Prograding Wedge Research Articles

Miocene facies associations and sedimentary evolution of the Southern Transylvanian Basin (Romania): Implications for hydrocarbon exploration

The Transylvanian Basin is a mature hydrocarbon province of Romania characterized by two petroleum systems: Mesozoic (thermogenic) and Miocene (biogenic). An extensive outcrop-based sedimentological and micropaleontological study correlated to seismic and well data discusses the elements of the Miocene petroleum system. The facies associations are indicative of alluvial, fandelta, shallow- and deep-marine settings. These are grouped into four different depositional systems (evaporite, mud-carbonate, sand-mud and sand-gravel). Their evolution in time and space shows large differences between various parts of the basin that have important consequences for exploration. The Transylvanian gas is formed by more than 99% methane of bacterial origin. This is sourced by low quality (<1% TOC) deep-marine shales. The shales contain Type II and Type III kerogen. The organic material is thermally immature. The best source rocks were deposited during major transgressions in the central-eastern parts of the basin. In general, reservoir quality is the best (porosity < 20%, permeability < 1 D) in the basin center, where reservoirs are deep-marine turbidite sandstones. Lower quality reservoirs are conglomerate-rich slope channels and various shallow-marine sandstones located near the basin margins. The seals are formed by shales that hold gas columns of up to 60 m. The most common structural traps are in 4-way dip closures related to salt-cored folds. Their timing is coeval with the late (post-Pannonian) exhumation of the basin and strongly linked to coeval salt tectonics. This requires a late charge and migration. The largest traps typically have multistory (up to 20) pay zones with a total of 100 BCF to 1 TCF reserves. Exploration to date has focused on structural traps, but most of the obvious structures have been drilled. It is argued that significant exploration potential lies in stratigraphic plays, including confined submarine fans, slope channels, detached lowstand prograding wedges, incised valleys, diapir flanks, salt-tectonics related unconformities and various sub-volcanic plays. Risks of the petroleum system elements associated to these plays in different areas of the basin are discussed.

Sequence stratigraphy and architecture of a late Early–Middle Aptian carbonate platform succession from the western Maestrat Basin (Iberian Chain, Spain)

The attributes of a ‘four-systems-tract’ sequence are at times difficult to identify in outcrop-scale carbonate successions. Poor exposure conditions, variable rates of sediment production, erosion and/or superposition of surfaces that are intrinsic to the nature of carbonate systems frequently conceal or remove its physical features. The late Early–Middle Aptian platform carbonates of the western Maestrat Basin (Iberian Chain, Spain) display facies heterogeneity enabling platform, platform-margin and slope geometries to be identified, and provide a case study that shows all the characteristics of a quintessential four systems tract-based sequence. Five differentiated systems tracts belonging to two distinct depositional sequences can be recognized: the Highstand Systems Tract (HST) and Forced Regressive Wedge Systems Tract (FRWST) of Depositional Sequence A; and the Lowstand Prograding Wedge Systems Tract (LPWST), Transgressive Systems Tract (TST) and subsequent return to a highstand stage of sea-level (HST) of Depositional Sequence B. An extensive carbonate platform of rudists and corals stacked in a prograding pattern marks the first HST. The FRWST is constituted by a detached, slightly cross-bedded calcarenite situated at the toe of the slope in a basinal position. The LPWST is characterized by a small carbonate platform of rudists and corals downlapping over the FRWST and onlapping landwards. The TST exhibits platform backstepping and marly sedimentation. Resumed carbonate production in shelf and slope settings characterizes the second HST. A basal surface of forced regression, a subaerial unconformity, a correlative conformity, a transgressive surface and a maximum flooding surface bound these systems tracts, and are well documented and widely mappable across the platform-to-basin transition area analyzed. Moreover, the sedimentary succession studied is made up of four types of parasequence that constitute stratigraphic units deposited within a higher-frequency sea-level cyclicity. Ten lithofacies associations form these basic accretional units. Each facies assemblage can be ascribed to an inferred depositional environment in terms of bathymetry, hydrodynamic conditions and trophic level. The architecture of the carbonate platform systems reflects a flat-topped non-rimmed depositional profile. Furthermore, these carbonate shelves are interpreted as having been formed in low hydrodynamic conditions. The long-term relative fall in sea-level occurred during the uppermost Early Aptian, which subaerially exposed the carbonate platform established during the first HST and resulted in the deposition of the FRWST, is interpreted as one of global significance. Moreover, a possible relationship between this widespread sea-level drop and glacio-eustasy seems plausible, and could be linked to the cooling event proposed in the literature for the late Early Aptian. Because of the important implications in sequence stratigraphy of this study, the sedimentary succession analyzed herein could serve as an analogue for the application of the four-systems-tract sequence stratigraphic methodology to carbonate systems.

