Delta-front Deposits Research Articles

Rapid subsidence and preservation of fluvial signals in an otherwise wave-reworked delta front succession: Early-mid Pliocene Orinoco continental-margin growth, SE Trinidad

Outcrops of the early-mid Pliocene Orinoco Delta succession on the Atlantic-facing Trinidad continental margin demonstrate facies and stratigraphic architecture of a shelf margin developed in a setting of high sediment supply (Orinoco River) and unusually rapid, sediment accumulation/subsidence (averaging >1 km/My). The studied Moruga clastic wedge of the Orinoco margin consists of a series of topset to deepwater clinoforms with channelized sandstones, river- and mixed-process delta front, wave-dominated delta front, shoreface, prodelta-offshore transition-shelf, upper-slope turbidite channels and mass-transport deposits. The succession is sand-prone and generally consists of coarsening- and thickening-upward delta-front lobes with hummocky/swaley and wave-ripple stratified sandstones indicating dominant storm wave reworking of the delta, particularly when the delta lobes had reached outer shelf sites after long shelf transits. The lower parts of distributary channels are well preserved and characterized by cross-bedded to parallel-laminated sandstones with mud clast lags. The upper parts of channel fills were wave reworked during transgressions, creating hummocky/swaley cross-stratified strata. River-dominated and mixed-process (wave and tide) delta-front deposits occur commonly within stacked, coarsening-upward units. The river-dominated delta front deposits exhibit parallel-laminated to current ripple-laminated intervals with normally graded thin beds changing upward to parallel and low-angle cross lamination, but mixed-process delta front deposits also contain wave and bidirectional current ripples as well as discrete intervals of hummocky/swaley cross-stratification. The preservation of the channel-base as well as river- and mixed-process delta-front deposits in the otherwise storm wave-dominated Atlantic-facing setting suggests some sheltering of the coastal morphology, and possibly low-frequency of large storms, but it is more likely that the unusually rapid subsidence (at rates competing well with Icehouse rates of eustatic sea-level change) and burial of deposits may have prevented the fluvial and tidal signals from being completely reworked by the storm waves in the studied outcrop area. This study demonstrates an example of how the interactions of river-, tide- and wave-generated facies impacted stratigraphic architecture of a rapid subsiding shelf margin built by shelf-edge deltas with high sediment supply.

Characterization of effective permeability in heterolithic, distal lower-shoreface sandstone reservoirs: Rannoch Formation, Brent Group, UK North Sea

The reservoir properties of distal lower-shoreface and distal wave-dominated delta-front deposits, which consist of sandstone beds with locally scoured bases and mudstone interbeds, are poorly understood. The lower Rannoch Formation (Middle Jurassic Brent Group) forms an interval of such heterolithic sandstones in many North Sea reservoirs, and is used to illustrate a workflow for rapid estimation of reservoir properties and their sensitivity to key parameters. Mudstone-interbed thickness distributions in cored reservoir successions are compared to the thickness distribution of sandstone scour-fills in an outcrop analogue(s) in order to identify mudstones with the potential to form laterally extensive barriers to vertical flow. Effective k v / k h at the scale of several typical reservoir-model grid cells (200 × 100 × 20 m) is estimated in intervals bounded by these mudstone barriers via a simple analytical technique that is calibrated to previously documented reservoir-modelling experiments, using values of sandstone proportion measured in cored reservoir successions. Using data from the G2 parasequence (Grassy Member, Blackhawk Formation, east-central Utah, USA) outcrop analogue, mudstones bounding 3–8 m-thick, upwards-coarsening successions in the lower Rannoch Formation may define separate stratigraphic compartments in which grid-cell-scale effective k v / k h is estimated to be 0.0001–0.001 using a streamline-based analytical method.

Fluvial shoal water deltas: pre‐vegetation sedimentation through the fluvial–marine transition, Lower Cambrian, English Channel region

