A distinctive second-order (5–20 m.y.) stratigraphic signature is present within upper Paleozoic, evaporite-related, carbonate-platform successions of the Paradox and Permian basins in the southwestern United States. This supersequence scale of cyclicity occurs under icehouse and transitional conditions and in synorogenic and postorogenic phases of basin development and appears to control the depositional and stratigraphic-trapping components of petroleum systems in these basins. Primary controls on the development of second-order architecture include basin geometry and subsidence patterns, sediment supply, and long-term sea-level changes. Climate and paleoceanography are important controls on facies development within sequences. Supersequences are defined as long-term transgressive-regressive cycles that may be symmetric or asymmetric and that are composed of third-order sequences.
The most normal, open-marine depositional conditions occur during the trasgressive phase of second-order cycles. Organic-rich, source-rock-quality (source-prone) lime mudstones are deposited across shelves and in the basin. Carbonate reservoirs are best developed during second-order transgression and turnaround to highstand. Open-marine conditions and diverse skeletal-grain bank deposition occur during third-order highstands of second-order transgressions. Early marine cement is relatively minor in occurrence, and reservoir quality is excellent. During the second-order turnaround and early regression, shelf areas become more restricted, and third-order highstands are dominated by nonskeletal-grain (ooid and peloid) bank-margin deposition. During the later stages of second-order regression, if platform progradation accesses a deeper basin or encounters elevated salinities, or if siliciclastic input increases, the biota become more specialized, marine cementation increases, and reservoir quality is reduced.
The Lower Pennsylvanian (Morrowan-lower Missourian) supersequence, in the Paradox basin of the Four Corners region, United States, is a symmetrical cycle of approximately 20–22 m.y. duration and is composed of third-order composite depositional sequences, six of which are present in the Paradox basin. Third-order transgressive systems tracts contain organic-rich, black, laminated lime mudstones. Third-order highstand systems tracts of the second-order transgression are characterized by thick mound facies containing a diverse biota and skeletal grainstone banks. Third-order highstands of the second-order turnaround are composed of thick aggradational to progradational stacks of cyclically deposited, relatively thin, phylloid-algal buildups and areally extensive, porous nonskeletal grainstone banks. Third-order highstands of the second-order late-regressive phase are characterized by abundant siliciclastics and nonporous crinoid-rich, skeletal grainstone banks.
The Upper Permian (Guadalupian) supersequence in the Permian basin, west Texas, United States, is an asymmetrical cycle, consisting of 11 third-order depositional sequences; it was deposited over a period of about 10 to 12 m.y. Normal-marine conditions existed over a wide area during second-order transgression, and skeletal-grain bank margins exhibit the most diverse biota. Deep-water lime mudstones are source prone. The third-order highstands of the initial second-order regressive phase are characterized by prograding ooid and peloid ramp margins with excellent reservoir quality. Cemented, upright margins of organic-cement boundstone with steep (35°–40°) foreslopes and poorer reservoir quality developed during the late stages of second-order regression.
Postdepositional compactional drape of turnaround third-order highstand bank margins over underlying structural blocks, combined with shelfward facies changes to nonporous inner-shelf strata, provides a combined structural and stratigraphic trapping configuration for large petroleum accumulations. Stratigraphic traps form (1) during second-order transgressions where isolated platforms and buildups develop and are draped by late transgressive mudstones and (2) in shelf settings during the transgression and turnaround at facies changes from porous carbonate sandstones to nonporous lagoonal dolomites and evaporites.