Sequence Stratigraphic Model Research Articles

Comparative sequence stratigraphy and organic geochemistry of gas shales: Commonality or coincidence?

Comparison of some Paleozoic and Mesozoic gas shales has revealed a generally common sequence stratigraphy, consisting of, from the base upward: combined sequence boundary/transgressive surface of erosion (SB/TSE) upon which sits a transgressive systems tract (TST) enriched in organic matter, then a somewhat ‘cleaner gamma-ray’ highstand or regressive systems tract (HST/RST). When resolvable, this stratigraphy occurs at 2nd, 3rd, and 4th order relative sea level cyclicity, forming couplets of relatively organic-rich (TST and condensed section-CS) and organic-poor (HST/RST) strata at these time scales. A higher order sequence stratigraphy can be developed for Mesozoic shales than for Paleozoic shales owing to greater biostratigraphic age resolution of the former. The higher frequency cycles are superimposed upon lower frequency cycles, giving rise to a complex stratigraphy of several couplets of differing thickness and distribution. These documented common characteristics provide the basis for a unifying, general sequence stratigraphic model for shales which can be utilized to identify, predict and map the most productive facies within a shale stratigraphic interval.There also are several commonalities in geochemistry of gas shales. Prolific gas shales are usually >200 ft (65 m) thick, contain >3% TOC, have Hydrogen Index values >350 mg HC/g, contain Type II kerogen and have organic maturity values >1.1% Ro. Organic-rich shales typically contain minerals such as pyrite and phosphates (apatite) and show biomarker ratios (pristane/phytane, steranes distribution, C13–C20 regular isoprenoids and C18 aryl isoprenoids) indicative of anoxic conditions during source rock deposition. In high maturity areas, there is a reversal in ethane isotope abundance owing to secondary cracking of hydrocarbons; such a reversal is normally associated with best gas production. Relative hydrocarbon potential (S1 + S2/TOC) is a geochemical parameter which reflects oxygenation conditions in the depositional environment. It can be related to relative sea level fluctuations within a sequence stratigraphic context.Understanding the sequence stratigraphy of shale provides a powerful tool not only for regional-to-local stratigraphic correlations from well logs and seismic reflection records, but also for high-grading stratigraphic intervals most favorable for preservation of organic matter and gas generation and storage.

Depositional architecture and sequence stratigraphic correlation of Upper Ordovician glaciogenic deposits, Illizi Basin, Algeria

Abstract Upper Ordovician glaciogenic deposits are profoundly important as hydrocarbon reservoirs across North Africa, such as within the Illizi Basin of SE Algeria. In this study we present a new sedimentological and sequence stratigraphic model for Upper Ordovician glaciogenic deposits based on the analysis of core descriptions and wireline logs from 25 wells in the Tiguentourine Field. Within the glaciogenic succession, two ice advance–retreat cycles can be defined, consisting of glaciomarine ice-contact fan deposits and tillites. Deposits of the marine ice-contact fan systems generally show a retrogradational stacking pattern from ice-proximal to ice-distal deposits. This pattern is attributed to the deposition in front of a retreating ice sheet. The proximal marine ice-contact fan deposits consist of massive or low-angle cross-bedded pebbly sandstone. They are interpreted as the deposits of turbulent, high-energy plane-wall jets, emerging from subglacial meltwater conduits. These jet-efflux deposits are up to 60 m thick and interbedded with deposits of cohesive and non-cohesive debris flows. The jet-efflux deposits are overlain by fine-grained, thick-bedded massive sandstone. These mid-fan deposits build up the bulk of the glaciomarine fans and are interpreted as deposits of underflows, generated at the point of flow-detachment, where marine meltwater jets become buoyant and large volumes of sediment fall-out from suspension. In the upper part of the fan succession massive sandstones pass upwards into mud-prone massive sandstones, interpreted as deposits of cohesive sandy debris flows. The most ice-distal deposits are muddy sandstones and mudstones deposited by waning low-density turbulent flows and suspension fall-out. The best reservoir properties within the glaciogenic succession are attributed to the proximal and medial deposits of the ice-contact fans such as coarse-grained jet-efflux deposits and sustained high-density turbulent flow deposits. However, the mud content within the massive sandstones is highly variable and influences the reservoir quality. Both glacial depositional sequences infill 60–175 m deep, elongated depressions, which are interpreted as subglacial tunnel valleys. These tunnel valleys acted as depocentres for the glaciomarine fan deposits. After final deglaciation and post-glacial transgression, organic-rich shale was preferentially deposited in underfilled tunnel valleys.