Icehouse, cool-water carbonate ramps: the case of the Upper Pliocene Capodarso Fm (Sicily): role of trace fossils in the reconstruction of growth stages of prograding wedges

An integrated approach, based on the use of trace fossils combined with analysis of physical and biogenic structures, identification of key surfaces, and reconstruction of stratigraphic architecture, proved to be of critical value in defining the depositional environments, elucidating the dynamics of progradation, and characterizing the various systems tracts of Upper Pliocene progradational wedges (Capodarso area, Sicily) generated by cool-water carbonate ramps. The studied succession consists of a stack of six cycles, consisting of terrigenous mudstones passing upwards into biocalcarenite wedges with distinct clinoforms. The prograding biocalcarenite bodies show seaward-increasing height and steepness of the clinoformed front as a result of development into increasingly deeper water, and may be regarded as the record of distally steepened ramps, dominated by storm-induced downwelling flows. Evidence of forced-regressive progradation is provided by stratal geometries, physical structures, and trace fossil assemblages existing at the top of the bodies, which attest to a gradual increase in energy level at the top of the ramp, concurrently with the progression of seaward outbuilding of the bodies. Three trace-fossil suites (Thalassinoides/Piscichnus, Scolicia, Dactyloidites) are shown to be linked with the successive growth stages of individual prograding wedges, whereas abandonment stages, characterized by starvation of sediment input, are marked by the Ophiomorpha suite. The Capodarso ramps, like other Plio-Pleistocene equivalents of the Mediterranean area, are small, high-gradient ramps, with stratigraphical architecture controlled by high-amplitude, orbitally driven glacio-eustatic changes. An ecologically and bathymetrically based subdivision of the ramps into inner, mid and outer ramp environments is hardly applicable, as most of the wedge progradation occurs in highly dynamic conditions, dominated by physical processes of transport and resedimentation of skeletal material, which result in faunal composition dominated by allochthonous material. The use of trace fossils is of critical value in this context, due to the scarcity of ecological information provided by faunal elements.

Lateglacial–Holocene shoreface progradation offshore eastern Scotland: a response to climatic and coastal hydrographic change

Seismic facies, provenance and marine faunal associations of a nearshore prograding sediment wedge offshore eastern Scotland are studied to investigate environmental changes in the adjacent North Sea during Lateglacial–Holocene time. The sediments form part of the St. Andrews Bay Member (Forth Formation), which is divided into four lithozones (L1–L4) that represent distinct pulses of sedimentation during the sequential growth of the sediment wedge. Radiocarbon dates, combined with the local curve of relative sea level change, indicate that progradation was initiated as a fluvio‐deltaic deposit (L1) during the Younger Dryas Stadial. Further construction of the sediment package took place during the mid‐ to late Holocene by sublittoral tidal processes that deposited three discrete, highstand shoreface wedges (L2–L4), which display both progradation and longshore migration (to the NE), and may have experienced episodic brackish marine conditions. A depositional cyclicity of about 1000 years is proposed for lithozones L2–L4, separated by hiatuses of 1000–2000 years. We tentatively suggest that the Holocene development of the prograding wedge offshore eastern Scotland was a response to phases of strong westerly winds driving an enhanced influx of Atlantic Water into the North Sea. A concomitant increase in rainfall may account for the freshening of the coastal zone at this time. However, correlation with the recently postulated global periods of Holocene rapid climate change remains unclear.

The underestimated Proterozoic component of the Canadian Cordillera accretionary margin

Abstract Analysis of several types of seismic and potential field geophysical data consistently indicate that the majority of the crust underlying the Canadian Cordillera and much of western Canada was originally Proterozoic sedimentary rocks shed off the Canadian Shield into rift or basin structures between 1.84 and 0.54 Ga. These variably metamorphosed strata were primarily quartz- and limestone‐rich sediments and thus have distinctive geophysical signatures because of their lower density, lower magnetization, and lower Poisson's ratio compared with more mafic rocks. The sediments formed a prograding wedge that has a distinctive, internally reflective, seismic stratigraphy. In the east, these Proterozoic sedimentary rocks thicken at a ‘hinge line’ defined by the margin of the pre-1.84 Ga crystalline basement of the Canadian Shield; previous work mapped this hinge line locally using deep reflection profiles and regionally using distinctive gravity gradients. Here we assemble previously published results of several geophysical methods to define the overall shape of the wedge along the margin and westward to where it pinches out at the modern Moho beneath the crustal collage of exotic and suspect terranes accreted onto North America during the Mesozoic. The volume of crust occupied by this wedge limits the thickness of most accreted terranes to several kilometres and suggests that deeper portions of the accreted blocks detached or underthrust the wedge during accretion and are no longer contiguous to crust exposed at the surface. This type Cenozoic accretionary orogen thus spent most of its prior geological history as a passive or extensional margin punctuated by only a few, brief convergent or accretionary events.

