Submarine Fan Sequence Research Articles

Lithological typing, depositional environment, and reservoir quality characterization of the \u201cM-Field,\u201d offshore Douala Basin, Cameroon

Sedimentary features such as channel sands and submarine fan sequences are important hydrocarbon reservoirs throughout the world and may be interpreted from bed thickness trends and log motifs. The log motifs of four well logs were employed during this study to delineate reservoir architectural depositional features and environment, determine their lithological attributes, and evaluate reservoir properties. Gamma ray log signatures were utilized to infer depositional features, discriminate litho-units, and define depositional environment; resistivity log was employed to identify reservoir zones and formation fluids, while the combined density and neutron logs aided in the estimation of reservoir porosity and lithological typing. Results of analysis show seven laterally discontinuous reservoirs: one in Well X (X2), three in Well Y (Y1, Y2, Y3), and three in Well W (W1, W2, and W3) with thicknesses of 6.2 m, 19.2 m, 7.6 m, 78.7 m, 100.9 m, 38.1 m, and 41.6 m, respectively. Reservoirs architecture reveal distributary channel sands, submarine fan lobes, and deep tidal channel-fills deposited within the deep marine mud-rich N’kapa Formation. Average porosity, water saturation, and hydrocarbon saturation values of Well X and Well Y reservoirs are 20.8%, 30.8%, 69.2%, and 40.2%, 18.3%, 81.7%, respectively, indicating moderate-to-good reservoir quality. Also, the permeability values are moderate to good (74.89–12,584 md), except for Y3 and Well W reservoirs whose well log-derived permeabilities were unreliable due to their non-irreducible nature. Density-neutron crossplots indicate heterogeneous reservoir lithologies comprising of limestone, dolomite, and sandstones, with predominant calcite and subordinate dolomite as mineral matrix. Crossplot of water saturation and porosity revealed grain-size variation ranging from fine-grained to silty sediments with bulk volume water values suggesting Well X and Y reservoirs to be homogeneous and will produce water-free hydrocarbons, while Well W reservoirs are heterogeneous water-prone reservoirs.

Refining accretionary orogen models for the Tasmanides of eastern Australia

The well-known southwest-to-northeast younging of stratigraphy over a present-day cross strike distance of >1500 km in the southern Tasmanides of eastern Australia has been used to argue for models of accretionary orogenesis behind a continually eastwards-rolling paleo-Pacific plate. However, these accretionary models need modification, since the oldest (ca 530 Ma) outcrops of Cambrian supra-subduction zone rocks occur in the outboard New England Orogen, now ∼900 km east of the next oldest (520–510 Ma) supra-subduction zone rocks. This is not consistent with simple, continuous easterly rollback. Instead, the southern Tasmanides contain an early history characterised by a westwards-migrating margin between ca 530 and ca 520 Ma, followed by rapid eastwards rollback of the paleo-Pacific plate from 520 to 502 Ma that opened a vast backarc basin ∼2000 km across that has never been closed. From the Ordovician through to the end of the Carboniferous, the almost vertical stacking of continental margin arcs (within a hundred kilometres of each other) in the New England Orogen indicates a constant west-dipping plate boundary in a Gondwana reference frame. Although the actual position of the boundary is inferred to have undergone contraction-related advances and extension-related retreats, these movements are estimated to be ∼250 km or less. Rollback in the early Permian was never completely reversed, so that late Permian–Triassic to Cretaceous arcs lie farther east, in the very eastern part of eastern Australia, with rifted fragments occurring in the Lord Howe Rise and in New Zealand. The northern Tasmanides are even more anomalous, since they missed out on the middle Cambrian plate boundary retreat seen in the south. As a result, their Cambrian-to-Devonian history is concentrated in a ∼300 km wide strip immediately west of Precambrian cratonic Australia and above Precambrian basement. The presence in this narrow region of Ordovician to Carboniferous continental margin arcs and backarc basins also implies a virtually stationary plate boundary in a Gondwana frame of reference. This bipolar character of the Tasmanides suggests the presence of a segmented paleo-Pacific Plate, with major transform faults propagating into the Tasmanides as tear faults that were favourably oriented for the formation of local supra-subduction zone systems and for subsequent intraplate north–south shortening. In this interpretation of the Tasmanides, Lower–Middle Ordovician quartz-rich turbidites accumulated as submarine fan sequences, and do not represent multiple subduction complexes developed above subduction zones lying behind the plate boundary. Indeed, the Tasmanides are characterised by the general absence of material accreted from the paleo-Pacific plate and by the dominance of craton-derived, recycled sedimentary rocks.

