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AL-TAWIL, AUS A. and J. FRED READ, Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-025O.

ABSTRACT: Regional Characteristics of 5-10 M Scale Previous HitCarbonateNext Hit Previous HitCyclesNext Hit of Late Mississippian Greenbrier Depositional Previous HitSequencesNext Hit, West Virginia

The 0-500m thick Late Mississippian Greenbrier Group (Big Lime) is a major hydrocarbon reservoir in the Appalachians. It consists of ramp carbonates that contain several depositional Previous HitsequencesNext Hit (tens of meters thick) of transgressive shaly/muddy carbonates, skeletal/oolitic grainstones and red beds. Sequence boundaries updip have erosional unconformities, caliches, breccias and channelling, whereas downdip they are relatively conformable and overlain by lowstand red beds. The Previous HitsequencesNext Hit contain high frequency, 5-10 m Previous HitcarbonateNext Hit Previous HitcyclesNext Hit, possibly of 100 to 400 KY duration. Updip, few Previous HitcyclesNext Hit are developed. These consist of grainstone-dominated facies with sharp/erosional tops, locally developed beach facies, eolianites and caliches. Tidal flat facies are rare and thin. In the thicker downdi sections, there are more Previous HitcarbonateNext Hit Previous HitcyclesNext Hit. These are dominated by skeletal packstone grading up into skeletal/oolitic grainstone, and some thick (up to 8 m) tidal flat laminated dolomites. The oolites likely are sheets to tidal bar sands and channelled bodies. Oolites are rare in the furthest downdip sections on the ramp. Downdip cycle boundaries rarely have caliches, but erosional and microkarstic surfaces are common. Comparison of the observed sections with computer generated synthetic cross sections suggests that the Previous HitcyclesNext Hit were developed under moderate amplitude (tens of meters) high frequency Milankovitch eustatic Previous HitseaNext Hit Previous HitlevelNext Hit fluctuations dominated by 100-400 KY periods, which mark the onset of the Late Carboniferous Gondwana glaciation. The modelling closely simulates limited updip ycle development, and abundant cycle development downdip, along with locally well developed lowstand tidal flats. Even though high frequency Previous HitseaNext Hit Previous HitlevelTop changes exposed the platform many times, the semi-arid climate limited diagenesis to minor, near surface dolomitization and little subaerial cementation. Consequently, the oolite porosity is dominantly interparticle, supplemented by microporosity.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.