--> --> Abstract: Where are All Those Darn Reefs? An Appraisal of the Potential for More Silurian Reef Discoveries in the Illinois Basin, by R. M. Cluff and C. M. Murphy; #90926 (1999)

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CLUFF, ROBERT M. and CATHERINE M. MURPHY , The Discovery Group, Inc., 1560 Broadway, Ste 1470, Denver, CO 80202-5150 ([email protected])

Abstract: Where are All Those Darn Reefs? An Appraisal of the Potential for More Silurian Reef Discoveries in the Illinois Basin

Intensive subsurface and seismic exploration in southern Illinois has resulted in at least 26 known reef discoveries over a 90-year period. Many other small oil fields in Illinois are possibly reef-related, but have not been proven by drilling or seismic to be underlain by Silurian reefs. The last significant reef discoveries were made in 1973-74 after an intensive 2-D CDP seismic effort by several major oil companies. These data have subsequently been re-sold and re-interpreted numerous times, but no additional large reefs have been found.

The average recovery from Silurian reef fields is (approx.) 5800 bbls/ac or 116 MBO per 20-ac location, which is substantially above the typical well recovery in this state. The compelling economics of reef production draws many new players to the reef play. With a steady stream of new explorers and new ideas, it begs the question: "Where are all those dam reefs, and why aren't we finding new reefs all the time?"

Consideration of the 2-D seismic density over the reef trend suggests most if not all reefs over 160 acres in size have been discovered, and the remaining target size is < 80 acres. This is based on: 1) average shooting density on 2 x 2 mile CDP grid; 2) reefs are roughly circular in shape; 3) reefs are randomly distributed with respect to the seismic grid; 4) 1970 vintage 2-D data would detect a reef if it crossed the reef. From simple geometric analysis, this exploration effort should have found 100% of the reefs >640 ac in size, 80% of the 320 ac reefs, 60% of the 160 ac reefs, and 40% of the 80 ac reefs. In fact the 1970's efforts found three fields: Nashville (~620 ac), Brubaker (70 ac) and Lillyville North (50 ac). Reefs become increasing difficult to identify on noisy seismic data as they get smaller, so the probability of detection goes down and assumption #4 is violated. In our opinion the industry is and has always been finding reefs, but they are mostly very small fields. The exploratory effort associated with defining such tiny reef targets is an economic problem. 

AAPG Search and Discovery Article #90926©1999 AAPG Eastern Section Meeting, Indianapolis, Indiana