Sequence Stratigraphic Control on Reservoir Development: The Middle Silurian Racine Formation in the Sangamon Arch, West-Central Illinois
Several prolific dolomite petroleum reservoirs occur in the upper part of the Middle Silurian Racine Formation in the Sangamon Arch, west-central Illinois. Detailed facies and sequence stratigraphic analyses, and well to well correlation of the Racine Formation, along the southeastern flank of the Sangamon Arch, have indicated that the middle part of the Racine Formation is erosionally truncated and subsequently overlapped by the younger Silurian deposits throughout the study area, signifying a pronounced unconformable boundary. This prominent intra-Racine unconformity subdivides the Racine Formation into two depositional sequences comprising several carbonate reservoir units within the highstand systems tracts.
The reservoirs are characterized by porous dolomitized lime mudstone to grainstone and reef/reef rudstone facies. They contain partially dolomitized echinoderm fragments and molds of crinoids, brachiopods and corals, suggesting deposition in a normal shallow marine environment. The reservoir facies were deposited along a southwest trending ramp margin that was roughly parallel to the Sangamon Arch trend and graded basinward into muddy open marine carbonates below wave base. The dolomite reservoirs are generally interlayered with laterally extensive, impermeable limestone beds displaying cyclic reservoir- non-reservoir packages. They constitute the upper parts of small scale transgressive-regressive cycles within the highstand systems tracts, suggesting sea level fluctuations and percolating seawater as the primary controls for early dolomitization. This interpretation is supported by: (1) gradual loss of porosity laterally; (2) the sharp upper contacts and gradational lower contacts of the reservoir bodies, and (3) resistance of the early-formed dolomite to chemical compaction during burial diagenesis, thus preserving the early original porosity. Depositional, diagenetic and sequence stratigraphic models developed in this study provide important predictive tools for petroleum exploration.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California