Third-order Composite Sequences Research Articles

Lower Visean sea-level changes in the northern Precaspian Basin

This study presents the sequence stratigraphic interpretation of subsurface data in a Lower Carboniferous lagoon setting in the northern boundary zone of the Precaspian Basin. The proposed stratigraphic and facies distribution model of the lagoon fill enables the reconstruction of the relative sea-level history and refines the understanding of the geological evolution and eustatic changes in the Caspi region during the Lower Carboniferous. Semi-circular carbonate mounds were built in the Devonian period and Tournaisian stage enclosing a lagoon with an opening towards the Caspi Sea in a southeast direction. A sea-level fall at the end of the Tournaisian stage terminated the growth of the carbonate buildups; during the Lower Visean (Lower Carboniferous) time, the lagoon was filled with mixed siliciclastic and carbonate sediments. The lagoon fill is interpreted as a third-order composite sequence composed of three fourth-order sequences representing sea-level changes with an increased magnitude and frequency relative to the quiet conditions that prevailed during the previous carbonate buildup in Famennian-Tournaisian. The highstand systems tracts of the fourth-order sequences are likely Waulsortian-type carbonate banks, while the falling-stage and lowstand systems tracts are mostly clastic progradational wedges. The transgressive systems tracts include retrograding shale and grain shoal facies. A prominent falling-stage systems tract was detected in the third fourth-order sequence in the lagoon fill representing a significant sea-level fall in lower Visean time. The falling-stage systems tract is composed of three downstepping-prograding lobes indicating a continuously falling sea level punctuated by minor pauses. Seismic imaging revealed a series of distinct prograding wedges inside the falling stage lobes that are either accretionary bedset or high-frequency nested sequences. The observed fall in sea level during the Lower Visean may be associated with, and it may provide stratigraphic evidence for the beginning of Paleozoic glaciation on Gondwana.

High-resolution sequence stratigraphy of the Upper Ordovician Montoya Group, southern New Mexico and western Texas: Outcrop analog of an unconventional chert and carbonate reservoir

The Upper Ordovician Montoya Group crops out in southern New Mexico and westernmost Texas and records predominantly subtidal deposition on a gently dipping carbonate ramp that was subsequently almost entirely dolomitized. The Montoya Group is a third-order composite sequence composed of six regionally correlative, shallowing-upward, third-order depositional sequences (M0–M5). Sequence M0 has sandstone at its base that is overlain by skeletal packstone-grainstone. Sequence M0 occurs only locally and was likely deposited in a topographic low formed during regional development of the unconformity following El Paso Group deposition. Sequence M1, marking the initial widespread transgression over the Ellenburger unconformity, consists of sandstone updip that passes downramp into skeletal packstone. The highstand systems tract (HST) of M1 consists of a prograding skeletal grainstone that was subaerially exposed upramp. Sequence M2, which contains the second-order maximum flooding surface, has abundant subtidal cherty carbonate at its base, which shallows upward into a widespread, prograding coral packstone-grainstone in the HST. Sequence M3 also contains abundant downramp chert that passes upramp into an aggrading crinoidal shoal and farther upramp into peritidal mudstone. Sequence M4 records an extensive basinward shift in facies as peritidal burrowed and cryptalgalaminated mudstone prograded over subtidal carbonate. Sequence M5 is only locally developed downramp and consists of crinoidal grainstone with abundant evidence of subaerial exposure. A regional unconformity separates the Montoya Group from the Silurian Fusselman Dolostone or younger units. Parasequences (meter-scale cycles) recording low- to moderate-amplitude relative sea level fluctuations are ubiquituous features at individual outcrops but are difficult to correlate regionally. The abundance of syn- or early depositional chert in the subtidal facies indicates that the Montoya Group was deposited within a region of strong regional upwelling along southern Laurentia. This early formed chert was the reservoir facies in a successful Upper Ordovician gas play in Ward and Reeves Counties, Texas.

Discrimination of Local and Global Effects on Upper Mississippian Stratigraphy, Illinois Basin, U.S.A.

