1999 Conference Abstracts

back to list...

1999 Abstract: Ahmad and Saller

Geological Heterogeneities and Reservoir Model Based on Facies Architecture of Middle Mississippian Salem Grainstone Shoal, Illinois Basin

Nadeem Ahmad
Department of Geological Sciences
Indiana University
Bloomington, IN 47405 (USA)

Arthur H. Saller
Unocal Corporation
14141 Southwest Freeway
Sugarland, TX 77478 (USA)


Internal architecture, three scales of reservoir heterogeneity, and porosity-permeability compartments can be reconstructed for Middle Mississippian Salem Limestone reservoirs of the Illinois Basin by integrating bedform architecture, depositional fabrics, and trends of porosity development. In sawed quarry walls of south-central Indiana, excellent exposures of the Salem Limestone allow detailed mapping of a wide range of bedforms representative of an extremely dynamic hydrographic regime similar to that of the shoreface zones of siliciclastic shelves. Echinoderm- and bryozoa-rich grainstones and fossiliferous oolitic grainstones are the predominant depositional facies and were deposited as subtidal shoals in an upper shoreface setting. Vertical and spatial relationships of facies and bedforms reveal hierarchies of depositional units and bounding surfaces that have been used to reconstruct four architectural packages (AP’s) of the scale of mesoforms.

AP-I is a channelized mesoform that defines a NNE- SSW intrashoal channel. AP-II is a large storm-deposited sandwave complex that filled the channel. AP-III is a dark- gray grainstone/packstone that grades laterally into packstones and was deposited in a relatively low energy setting. AP-IV consists of oscillatory trough cross-stratified megaripples of oolitic grainstones. Numerous stylolites occur and add to the mesoscale heterogeneity. Together, microfacies, bedforms, and bounding surfaces compartmentalize porosity and permeability and lead to micro- and meso- scale heterogeneities within the mesoform scale architectural packages.

Lessons learned from such outcrop models can be applied to the producing Salem reservoirs in the subsurface. Here, in the distal parts of the inner ramp, the shoal complex is even more compartmentalized because each AP is represented by a "complete" suite of upward-shallowing facies: a basal outer shelf sponge-spicular wackestone, a main shoal grainstone, and an overlying restricted marine lagoonal wackestone. When integrated with mercury-injection capillary-pressure and production data, these packages and respective scales of reservoir heterogeneity help delineate flow units for reservoir characterization and simulation purposes and determine how the reservoir facies will drain during primary and secondary recovery. The results can also be used to reduce uncertainty (geologic risk) during the pre-drill economic assessment, field appraisal, and development phases.