Downfan Transformation from Turbidity Currents to Debris Flows at a Channel-to-Lobe Transitional Zone: The Lower Pleistocene Otadai Formation, Boso Peninsula, Japan

Transformation of turbidity currents into debris flows across a channel-to-lobe transition zone was recognized on the basis of lateral mapping of a falling-stage-systems-tract deposit in the lower Pleistocene Otadai Formation submarine-fan sequence on the Boso Peninsula, Japan. The most proximal outcrops comprise 1 to 4 m thick turbidites, which are laterally equivalent to debrites encased in turbidites in outcrops farther down flow. Such turbidites and encased debrites were deposited from single flow events. The most distal outcrops comprise only thinner (2 to 40 cm thick) turbidites. Debrites contain many siltstone clasts and finer-grained sediments, which are characterized by deep-water faunas and clay-mineral composition similar to those of interbedded hemipelagites. Therefore, the transformation of turbidity currents into debris flows is interpreted to have occurred in response to the incorporation of many siltstone clasts and finer-grained sediment particles into the precursor turbidity currents through erosion of muddy substrates. Subsequently, turbulence in the precursor turbidity currents was suppressed and near-bed flow with higher sediment concentration developed. Intense erosion of muddy substrates is interpreted to have occurred in response to the increase in intensity of turbulence in the precursor turbidity currents at the mouths of middle-fan channels. The precursor turbidity currents (or turbidity currents generated by dilution of debris flows) produced a turbidite deposit, which was subsequently overrun by the debris flow in a proximal lobe environment. The debris flows are interpreted to have subsequently been transformed into turbidity currents, which are recorded by turbidite deposition in the more distal-lobe environment. Debrites can characterize the proximal portion of an attached-lobe deposit in a channel-to-lobe transition zone. Such debrites can be significant heterogeneities for fluid flows in sand-prone successions from middle-fan channel to attached-lobe deposits.

Geodynamic evolution of the outer-arc–forearc belt in the Andaman Islands, the central part of the Burma–Java subduction complex

The Andaman Islands, the central part of Burma–Java subduction complex, expose tectonostratigraphic units of an accretionary prism in an outer-arc setting and turbidites of a forearc setting. A number of N–S-trending dismembered ophiolite slices of Cretaceous age, occurring at different structural levels with Eocene trench-slope sediments, were uplifted and emplaced by a series of E–dipping thrusts. Subsequently, N–S normal and E–W strike-slip faults resulted in the development of a forearc basin with deposition of Oligocene and Mio-Pliocene sediments. Metapelites and metabasics of greenschist to amphibolite grade occur in a melange zone of ophiolites. The Eocene Mithakhari Group represents pelagic trench sediments and coarser clastics derived from ophiolites. Evidence of frequent facies changes, predominance of mass flow deposits, syn-sedimentary basinal disturbance and wide palaeogeographic variation indicate deposition of Eocene sediments in isolated basins of an immature trench-slope setting. Deposition of the Oligocene Andaman Flysch Group in a forearc setting is indicated by the large-scale persistence of beds, lack of small-scale lithological variation, bimodal provenance, less deformation, a well-defined submarine fan sequence and development predominantly on the eastern part of the outer arc. The Mio-Pliocene Archipelago Group includes alternations of siliciclastic turbidites and subaqueous pyroclastic flow deposits in the lower part and carbonate turbidites in the upper part, suggesting its deposition in the shallower forearc compared to the siliciclastic Oligocene sediments.

