Evaluating the Reservoir Potential of Mississippian Structures

AHR, WAYNE M., Texas A&M University, College Station, TX

At least five outcomes are possible from drilling a Mississippian structure in north Texas: no porosity or four different kinds of porosity that could have originated as the result of depositional, diagenetic, or tectonic processes. Reservoir porosity may or may not be facies selective. Examples of facies-selective reservoirs are found at Conley, Thrash, Dick Scott, and Northeast Shackelford fields in Texas. Lisbon field, Utah, and Quanah City field, Texas, are non-facies-selective reservoirs. A successful evaluation strategy will involve the following steps: (1) mapping present-day structure; (2) creating paleostructural overlays; (3) having some knowledge of tectonic and sedimentary trends within the basin; and (4) having some knowledge of diagenetic style at target level within the basin. End-member pore types in Mississippian reservoir include (1) facies-selective, depositionally controlled pores in bryozoan-crinoid sands; (2) facies-selective porosity in mud-mound reefs; (3) nonselective fracture porosity that is best developed in dolomitic mudmounds, and (4) diagenetically formed porosity that may or may not be facies selective. All four reservoir types have one thing in common: they are associated with paleostructural highs. Present structural highs that fail the paleostructure overlay test will almost surely be dry holes. Predicting the pore type after passing the paleostructure overlay test requires some knowledge of subsurface geology from lithologic logs, the best of which come from core studies.

AAPG Search and Discovery Article #91011©1991 AAPG Southwest Section Meeting, Abilene, Texas, February 9-12, 1991 (2009)