AbstractLower Palaeozoic fluvial systems tend to be more sand‐prone than those of later eras and the nature of coastal environments less certain. Field studies are presented that characterize the fluvial to marine transition over a distance of 80 km, in the Lower Cambrian of the Cotentin Peninsula, northern France. The sedimentary rocks are divided into six facies associations which represent deposition in proximal fluvial, distal fluvial, delta plain, delta front, pro‐delta and offshore carbonate bank environments. The basin fill is sandstone‐dominated and subdivided into three stratigraphic intervals. A 200 to 300 m thick basal interval contains very coarse‐grained fluvial sandstones deposited during a relative sea level lowstand. An overlying interval, 250 to 1500 m thick, is a facies mosaic. Fluvial strata in the north‐west pass laterally south‐east into deltaic and shallow marine pro‐delta sediments. The delta front deposits show repetitively stacked, upward‐coarsening parasequences, 8 to 10 m thick, which reflect the repeated progradation of lobate, fluvially‐dominated deltas onto a shallow marine shelf. The deltas formed following marine transgression and accumulated during a period of gradually rising relative sea level. An upper unit, 130 m thick, containing offshore stromatolitic and oolitic limestones, caps the study interval and represents deposition during a relative sea level highstand. The fluvial and delta distributary channel sandstones of the middle unit contain <1% mudstone. The cohesionless substrate determined that deltaic distributaries were predominantly braided in character and subject to common bifurcations which resulted in an ordered diminution of channel size and competence in a seaward direction. Terminal distributary channels show evidence of migratory levées and mouth‐bars and consistently delivered fine to medium‐grained sand to the delta front. The study highlights an example of pre‐vegetation deltaic sedimentation that was hydraulically organized and predictable, despite being fed by braided fluvial systems with high levels of peak discharge.

Quantitative analysis of tight sandstone reservoir heterogeneity based on rescaled range analysis and empirical mode decomposition: A case study of the Chang 7 reservoir in the Dingbian oilfield

Of the characteristics of tight sandstone reservoirs, heterogeneity is one of the most difficult characteristics to analyze. Heterogeneity is mainly manifested in three properties: lithology, porosity and electricity. The Chang 7 reservoir in the Dingbian oilfield is a tight sandstone reservoir controlled by delta front deposits. In this sedimentary environment, the underwater distributary channel oscillates frequently, resulting in considerable heterogeneity of the Chang 7 formation in the study area. The rescaled range method can calculate the fractal dimension D of logging curves, thus quantitatively characterizing reservoir heterogeneity. After analyzing the relationship between average wave number and IMF number by EMD method, the reservoir heterogeneity can be quantitatively characterized by ρ value. By comparing D value, ρ value and F value of different heterogeneous types, it is found that F value can distinguish different heterogeneous types. The order of F value also satisfies the actual situation of formation heterogeneity. The new heterogeneity coefficient is used to study the heterogeneity of Chang 7 reservoir, it is found that the relationship between the lithological heterogeneity represented by the natural gamma (GR) log and oil production is a quadratic function that initially increases and then decreases, that the relationship between the pore heterogeneity represented by the interval transit time (AC) log and oil production is linear, and that the relationship between the reservoir electrical heterogeneity represented by the deep induction log (LLD) and oil production is linear too.

Stratigraphic architecture, shelf-edge delta and constraints on the development of the Late Oligocene to Early Miocene continental margin prism, the Pearl River Mouth Basin, northern South China Sea

The early-stage shelf-margin prism in the Pearl River Mouth Basin (PRMB), northern South China Sea, formed from the Late Oligocene to Early Miocene, comprises a set of shelf-edge delta and slope fan wedges of the Zhuhai Formation and records the development of the continental slope in response to the interaction of sea-level change, tectonic subsidence and sediment supply. Based on integrated analysis of seismic, well logging and core data, the stratigraphic architecture, depositional systems and controlling processes are documented. The Zhuhai Formation is a composite sequence bounded by regional unconformity, and can be further divided into five sequences (CS3-1-CS3-5). The depositional genetic units (transgression, ascending normal regression and descending forced regression) are identified within sequences based on variation in shelf edge trajectories. The shelf deltaic clinoforms are characterized by oblique-tangential reflections in seismic profiles with a thickness of 100–150 m. In contrast, the shelf-margin delta and slope clinoforms generally are 300–1200 m thick and mainly display as sigmoid or sigmoid-tangential seismic reflections. The delta deposits usually consist of thick beds of delta front deposits and prodelta slump and deformed sediments, and are associated with sandy slope fan systems. Eleven delta complexes are distinguished by tracking the distribution of the deltaic clinoforms in each sequence, and these deltaic deposits tend to extend along their strike direction which may be related to the reworking of strong waves and coastal currents. Slope fan systems consist mainly of turbidite channel fills, frontal splay and debris flow deposits, and generally develop along the slope in CS3-3-CS3-5, which may be triggered by steep shelf margin slope and fault activity. The stratigraphic pattern of the shelf margin prism and depositional evolution of the deltaic systems from the Late Oligocene to Early Miocene were controlled by the interaction of sea level changes, sediment supply and tectonic subsidence. The ascending trajectory from the CS3-1 (30 Ma) to the top of the CS3-5 indicates that the relative sea level continued to rise generally but the shelf margin prograded basinward rapidly due to the great amount of sediment supply and lower subsidence rate. Tectonic uplift of the source area and strengthened physical erosion during this period may be the reason for the large sediment supply.