The Central Brae Field, Blocks 16/07a, 16/07b, UK North Sea

abstract The Central Brae Oilfield is the smallest of three Upper Jurassic fields being developed in UK, Block 16/07a. The field was discovered in 1976 and commended production in September 1989 through a sub-sea template tied back to the Brae 'A' platform in the South Brae Oilfield. The field Stooip is 244 MMBBLs, and by May 1999 cumulative exports of oil and NGL reached 44 MMBBLs. The Central Brae reservoir is a proximal submarine fan sequence, comprising dominantly sand-matrix conglomerate and sanstone with a minor mudstone units. The sediments were shed eastwards off the Fladen Ground Spur and were deposited as a relatively small and steep fan at the margin of the South Viking Graben. Mudstone facies border the submarine fan deposits to the north and south, forming stratigraphic seals. The structure is a faulted anticline developed during the latest Jurassic and early Cretaceous, initially formed as a hangingwall anticline during extension but subsequently tightened during compressional phases. The western boundary of the field is formed by a sealing fault, whilst to the east, there is an oil-water contact at 13426 ft TVDss. The overlying seal is the Kimmeridge Clay Formation, which also interdigitates with the coarser facies basinwards and provides the source of the hydrocarbons.

Fe‐Illites in A Proterozoic Deep Marine Slope Deposit in the Penganga Group of the Pranhita Godavari Valley: Their Origin and Environmental Significance

Ferric illite (Fe‐illite) developed extensively in a siliciclastic submarine fan sequence within a lime‐mud succession in the Proterozoic Penganga Group at Adilabad, South India. The minerals formed as de novo by preferentially replacing high‐K minerals like alkali feldspar and mica. It is compositionally intermediate between glauconite and illite in a continuously varying chemical spectrum. The mineral exhibits an R1 or R3 type stacking sequence, with low (<10%) interlayering of illite and smectite (I/S). All the analyzed grains are characterized by high potash content, and it has been inferred that the maturity is dictated by high ak in the microenvironment. This occurrence strongly calls into question the environmental significance that has so far been attributed to glauconitic minerals.

Abstract: Inner and Middle Submarine Fan Sequences in the German Rancho Formation (Paleocene-Eocene) on the Gualala Block, Sonoma and Mendocino Counties, California

Abstract: Inner and Middle Submarine Fan Sequences in the German Rancho Formation (Paleocene-Eocene) on the Gualala Block, Sonoma and Mendocino Counties, California