Sequence Stratigraphic Appraisal: Coastal Swamp Depobelt In The Niger Delta Basin Nigeria

Mid-Lower Miocene Agbada sedimentary intercalations of “AB” Field in the coastal swamp depobelt, Western Niger-Delta, were evaluated to determine their sequence stratigraphic character. The analysis was based on a combination of data sets including logs of six wells to describe lithic variations of the Agbada Formation within the ‘AB' Field. These well logs were integrated with biostratigraphic data to develop a sequence stratigraphic framework. The results revealed two maximum flooding surfaces dated 10.4Ma and 9.5Ma. A sequence boundary dated at 10.35Mya was also identified across the field. The strata in the study area were divided into lowstand system tracts (basin floor fan, slope fan and prograding wedge), transgressive and highstand systems tracts. Biostratigraphic data suggested that these sediments were deposited in coastal deltaic to bathyal marine environments. Basal deposits directly overlying sequence boundaries where formed by the migration of large distributary channels. Upward coarsening sets of inclined beds, hundreds of feet thick, record progradation of deltas into slope. Blocky and upward fining well-log patterns are interpreted to reflect deposition in shoreline, paralic, and fluvial environments. The thick sequences of highly microfossiliferous are the probable petroleum source rocks. The massive sand formation of the basin floor fan, the sand-rich prograding wedge and the highstand sands as well as the transgressive sands constitute good reservoirs. The distal shale toes of the prograding wedge and transgressive shales as well as highstand shales form seals for the stratigraphic traps formed in the study area. Keywords: Sequence stratigraphic tool, system tract deposits. Global Journal of Geological Sciences Vol. 6 (2) 2008: pp. 129-137

High frequency sea level fluctuations recorded in the Black Sea since the LGM

This paper presents a 3D geometric interpretation of very high resolution seismic Chirp profiles acquired on the Romanian shelf during ASSEMBLAGE European Project. The results provide a solid record of the Black Sea Last Glacial Maximum (LGM) water level fluctuations. This pseudo-3D seismic interpretation shows that the Black Sea lacustrine shelf deposits form a significant basinward-prograding wedge system. On top of these prograding sequences is a set of sand dunes that delineates a wave cut-terrace like feature around the isobath − 100 m. Landward of this dune field are small depressions containing barkhan-like bodies. The upper part of the last prograding sequence is incised by anastomosed channels which end in the Danube (Viteaz) canyon which are also built on the lacustrine prograding wedge. Ten stratigraphic units were distinguished and correlated with analyses of cores retrieved from this area. The results demonstrate that the first eight sequences represent lacustrine prograding wedges, the ninth sequence is the dune system itself and the tenth is a marine mud drape which covers the entire continental shelf. The lacustrine prograding wedges represent a lowstand deposit characterised by forced regression-like reflectors mapped from the pseudo-3D seismic data. Their hinge point corresponds to the wave erosion surface mapped around − 100 m isobath on the multibeam mosaic. Dated cores give age control on this lowstand period, which lasted from 11 to 8.5 kyr 14C BP as implied by: (1) the continuously dry climatic conditions in the region inferred by high percentages of herbs and steppe elements determined from ASSEMBLAGE cores and, (2) the formation of dunes between 10 and 8.5 kyr 14C BP on the desiccated north-western Black Sea shelf at − 100 m and (3) all of these covered by a marine mud drape confirming that the dune system is no longer active. The buried, anastomosed fluvial channels that suddenly disappear below 90 m depth, and a unique wave-cut terrace between 95 and 100 mbsl on the outer shelf are also consistent with a major lowstand base-level at around − 100 m water depth. Preservation of sand dunes and the occurrence of small, buried incised valleys mark a rapid transgression within less than a century during which ravinement processes related to the water level rise had no time to significantly erode the seafloor.

Sedimentary response to climate and sea level changes during the past ∼400 ka from borehole PRAD1–2 (Adriatic margin)

Borehole PRAD1–2 was drilled in ∼186 m water depth on the upper slope of the central Adriatic, in the frame of Profiles across Mediterranean Sedimentary Systems (PROMESS1) European Union‐funded project. The borehole penetrated 71.2 m through a stratigraphic interval characterized by subparallel seismic reflections and uniform seismic units. According to an age‐depth model based on several independent proxies (including foraminifera and nannoplankton stratigraphy, ∂18O curves, and magnetostratigraphy) the cored interval records Marine Isotope Stages and Substages (MIS) from MIS1 to the top of MIS11, thus encompassing the past ∼370 ka. PRAD1–2 therefore represents an unprecedented continuous record through the last four glacial‐interglacial cycles from a proximal continental margin setting where depositional sequences are typically composed of progradational units. These progradational units record dominantly interglacial intervals (MIS5, MIS7, and MIS9) and appear composed of thicker highstand deposits (HST) formed during interstadials and thinner forced‐regression units (FSST) deposited during stadials above distinctive downward shift surfaces. The development of thicker highstand deposits with a distinctively thicker bottomset reflects enhanced shore‐parallel advection any time sea level rise leads to the drowning of the Adriatic shelf, triggering the formation of dense water and vigorous cyclonic circulation. This advection mechanism persisted in each cycle throughout the early phases of the sea level fall but progressively decreased as sea level fall proceeded approaching the maximum lowstand position, when most of the shelf became exposed. Relative sea level falls punctuating interglacials within each 100‐ka cycle were thus accompanied by a dearth in sediment flux on the outer shelf. The alternation of HST and FSST progradational wedges with markedly different thickness and downlap geometry of their bottomsets is the most evident stratigraphic signature, within each 100‐ka depositional cycle, of the impact on the shelf of higher‐frequency (∼20 ka) sea level cycles and concomitant supply fluctuations.