Research of Sequence Stratigraphic Models of Jiufotang Formation, Lower Cretaceous, the Luxi Sag, the Kailu Basin

With comprehensive utilization of drilling, logging and seismic data, research was conducted on the sequence stratigraphic models of Jiufotang Formation, Lower Cretaceous in the Luxi Sag, Kailu Basin. It was revealed that Jiufotang Formation could be divided into one three-order sequence, which including lowstand systems tract (LST), transgressive systems tract (TST), highstand systems tract (HST) and regressive systems tract (RST). Within the sequence, it developed five types of sedimentary facies, which were nearshore subaqueous fan, braided river delta, meandering river fan, turbidite fan and lacustrine facies, respectively. The LST mainly developed nearshore subaqueous fan, the TST mainly developed meandering river deltas while the HST and RST mainly developed meandering river delta, braided river delta and nearshore subaqueous fan. In addition, turbidite fan developed in all four systems tracts. Two types of sequence stratigraphic models developed: short-axis model and long-axis model.

A case study of microbial mat-related features in coastal epeiric sandstones from the Paleoproterozoic Pretoria Group (Transvaal Supergroup, Kaapvaal craton, South Africa); The effect of preservation (reflecting sequence stratigraphic models) on the relationship between mat features and inferred paleoenvironment

The Magaliesberg Formation sandstones within the upper part of the Pretoria Group are interpreted as a second-order highstand systems tract within an overall clastic epeiric marine setting, where episodic braided fluvial systems fed sediment directly into a tidally dominated coastline setting. Mat-related features are abundantly preserved within the inferred tidal flat and supratidal deposits. The Daspoort Formation sandstones at a lower stratigraphic level are ascribed to a closely analogous paleoenvironmental setting, yet have hardly any preserved mat features at all; they are ascribed to a second-order transgressive systems tract in a sequence stratigraphic framework. During highstand the depositional systems are inferred to have aggraded, thus preserving the mat-related features formed in Magaliesberg intertidal–supratidal environments. During second-order transgression, the formation of the wave/tidal ravinement surface in the subtidal/intertidal environments largely obliterated the features of the Daspoort intertidal and supratidal environments, leading to a paucity of mat-related structures in the preserved Daspoort succession. This case study thus supports the application of mat-related features to integrated paleoenvironmental and sequence stratigraphic analysis, while emphasizing the role of preservation.

Contourites within a deep-water sequence stratigraphic framework

Sequence stratigraphy has proven to be an extremely useful predictive tool in the search for hydrocarbons along the continental margins. However, of the several models in use, none includes the effects of alongslope processes in deep-water. This paper, therefore, is a first attempt to place contourite depositional systems (CDSs) firmly within a sequence stratigraphic framework, based on detailed examination of over 20 CDSs worldwide. It also presents a new view of how sea level variation influences bottom current generation and intensity. Two key controls on contourite drift formation are identified: sediment influx and bottom current velocity. Sea level directly influences the sediment influx to a basin and, therefore, the contourite response fits nicely into the downslope sequence stratigraphy model. Bottom current velocity variations in response to sea level are more complex, and two key controls are identified: (1) oceanic gateways can effectively constrict and accelerate water masses and are therefore closely associated with CDS evolution; fluctuating sea level will affect the water exchange through a gateway; (2) changing rates of bottom-water generation: some water masses appear more vigorous during periods of lowstand, whereas others appear more sluggish. In order to accommodate this variation, two new sequence stratigraphy models are herein presented, comprising both downslope and alongslope processes. The first model reveals a CDS where bottom current activity is markedly more vigorous during times of sea level highstand, whereas the second model indicates margin evolution where bottom-water currents are most vigorous during times of sea level lowstand. It is recognised that there are additional controlling factors linked to sea level variation which can significantly modify the distribution and development of contourite elements.

Glaucony from the Cretaceous of the Sierra de Guadarrama (Central Spain) and its application in a sequence-stratigraphic context

Detailed sedimentological and geochemical analyses of Upper Cretaceous glaucony-bearing deposits from the middle portion of the Castro de Fuentiduena Formation, in Central Spain, enable identification of a multiphase history of glaucony accumulation. Despite its relatively high maturity (K2O > 7%), glaucony from the transgressive subtidal sandstones has anomalously low concentration in the host rock (generally <10%), suggesting remobilization from a different source. The remarkable thickness of the glaucony-bearing horizons, concurrently with concentration of glaucony in laminae, small size, and high degree of roundness and sorting provide further evidence for an allochthonous origin of the green grains. In contrast, authigenic glaucony from the overlying offshore clays exhibits higher abundance (up to 45%) and lower maturity (K2O < 7%) and is interpreted to have formed in situ. Allochthonous glaucony originally developed in more distal areas during a prolonged period of sediment starvation. The green grains were then swept away from their place of origin and concentrated into tidal bars and channels within the upper transgressive systems tract of the third-order depositional sequence. Maximum concentration of autochthonous glaucony is recorded at the transition from tidal-influenced to offshore deposits: this glaucony, which is relatively less evolved due to lower time available for maturation, is interpreted to reflect the turnaround from transgressive to highstand conditions, marking the condensed section of the depositional sequence. As postulated by previous sequence-stratigraphic models, this study documents that allochthonous glaucony can be widespread throughout the transgressive systems tract (TST), while the condensed section (CS) typically hosts autochthonous glaucony. Contrary to the existing literature, however, this study shows that glaucony from the TST may be even more mature than glaucony from the CS, if enough time for maturation during transgression is allowed before the ultimate concentration of the green grains. Identification of spatial and temporal characteristics of glaucony, thus, is critical to a reliable sequence-stratigraphic interpretation of the glaucony-bearing deposits.