The Forties Field - 20 Years Young

Summary This paper describes how the development of the Forties field has evolved over the past 20 years; the current issues and activities; and the direction for the future. Introduction In October 1970, BP' s semi-submersible drilling rig SeaQuest struck oil in Upper Palaeocene (Tertiary) sandstones of UKCS Block 21/10 (Figure 1). Four appraisal wells were drilled in 1970 and 1971, which delineated a large oil reservoir at a depth of 2135 metres, covering an area of 93 km2. The maximum thickness of the oil-bearing sandstone was estimated at 187 metres. Reserves were calculated to be 1800 mmstb from an oil-in-place of 4600 mmstb, and production was projected to continue until the year 2000. In 1971 Shell/Essodrilled a successful well, 22/6-1, which demonstrated the south-eastern extension of the field into Block 22/6. The field is a four way dip closed anticline which overlies the Forties-Montrose Ridge and is situated close to the junction of the Central, South Viking and Witch Ground Grabens (Figure 2). The reservoir occurs in thick Upper Palaeocene sandstones deposited as a major sand-rich submarine fan sequence. The field is underlain by a large active aquifer. The development plan evolved during 1972-75. The first major oil field development in the North Sea, it comprised four fixed steel drilling/production platforms, installed in Block 21/10 in 1974 and 1975, with a total capacity for108 wells, a production system capable of handling 500 mbd of oil, later increased to 600 mbd, with 100 mbd of produced water, and a sea water injection system with a capacity of 600 mbd. A schematic diagram of the production system is shown in Figure 3. Development drilling began in June 1975 and production started on 12th September 1975. Plateau production of 500 mbd was reached in1978 and lasted until 1981. A fifth platform, smaller and without processing facilities, was added over the south-eastern extension (South East Forties -SEF) during 1985, with production beginning in March 1987. For more detail on the reservoir reasoning behind the original development scheme see Reference1. Forties is currently producing 111 mbd, the field water cut has only recently passed 60% and 90% of the reserves have been produced. Reservoir Management Reservoir performance has been characterised by excellent vertical sweep through clean sands by the underlying, very mobile aquifer, supported by peripheral sea water injection. There are sufficient permeability restrictions between the three parts of the field, namely SEF, the Main Sand and the Charlie Sand, to cause significant pressure differences between them (Figure 4). Other localised isolated regions have been identified in the FD area during recent infill drilling. The management of the reservoir has been based on treating the three regions separately in terms of pressure support requirements. The primary thrust of reservoir management, common to all areas, has been to track the movement of water within the field to maximise vertical and areal sweep. This has been achieved by performing water shut-off when advantageous to oil flow rate, applying artificial lift when possible, and targeting infill wells at poorly swept areas. With the onset of sea water breakthrough in production wells, bariumsulphate scale deposition has increased in down-hole tubulars and surface facilities (Reference 2). Extended life scale inhibitor squeeze treatments into the formation have been required to inhibit scale deposition. Calcium carbonatescale has also been a problem in gas lifted wells. Generally the deposition of scale is inhibited, but in some cases inhibitor deployment has not been successful. Acid (for carbonate) or chelator treatments have had limited success, while in severe cases wells have been successfully restored to production by milling with coiled tubing (Reference 3).

Turbidite deposition from multiple sources; Quaternary Paola Basin (eastern Tyrrhenian Sea)

ABSTRACT The western margin of the Calabrian Arc is underlain by a thick (as much as 5 km) post-Miocene sedimentary succession. The uppermost part of this margin sequence is referred to as the Paola Basin System (Quaternary in age) and partially fills a present-day physiographic slope basin. Seismic and high. resolution reflection profiles reveal that a prominent unconformity marks the base of the Paola Basin System (PBS). The unconformity connects upslope to an area of recurrent shelf. edge instability and is overlain by large mass-failure deposits over much of the basin area. Basin-wide sedimentary drape deposits form stratigraphic markers that subdivide the Paola Basin System into three stages, each with unique patterns of slope erosion, including mass wasting, and lower.slope to basin-floo turbidite deposition. Three major sediment pathways elongated perpendicular to the margin developed between the unconformity and the first basin-wide drape. Upsection, sediment input for turbidite sedimentation in Paola Basin has involved many, commonly ephemeral, point sources. Most of the turbidite deposits do not form complete submarine fan sequences but consist of only a few turbidite elements. Changes in the size and type of turbidite elements through time suggest a decrease in the size and/or duration of individual sediment sources in the latest Pleistocene. The Paola Basin System is an example of a complex fill of a tectonically controlled basin where the effects of cyclic eustatic changes in sea level modify the effects of longer-term changes in sedimentation driven by tectonic ctivity in the source region and structural deformation in the depositional area.