Growth patterns of a proximal terrigenous margin offshore the Guadalfeo River, northern Alboran Sea (SW Mediterranean Sea): glacio-eustatic control and disturbing tectonic factors

The shelf-upper slope stratigraphy offshore and around the Guadalfeo River on the northern continental margin of the Alboran Sea, Western Mediterranean Basin, has been defined through the interpretation of a grid of Sparker seismic profiles. We tried to identify evolutionary trends in shelf growth, as well as to determine the regional/local factors that may modify the influence of glacio-eustatic fluctuations. Four major depositional sequences are identified in the sedimentary record by a detailed seismic interpretation, which defines three significant intervals of shelf-upper slope progradation, dominated by deposition of shelf-margin wedges, which resulted in uniform patterns of shelf-margin growth in response to significant sea-level falls. In contrast, the record of transgressive intervals is more variable, mainly as the result of distinct patterns of regressive-to-transgressive transitions. Major progradational wedges are internally composed of seaward-prograding, landward-thinning wedges, interpreted to represent shelf-margin deltaic deposits. In contrast, the last aggradational interval is composed of shelf-prograding wedges that show distinct characteristics, in terms of seismic facies, morphology and distribution when compared with previous shelf-margin wedges. These shelf wedges are thought to represent the particular case of Regressive Systems or Shelf Margin Systems Tracts, and their development seems to be controlled by a drastic change in main depocenter location, which moved from the upper slope to the shelf during the Pleistocene. The stacking pattern of seismic units, the shallowness of the acoustic basement and the migration of the shelf break are used to infer spatial and temporal changes in tectonic subsidence-uplift rates, which interact with low-order glacio-eustatic changes. For much of the Pliocene-Quaternary, uplifted sectors alternated laterally with sectors experiencing more subsidence. Subsequently, a significant change from lateral outgrowth to vertical accretion is recognised. This stratigraphic change could be related to the combined influence of increased subsidence rates on the shelf and the onset of higher-frequency glacio-eustatic cyclicity after the Mid Pleistocene Revolution that occurred around 1 Ma.

Land–sea correlation between Late Holocene coastal and infralittoral deposits in the SE Iberian Peninsula (Western Mediterranean)

The well-exposed systems of prograding beach ridges on the Carchuna–Calahonda (Granada) and Campo de Dalías–Roquetas (Almería) coastal plains continue offshore as infralittoral prograding wedges (IPW). The Holocene IPW is a narrow morpho-sedimentary unit up to 2.5 km wide which develops seaward from the lower edge of the shoreface to 15–20 m depth, extending to a well-defined break of slope at water depths of 35–40 m. These IPWs have been recognized and studied using very high-resolution seismic profiles (TOPAS) and multibeam data (EM-3000D). In detail they are complex morpho-sedimentary units in which internal structures are closely linked to the pattern of progradation of the adjacent coastal plains. When longshore currents produce significant littoral drift, the IPWs are composed of several minor units arranged in offlap, which accrete parallel or oblique to the main shoreline. Therefore, it is possible to correlate progradational units in the coastal plain (H-units, sensu[Goy, J.L., Zazo, C., Dabrio, C.J., 2003. A beach-ridge progradation complex reflecting periodical sea-level and climate variability during the Holocene (Gulf of Almería, Western Mediterranean). Geomorphology 50, 251–268]) and subunits in the IPW, but special care is required depending on the local arrangement of morpho-sedimentary units. Besides, it is not realistic to draw conclusions regarding the age of the subunits inside a given IPW without adequate dating, as the number of subunits will greatly vary from place to place depending on local factors, magnitude of sea-level oscillations, and sediment supply.

