The Second-Order Cycle, Carbonate Platform Growth, and Reservoir, Source and Trap Prediction
SARG, RICK, JIM R. MARKELLO, and L. JIM WEBER
A distinctive 2nd-order (10-20 MY) stratigraphic signature is present within ancient, evaporite-related, carbonate platform successions. Dominant controls include basin geometry, subsidence, sea level and paleoceanography. The most normal, open marine conditions occur during the transgressive component of second-order cycles. Source-prone, organic-rich lime mudstones are deposited across shelves and in the basin. As shelfal areas become more restricted during the 2nd-order regression, highstand platforms are dominated by nonskeletal bank margin deposition. Reservoir and trap conditions are best developed during the transgressive and turnaround period. When platform progradation accesses a deeper basin or evaporitic conditions, the biota changes, marine cementation increases, and reservoir-quality is poor.
The Late Permian supersequence (10-12 My) in the Permian basin, West Texas, USA is an asymmetrical cycle. Normal marine conditions occurred over a wide area during transgression, and skeletal grain bank margins exhibit the most diverse biota. Deepwater, draping lime mudstones are source-prone. The initial regressive highstands are characterized by prograding nonskeletal ramp-margins with excellent reservoir-quality. Late regressive highstands platforms developed cemented, upright margins and have poor reservoir quality.
The middle Pennsylvanian (Desmoinesian) in the Paradox basin of the Four Comers region, USA is a symmetrical cycle (8-10 My). Transgressive systems tracts contain black laminated, organic-rich lime mudstones. Highstand systems tracts of the 2nd-order transgression are characterized by thick, porous mound facies and skeletal grain banks. Highstands of the regressive turnaround contain thin algal buildups, and abundant, porous nonskeletal grain banks.