Ordovician contourites in the Lachlan Fold Belt, eastern Australia

The Ordovician sequence in the northeastern Lachlan Fold Belt consists of three lithologic units representing an overall fining-upward succession from turbidites to black shale. The lowest unit, dominated by turbidite beds, represents a small segment of an extensive Ordovician submarine fan sequence that covered much of southeastern Australia. The overlying unit consists predominantly of ripple-bedded sandstone with beds 3–5 cm thick. These beds are texturally mature, containing heavy minerals, mica and silt concentrated on the cross-laminae. The ripple-bedded unit has a mean palaeocurrent trend to 321° which is at 60° to sole marks and almost normal to the trend of ripple cross-bedding in the underlying and associated turbidite beds. This unit is here interpreted as a sandy contourite sequence that can be attributed to contour currents moving northwards along the eastern margin of Gondwana from areas of subpolar downwelling off southern Africa and America. The two major fining-upward successions in the Ordovician sequence in southeastern Australia may reflect eustatic sea level rises during the Middle and Late Ordovician and thus provide evidence of interglacial periods in the Darrawilian and late Gisbornian to Bolindian. The contourites would have accumulated in the latter stages of the glacial periods after sea level had risen sufficiently to cut off major sediment supply to the submarine fan but before the temperature had risen to the extent of preventing deep thermohaline circulation.

The Magnus Field, Block 211/7a, 12a, UK North Sea

abstract Magnus is the most northerly producing field in the UK sector of the North Sea. The oil accumulation occurs within sandstones of an Upper Jurassic submarine fan sequence. The combination trap style consists of reservoir truncation by unconformity at the crest of the easterly dipping fault block structure and a stratigraphic pinchout element at the northern and southern limits of the sand rich fan. The reservoir is enveloped by the likely hydrocarbon source rock, the organic rich mudstones of the Kimmeridge Clay Formation.

The Central Brae Field, Block 16/7a, UK North Sea

Abstract Central Brae Oilfield is the smallest of three Upper Jurassic fields being developed in UK Block 16/7a. The field was discovered in 1976 and commenced production in September 1989 through a subsea template tied back to the Brae 'A' platform in the South Brae Oilfield. Recoverable reserves are estimated as 65 MMBBL of oil and 6 MMBBL of NGL. The Central Brae reservoir is a proximal submarine fan sequence, comprising dominantly sand-matrix conglomerate and sandstone with minor mudstone units. The sediments were shed eastwards off the Fladen Ground Spur and were deposited as a relatively small and steep sedimentary cone at the margin of the South Viking Graben. Mudstone facies border the submarine fan deposits to the north and south, forming stratigraphic seals. The structure is a faulted anticline developed during the latest Jurassic and early Cretaceous possibly in response to large-scale rotational slump movement within the easterly-dipping graben margin sequence. The western boundary of the field is formed by a sealing fault, whilst to the east, there is an oil-water contact at 13 426 ft TVSS. The overlying seal is the Kimmeridge Clay Formation, which also interdigitates with the coarser facies basinwards, and provides the source of the hydrocarbons.

Mixing of Biogenic Siliceous and Terrigenous Clastic Sediments: South Belridge Field and Beta Field, California: ABSTRACT

The intermixing and interbedding of biogenically derived siliceous sediment with terrigenous clastic sediment in reservoirs of upper Miocene age provides both reservoir rock and seal and influences productivity by affecting porosity and permeability. Miocene reservoirs commonly contain either biogenic-dominated cyclic diatomite, porcelanite, or chert (classic Monterey Formation) or clastic-dominated submarine fan sequences with interbedded or intermixed siliceous members of biogenic origin. Biogenic-clastic cycles, 30-180 ft thick, at South Belridge field were formed by episodic influx of clastic sediment from distant submarine fans mixing with slowly accumulating diatomaceous ooze. The cycles consist of basal silt and pelletized massive diatomaceous mudstone, overlain by burrowed, faintly bedded clayey diatomite and topped by laminated diatomite. Cycle tops have higher porosity and permeability, lower grain density, and higher oil saturation than clay and silt-rich portions of the cycles. Submarine fan sediments forming reservoirs at the Beta field are comprised of interbedded sands and silts deposited in a channelized middle fan to outer fan setting. Individual turbidites display fining-upward sequences, with oil-bearing sands capped by wet micaceous silts. Average sands are moderately to poorly sorted, fine- to medium-grained arkosic arenites. Sands contain pore-filling carbonate and porcelaneous cements. Porcelaneous cement consists of a mixture of opal-A, opal-CT,more » and chert with montmorillonite and minor zeolite. This cement is an authigenic material precipitated in intergranular pore space. The origin of the opal is biogenic, with recrystallization of diatom frustules (opal-A) into opal-CT lepispheres and quartz crystals. Porcelaneous cement comprises 4-21% of the bulk volume of the rock. Seventy percent of the bulk volume of the cement is micropore space.« less