Flow unit characteristics of fan delta front deposits and its influence on reservoir development - Taking yulou oil bearing sets in some experimental area in west depression in Liaohe Basin in China as an example

Taking some steam flooding experimental area in the western depression of the Liaohe Basin in China as an example, it quantitatively evaluates the flow units of the sedimentary facies in fan delta front and analyzes the influences of the flow units on the development of the oilfields. Comprehensive analysis of multifarious data such as geology, core analysis and well logs indicates that porosity, permeability and FZI are key parameters for classification and evaluation of the reservoir flow units. Then using SPSS cluster analysis software platform, cluster analysis are conducted on reservoir physical properties of cored wells, and based on which discriminant analysis are performed to obtain the discriminant formula. The discriminant analysis of reservoir physical properties of the wells without core data is based on fine interpretation of well logging data. Combined with the results of cored wells, the classification and evaluation of the flow units has been completed in the target layer of ​​the whole study area. Finally the flow units are divided into four classes, namely, Class A, Class B, Class C and Class D. Classes A and B flow units represent high-quality reservoirs. The flow units are mainly controlled by sedimentation. Classes A and B are mainly sedimentary microfacies of underwater distributary channel and distributary mouth sand bar. The paper proposes five principles of judging the rationality of flow units, describes the development characteristics of different classes of flow units on section and plane, analyzes the influence of flow units on oilfield development and suggests countermeasures. Specially, in the process of steam flooding, target injection layers should be optimized and steam injection pressure should be effectively controlled to prevent steam channeling; when planning infill wells, injection and production wells should be deployed in the same class of flow unit to ensure good connectivity; the development of remaining oil should focus on Classes A and B flow units, and meanwhile pay more attention to the fine interpretation results of well logging data.

Development and exploration practice of the concept of hydrocarbon accumulation in rifted-basin troughs: A case study of Paleogene Kongdian Formation in Cangdong sag, Bohai Bay Basin

Abstract Based on the merged 3D seismic data, well logging, formation testing, analysis and testing data, the structural evolution, sedimentary reservoirs, thermal evolution of source rocks were investigated of Paleogene Kongdian Formation in the trough area of Cangdong sag, Bohai Bay Basin. A conventional-unconventional hydrocarbon accumulation pattern in the trough area of rifted basin was revealed. The reservoir forming elements in the trough area of Cangdong sag have a zonation feature in terms of reservoirs and source rocks. There are two types of reservoir forming models, primary trough and reformed trough. The formation and evolution of trough controlled the orderly distribution of conventional oil to unconventional oil in the trough. Particularly, structural reservoirs occur in the upper part of the trough, stratigraphic-lithologic reservoirs are likely to form in the delta front deposits at the outer ring of trough, the middle ring transitional belt is the favorable site for tight oil reservoirs, while the fine grain deposits zone in the inner ring is shale oil and gas exploration area. The study has pointed out the new domains and directions for searching reserves in the secondary exploration of mature oilfields.

Depositional sequence response to Paleogene Lake level and Paleoclimate changes in the northern Liaoxi depression, eastern China

Bohai Bay Basin is a Cenozoic rift basin containing abundant oil and gas resources in northeastern China. By combining 3D seismic data and well logs, three composited sequences are identified from regional unconformity relationships, and seven sequences can be further identified from local unconformities. Major depositional systems include fan delta, braided delta, fluvial delta, shallow lake, deep lake, and sublacustrine fan. Through identifying onlap points in the ramp of the Liaoxi depression, a lake level change curve is constructed. Pollen and algae data are used to analyze paleoclimate which shows two large cool-warm cycles in Paleogene Liaoxi depression. Lake level change is controlled by both tectonic subsidence and climate change. Paleoclimate change is the main factor controlling lake level during stable tectonic periods, while tectonic subsidence is the main controlling factor controlling lake level during active tectonic periods. Depositional evolution corresponds with lake level change, which indicates that lake level stays relatively high with deep or shallow lacustrine and delta front deposits developed, and lake level stays relatively low with alluvial fan, braid delta plain, fan delta, and shallow lacustrine deposits developed.

