Deposition Analysis Based on Electric-Log Character of Cypress Sandstone (Chesterian) in Illinois

Beverly Seyler

Four major deposystems were mapped in the Cypress Sandstone using SP (spontaneous potential) and resistivity log character in a state-wide cross-section grid. Illinois was a shallow, gently sloping embayment subject to periodic influx of sediment and changes in sea level during Late Mississippian (Chesterian) time. Cypress log character varies predictably from the eastern margin of Illinois to the western erosional limit. Deposystem interpretations based on log character have identified (1) a deltaic zone characterized by thin sandstones interbedded with shale and siltstone in easternmost central Illinois, (2) an offshore marine zone characterized by sand bodies 50-150 ft thick, displaying blocky SP in a 20-mi wide band in east-central Illinois, (3) an offshore marine zon characterized by sandstone bodies 15-40 ft thick in central Illinois, and (4) an offshore marine zone characterized by predominantly shale log character in west-central Illinois. Presence of the shale coincident with the DuQuoin monocline suggests that Cypress deposition was influenced by structural expression of the monocline.

Most petroleum produced in the Cypress is from facies within the deltaic environment. Detailed log analysis enables interpretation of prodelta, delta front, distributary mouth bar, distributary channel, crevasse splay, interdistributary bay, and reworked marine facies within the deltaic environment. The most productive of these facies are distributary mouth bar, sand-filled distributary channels, and reworked marine sandstones where greater porosities and permeabilities are found. Other deltaic facies may be productive but possess less favorable reservoir qualities due in part to greater clay content. Maps of individual delta lobes may predict the location of productive facies. This type of mapping is proposed as an exploration strategy for stratigraphically entrapped Cypress reservoi s.

AAPG Search and Discovery Article #91023©1989 AAPG Eastern Section, Sept. 10-13, 1989, Bloomington, Indiana.