Development of depositional systems in the southeastern Yellow Sea during the postglacial transgression

This study focuses on the development of depositional systems in the southeastern Yellow Sea during the postglacial transgression, based on the analysis of closely spaced high-resolution seismic profiles (Chirp) and piston cores. Transgressive deposits comprise the paralic component of estuarine channel fills, seaward prograding wedge, and along-shore prograding mound, and the marine component of sand veneer, mud blanket, and offshore ridge. These transgressive deposits were developed through three distinctive stages in terms of possible sea-level position. During the early stage, tidal sand ridges and sand veneer formed south of Jeju Island, resulting from shoreface erosion associated with a relatively slow transgression. The following transgression, in the middle stage, inundated subaerially formed interfluves along the central part of the sea, and affected development of estuarine environments with increasing tidal regime in the eastern part of the sea. Combined with the strong tidal reworking, insufficient sand supply enhanced the formation of erosional ridges. Short-period increase in fluvial discharge affected the deposition of coastal wedge in the western part. Fine-grained sediments from either fluvial discharge or tidal reworking were dispersed seaward to form mud blanket. The late stage was characterized by the deposition of veneered sands off the Jiangsu coast, which was due to relatively slow rise in sea level. Further offshore, along-shore prograding mound was constructed by Huanghe-derived sediment influx, and subsequently eroded and redeposited by wave and tide activities, thus detached from the proximal source. The depositional processes and variability of the transgressive deposits in the southeastern Yellow Sea were largely controlled by changes in rate of sea-level rise during the transgression, most likely related to the global flooding events triggered by melt water pulses. The changes in shoreline configuration and oceanographic regime have also influenced the distribution and morphology of the transgressive deposits. A short-period increase in riverine discharge caused the temporal sediment progradation off the Jiangsu coast.

Sedimentary processes in the Wilkes Land margin: a record of the Cenozoic East Antarctic Ice Sheet evolution

Multichannel seismic data collected off Wilkes Land (East Antarctica) reveal four main units that represent distinct phases in the evolution of the Cenozoic depositional environment. A Cretaceous synrift succession is overlain by hemipelagic and distal terrigenous sequences deposited during Phase 1. Sediment ridges and debris-flow deposits mark the transition to Phase 2. Unit 3 records the maximum sediment input from the continent and is characterized by the predominance of turbidite deposits. During Phase 4 the sediment supply from the continental margin was reduced, and draping and filling were the dominant processes on the continental rise. Unit 4 also contains the deposits of sediment wave fields and asymmetric channel-levee systems. These four units are a response to the Cenozoic evolution of the East Antarctic Ice Sheet. During Phase 1, small ice caps were formed in the innermost continental areas. The ice volume increased under temperate glacial regimes during Phases 2 and 3, when large volumes of melt-water production led to high sediment discharge to the continental rise. Change to a polar regime occurred through Phase 4, when a thick prograding wedge developed on the continental shelf and slope and the sediment transport to the rise diminished, producing general starvation conditions.

Eocene Turbidite-Population Statistics from Shelf Edge to Basin Floor, Spitsbergen, Svalbard

Abstract There is a need for better tools in the interpretation of depositional sub-environments of deep-marine facies. Using seismic-scale outcrops of Eocene shelf-margin clinoforms in Spitsbergen, we have statistically characterized the turbidite beds in terms of grain size, bed thickness, and dominant sedimentary structure at four different lowstand sites in individual, mappable clinoforms. These sub-environments include: (1) upper-slope canyon and gully fill, (2) upper to middle-slope late prograding wedge, (3) lower-slope channel–levee complex, and (4) basin-floor fan. The falling-stage basin-floor fan and slope canyon/gully deposits are generally coarser grained than the rising-stage sub-environments. They also have far fewer siltstone and mudstone beds and have significantly more beds of upper-medium to very coarse grain size compared to the rising-stage channel–levee and prograding-wedge deposits. Thin beds are particularly voluminous in the late prograding-wedge and channel–levee systems, whereas only the basin-floor fans and canyon fill have large numbers of beds thicker than 10 cm. Turbidity currents deposit sediment at distinctly different sites between shelf edge and basin floor at different times during the development of the base-level cycle. Differences between these sub-environments are both qualitative and quantitative and are demonstrable using statistical analysis. The results of this statistical analysis from well-exposed, easily identifiable architectural elements can be used to reconstruct the shelf margins of analogous basins, where only sparse outcrop or well data are available.

Understanding growth-faulted, intraslope subbasins by applying sequence-stratigraphic principles: Examples from the south Texas Oligocene Frio Formation: Discussion