Influence of sedimentary facies on reservoir quality and distribution of diagenetic features in the Funing Formation, Wanglongzhuang Oilfield, Subei Basin, Eastern China

The Paleogene clastic sediments of the Funing Formation, which is characterized by slump-fan, delta-front, and beach-bar deposits, are important targets for oil and gas exploration in the Wanglongzhuang Oilfield, Subei Basin, eastern China. Three categories of reservoir quality are distinguished on the basis of petrographic and petrophysical experiments such as nuclear magnetic resonance (NMR) and mercury injection (MI). A paragenetic sequence for diagenetic evolution of each sedimentary facies has been reconstructed to determine the mechanisms for the variations in reservoir quality of the formation and the influence of the sedimentary facies on the distribution of diagenetic features in each of its members. The maturity of the organic material, textural relationships, and the distribution pattern of diagenetic minerals suggest that the study area underwent mainly mesodiagenesis. The mesodiagenesis A and mesodiagenesis B stages induced different diagenetic features in the target reservoir; the beach-bar sand of the E1f2 unit and the delta-front sand of the E1f4 unit underwent mostly only cementation; the delta-front deposits of the E1f3 unit, however, suffered more from mechanical and chemical compaction than from cementation. The loss of porosity in the fluxoturbidites of E1f4 due to compaction was just as large as the loss by cementation. The gravity-flow channel, sand-bar, and distributary-channel deposits have a higher porosity and permeability than the sand-beach and estuarine-bar deposits.

Characterization of middle Eocene tide-influenced delta: A study from core samples of Hazad Member, Ankleshwar Formation, South Cambay Basin, India

The Hazad Member (Middle Eocene) of the Ankleswar Formation in Cambay Basin, India, is traditionally reported as deltaic system. Present work documents three major facies associations, namely, (i) sandstone-rich upper delta plain (FA-1) deposits, (ii) sandstone-mudstone heterolithic lower delta plain–delta front (FA-2) deposits, and (iii) shale-dominated prodelta (FA-3) deposits, in an overall coarsening-up to fining-up succession. Tidalites are well preserved in FA-2 and are represented by laterally accreted tidal bundles, tidal beddings and vertically accreted tidal rhythmites, described from drill core samples in this communication. Laterally accreted tidal bundles with reactivation surfaces in sand-dominated heterolithic units indicate time-velocity asymmetry in subtidal condition. Tidal beddings and tidal rhythmites in mud-dominated heterolithic units, associated with asymmetric/symmetric ripple forms and desiccation cracks, indicate periodic subaerial emergence in intertidal flat depositional setting. Systematic analysis of the architecture of the tidalites in different parts of the basin signifies rapid shift in sedimentation from subtidal to intertidal flat within the lower delta plain. Transitions from prodeltaic to tidally (subtidal-intertidal) affected delta front to lower delta plain and fluvial-dominated upper delta plain depositional systems attest to high frequency transgressive-regressive cycles in response to changing accommodation, as a result of sea level fluctuations and basinal tectonisms in the Cambay Basin.

Facies delineation and sandstone prediction using seismic sedimentology and seismic inversion in the Eocene Huizhou Depression, Pearl River Mouth Basin, China

Recently, the rift-related successions in the Eocene Huizhou (HZ) Depression have been considered crucial intervals for hydrocarbon exploration in the Pearl River Mouth Basin, China. However, the facies delineation and sandstone distribution are poorly understood due to the limited well data. Our study area is the HZ25 Block in the HZ Depression, which consists of a typical overlapped synthetic transfer zone connecting the HZ26 and XJ30 Sags. Integrated analysis of seismic and logging data indicates that the Eocene Wenchang Formation can be subdivided into four third-order sequences (SQ1–4). Well-based observations reveal that the braided deltaic system is mainly characterized by distributary channels, subaqueous distributary channels, mouth bars, and sheet-sand deposits. The sandstone isopach map acquired by seismic inversion indicates that the sandstone could be 340 m thick in the SQ2 but sharply decreases in the SQ3 in the transfer zone area. Stratal slices make it possible to delineate the facies maps by extrapolating the well observations to the entire 3D seismic survey; it is suggested that the SQ2 deltaic package largely comprises braided delta plain (distributary channels) and front deposits (subaqueous distributary channels and mouth bar) with a broad distribution of [Formula: see text], whereas the SQ3 deltaic package is dominated by delta-front deposits only [Formula: see text] in extent. This study suggests that small-scale fan-delta deposits are confined to the central part of the boundary fault, whereas the transfer zone captures large-scale braided delta sediments with a long transportation distance up to 8 km. The sandstones developed in the northwestern part of the transfer zone within the SQ2 sequence are expected to form potential hydrocarbon reservoirs for future oil exploration. Furthermore, this study may have implications for reservoir prediction and hydrocarbon exploration in other areas lacking well data, such as the HZ Depression.