Submarine fans: Characteristics, models, classification, and reservoir potential

Submarine-fan sequences are important hydrocarbon reservoirs throughout the world. Submarine-fan sequences may be interpreted from bed-thickness trends, turbidite facies associations, log motifs, and seismic-reflection profiles. Turbidites occurring predominantly in channels and lobes (or sheet sands) constitute the major portion of submarine-fan sequences. Thinning- and thickening-upward trends are suggestive of channel and lobe deposition, respectively. Mounded seismic reflections are commonly indicative of lower-fan depositional lobes. Fan models are discussed in terms of modern and ancient fans, attached and detached lobes, highly efficient and poorly efficient systems, and transverse and longitudinal fans. In general, depositional lobes are considered to be attached to feeder channels. Submarine fans can be classified into four types based on their tectonic settings: (1) immature passive-margin fans (North Sea type); (2) mature passive-margin fans (Atlantic type); (3) active-margin fans (Pacific type); and (4) mixed-setting fans. Immature passive-margin fans (e.g., Balder, North Sea), and active-margin fans (e.g., Navy, Pacific Ocean) are usually small, sand-rich, and possess well developed lobes. Mature passive-margin fans (e.g., Amazon, Atlantic Ocean) are large, mud-rich, and do not develop typical lobes. However, sheet sands are common in the lower-fan regions of mature passive-margin fans. Mixed-setting fans display characteristics of either Atlantic type (e.g., Bengal, Bay of Bengal), or Pacific type (Orinoco, Caribbean), or both. Conventional channel-lobe models may not be applicable to fans associated with mature passive margins. Submarine fans develop primarily during periods of low sea level on both active- and passive-margin settings. Consequently, hydrocarbon-bearing fan sequences are associated generally with global lowstands of sea level. Channel-fill sandstones in most tectonic settings are potential reservoirs. Lobes exhibit the most favorable reservoir quality in terms of sand content, lateral continuity, and porosity development. Lower-fan sheet sands may also make good reservoirs. Quartz-rich sandstones of mature passive-margin fans are most likely to preserve depositional porosity, whereas lithic sandstones of active-margin fans may not.

A sequential back-arc and foreland basin thrust duplex model for the Southern Uplands of Scotland

Thrust imbrication of Ordovician and Silurian submarine fan sequences overlapping pelagic deposits in the Southern Uplands has been interpreted in terms of an accretionary prism formed above a NW-directed subduction zone. Structural features invoked to support accretion are not definitive and could be explained in terms of a thin-skinned thrusting model. New palaeocurrent and compositional evidence from Llandeilo to mid-Llandovery age turbidites in the northern part of the Southern Uplands proves interdigitation of sediments with strongly contrasting petrography. Turbidites derived from the south contain significant quantities of fresh andesitic detritus whereas those from the north form more mature quartz-rich formations. This implies a back-arc situation; the turbidites being deposited in a basin with a relatively mature continental landmass to the north and a rifted continental fragment containing an active volcanic arc to the south. Oblique collision of the opposing continental margins of the Iapetus Ocean during the Llandovery caused the cessation of subduction. Underthrusting of the southern margin initiated a SE-propagating thrust stack which deformed the back-arc basin sequence and may eventually have ramped over the eroded and faulted remains of the volcanic arc. A southward-migrating foreland basin formed ahead of the rising thrust stack and is now represented by the late Llandovery Hawick Group and Wenlock sequences. Mid- to end-Silurian sinistral strike-slip resulted from oblique collision and produced a transpressional regime during which reactivation of deep-seated structures allowed the intrusion of lamprophyre dykes and granites.