Brown et al. (2004) recently elaborated on previous sequence-stratigraphic models (e.g., Mitchum et al., 1990) to interpret large rollovers in the Frio Formation of Texas as depositional basins, in contrast to earlier interpretations of structural basins, filled primarily with shallow-water deposits (e.g., Ewing, 1986; Galloway and Morton, 1989). This discussion describes fundamental shortcomings in their observations and interpretations, specifically 1. Deep-water Frio facies are farther downdip and deeper than predicted by Brown et al. (2004). 2. Their model ignores the tenfold magnitude difference between the gradients of growth faults and clinoforms. 3. The comparatively thicker section on the downthrown block reflects greater subsidence rates associated with focused structural extension and not the development of steeply dipping fault scarps at the shelf margin. 4. Multiple methods of correlating well logs in growth-faulted regions are essential, including those disparaged as “traditional” by Brown et al. (2004), particularly in a “sequence-stratigraphic” framework. 5. Their sequence-stratigraphic model makes no reference to the past 15 yr of work on progradational components (i.e., highstand, lowstand prograding wedge, forced regressive wedge, and falling stage systems tracts); this especially hampers the stratigraphic interpretation of the progradational facies that dominate these growth-faulted zones. The disruption of stratigraphy and structure that occurs across growth faults has challenged geologists for many years. Which changes in thickness, age, depositional environments, and facies are related to growth faulting? Strata in growth-faulted settings are commonly characterized by shoreline environments and their resulting coarsening-upward regressive units or parasequences. In many situations, it is relatively straightforward to recognize parasequences in well logs, clearly bounded by marine flooding surfaces, expanding downdip across several growth faults, and it thus seems that an increase in subsidence rate was the cause of the thickening (e.g., figure 13 of Edwards, 1981). These parasequences display concomitant facies changes from proximal to distal (e.g., figures …

Development of Incisions on a Periodically Emergent Carbonate Platform (Natih Formation, Late Cretaceous, Oman)

Abstract Subaerial exposure of carbonate platforms is generally recorded by karstification and pedogenesis, whereas erosion features such as incisions along emersion surfaces have seldom been observed and studied. However, the recognition of incisions and the characterization of their fill may facilitate definition and hierarchy of sequence boundaries, especially along exposure surfaces that do not otherwise present clear petrographic evidence for emersion. In the Natih Formation (Late Albian–Early Turonian, Oman), two successive incision surfaces are present in the upper part of the first third-order depositional sequence. Seismic interpretation of regional subsurface data has allowed the quantification of progradational geometries within the Natih sequence I carbonate platform and the correlation of incision surfaces with forced regressive prograding wedges on the margins of the intrashelf basin. Detailed geological sections and correlations have been made from three outcrop localities, allowing a precise description of geometries and facies of incision fills. Morphology, orientation, and extent of these incisions have been assessed from detailed seismic interpretation coupled with forward seismic modeling. The integration of outcrop and seismic data sets at the regional and local scales allows the interpretation and discussion of the origin and factors controlling the development of these incisions and a refinement of the stratigraphic model. These two incisions record rapid sea-level variations (500 ky) with a magnitude of approximately 20 and 30 m that occurred during the early Cenomanian. This is probably not a unique case, since for the same time interval, incisions have been observed in siliciclastic systems in western Canada and India and in carbonate systems on the Arabian plate.

Potential Eocene and Oligocene stratigraphic traps of the Rockall Plateau, NE Atlantic Margin

Abstract Following thermal uplift during the late Paleocene to early Eocene, the denudation of the subaerial hinterland provided a massive sediment supply that led to the development of a number of large, prograding sedimentary wedge systems flanking the Hatton and Rockall basins. Regional seismic data mapping and borehole data indicate that the wedges are Eocene in age and have a high percentage of coarse clastic material typical of high-energy, fluvial or near-shore marine environments. The prograding wedges have been mapped and can be viewed as large, clastic fairways within which trapping at a number of scales exists. Seismic interpretation suggests that the wedges are present at various stratigraphic levels within the Eocene and are locally separated by unconformities. However, all pre-date the margin-wide late Eocene unconformity (C30), which resulted in subsidence and deepening of the Rockall and Hatton basins. A marine transgression inundated most former land areas, and a marked change occurred in basinal facies; a change from fluvial/near-shore clastic sedimentation to deep-water mud and ooze deposition influenced by bottom-currents. These conditions persisted throughout most of the Oligocene and Neogene and hence provided a seal for potential hydrocarbon-bearing sand-prone Eocene reservoirs internal to the wedge-systems. Additional sealing potential may be provided by shale layers internal to the wedges. Buried Eocene pinchout lobes, submarine fans at the base of basalt scarp faces and Oligocene slump deposits also provide potential trapping mechanisms. High, and probably unacceptable, risks include biodegradation and poor seal development due to the typically shallow depth of burial of the wedges. However, the majority of the wedges should be treated as analogues, with some of the deeper examples providing some scope for consideration as exploration targets. The scale of the prograding wedge play fairway is massive, with volumes measured in tens of cubic kilometres.