A location-based multiple point statistics method: modelling the reservoir with non-stationary characteristics

AbstractIn this paper, a location-based multiple point statistics method is developed to model a non-stationary reservoir. The proposed method characterizes the relationship between the sedimentary pattern and the deposit location using the relative central position distance function, which alleviates the requirement that the training image and the simulated grids have the same dimension. The weights in every direction of the distance function can be changed to characterize the reservoir heterogeneity in various directions. The local integral replacements of data events, structured random path, distance tolerance and multi-grid strategy are applied to reproduce the sedimentary patterns and obtain a more realistic result. This method is compared with the traditional Snesim method using a synthesized 3-D training image of Poyang Lake and a reservoir model of Shengli Oilfield in China. The results indicate that the new method can reproduce the non-stationary characteristics better than the traditional method and is more suitable for simulation of delta-front deposits. These results show that the new method is a powerful tool for modelling a reservoir with non-stationary characteristics.

Stratigraphy of the marine Lower Triassic succession at Kap Stosch, Hold with Hope, North-East Greenland

The classical marine uppermost Permian – Lower Triassic succession exposed on the north-east coast of Hold with Hope in East Greenland, south-east of Kap Stosch, is placed in the Wordie Creek Group. A new lithostratigraphic subdivision of the group is proposed here. The group comprises the revised Kap Stosch Formation overlain by the new Godthåb Golf Formation. The Kap Stosch Formation is dominated by alternating fine- and coarse-grained, cliff-forming units that constitute the basis for the erection of eight new members. They are (from below): 1. The Nebalopok Member, uppermost Permian, Hypophiceras triviale ammonoid zone, and lowermost Triassic, lower Griesbachian, Hypophiceras triviale – H. martini ammonoid zones, composed of basinal and base-of-slope siltstones and turbiditic sandstones. 2. The conglomeratic Immaqa Member (H. martini ammonoid zone), consisting of a thick clinoform-bedded unit commonly overlain by horizontally bedded deposits, representing the foreset and topset, respectively, of a Gilbert-type delta. 3. The fine-grained Fiskeplateau Member (H. martini ammonoid zone), composed of siltstones and fine-grained sandstones, representing basinal and delta front deposits. 4. The conglomerate-dominated Knolden Member (H. martini ammonoid zone), comprising a clinoform-bedded unit overlain by horizontally-bedded deposits, representing foreset and topset, respectively, of a Gilbert-type delta. 5. The fine-grained Pyramiden Member, (lower–upper Griesbachian Metophiceras subdemissum, Ophiceras commune and Wordieoceras decipiens ammonoid zones), composed of variegated siltstones and sandstones deposited in proximal basin and slope environments. 6. The Naasut Member (top Griesbachian, probably Wordieoceras decipiens ammonoid zone), dominated by thick structureless coarse-grained sandstones commonly showing clinoform bedding, deposited in slope, base-of-slope and proximal basin environments. 7. The Falkeryg Member (lowermost Dienerian, Bukkenites rosenkrantzi ammonoid zone), comprising thick, commonly pebbly sandstones deposited in shelf, slope and base-of-slope environments. 8. The Vestplateau Member (lower Dienerian, Bukkenites rosenkrantzi ammonoid zone) composed of siltstones and fine-grained sandstones deposited in basinal environments. The overlying Godthåb Golf Formation (Dienerian, Anodontophora breviformis – A. fassaensis bivalve zones) is dominated by shallow marine sandstones with several coarser grained levels. The rich ammonoid faunas of the Wordie Creek Group allow a biostratigraphic zonation which can be correlated with schemes from other parts of the Arctic region. This zonation is complemented with information on palyno, conodont, fish and isotope stratigraphy.