Recognition and depiction of special geologic bodies of Member 3 of Dongying Formation in Littoral Slope Zone, Qikou Sag

The purpose of this study is to forecast the profitable oil and gas reservoir, which is the key of finding hydrocarbon, based on the conception of special geologic bodies. With the guiding methodology of the research thinking of integration of point-line-surface by using the methods and techniques of logging, seismic, seismic attribute, and logging constrained inversion in 3D data volume, the special geologic bodies of Member 3 of Dongying Formation in Littoral Slope Zone of Qikou Sag, which has important hydrocarbon exploration potential, are recognized and described under the constraint of sequence stratigraphic framework. As a result, the developed scale, geometric shape and space distribution feature of the special geologic bodies are forecasted; the inner structure and sequence structure patterns of the geologic bodies are also ascertained. From the lowstand system tract (LST) and lacustrine expanding system tract (EST) to the highstand system tract (HST), the geologic bodies have evolved from relative centralization of lake basin reducing period to three relative dispersive isolated parts of broad lake basin period. According to the relevance and regularity of the development of geologic bodies, the conclusions can be obtained that three types of potential profitable reservoir traps, including the lithologic lens traps, lithologic updip pinchout traps and structural-lithologic composite traps, are forecasted. In addition, scientific basis for further hydrocarbon exploration in new area (few-well area and no-well area) is offered in the guidance of sequence stratigraphic model.

Terminal Ordovician carbon isotope stratigraphy and glacioeustatic sea-level change across Anticosti Island (Quebec, Canada)

Globally documented carbon isotope excursions provide time-varying signals that can be used for high-resolution stratigraphic correlation. We report detailed inorganic and organic carbon isotope curves from carbonate rocks of the Ellis Bay and Becscie Formations spanning the Ordovician-Silurian boundary on Anticosti Island, Quebec, Canada. Strata of the Anticosti Basin record the development of a storm-dominated tropical carbonate ramp. These strata host the well-known Hirnantian positive carbon isotope excursion, which attains maximum values of ~4.5‰ in carbonate carbon of the Laframboise Member or the Fox Point Member of the Becscie Formation. The excursion also occurs in organic carbon, and δ^(13)C_carb and δ^(13)C_org values covary such that no reproducible Δ^(13)C (= δ^(13)C_carb – δ^(13)C_org) excursion is observed. The most complete stratigraphic section, at Laframboise Point in the west, shows the characteristic shape of the Hirnantian Stage excursion at the global stratotype section and point (GSSP) for the Hirnantian Stage in China and the Silurian System in Scotland. We therefore suggest that the entire Hirnantian Stage on Anticosti Island is confined to the Laframboise and lower Fox Point Members. By documenting discontinuities in the architecture of the carbon isotope curve at multiple stratigraphic sections spanning a proximal to distal transect across the sedimentary basin, we are able to reconstruct glacioeustatic sea-level fluctuations corresponding to maximum glacial conditions associated with the end-Ordovician ice age. The combined litho- and chemostratigraphic approach provides evidence for the diachroneity of the oncolite bed and Becscie limestones; the former transgresses from west to east, and the latter progrades from east to west. The sea-level curve consistent with our sequence-stratigraphic model indicates that glacioeustatic sea-level changes and the positive carbon isotope excursion were not perfectly coupled. Although the start of the isotope excursion and the initial sea-level drawdown were coincident, the peak of the isotope excursion did not occur until after sea level had begun to rise. Carbon isotope values did not return to baseline until well after the Anticosti ramp was reflooded. The sea-level–δ13Ccarb relationship proposed here is consistent with the “weathering” hypothesis for the origin of the Hirnantian δ^(13)C_carb excursion.

Facies Associations, Paleoenvironment, and Base-Level Changes in the Upper Cretaceous Wahweap Formation, Utah, U.S.A.