Seismic and well evidence for the early development of the Faeroe–Shetland Basin

Synopsis The Faeroe–Shetland Basin occurs between the Faeroe and Shetland Islands north of the United Kingdom mainland. Most geologists currently favour an early Cretaceous rifting event for the origin of this basin. However, data from cores, cuttings, geophysical logs and seismic reflection profiles pertaining to well 206/5–1 show that Lower, Middle and Upper Jurassic marine sediments were deposited within the Faeroe–Shetland Basin. Therefore this basin had formed by the early Jurassic. The early and mid-Jurassic sediments probably record submarine fan sequences; the late Jurassic sediments are interpreted as fault scarp talus marginal to a deep muddy marine basin. As well 206/5–1 did not reach metamorphic basement, older sediments must also be present within the Faeroe-Shetland Basin. Southeast of the Faeroe-Shetland Basin, the West Shetland Basin accumulated a wedge of Middle and Upper Jurassic nearshore sediments showing that both basins were then subsiding under the same regional tectonic influence. Three wells in the West Shetland Basin terminated at approximately 2.2 km, 2.6 km and 2.7 km within Triassic sediments. Below this, seismic data demonstrate that up to 3.9 km of older sediment remains undrilled above metamorphic basement. We therefore confidently infer a Permian or older origin for the West Shetland Basin. Seismic data shows the Jurassic and pre-Jurassic shapes of the West Shetland Basin to be almost identical. If the same tectonic control also influenced the Faeroe-Shetland Basin, we must infer a Permian or older age for the Faeroe-Shetland Basin. A long-lived deep marine Faeroe-Shetland Basin implies the presence of pre-Mesozoic hydrocarbon source rocks, plus the possibility of deeply buried reservoirs.

THE GEOLOGY AND EVOLUTION OF THE SOUTH PEPPER HYDROCARBON ACCUMULATION

The South Pepper field, discovered in 1982, is located 30 km southwest of Barrow Island in the offshore portion of the Barrow Sub-basin, Western Australia. The oil and gas accumulation occurs in the uppermost sands of the Lower Cretaceous Barrow Group and the overlying low permeability Mardie Greensand Member of the Muderong Shale.The hydrocarbons are trapped in one of several fault closed anticlines which lie on a high trend that includes the North Herald, Pepper and Barrow Island structures. This trend is postulated to have formed during the late Valanginian as the result of differential compaction and drape over a buried submarine fan sequence. During the Turonian the trend acted as a locus for folding induced by right-lateral wrenching along the sub-basin edge. Concurrent normal faulting dissected the fold into a number of smaller anticlines. This essentially compressional tectonic phase contrasted with the earlier extensional regime which was associated with rift development during the Callovian. A compressional tectonic event in the Middle Miocene produced apparent reverse movement on the South Pepper Fault but only minor changes to the structural closure.Geochemical and structural evidence indicates at least two periods of hydrocarbon migration into the top Barrow Group - Mardie Greensand reservoir. The earlier occurred in the Turonian subsequent to the period of wrench tectonics and involved the migration of oil from Lower Jurassic Dingo Claystone source rocks up the South Pepper Fault. This oil was biodegraded before the second episode of migration occurred after the Middle Miocene tectonism. The later oil is believed to have been sourced by the Middle to Upper Jurassic Dingo Claystone. Biodegradation at this stage ceased or became insignificant due to temperature increase and reduction of meteoric water flow. Gas-condensate, sourced from Triassic or Lower Jurassic sediments may have migrated into the structure with this second oil although a more recent migration cannot be ruled out.The proposed structural and hydrocarbon migration history fits regional as well as local geological observations for the Barrow Sub-basin. Further data particularly from older sections of the stratigraphic column within the area are needed to refine the interpretation.