Margin architecture reveals the transition to the modern Antarctic ice sheet ca. 3 Ma

Research Article| April 01, 2006 Margin architecture reveals the transition to the modern Antarctic ice sheet ca. 3 Ma Michele Rebesco; Michele Rebesco 1Istituto Nazionale di Oceanografia e di Geofisica Sperimentale, Borgo Grotta Gigante 42/C, 34010 Sgonico (TS), Italy Search for other works by this author on: GSW Google Scholar Angelo Camerlenghi; Angelo Camerlenghi 2Istituto Nazionale di Oceanografia e di Geofisica Sperimentale, Borgo Grotta Gigante 42/C, 34010 Sgonico (TS), Italy, and Institució Catalana de Recerca i Estudis Avançats, c/o GRC Geociències Marines, Departament d'Estratigrafia, P. i Geociències Marines, Universitat de Barcelona, C/ Martí i Franquès, s/n, E-08028 Barcelona, Spain Search for other works by this author on: GSW Google Scholar Riccardo Geletti; Riccardo Geletti 3Istituto Nazionale di Oceanografia e di Geofisica Sperimentale, Borgo Grotta Gigante 42/C, 34010 Sgonico (TS), Italy Search for other works by this author on: GSW Google Scholar Miquel Canals Miquel Canals 4GRC Geociències Marines, Departament d'Estratigrafia, P. i Geociències Marines, Universitat de Barcelona, C/ Martí i Franquès, s/n, E-08028 Barcelona, Spain Search for other works by this author on: GSW Google Scholar Author and Article Information Michele Rebesco 1Istituto Nazionale di Oceanografia e di Geofisica Sperimentale, Borgo Grotta Gigante 42/C, 34010 Sgonico (TS), Italy Angelo Camerlenghi 2Istituto Nazionale di Oceanografia e di Geofisica Sperimentale, Borgo Grotta Gigante 42/C, 34010 Sgonico (TS), Italy, and Institució Catalana de Recerca i Estudis Avançats, c/o GRC Geociències Marines, Departament d'Estratigrafia, P. i Geociències Marines, Universitat de Barcelona, C/ Martí i Franquès, s/n, E-08028 Barcelona, Spain Riccardo Geletti 3Istituto Nazionale di Oceanografia e di Geofisica Sperimentale, Borgo Grotta Gigante 42/C, 34010 Sgonico (TS), Italy Miquel Canals 4GRC Geociències Marines, Departament d'Estratigrafia, P. i Geociències Marines, Universitat de Barcelona, C/ Martí i Franquès, s/n, E-08028 Barcelona, Spain Publisher: Geological Society of America Received: 27 Jun 2005 Revision Received: 16 Nov 2005 Accepted: 16 Dec 2005 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2006) 34 (4): 301–304. https://doi.org/10.1130/G22000.1 Article history Received: 27 Jun 2005 Revision Received: 16 Nov 2005 Accepted: 16 Dec 2005 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Michele Rebesco, Angelo Camerlenghi, Riccardo Geletti, Miquel Canals; Margin architecture reveals the transition to the modern Antarctic ice sheet ca. 3 Ma. Geology 2006;; 34 (4): 301–304. doi: https://doi.org/10.1130/G22000.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Seismic reflection data collected primarily on the continental slope of the Pacific margin of the Antarctic Peninsula after drilling Ocean Drilling Program (ODP) Leg 178 allow us to date a regional change in the style of margin accretion to ca. 3 Ma. Late Pliocene deep erosion of continental shelf and slope, which likely included the emplacement of a megadebris-flow deposit, was followed by prominent growth of steep, relatively stable continental slope prograding wedges, while the deep distal margin sedimentation rate was significantly reduced. A review of the available stratigraphy from Deep Sea Drilling Project–ODP drilling, and correlation with available seismic stratigraphic information, allowed us to recognize comparable changes in terms of age and trends in continental margin evolution in several places around Antarctica. We argue that this late Pliocene architectural change of the Antarctic margin reflects a change in the texture and water content of the sediment delivered by the Antarctic ice sheet following the transition from wet- to dry-based ice regimes. This circum-Antarctic change coincides with the late Pliocene (ca. 3 Ma) global cooling and is proposed as the marker event of the transition to the modern cold polar dry-based Antarctic ice sheet. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Success Factors in Troll Geosteering

