Mississippian Cypress Sandstone Architecture in the Illinois Basin

B. Seyler and J. P. Grube
Illinois State Geological Survey, Champaign, IL

The Cypress Sandstone has been the focus of considerable research. It is the most widespread siliciclastic unit and the most prolific horizon in the Illinois Basin with production exceeding a billion barrels of oil to date. Results of reservoir characterization research sponsored by the Department of Energy through Plains Illinois, Inc. show regional sandstone trends, portions of which reveal paleotectonic influences. Detailed reservoir architecture of select oil fields shows compartmentalization plays a significant role in development and recovery programs.

The regional Cypress Sandstone architecture was established by constructing a network of cross sections in a statewide grid of more than 2000 wells. Minimum data density was four wells per township. A Cypress net sandstone thickness map shows significant sandstone depositional trends concentrated in and aligned with the present structural low of the Basin. On a regional scale, the trend of thick sandstones appears to have a strong syndepositional structural control. There is evidence that many of the current major structural features in the Illinois Basin also were active during or prior to deposition of lower Chesterian strata.

Cypress Sandstone reservoir architecture at Lawrence Field and many other fields is characterized by multiple, less than 10 foot thick, vertically stacked or shingled, sandstones that range in overall thickness from less than 10 feet to 40 feet. These sandstones form as linear ridges, oriented northeast-southwest and are commonly very compartmentalized. The linear sandstone ridges are analogous to modern- day tidal shoals. The channel-like sandstones that exceed a thickness of 100 feet, are rarely productive. Successful field development and implementation of secondary and tertiary recovery programs in these compartmentalized reservoirs requires detailed reservoir characterization.

AAPG Search and Discovery Article #90900©2001 AAPG Eastern Section Meeting, Kalamazoo, Michigan