The Wahweap Formation is an ~ 400-m-thick succession of fluvial and estuarine channel sandstones and floodbasin mudstones divided into lower, middle, upper, and capping sandstone members. Facies analysis of the Wahweap Formation on the Kaiparowits Plateau reveals the presence of ten facies associations grouped into channel and floodbasin deposits. Facies associations (FAs) from channels include: (1) single-story and (2) multistory lenticular sandstone bodies, (3) major tabular sandstone bodies, (4) gravel bedforms, (5) low-angle heterolithic cross-strata, and (10) lenticular mudrock, whereas floodbasin facies associations include: (6) minor tabular sandstone bodies, (7) lenticular interlaminated sandstone and mudrock, (8) inclined interbedded sandstone and mudrock, and (9) laterally extensive mudrock. The lower and middle members are dominated by floodbasin facies associations. The lower member consists dominantly of FA 8, interpreted as proximal floodbasin deposits including levees and pond margins, and is capped by a persistent horizon of FA 3, interpreted as amalgamated channel deposits. FAs 4 and 6 are also present in the lower member. The middle member consists dominantly of FA 9, interpreted as distal floodbasin deposits including swamp, oxbow-lake, and waterlogged-soil horizons. FAs 1, 2, 5, 6, 7, 8, and 10 are present in the middle member as well, which together are interpreted as evidence of suspended-load channels. The upper member is sandstone-dominated and consists of FAs 1, 2, 3, 5, 7, and 8. FAs 5 and 7, which occur at the base of the upper member, are interpreted as tidally influenced channels and suggest a marine incursion during deposition of the upper member. The capping sandstone is characterized by FAs 3, 4, and 6, and is interpreted to represent a major change in depositional environment, from meandering river systems in the lower three members to a low-accommodation, braided river system. Improved age constraints on the Wahweap Formation indicate that the middle and upper members were deposited during the eustatic Claggett transgression (T8 of Kauffman 1977) in the adjacent Western Interior Seaway. Additionally, facies analysis of the Wahweap Formation has revealed an increase in both the sand:mud ratio and the degree of sandstone amalgamation from the upper part of the middle member to the base of the capping sandstone, suggesting a gradual reduction in generation of accommodation in the basin. Following recent alluvial sequence stratigraphic models, the middle member is interpreted as the isolated fluvial facies tract, while the upper member represents the tidally influenced and highstand facies tracts. Maximum transgression occurred during deposition of the lowest part of the upper member, synchronous with the putatively eustatic Claggett highstand in other parts of the Western Interior Basin. The sequence boundary is placed at the base of the overlying capping sandstone member, diagnosed by a major shift in petrography and paleocurrent direction, as well as up to 4 m of fluvial incision into the underlying upper member. The capping sandstone member is interpreted as the amalgamated fluvial facies tract, and the sequence boundary at the base of the capping sandstone is regarded as tectonically induced. Improved geochronology in the formation now permits high-resolution correlation with marine units to the east and along-strike correlation with contemporaneous alluvial units up and down the western margin of the Western Interior Basin.

Sequence stratigraphy of a glaciated basin fill, with a focus on esker sedimentation

A large integrated data set of cores, outcrop data, and seismic transects from the mud-buried Vars-Winchester esker in the Champlain Sea basin, Canada, was studied to gain insight into how muddy glaciated basins fill with sediment, and how esker sedimentary systems contribute to this process. Three stratigraphic units—a till sheet over carbonate bedrock, the Vars-Winchester esker , and overlying Champlain Sea mud—are identified in the data set. The till is massive, mud rich, carbonate rich, and drumlinized. The esker is also carbonate rich, and rests erosively on till or bedrock. It consists of two elements, a narrow gravelly central ridge and a broad sandy carapace. Three units comprise the overlying mud package: gray carbonate-rich rhythmites, massive bioturbated mud, and carbonate-poor, red-and-gray rhythmites. A sequence stratigraphic model is proposed to explain these observations. Emphasis is placed on gradual ice-front translation superimposed by rapid meltwater events. The esker is interpreted to have been derived from the underlying till by water that flowed through a subglacial conduit (R-channel), within which the narrow gravelly central ridge was deposited. Most mud and finer sand bypassed the conduit and was deposited proglacially on the floor of the Champlain Sea, first as sandy outwash and, farther basinward, as muddy carbonate-rich rhythmites. Gradual ice-front retreat superposed distal facies over proximal facies, generating the upward-fining succession that starts with the esker gravel and ends with muddy rhythmites. Most esker sediment appears to have been deposited during rapid, jokulhlaup-like floods that punctuated gradual retreat. Discharges are estimated to have been high, possibly on the order of several hundred to, perhaps more commonly, several thousand cubic meters per second. The chaotic and random-looking appearance of the resultant sedimentological signatures in the esker sensu stricto is sharply contrasted with the regularity of the muddy rhythmites. If the rhythmites are indeed correlative to the esker, which seems reasonable given their geochemistry and the fact that their volume scales to the volume of mud in the till, the flood events that deposited the esker must have been seasonally mediated, and the basin water must have attenuated the flood signal, resulting in a rhythmic “on-off” signature in more distal portions of the system. The regularity of the rhythmites does not betray the chaotic nature of the esker sensu stricto, and vice versa. Studying either one in isolation would lead to a very different “end-member” impression of how eskers form and how esker sedimentary systems operate during the infilling of glaciated basins.