Identifying the source of natural gamma-rays in shallow-marine siliciclastic strata and their significance for shale evaluation: A case study of the CO2 storage aquifer at the Nagaoka site, Japan

Gamma-ray (GR) logging measures naturally occurring gamma radiation in strata, and is a widely used method for reservoir characterization. The objectives of this study are to identify the source of total GR, and to discuss criteria for reasonable shale evaluation by the combined use of sediment core and GR logging analysis using a case study of the CO2 storage aquifer at the Nagaoka site in Japan. The aquifer is comprised of shallow-marine siliciclastic deltaic deposits characterized by prodelta to delta front facies. The profile of total GR output is consistent with that of mud content, suggesting that the main source of GR can be attributed to minerals in the mud fraction. Prior to comparison of the radioactive elements measured by spectral gamma-ray (SGR) logging and elements from core analysis with total GR, elements were transformed to centered log-ratios to remove constant-sum constraints. Evaluation of the total GR profile with radioactive elements from SGR logging indicates that the distribution of thorium (Th) exhibits a high contrast between prodelta and delta front deposits. Th content has a significant positive correlation with total GR, implying that Th-bearing minerals mostly appear in fine-grained fraction. Conversely, the distribution of potassium (K) has a significant negative correlation with total GR, implying that K-bearing minerals appear in coarse-grained fraction. Based on geochemical analysis of the sediment core, minerals composed of Mg, Ti, Fe, Zr, and Th are likely to be radioactive carriers in the fine-grained fraction, and those containing Na, Al, Si, K, Rb, and Sr are considered to be the radioactive carriers in the coarse-grained fraction. The former can be attributed to detrital zircon grains and clay minerals, including smectite and chlorite, which contain or adsorb the radioactive Th, and the latter can be attributed to K-feldspar. The dispersion diagram between total GR and radioactive elements at observation wells emphasizes the differences in the endmembers of total GR between the two depositional sequences. The estimated shale volume is concordant with measured values if we select the endmembers at each depositional sequence. This means that the endmembers should be selected independently for each depositional sequence to enhance the reliability of shale evaluation. For improving shale evaluation, the combined use of GR logging and detailed core analysis in terms of sedimentology and geochemistry appears to be effective for reliably assessing shale volume in the target strata.

Geological Reservoir Characterization and Modelling of a CO2 Storage Aquifer: A Case Study of the Nagaoka Site, Japan

Site characterization is critical for safe and effective geological storage of CO2 because geological heterogeneity affects reservoir quality, plume migration, and trapping. This study shows geological characterization and modeling of a CO2 storage aquifer near Nagaoka, Japan. Core and well-logging analysis show that the target aquifer consists of deltaic deposits, which are characterized by a fining-upward to coarsening-upward succession that has developed on a ravinement surface. The target aquifer is attributed to prodelta and deltafront deposits. A geostatistical method known as sequential Gaussian simulation with collocated cokriging was applied to generate a spatial distribution of petrological and physical properties including shale volume, porosity, and permeability. The simulation was performed using well-log data as a primary variable and seismic-derived porosity data as a secondary variable under the constraints of correlation coefficient between variables. Simulated models showed that impermeable fine-grained deposits are distributed toward the east. This trend can explain the fact that breakthrough has not been observed at an observation well located farthest to the west from the injection well, IW-1. This result is consistent with previously proposed paleogeographic interpretations that show that detrital particles are transported from east to west at the Nagaoka site.

The Lower Cretaceous succession of the northwestern Barents Shelf: Onshore and offshore correlations