Lower Paleozoic Carbonate Slope-Sequence, Northern Seward Peninsula, Alaska: ABSTRACT

Examination of a lower Paleozoic carbonate unit exposed along the northern coast of the Seward Peninsula revealed an undeformed section at Cape Deceit, greater than 160 m (525 ft) thick, that represents part of a prograding carbonate submarine-fan sequence. The top 60 m (200 ft) of the sequence is a massive, disorganized, carbonate conglomerate interpreted to be debris flows deposited in large feeder channels of an inner-fan complex. Stratigraphically below this interval are channelized conglomerates and calcarenites, 40-50 m (131-165 ft) thick. Overall, this section thickens and coarsens upward but is composed of many thinning- and fining-upward cycles, 2-8 m (7-26 ft) thick. This section probably represents channel deposits of the midfan complex. The bottom part of the equence consists of a coarsely crystalline, thinly bedded limestone section overlying a calcareous shale section. The beds within this section are laterally extensive and represent outer-fan, fan-fringe, and basin-plain deposits. Although no recognizable megafossils were found in the Cape Deceit section, Silurian conodonts have been reported by other workers. Rugose coral fragments of indeterminate age were found in an undeformed section, 19 km (10 mi) to the east, which appears to be correlative with the Cape Deceit section. End_of_Article - Last_Page 677------------

A RIFT‐RELATED STRATIGRAPHIC TRAP

During the prospectivity analysis of the 40 blocks of the 8th Norwegian round of licensing, a potential “giant” stratigraphic trap was discovered. Its evolutionary histroy makes it especially interesting from a regional point of view.Well information from neighbouring blocks and high quality seismic data has made this possible.In block 16/4 in the southern part of the Viking Graben, a combined stratigraphic/structural trap was formed during the Late Paleocene‐Early Eocene.A submarine fan sequence reached across the viking Graben from the Shetland Platform onto the western flank of the Utsira High of block 16/4 during the Early Paleocene. Later, the Tertiary basins sank with the depo‐centers centered over the old graben. This sinking, together with reactivation of old faults in the southern part of block 16/1 and the northern part of block 16/4 during the Late Paleocene‐Early Eocene, created a large trap. This type of trap is typical for early post‐rift sequences where the post‐rift basin development is governed by the old lines of weakness inherited from the underlying aborted rift.

Hemipelagic chalks in a clastic submarine fan sequence: Miocene SW Turkey

Summary The Miocene Salir Formation in SW Turkey is a 500 m thick dominantly terrigenous clastic succession deposited by turbidity currents and other mass-flow mechanisms on a series of small submarine fans. During intervals of non-clastic deposition hemipelagic chalk horizons up to 60 mm thick accumulated. The chalks comprise disseminated planktonic and small benthonic foraminifera dispersed in a micrite matrix with abundant poorly preserved coccolith plates and other nannoplankton fragments. Biogenic sand and silt-sized particles form up to 20% of the rock. Carbonate content ranges from 55% to 95%. The non-carbonate component comprises dispersed clay platelets and in some beds silt-sized particles. The chalks range from structureless homogeneous units, to beds which show a wide range of sedimentary structures which suggest reworking of the slowly deposited hemipelagic ooze by bottom currents of fluctuating velocity. The lateral and vertical distribution of the hemipelagic chalks reflects the area on the submarine fan on which they were deposited. Areas away from the main locus of deposition, for example interchannel areas in proximal sequences, and inactive mid-fan areas are characterized by frequent and often thick chalk beds. In active mid-fan and inner-fan channel areas hemipelagic chalks are scarce or absent.