Stratigraphy of a Cretaceous Coastal-Plain Fluvial Succession: The Campanian Masuk Formation, Henry Mountains Syncline, Utah, U.S.A.

Abstract The internal architecture and genetic-stratigraphic analysis of coastal-plain fluvial successions has applications to both academic understanding of continental-margin stratigraphy and to the economic evaluation of subsurface hydrocarbon and water reservoirs. Despite the abundance of exposed ancient examples of such coastal-plain to shallow marine transitions, however, the lateral facies changes and stratigraphic stacking patterns of such transitions remain largely unresolved in detail. This study presents a sedimentological and stratigraphic analysis of the Campanian Masuk Formation in the Henry Mountains syncline of Utah, western USA, and proposes a sequence-stratigraphic model to explain the geometries of fluvial-channel lithosomes found in the unit. At its type section, the Masuk Formation is an ~ 220-m-thick succession of interbedded sandstone, mudstone, and coal with minor intraformational breccias. Six facies are defined: Facies 1, coal and carbonaceous shale, records accumulation of organic sediments in coastal-plain mires; Facies 2, mudrock, records coastal-plain floodbasin settings; Facies 3, thinly interbedded sandstone and siltstone, represents estuarine-basin deposits with one interval of marine flooding; Facies 4, admixed siltstone-sandstone with intraformational breccia, represents chaotically filled coastal plain channels; Facies 5, interbedded sandstone-siltstone with inclined heterolithic stratification (IHS), records deposits of coastal-plain channels with variable sediment supply; and Facies 6, trough-cross-bedded sandstone, records sand-dominated coastal-plain channel deposits. Facies 4, 5, and 6 all show evidence of coastal and tidal influence on sediment accumulation (IHS, low-diversity trace-fossil assemblages, bimodal to bipolar paleocurrent distributions, mud drapes on small-scale sedimentary structures) to varying degrees. Facies analysis and tracing of key surfaces such as regionally extensive erosion surfaces (sequence boundaries) and flooding surfaces suggest that the entire Masuk Formation (and the uppermost part of the underlying Muley Canyon Sandstone) is a single depositional sequence. The typically multilateral channel bodies are traceable in a depositional strike direction over distances much greater than empirically calculated estimates of channel-belt width. This pattern suggests an allogenic control on stacking patterns. Multistory and multilateral channel bodies typically have a Facies 4 unit at the base, overlain by one or more stories of Facies 5, and in turn capped by Facies 6. This stacking pattern could reflect cycles of incision, initial lowstand channel filling, and subsequent backfilling of estuarine channels during transgressions. This pattern further hints at the possibility of preservation of high-frequency sequences nested within the longer-term sequence. This study provides new insights into the stacking patterns of coastal-plain fluvial successions and their interpretation.

Important geological properties of unconventional resource shales

AbstractThe revelation of vast global quantities of potentially productive gas and oil-prone shales has led to advancements in understanding important geological properties which impact reservoir performance. Based upon research on a variety of shales, several geological properties have been recognized as being common and important to hydrocarbon production. (1) transport/depositional processes include hemipelagic ‘rain’, hyperpycnal flows, turbidity current flows, tempestites, wave-reworking, and contour currents in both shallow and deep water settings. (2) Common shale minerals include clays, quartz, calcite, dolomite, apatite, and pyrite; organic constituents include spores (Tasmanites), plant remains, biogenic quartz and calcite, and arenaceous foraminifera. (3) Porosity and permeability are characteristically low with pore sizes ranging down to the nanoscale. Main pore types include intergranular (including pores within clay floccules), porous organic matter, porous fecal pellets, and microfractures. (4) Important geochemical characteristics include organic richness (&gt;3%), maturity (&gt;1.1%Ro for shale gas and 0.6–0.9% for shale oil) and type (I–IV), in addition to certain biomarkers which are indicators of bottom water oxicity during deposition. Remaining hydrocarbon potential [RHP = (S1 + S2)/TOC] also reflects temporal environmental changes. ‘Isotopic reversals’ can be used to detect best producing areas in shale-gas plays. (5) Lithofacies stacking patterns and sequence stratigraphy are the result of eustatic depositional history. A general sequence stratigraphic model is presented here that highlights this commonality. (6) Geomechanical properties are key to drilling, fracturing and production of hydrocarbons. Brittle-ductile couplets at several scales occur in shale sequences. (7) Geophysical properties, when calibrated to rock properties, provide a means of regionally to locally mapping the aforementioned properties. (8) Economic and societal considerations in the exploration and development of resource shales are garnering attention. Many potentially economic shale-gas and shale-oil plays are being identified globally. Risks and uncertainties associated with gas- and oil-rich shales include the lack of long-term production histories, environmental concerns related to hydraulic fracturing, uncertainty in calculating hydrocarbons-in-place, and fluctuations in supply, demand, and price.