ABSTRACT Interpretive cross sections based on detailed descriptions of 33 outcrops and cores are used to better understand the relative effects of tectonics, eustasy, tides, and climate on Upper Mississippian (middle Chesterian) stratigraphy in the tectonically active, tide-dominated Illinois basin. The cross sections show that five mixed carbonate-siliciclastic, high-frequency sequences in the Bethel through Glen Dean formations can be correlated around the outcrop belt through areas with very different subsidence histories. The sequence boundaries are marked by paleosols and incised valleys and can be correlated basin-wide within a framework of distinctive marker beds. Because of its updip position, lowstand systems tracts are not preserved in the Illinois basin. The transgressive systems tracts generally consist of one or two parasequences that are typically composed of tidally influenced quartz sandstone filling incised valleys at the base overlain by open-marine skeletal limestone, shallow-marine shale, and heterolithic siliciclastic tidal-flat deposits. The maximum flooding surface (MFS) for the sequences is picked at the base of the deepest water limestone facies. Highstand systems tracts are composed of 1 to 6 regressive parasequences that consist of basal offshore skeletal limestone capped by laterally extensive shale and heterolithic siliciclastic tidal-flat facies. The basin-wide extent of the sequence boundaries and maximum flooding surfaces across tectonic highs and lows suggests that the sequences were produced by eustatic sea-level changes rather than local tectonics or autogenic processes. The sequences were likely produced by moderate- to high-amplitude (30-100 m) fourth-order ( 400 ky) glacio-eustatic sea-level changes driven by the transition from the greenhouse conditions of the Early Mississippian to the icehouse conditions of the late Paleozoic. The lateral extent and frequency of component parasequences suggests that they were likely produced by fifth-order sea-level changes (10-100 ky). The sequences may be bundled into third-order composite sequences, but the third-order signal is obscured by the magnitude of the fourth-order sea-level changes a feature typical of ice-house stratigraphies. The sequences can be used as time slices to identify spatial and temporal variations in differential subsidence between the Cincinnati Arch and the more rapidly subsiding Basin Interior. Episodes of high and low differential subsidence occurred every two to three sequences. These subsidence variations had a major impact on lithofacies distribution and onlap and offlap geometries in sequences and parasequences. The occurrence of some widespread seismically disturbed beds suggests that active faulting occurred during deposition. Normal faulting appears to have occurred during periods of high differential subsidence and reverse faulting during periods of low differential subsidence. Differential subsidence and related normal and reverse faulting may have occurred in response to phases of thrust loading and quiescence in the Appalachian orogenic belt to the east. Even in this tectonically active setting, however, it is the eustatic signal that generates basin-wide, mappable stratigraphic sequences.

Reservoir Anatomy within a Sequence Stratigraphic Framework: The Pennsylvanian Breathitt Group of Eastern Kentucky

Detailed quantitative sequence stratigraphic analysis and three-dimensional (3-D) modeling of the fluvio-deltaic Pikeville, Hyden, and four Corners formations of the Breathitt Group, eastern Kentucky, indicate that it is possible to make confident deterministic correlations of sand bodies at offshore development well spacings. The key to high-confidence correlation is the delineation of the 3-D architecture of third-order composite sequences, which are seismically mappable, and the recognition of internal fourth-order sequences, which are stacked into lowstand, transgressive, and highstand sequence sets. Volumetric partitioning of reservoir sand bodies between systems tracts and sequences allows prediction of channel sandstone geometries and channel: crevasse splay ratios. Furthermore, channel-fill type is predictable from the sequence stratigraphy. Within 13 studied fourth-order sequences, incised valley fills represent the main reservoirs, which are laterally continuous over tens of kilometers. Sand bodies within transgressive and early highstand systems tracts are usually filled by siltstones or heteroliths and have geometries less than 1.0 km in width. Within fourth-order sequence sets, incised valley fills are sometimes vertically stacked to produce anaomalously thick fluvial sand bodies. These are the prime potential producing zones within coastal plain reservoirs. High-resolution sequence stratigraphy thus provides a deterministic approach to correlation and 3-D modeling. Use of themore » Stratamodel geocellular modeling system permits circulation of connectivities for different systems tracts and sequence sets, thus allowing the development of conceptual models for perforation policy in complex [open quotes]jigsaw-type[close quotes] reservoirs.« less

