Contrasting Styles and Common Controls on Middle Mississippian and Upper Pennsylvanian Carbonate Platforms
in the Upper Midcontinent, U.S.A.

W. Lynn Watney, Evan K. Franseen, Alan P. Byrnes, and Susan E. Nissen
Energy Research Section, Kansas Geological Survey, The University of Kansas
1930 Constant Avenue - Campus West, Lawrence, Kansas 66047

Abstract

Middle Mississippian Osagean shelf and shelf margin lithofacies bordering the northern Anadarko Basin are dominated by heterozoan carbonates including sponge-spicule rich facies and early formation of chert. Chert increases southward from nodules in shallow, restricted inner shelf areas to extensive, thick chert beds deposited on an open marine shelf margin. Decameter-scale sequences are commonly delineated by glauconite beds and subaerial exposure. Early to post-Mississippian subaerial exposure diagenesis resulted in sponge spicule dissolution, vuggy porosity in moldic-rich rocks, and autobrecciation.

In contrast, Upper Pennsylvanian carbonates contain abundant photozoan carbonates and are dominated by decameter-scale, unconformity-bounded sequences deposited on a shallow inclined ramp and shelf margin. Widespread oolite and grainstone shoals were commonly deposited during punctuated, high frequency forced regressions where early subaerial exposure resulted in pervasive moldic lithofacies and paleosol development.

 

Both carbonate settings were affected by block fault movement at a scale of 1-10's of km, linked to basement lineaments. Block faulting influenced locations of shelf edges and caused segmentation of the ramp/shelf profile, which created differences in lithofacies, thicknesses, and stratal architecture across segments. Recurrent structural movement created blocks where strata were preferentially preserved, locus areas for thick accumulations of sponge spiculite and oolite reservoir deposits, and topographic highs that were subjected to subaerial exposure. Later meteoric water influx and possible basinal fluid migration are also closely associated with lineaments. Integrated geological, petrophysical, and geophysical studies are providing a clearer picture of the complex creation of hydrocarbon plays and the variable reservoir properties of each system.

Importance/Implications

A goal of this poster is to demonstrate the importance of continuing reactivation of structural elements in shelf-shelf margin areas throughout the Paleozoic that impacted deposition and diagenesis and resultant reservoir properties. Continued recognition of the impact of these reactivated structures during Paleozoic deposition is improving our understanding and prediction of hydrocarbon pay in these reservoirs.

1. Structural reactivation is apparently common and leads to stacked pays at field scale and the coincidence of hydrocarbon plays.
2. True stratigraphic traps of economically producible hydrocarbons are probably fewer than believed and accordingly structural constraints set limits and additional guidelines for future resource play development and resource assessment.
3. Reservoir prediction necessarily involves factoring in complexities of linked structure, sedimentation, diagenesis, and post-depositional preservation of pore space.
4. Structural controls on reservoirs at all scales will be better modeled when strata are quantitatively characterized as mechanical units.
   

The relative roles of processes including deposition, diagenesis, and structure probably need to be refined to improve prediction of ever-decreasing field size in spite of the use of more efficient and effective technologies. Moreover, refined geologic models are necessary to keep up with technology.

 

Summary

Middle Mississippian Osagean

• Middle Mississippian Osagean shelf and shelf margin lithofacies dominated by heterozoan carbonates including sponge-spicule rich facies and early formation of chert
• Chert increases southward from nodules in shallow, restricted inner shelf areas to extensive, thick chert beds deposited on an open marine shelf margin
• Decameter-scale sequences are commonly delineated by glauconite beds and subaerial exposure
• Early to post-Mississippian subaerial exposure diagenesis resulted in additional sponge spicule dissolution, vuggy porosity in moldic-rich rocks, and autobrecciation

Middle and Upper Pennsylvanian carbonates

• Abundant photozoan carbonates
• Dominated by decameter-scale, unconformity-bounded sequences
• Deposited on a shallow inclined ramp and shelf margin
• Widespread oolite and grainstone shoals were commonly deposited
• Punctuated, high frequency forced regressions where early subaerial exposure resulted in pervasive moldic lithofacies and paleosol development

Block Fault Movement

• Mississippian and Pennsylvanian carbonate settings were affected by block fault movement at a scale of 1-10's of km
• Deformation closely linked to reactivation of basement lineaments
• Block faulting influenced locations of shelf edges and caused segmentation of the ramp/shelf profile
• Distinct localized differences in lithofacies, thicknesses, and stratal architecture across segments
• Preferentially preserved, locus areas for thick accumulations of sponge spiculite and oolite reservoir deposits
• Topographic highs that were subjected to subaerial exposure
• Later meteoric water influx and possible basinal fluid migration are also closely associated with lineaments
• Reactivation often leads to stacked pays at field scale and plays at regional level
  

Integrated geological, petrophysical, and geophysical studies are providing a clearer picture of the complex creation of hydrocarbon plays and the variable reservoir properties of each system.

 

Controls on Carbonate Platform and Reef Development II: A Session in Honor of Dr. Wolfgang Schlager, 2005 AAPG Annual Convention (June 19-22, 2005) Technical Program

Lead-in to Rest of Poster

Mississippian and Pennsylvanian reservoir strata in the Midcontinent have been studied extensively, and the above documented controls are fairly well understood and have aided in production of these intervals. However, there are patterns, questions, and production problems that are still not fully understood. In addition to the above controls on reservoir character of these strata, our ongoing studies indicate that local structures (e.g. faults, fractures, lineaments) that segmented these broad shelf and shelf-margin areas, and that were variously re-activated throughout the Paleozoic, are important elements that controlled depositional patterns, paleotopography, weathering intensity, and movement of fluids. The contribution of re-activated (and syn-depositional) structural elements to reservoir character are as critical as commonly thought of processes such as sea-level history and post-depositional tectonism.