This article, written by Technology Editor Dennis Denney, contains highlights of paper OTC 17110, “Success Factors in Troll Geosteering,” by S. Leiknes and I. Osvoll, Norsk Hydro ASA, prepared for the 2005 Offshore Technology Conference, Houston, 2–5 May. Copyright 2005 Offshore Technology Conference. Reproduced by permission. Hydro has produced oil from horizontal wells in the thin oil layer of the Troll West field offshore Norway since 1995. The main challenge is to drain the thin target sands efficiently, within a remain-ing oil column less than 20 m thick covering an area of approximately 500 km2. New horizontal producers are continually being drilled. Vertical geosteering is performed on a centimeter scale, and quick decision making within the well teams is needed to optimize the well paths while drilling. Introduction The Troll field is the largest producing oil and gas field on the Norwegian continental shelf. The field is in the northern North Sea on the margin of the Viking graben and is divided by two major north/south trending faults separating the field into three provinces: Troll West oil province, Troll West gas province, and Troll East. Oil has been produced from horizontal wells in the Troll West oil and gas provinces where the initial oil-column thickness was 22 to 27 m and 11 to 13 m, respectively. More than 150 horizontal producers have been drilled in Troll West, and approximately two-thirds of the total oil reserves of 1.4 billion bbl has been produced. The oil column in Troll East is 1 to 4 m thick, and only the huge gas cap has been considered commercial. New horizontal producers are drilled in the progressively thinning oil layer. As shown in Fig. 1, well planning and drilling are increasingly difficult because of the ever-larger number of existing producers, the consequent decrease in remaining targets, and movement of the fluid contacts resulting from production. Collaboration between geophysicists, geologists, and reservoir and production engineers is crucial to meet this challenging task. Reservoir Framework The reservoir consists of stacked, off-lapping, shallow marine sandstones and intervening siltstones mostly in the Upper Jurassic (Oxfordian) Sognefjord formation. Reservoir zones correspond to the finer and coarser portions of the prograding wedges (parasequences) and can be mapped with good confidence over most of the field. For geomodeling and well-planning purposes, the depositional succession is simplified into clean sand and micaceous sands. The clean sands generally are well sorted, with permeabilities ranging from 1 to 30 darcies, while the micaceous sands are characteristically finer and silty, with permeabilities ranging from 10 to several hundred millidarcies.

Characterization of deltaic sediment bodies based on in situ CPT/CPTU profiles: A case study on the Llobregat delta plain, Barcelona, Spain

This article describes a preliminary study on the Llobregat delta, Spain, which includes the construction of a 3D model from cone penetration tests (CPT) and piezocone tests (CPTU) in order to establish the architectural stacking pattern of deltaic sediment bodies. The sediment facies identified from boreholes have been characterized by using their mechanical behaviour and have been used to identify the depositional sequence of the Llobregat delta. Within the general architectural frame, flood plain silts and silty sands, crevasse sands, fluvial channel sands, sand ridges, delta front silts and clays, and prodelta silts and clays have been characterized as a function of cone tip resistance ( q c) and sleeve friction ( f s). From the sediment facies characterization, a depositional sequence constituted by three systems tracts is proposed. The sequence is divided into: a lowstand systems tract (LST) represented by fluvial gravels, a transgressive systems tract (TST) constituted by a fining-upward sequence with a thin sand sheet below silts and clays, and the highstand systems tract (HST) represented by three prograding wedges displaying coarsening-upward sequences. We emphasize the simplicity, speed, high spatial resolution and low cost of CPT/CPTU grids as a new tool enabling us to reconstruct the three-dimensional structure of deltaic environments for sedimentological and stratigraphical purposes.

Late Cenozoic prograding wedges on the NW European continental margin: their formation and relationship to tectonics and climate

During late Pliocene to Pleistocene times, prominent prograding wedges were deposited along the continental margin of NW Europe, resulting in seaward shelf break migration of up to 150 km. Much of the sediment accumulation occurred marginal to the former mid- to high-latitude ice sheets. The geographical distribution, and stratigraphical and chronological data may suggest that the instigation of the wedges was variously related to tectonic uplift as well as a response to the late Pliocene to Pleistocene climate deterioration and onset of major northern hemisphere glaciations. The onset of wedge growth on the NW UK and Irish margins was initiated at about 4 Ma in response to tectonic tilting of the margin in that region. However, glacially derived sediments here comprise a significant proportion of the wedges, especially since 0.44 Ma. For the Faroe margin, no detailed chronology is available; however, it may be inferred that onset of glacigenic wedge growth here did not post-date that observed on the NW UK and Irish margins. Offshore Norway, wedge growth has largely occurred since ca. 2.7 Ma in response to northern hemisphere glaciations, also recording a major change in sediments transport routes at 0.8–1.1 Ma (reflecting larger Fennoscandian Ice Sheets). Presently, it is uncertain whether the glacigenic wedge growth was preceded by a fluvial phase (in response to uplift) in this area. In the western Barents Sea, an early phase of wedge growth was (glacio) fluvial in character. Off western Spitsbergen, the development was similar to that of the Barents Sea although the glacigenic wedge-growth phase may have started somewhat earlier. The wedges commonly display gently inclined seaward prograding clinoforms, and transparent to chaotic internal acoustic facies. Sampling of their sediments reveals that they are mainly composed of glacigenic diamicton interbedded with marine and glaciomarine sediments that, to various extents, have been affected by bottom-current action. The clinoforms of these wedges vary in geometry from oblique to sigmoidal, and they also show varying degrees of aggradation throughout their development. The resulting stratal stacking pattern can be attributed to a combination of variations in sediment supply, sedimentary processes, and accommodation space, the latter being a function of tectonic movements and/or loading induced subsidence as well as eustatic sea-level fluctuations.