Inferred glacio-eustatic control for high-frequency depositional sequences of the Plio-Pleistocene Kazusa Group, a forearc basin fill in Boso Peninsula, Japan

The Plio—Pleistocene forearc basin fill of the Kazusa Group, as much as 3000 m thick, is represented by a series of marine environments ranging from deep-sea basin plain to shallow sea. It developed during about 2.4 to 0.45 Ma, and is well exposed in the Boso Peninsula, Japan. Chronostratigraphic resolution of the Kazusa Group is partly within about 0.05 m.y., based on the combination of fission track dating of volcanic ash layers, and bio- and magneto-stratigraphy. Many volcanic ash layers are intercalated in the Kazusa Group and provide distinct chronostratigraphic surfaces within the heterogeneous lithologies. Detailed outcrop studies on depositional facies discontinuities and lateral terminations of volcanic ash layers allow high-resolution sequence stratigraphic interpretation of the Kazusa Group. Seventeen depositional sequences were identified in the Kazusa Group. Each depositional sequence was tentatively correlated with the stable oxygen isotope sea-level index, integrated with the age framework of the Kazusa Group. This correlation indicates that frequencies of the depositional sequences can be estimated to be in the range of 0.2-0.02 m.y. and are within fourth- and fifth-order stratigraphic cyclicities. Depositional sequences constitute third-order transgressive and highstand sequence sets and the Kazusa Group is interpreted to represent a third-order composite sequence with a duration of about 1.95 m.y. Development of the third-order composite sequence was also controlled by an overall rise and fall of glacio-eustatic sea levels, overprinted by tectonic events related to fluctuations in oceanic plate motion which occurred at about 2.6 Ma and 0.5 Ma, respectively.

High-frequency sequences and their stacking patterns: sequence-stratigraphic evidence of high-frequency eustatic cycles

A hierarchy of interpreted eustatic cyclicity in siliciclastic sedimentary rocks has a pattern of superposed cycles with frequencies in the ranges of 9–10 m.y., 1–2 m.y., 0.1–0.2 m.y., and 0.01–0.02 m.y. (second- through fifth-order cyclicity, respectively). Stratigraphic units displaying this cyclicity include composite sequences, sequences, and parasequences. On the Exxon global cycle chart, fundamental third-order cycles (1–2 m.y. average duration) stack into related groups (second-order cycles: 9–10 m.y. duration). A much larger pattern (about 200 m.y.) is interpreted as tectonically controlled eustasy probably related to sea-floor spreading rates.One and probably two higher orders of cyclicity (fourth-order: 0.1–0.2 m.y.; and fifth-order: 0.01–0.02 m.y.) are now observed in work with well logs, cores, and outcrops in areas of very rapid deposition. These frequencies are in the range of Milankovitch cycles, and may represent part of the Milankovitch hierarchy which has been widely interpreted for cyclical units in carbonate rocks.High-frequency (fourth-order) sequences, which form at a 0.1–0.2 m.y. cyclicity, have all the stratal attributes of conventional sequences, including constituent parasequences and systems tracts, and play a dominant role controling reservoir, source, and sealing rock distribution. A consistent hierarchy of stratigraphy is observed. Parasequences (probable fifth-order cyclicity) stack into sets to form systems tracts in fourth-order sequences. Groups (sets) of fourth-order sequences are deposited between major third-order boundaries within third-order composite sequences. Sequences in these sets stack in prograding and backstepping patterns to form third-order lowstand, transgressive, and highstand sequence sets.Third-order sequence boundaries are marked by greater basinward shifts in facies, by larger more widespread incised valleys, and by more extensive onlap than are fourth-order sequence boundaries. Third-order condensed sections commonly are widespread, faunally rich, and widely correlated biozone and mapping markers. Fourth-order sequence analysis helps to understand reservoir, source, and seal distribution at the play and prospect scale. An example from the Gulf of Mexico is discussed.