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Advances in Understanding Devonian and Silurian Subcrop Plays in Illinois

Joan E. Crockett and Randolph D. Lipking

Illinois State Geological Survey, Champaign, IL 61820

Difficulties in correlating old electric logs coupled with paucity of core and sample material has led to generic naming of pays or mis-correlation of pay zones. This practice may have led to misunderstanding of reservoir geometries and compartments in strata at or near the sub-Kaskaskia unconformity, or bypassed pays. Accurate correlation can be an effective tool in exploring for and exploiting productive facies and reservoir compartments. Case studies will be used to show where multiple pays and missed pays may exist. Cross sections will examine the sub-Kaskaskia unconformity’s role in oil play analysis.

Correlation of productive zones at and near the sub-Kaskaskia unconformity has identified areas where generic naming of pays may have led to missed or bypassed pays. For example, both the Middle Devonian Lingle and the Grand Tower producing zones may be present in the same field and contain similar dolomite facies that are productive but are labeled "Devonian" or "Geneva," which can lead to misunderstanding of the nature and distribution of reservoir compartments. In another case study, it appears that production designated as "Devonian" is from at least three distinct Devonian units; the basal Middle Devonian Dutch Creek, and Lower Devonian Clear Creek and Grassy Knob formations. This paper will examine these and other examples that may lead to further field development and discoveries in Illinois, and may be applicable elsewhere.

AAPG Search and Discovery Article #90059©2006 AAPG Eastern Section Meeting, Buffalo, New York