The Lower Cretaceous succession in the Barents Sea is listed as a potential play model by the Norwegian Petroleum Directorate. Reservoirs may occur in deep to shallow marine clastic wedges located in proximity to palaeo-highs and along basin margins. In addition, shelf-prism-scale clinoforms with high amplitude anomalies in their top- and bottomsets have been reported from reflection seismic but they have never been drilled. In Svalbard, the exposed northwestern corner of the Barents Shelf, Lower Cretaceous strata of shelfal to paralic origin occur, and includes the Rurikfjellet (Valanginian–Hauterivian/lowermost Barremian), Helvetiafjellet (lower Barremian–lower Aptian) and Carolinefjellet formations (lower Aptian–middle Albian). By combining sedimentological outcrop studies and dinocyst analyses with offshore seismic and well ties, this study investigate the link between the onshore strata and the offshore clinoforms. Age-vise, only three (S1–S3) of the seismic sequences defined in the offshore areas correlate to the onshore strata; S1 correspond to the Rurikfjellet Formation, S2 to the Helvetiafjellet Formation and the lower Carolinefjellet Formation, and S3 to the upper Carolinefjellet Formation. Offshore, all three sequences contain generally southward prograding shelf-prism-scale clinoforms. A lower Barremian subaerial unconformity defines the base of the Helvetiafjellet Formation, and its extent indicates that most of the Svalbard platform was exposed and acted as a bypass zone in the early Barremian. Onshore palaeo-current directions is generally towards the SE, roughly consistent with the clinoform accretion-direction towards the S. The local occurrence of a 150 m thick succession of gravity flow deposits transitionally overlain by prodelta slope to delta front deposits in the Rurikfjellet Formation, may indicate that shelf-edges also developed in Svalbard. The late Hauterivian age of theses deposits potentially highlights the inferred offlapping nature of the Lower Cretaceous strata as they predate the lower Barremian unconformity, and thus record a hitherto unknown regression in Svalbard. The presence of the lower Barremian subaerial unconformity in Svalbard, the general southeastward palaeo-current directions, and the age-equivalent clinoform-packages south of Svalbard, suggests that the onshore and offshore strata is genetically linked and was part of the same palaeo-drainage system.

Interplay of tidal and fluvial processes in an early Pleistocene, delta-fed, strait margin (Calabria, Southern Italy)

The architecture and morphodynamics of modern and ancient tidal straits and in particular the deposits of strait-margin zones, have been significantly understudied compared to other marginal marine settings, even though many reservoirs in the North Sea and the Norwegian Continental Shelf are developed in narrow grabens or seaways. This paper presents a detailed sedimentological and stratigraphic analysis of an early Pleistocene marginal-marine succession deposited along the northern margin of the Siderno paleostrait (southern Italy). This area represents an excellent case study of sedimentation along a tidal strait margin, interpreted to record the interaction of fluvial and tidal processes. Here, syn-depositional tectonics produced a complex coastal morphology, significantly influencing sedimentation and hydrodynamic processes. Along the strait margin, the emplacement of an isolated tectonic high (Piano Fossati) created a ca. 3.5 km-wide local passageway. This morpho-structural element induced interplays between fluvio-deltaic processes (fed from the northern strait margin) and tidal current reworking (active within the marine strait).The field-based facies analysis reported here documents an initial stage of non-tidal shallow-marine sedimentation across the strait. A subsequent regression caused river-generated hyperpycnal flows and the transfer of large volumes of pebbly and shelly sandstones into deeper water. Tidal currents became amplified in the strait, and, in the delta-front area, they were able to rework river-derived sediments generating large dune fields. Following the local tidal transport pathway, strong tidal currents skewed the delta front (causing it to be asymmetrical) and elongated sand bodies in a direction parallel to the marine strait axis. Differently from the classical tide-influenced deltas in which onshore-offshore tidal flow predominates, coast-parallel deflection and strong asymmetry of delta-front deposits is a typical feature of deltas entering tide-dominated seaways and straits, where strong tidal currents are capable of dispersing large volumes of sand for significant distances along the coast and along the strait axis. This process became progressively enhanced during the following transgression, when tide-modulated currents reworked biocalcarenitic sands over the previous delta deposits, generating southeasterly migrating dunes. At the end of the transgression, strandplain deposits formed in this area. This last stage of sedimentation was followed by a dramatic regional-scale structural uplift, which ended any marine circulation within the strait. This work provides new insights on sedimentation in a tide-dominated strait, and helps to predict sandbody distribution along the strait margin and axis. These findings can be applied to any other setting characterized by a narrow (possibly structurally-confined) basin dominated by tidal currents.

Facies analysis and its relation to point-sourced growth faults in river-dominated prodeltaic delta front deposits of the Cretaceous Ferron Notom Delta, Utah, USA

