feet. Measured core porosities ranged from 3.9% to 26.5%. Permeabilities ranged from 2.5 md to 6,670 md. A total of 45 wells have been drilled in the productive portion of the field with only four wells penetrating the complete reservoir interval. Less than half of these wells penetrated greater than 10 feet into the reservoir. There has been limited pressure support from the underlying aquifer. The reservoir facies consists of a dolomitized reefal facies similar to those in Silurian reef reservoirs of the Michigan Basin. Erosional truncation at the top of the Silurian in the East Wapella field is interpreted to have been significant. Silurian reef field complexes of the Illinois Basin Western Shelf are very different from the Wapella East Field model. They produce from depths ranging from 2,000’ – 3,000’. They ubiquitously consist of a reef core of limited extent (i.e., 10-40 acres) composed of coral-stromatoporoid boundstones with separate-vug porosity, some of which are connected by fractures. Primary reservoir quality in these facies is generally reduced as a result of early submarine cementation that occludes much of the initial pore space. Producing wells from these facies have low cumulative oil production. These reef masses are capped and rimmed by crinoid lime grainstones, all tightly cemented, that grade laterally and downslope into avalanche deposits of several fining-upward cycles of crinoid dolomitic lime-dolomite packstones and wackestones. These packstone/wackestone packages are the most productive zones in the reef complex, and production per well is upwards of 250,000 BO. A limited porosity and permeability data set from Germantown Field, the most detailed of all the reef studies we’ve done, indicate porosity and permeability range from 7 % to 25% and 2 and 66 md, respectively, for these facies. Dolomite in this reservoir system probably was formed early in the burial history of the reefs, but reservoir quality appears to have been enhanced by later-stage burial dissolution and multi-stage dolomitization. The internal architectures of the Silurian reef complex reservoirs in the western and northern Illinois Basin are very heterogeneous. Secondary recovery programs in the form of re-drilling the reservoir complex, and implementation of waterflood programs can potentially make these fields profitable again.

AAPG Datapages/Search and Discovery Article #90373 © 2019 AAPG Eastern Section Meeting, Energy from the Heartland, Columbus, Ohio, October 12-16, 2019