Thickness trends and sequence stratigraphy of the Middle Devonian Marcellus Formation, Appalachian Basin: Implications for Acadian foreland basin evolution

Analysis of more than 900 wireline logs indicates that the Middle Devonian Marcellus Formation encompasses two third-order transgressive-regressive (T-R) sequences, MSS1 and MSS2, in ascending order. Compositional elements of the Marcellus Formation crucial to the successful development of this emerging shale gas play, including quartz, clay, carbonate, pyrite, and organic carbon, vary predictably within the proposed sequence-stratigraphic framework. Thickness trends of Marcellus T-R sequences and lithostratigraphic units reflect the interplay of Acadian thrust-load-induced subsidence, short-term base-level fluctuations, and recurrent basement structures. Rapid thickening of both T-R sequences, especially MSS2, toward the northeastern region of the basin preserves a record of greater accommodation space and proximity to clastic sources early in the Acadian orogeny. However, local variations in T-R sequence thickness in the western, more distal, area of the basin may reflect the reactivation of inherited Eocambrian basement structures, including the Rome trough and northwest-striking cross-structural discontinuities, induced by Acadian plate convergence. Episodes of block displacement locally warped the basin into northeast-southwest–trending regions of starved sedimentation and/or erosion adjacent to depocenters in which regressive systems tract deposits were ponded. Block movement appears to have initiated in late Early Devonian time, resulting first in thinning and local erosion of the Oriskany sandstone in northwest Pennsylvania. This study, in addition to providing the basis for a predictive sequence-stratigraphic model that can be used to further Marcellus exploration, tells of a foreland basin more tectonically complex than accounted for by simple flexural models.

Distinguishing fluvio-deltaic facies by bulk geochemistry and heavy minerals: an example from the Miocene of Denmark

Interpretations of seismic profiles, gamma-ray logs and sediment descriptions were used to classify seven facies in Miocene fluvio-deltaic deposits of Denmark. An impartial approach was adopted by not including analytical data in the facies definition. This approach allowed identification of significant differences between facies, where each fluvio-deltaic facies can be distinguished by integrating geochemistry and mineralogy through a sedimentary succession. Variations in the heavy mineral assemblage between facies are caused primarily by sorting of hydraulically equivalent grains. Decreasing grain size of quartz and heavy minerals from the channel facies towards the delta shoreface facies and further along the coast to the spit shoreface facies is associated with an increase in sorting and textural maturity. This trend is related to longshore drift. Increasing heavy mineral grain size is found from the delta slope facies offshore to the delta toe and shelf facies. This trend is interpreted as a result of sorting by turbidity currents. The mixed origin of the transgressive lag facies is shown by the poorer sorting in this facies. By indicating the amount of alteration the sediments have been exposed to, the Ti-mineral maturity has proven useful in characterizing the facies. This systematic approach of tying depositional environments to a well-calibrated sequence stratigraphic model has generated analytical results which are valid as reference levels for future facies identifications.

Temporal Evolution of Fluvial Style in a Compound Incised-Valley Fill, Ferron "Notom Delta", Henry Mountains Region, Utah (U.S.A.)

Abstract Nonmarine sequence stratigraphic models hypothesize systematic changes in fluvial style within individual sequences and across sequence boundaries, based largely on mudstone–sandstone ratios. The main purpose of the paper is to evaluate the validity of these models by documenting facies relationships and detailed bedding architecture of bar and channel deposits as well as mudstone–sandstone ratio within a compound incised-valley fill at the top of the Cretaceous Ferron Sandstone Member of the Mancos Shale Formation in the Henry Mountains region, Utah, U.S.A. Field photomosaics, walking out of beds, and 42 measured sections document the stratal and facies organization in the incised-valley fill. Two erosional surfaces with erosional relief up to 17 m are traced continuously for up to 7 km. These erosional surfaces are defined by onlap and truncation and show a marked basinward shift in facies, with pebbly coarse-grained and conglomeratic fluvial sandstones overlying highly burrowed marine mudstones and sandstones. These major erosional surfaces partition the valley fill into two sequences, marked as V2 and V1 in ascending order. Each sequence comprises a multistory fill that reaches a maximum thickness of about 30 m. Within each sequence there is a vertical transition in facies from fluvial, to tidal, and finally back into fluvial facies. In the younger V1, there is a systematic vertical evolution of fluvial style from braided, to single-thread meandering, and finally to a low-sinuosity river system. In contrast, formative rivers in the older V2 were always meandering and show more marine and tidal influence. A major change in valley sedimentology is recognized from V2 to V1, as shown by a 30° eastward shift in main flow direction, overall increase in grain size and river discharge, and a clear change in fluvial style from meandering into braided streams. This is also shown by the increase in preserved dune height in fluvial bar and channel deposits across the boundary, which suggests that deeper rivers cut the younger V1 valley. The change in river style from braided to meandering in V1 is interpreted to reflect a gradual decrease in slope and discharge as the valley was filled. The change in fluvial style across the sequence boundary reflects an increase in grain size and discharge, probably driven by a high-frequency climate change, as well as local increase in valley slope.