This study evaluates the evolution of a small-scale growth fault within a deltaic parasequence of the Ferron Sandstone, Utah. Analysis is based on 4 stratigraphic logs, and a panorama covering 100 m of lateral exposure.Five growth-fault blocks are displayed in strike and dip views. The listric faults dip basinward, and sole out into bedding parallel decollement surfaces associated with deformed prodelta shales, which also form diapirs. Throw on individual faults averages 4 m and heave increases with depth from ∼2.5 to ∼5 m. Strikes on fault planes average 160°, and fault dip decreases with depth from 50° to 0°, where they become parallel to bedding.Pre-growth facies comprise heterolithics with abundant graded beds, current-ripples, wave-ripples, small-scale soft sediment deformation, and a low bioturbation index. This indicates deposition in a river-dominated, storm-influenced delta that was supplied primarily by storm-triggered flood events and prograded rapidly. Growth strata are primarily planar and quasi-planar fine sandstones deposited as upstream and downstream accreting mouth bars.Faulting initiates with the building of a “critical load” reflecting a low-angle delta front foreset, probably linked to deposition in a mouth bar. Mobile prodelta muds form diapirs that breached the sediment-water interface, providing accommodation for the fault blocks. Termination of faulting occurred when sandy growth strata contacted and locked into higher strength pre-growth heterolithics deeper in the section. Minor deformation of growth strata is demonstrated by synthetic microfaulting in growth sands.This study emphasizes a localized stress field (i.e., point sourced) associated with sandy mouth bar deposits, which acted as fault initiation points. This contrasts with the distributed stress regimes commonly associated with regional-scale growth faults along continental margins. This can lead to expanded, but localized, reservoir compartments within river-dominated deltaic systems, especially those in epeiric seas or other shallow-water sedimentary basins, such as Prudhoe Bay Field, Alaska.

Process regime variability across growth faults in the Paleogene Lower Wilcox Guadalupe Delta, South Texas Gulf Coast

The Wilcox Group in Texas is a 3000m thick unit of clastic sediments deposited along the Gulf of Mexico coast during early Paleogene. This study integrates core facies analysis with subsurface well-log correlation to document the sedimentology and stratigraphy of the Lower Wilcox Guadalupe Delta. Core descriptions indicate a transition from wave- and tidally-influenced to wave-dominated deposition. Upward-coarsening facies successions contain current ripples, organic matter, low trace fossil abundance and low diversity, which suggest deposition in a fluvial prodelta to delta front environment. Heterolithic stratification with lenticular, wavy and flaser bedding indicate tidal influence. Pervasively bioturbated sandy mudstones and muddy sandstones with Cruziana ichnofacies and structureless sandstones with Ophiomorpha record deposition in wave-influenced deltas. Tidal channels truncate delta front deposits and display gradational upward-fining facies successions with basal lags and sandy tabular cross-beds passing into heterolithic tidal flats and biologically homogenized mudstones.Growth faults within the lower Wilcox control expanded thickness of sedimentary units (up to 4 times) on the downdip sides of faults. Increased local accommodation due to fault subsidence favors a stronger wave regime on the outer shelf due to unrestricted fetch and water depth. As the shoreline advances during deltaic progradation, successively more sediment is deposited in the downthrown depocenters and reworked along shore by wave processes, resulting in a thick sedimentary unit characterized by repeated stacking of shoreface sequences. Thick and laterally continuous clean sandstone successions in the downthrown compartments represent attractive hydrocarbon reservoirs. As a consequence of the wave dominance and increased accommodation, thick (tens of meters) sandstone-bodies with increased homogeneity and vertical permeability within the stacked shoreface successions are created.

Event Sedimentation, Deposition Rate, and Paleoenvironment Using CrowdedRosseliaAssemblages of the Bluesky Formation, Alberta, Canada

Abstract Spectacular examples of well-preserved, stacked Rosselia occur in cores of the Lower Cretaceous Bluesky Formation from Alberta's Peace River oil-sands deposit. Stacked Rosselia segments reflect burrow readjustments of single tracemakers following erosion and sedimentation events. Where present, these traces help to refine paleoenvironmental interpretations and can be used as proxies for assessing the magnitude and frequency of depositional events. In this study, six cores were logged in detail. Two representative cores containing densely packed (crowded), stacked Rosselia were described sedimentologically and ichnologically and burrow lengths and numbers were measured. The Bluesky sections are interpreted to represent storm-influenced, wave-dominated delta-front deposits on the basis of modal sedimentation measurements and the presence of storm- and wave-generated sedimentary structures. The stacked Rosselia burrows record up to four post-depositional re-establishments per tracemaker, with each readjustment representing a response to abrupt sediment accumulation. Decimeter-length burrows occurring in the core datasets suggest multiple decimeter-scale depositional events in a relatively short time frame, i.e., months to perhaps a few years, depending on the lifespan and growth rate of the organism. The use of Rosselia in this study illustrates a method of evaluating sedimentation events within a brief temporal window, one lying beyond the resolution of more traditional dating methods. Furthermore, the assemblages of crowded, stacked Rosselia are presented as the first documented occurrence from the Mesozoic and the first recognized in a cored succession.