Reservoir characterization of Lower Pennsylvanian sandstones for the application of ASP flood technology in Lawrence Field, Illinois

The alkali-surfactant-polymer (ASP) enhanced oil recovery (EOR) method is currently being applied in the lower Pennsylvanian Bridgeport sandstone of Lawrence Field, a 400 million barrel, mature producing field in southeastern Illinois. Detailed reservoir characterization has delineated flow units and permeability barriers that identify target zones for the application of ASP flood technology. Siliciclastic rock dominates lower Pennsylvanian strata in Lawrence Field. Deposition occurred unconformably over older Mississippian strata in an area of active uplift along the southern reaches of the LaSalle Anticlinorium. Contemporaneous tectonic influence in the area on both the deposition of the Bridgeport sandstones and reservoir characteristics is just beginning to be clearly understood. Detailed mapping of the subsurface geometry of Bridgeport sandstone reservoirs, as well as knowledge of their petrophysical characteristics, are critical to the success of an ASP type flood. Mapping with thousands of geophysical logs of various vintages has revealed that throughout the northern part of the field, a cyclical series of lenticular sandstone bodies, typically around 30 feet thick, interdigitate with non-reservoir siltstones and shales to create confined reservoirs ideal for the successful application of the ASP EOR technique. Those lenticular reservoirs are further subdivided into compartmentalized flow units that are bounded by thin shaley intervals that may or may not permit communication between compartments. Elsewhere, these more lenticular reservoirs appear to be eroded and replaced with younger, much thicker channel fill deposits of clean sandstone in some areas and low energy, fine grained sediments in others. Examination of nearly 4000 feet of core has revealed sedimentological features of both the lenticular sandstone bodies and the channel fill facies. Petrographic analysis further illuminates reservoir characteristics by showing the importance of the diagentic overprint and areas where high permeabilities create potential thief zones. Finally, recasting the Bridgeport strata in a sequence stratigraphic framework by defining regionally extensive sequence boundaries is helping to clarify how the Bridgeport reservoirs relate to one another and in which environment they were deposited. Success of ASP flood technology in Lawrence Field would encourage similar EOR projects in comparable fields throughout the Illinois Basin and around the world.