The epicontinental subsiding margin of the Triassic in Northern Israel, North Arabian Plate

The Triassic sedimentary pattern of the north Arabian margin represents a transition from continental to deep-marine; we here present a transect part of this suite from southern to northern Israel. Cuttings from the Ramalla (S) and Devora (N) boreholes were studied by petrographic and cathodoluminescence microscopy. Biotic and sedimentological components were grouped into microfacies, upon which a carbonate-evaporitic depositional facies model was based. Mid-to-inner ramp conditions persisted along the transect, essentially following a S–N depositional strike, with outer-ramp to basinal facies to the west. Five low-order (LO) cycles were identified and integrated into a sequence-stratigraphic model with eustatic, tectonic, and differential subsidence components: LO cycle 1 is Late Anisian (Pelsonian), cycle 2 is Late Anisian to Longobardian, cycle 3 is Early Carnian, cycle 4 is Late Carnian, and cycle 5 is at the Carnian/Norian transition. Facies range from supratidal via lagoonal facies in late HST or LST, to calcarenitic shoals and short-term deepening to offshore at maximum flooding. Cycles 1 and 2 are non-evaporitic, with a clastic influx event in cycle 2. Cycles 3 and 4 contain sabkha or salina evaporites in the lagoonal facies that end in the lower part of cycle 5. These facies group into a higher-order cyclicity with discrete events that can be correlated between the boreholes. Depositional and burial history were correlated to Alpine and Arabian Plate stratigraphy over 20 Myr, from the Anisian (Pelsonian) to the Norian. Siliciclastic, carbonate and evaporite-dominated environments indicate climates ranging from relatively humid to extremely arid. Rapid subsidence in the Late Anisian is attributed to Neotethyan rifting of northeastern Gondwana. From latest Anisian to Early Carnian, reduced subsidence took place in the face of eustatic sea level rise. Renewed subsidence in the Carnian is attributed to further Neotethyan rifting. Subsidence terminated in the Norian with uplift to the north, attributed to pre-volcanic doming.

Towards the standardization of sequence stratigraphy: Is the parasequence concept to be redefined or abandoned?

After more than thirty years from the introduction of the sequence-stratigraphic concepts (Payton, 1977), and their refinement in the classic SEPM special publication 42 (Wilgus et al., 1988), sequence stratigraphy has becomemoremature and now represents a widely used tool to study sedimentary successions. More recently, review publications (e.g. Miall, 1997; Posamentier and Allen, 1999; Catuneanu, 2006; Catuneanu et al., 2009) have put order among the various sequence-stratigraphic models and attempted to standardize the terminology. Among the historical issues raised by sequence-stratigraphic concepts and terminology, one deals with the high-frequency cycles that compose sequences, which have been named parasequences. A parasequence was defined as ‘a relatively conformable succession of genetically related beds or bedsets bounded by marine flooding surfaces and their correlative surfaces... Parasequences are progradational and therefore the beds within parasequences shoal upward’ (Van Wagoner et al., 1987, 1988, 1990). The flooding surface was defined as ‘a surface across which there is an abrupt shift of facies that may indicate an increase in water depth or a decrease in sediment supply’ (Van Wagoner et al., 1988, 1990). Parasequences may be stacked to form progradational, aggradational and retrogradational parasequence sets, which typify systems tracts composing a sequence (VanWagoner et al., 1990), the latter being defined as ‘a stratigraphic unit composed of a relatively conformable succession of genetically related strata and bounded at its top and base by unconformities or their correlative conformities’ (Mitchum, 1977).

