SEDIMENTOLOGY OF TWO LOWER PENNSYLVANIAN FLUVIO-TIDAL RESERVOIR SANDSTONES IN THE ILLINOIS BASIN

Depositional environment is an important control on reservoir quality because it dictates the internal architecture and heterogeneity of reservoirs. In Lawrence Field in southeastern Illinois, two oil productive Lower Pennsylvanian sandstone reservoirs appear to occupy the same stratigraphic position, but were instead deposited in different depositional environments at different times. Through correlation of geophysical logs and detailed sedimentary facies analyses of several cores, the stratigraphic relationship and depositional environments of the two juxtaposed sandstones were determined.

The Griggs sandstone conformably overlies dark grey shale and consists of up to 10.5 m of fine-grained, planar laminated to cross bedded sandstone that is interpreted to be hyperpycnal flow and unidirectional current deposits indicating deposition into the basin from a nearby fluvial source. Ripple bedded sandstone with reverse cross laminations and rhythmic lenticular bedded sandstone occur at the top of the sandstone and indicate a tidal influence. A widely traceable paleosol and thin coal caps the sandstone. The Griggs sandstone is interpreted to have been deposited in a tidally influenced deltaic setting. Following Griggs deposition, sea level fall initiated fluvial incision of the Griggs sandstone. The resulting valley was filled with the Robins sandstone featuring pervasive lag gravel at its base and up to 44 m of dominantly cross bedded to massively bedded medium-grained sandstone. These facies were deposited under unidirectional flow and as fluidized sediment flows, respectively, and indicate deposition in a braided fluvial system. The Robins sandstone transitions upward into bioturbated, lenticular bedded sandstone, recording the transition to a tidally influenced estuary environment that developed during transgression.

The Griggs sandstone is more heterogeneous and generally poorer quality than the Robins sandstone, which is thicker, coarser-grained, and more homogeneous. The stratigraphic juxtaposition of these disparate facies leads to potential miscorrelation with implications for operators considering EOR techniques. Through detailed correlation and facies analysis, the pitfalls of correlating two sandstone of strikingly different reservoir quality can be avoided.