Sandstones in the Chesterian (Late Mississippian), including those in the Cypress, Ridenhower, and Bethel, are major oil producers in the Illinois basin. Significant oil reservoirs occur as structurally influenced stratigraphic traps along the northwest-trending La Salle anticlinal belt of Lawrence and Crawford Counties, Illinois. Analyses of electric logs, sand thickness maps, and sedimentary structures show that these sandstones were deposited in tide-dominated deltaic, tidal-flat, and subtidal environments.

Coarsening upward, tide-dominated deltaic sequences inferred from spontaneous potential (SP) log signatures are composed of prodelta shales, delta-front shales and silts, and distributarymouth bar sands.

Tidal-flat sand bodies are indicated by SP log signatures with blunt bases and tops. Cores from these 20 to 70-ft (6 to 21 m) thick, laterally discontinuous units contain: (1) fine to medium-grained, rippled sandstone with infrequent rippled shale laminations, (2) lenticular bedding with little bioturbation, (3) flaser bedding, (4) bioturbated sandstone and shale that were apparently horizontally bedded, (5) plant fragments in shale, and (6) channel lag consisting of deformed shale clasts and rounded carbonate mud pebbles.

Off the flanks of the anticlinal belt, the Cypress, Ridenhower, and Bethel coalesce into single massive sand units up to 200 ft (61 m) thick, distinguished by blocky SP log signatures with abrupt bases and tops. Sand thickness maps showing these units as long linear bodies aligned parallel with major anticlinal axes suggest that they are subtidal sand ridges.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not currently have access to this article.