Regional stratigraphic architecture and reservoir types of the Oligo-Miocene deposits in the Dezful Embayment (Asmari and Pabdeh Formations) SW Iran

Abstract A regional sequence stratigraphic model is proposed for the Oligo-Miocene Asmari and Pabdeh Formations in the Dezful Embayment of SW Iran. The model is based on both new detailed sedimentological observations in outcrops, core and well logs, and an improved high-resolution chronostratigraphic framework constrained by Sr isotope stratigraphy and biostratigraphy. A better understanding of the stratigraphic architecture distinguishes four, geographically separated types of Asmari reservoirs. Three Oligocene sequences (of Rupelian, early Chattian and late Chattian age) and three Miocene sequences (of early Aquitanian, late Aquitanian and early Burdigalian age) have been distinguished, representing a period of 15.4 Ma. The stratigraphic architecture of these sequences is primarily controlled by glacio-eustatic sea-level fluctuations, which determined the distribution of carbonates, sandstones and anhydrites in this sedimentary system. Tectonic control became important in the Burdigalian with a regional tilt down towards the NE. The lithological heterogeneity, the complex geometries, and both early and late diagenetic alterations are the basis for a classification of four main stratigraphic reference types for the Asmari Reservoirs: Type 1, sandstone dominated; Type 2, mixed carbonate-siliciclastic; Type 3, mixed carbonate-anhydrite; and Type 4, carbonate dominated. The sequence stratigraphic model predicts how and when these types change laterally from one to another.

Reappraisal of the sequence stratigraphy of the Humber Group of the UK Central Graben

Abstract Deposition of the Callovian–Ryazanian Humber Group of the UK Central Graben occurred during rifting and long-term relative sea-level rise, which acted to suppress the formation of eustatically forced Exxon-type sequence boundaries. The superposition of highly variable halokinetically controlled subsidence means that classic, passive-margin derived sequence stratigraphic models are not appropriate to describe stratigraphic evolution in this rift setting. The sequence stratigraphy of the Humber Group has been re-evaluated using a transgressive–regressive sequence model, where maximum regressive surfaces are employed as sequence bounding surfaces. The Humber Group comprises two megasequences which reveal distinct phases of evolution of the basin. The latest Callovian–Kimmeridgian megasequence comprises a conformable sequence stack which lacks significant internal unconformities and records progressive marine flooding and overall backstepping onto the basin flanks during a phase of active rifting. The Volgian–Ryazanian megasequence is condensed and highly fragmentary due to punctuation by a number of unconformities which are consistently recognizable throughout the basin. The onset of this change in architectural style corresponds to the oldest unconformity at the base of the Volgian–Ryazanian succession, termed the Base Volgian–Ryazanian Unconformity, of latest Kimmeridgian to earliest Volgian age. The patterns of erosion of the Callovian–Kimmeridgian megasequence and the intra Volgian–Ryazanian unconformities record the effects of dramatic redistribution of underlying salt accompanied by probable uplift of the Forties–Montrose High and J Ridge, resulting in major modification of the basin morphology and the severing of possible earlier links with the Fisher Bank Basin. The kinematics of this event are equivocal, but it is possible that restricted Volgian–Ryazanian depocentres resulted from localized salt collapse rather than basement extension. Widespread erosion of Callovian–Kimmeridgian Humber Group sediments may have occurred in some areas where Volgian–Ryazanian Kimmeridge Clay deposits now overlie pre-Jurassic strata, and exploration models must incorporate the effects of Volgian reconfiguration in order to accurately predict reservoir distribution.

Sequence stratigraphy of Cenomanian–Turonian carbonate platform margins (Sarvak Formation) in the High Zagros, SW Iran: an outcrop reference model for the Arabian Plate

Abstract A high resolution sequence stratigraphic model has been constructed for the mid-Cretaceous Sarvak Formation (in the High Zagros region of SW Iran) which was deposited close to the eastern margin of the Arabian Plate. The exceptional outcrop quality, displaying the detailed facies patterns in the transition zone from carbonate platform to intra-shelf basin, offers the rare opportunity to distinguish between the relative control of carbonate sediment supply (S) and accommodation (A) on the depositional geometries of third- and fourth-order depositional sequences. Four third-order sequences have been distinguished in the Sarvak Formation, with a duration varying between 1.5 and 3 Ma, and a thickness of 50–150 m. These are in turn composed of fourth- and fifth-order sequences that form the stratigraphic building blocks of this carbonate system. A significant distinction has been made in the third-order sequences between the early transgression (e-TST) when the system was still flat, and corresponds to a ramp setting, and the late transgression (l-TST) when the carbonate platform to intra-shelf basin topography was created. The rate of accommodation creation is identified as the dominant factor controlling the morphology of the depositional profile, and, as such, the driving motor behind the dynamics of this type of carbonate system. The dip angle of the depositional profile has a major influence on: (1) the hydrodynamics of the system; (2) the type of carbonate sediment; and (3) the volume of carbonate sediment produced. A good correlation with the third-order sequences of the Natih Formation in Oman is demonstrated, which supports a dominant control by eustatic sea-level changes and a similar response of the carbonate system to changes in the rate of sea-level rise on the southern part of the Arabian Plate. This outcrop analogue can be considered as a good reference model for the Cenomanian–Turonian carbonate platform margins of the Arabian Plate, but also as a textbook example of the response of carbonate systems to sea-level fluctuations (relative influence of accommodation and sediment supply).