Fan Sandstones Research Articles

Paleocene reservoirs of the Everest trend

The Everest trend of stratigraphically-trapped Paleocene gas accumulations was first recognized by the Amoco Group in the late 1970s and tested by the 22/10a-2 well in 1981. The well discovered gas–condensate pay in submarine fan sandstones of the Maureen and Andrew formations of the Montrose Group. These sandstones are absent in the 22/10-1 well, some 8 km to the east, where the entire Paleocene section above the Ekofisk Formation consists of shales of the Rogaland Group. A large stratigraphic trap was mapped, relying on up-dip shale-out to the east, and dip drape over a structural nose in the underlying Jaeren High basement block. No sandstones were found in the age-equivalent section of the Forties Formation in the 22/10a-2 well and so a further, down-dip, stratigraphic accumulation was predicted and subsequently successfully tested by the 22/10a-4 well in 1984. These discoveries are now named the Everest Complex. Simultaneously, the play concept was proven to the north by the Sleipner Øst discovery in 1981, where shale-out of the sandstone occurs over the Ling High. A third, similar, accumulation between these two fields was discovered in 1982 by Well 22/5b-2, which forms part of the Fleming accumulation. Reservoirs, which consist of poor to moderately sorted quartz-rich arenites, exhibit a wide variety of sedimentary facies produced by turbiditic and gravity flow processes. The distribution and form of the sandbodies is controlled by a combination of underlying palaeotopography, regional tectonism and global eustatic events. Correlation of reservoir units is accomplished by integration of wireline logs, detailed biostratigraphy and seismic facies analysis The commercial importance of the trend is reflected in the reserves proven to date, which are in excess of 4 trillion cubic feet (TCF) of wet gas.

THE SEDIMENTARY EVOLUTION OF THE RED SEA AND GULF OF ADEN

Onset of rifting, and flooding by marine waters, occurred in the late Oligocene in the Gulf of Aden and southern Red Sea. The northern part of the Red Sea may have been a largely continental rift at this initial stage, but continued rifting established marine conditions throughout the system by the early Miocene. Episodic isolation of the Red Sea system, leading to evaporite deposition in some basins, commenced in the mid‐Miocene and over two kilometres of salt had accumulated in most Red Sea basins by the end of the Miocene.Re‐establishment of persistently‐marine conditions occurred in the Pliocene, and marine recharge is now sufficiently high to permit vigorous carbonate build‐ups in shallow‐water areas.Clastic sediment textures suggest that marginal escarpments, which first developed during the onset of rifting, were strongly uplifted in the Pliocene‐to‐Recent period. Subsidence of basin floors seems to have been particularly rapid during the period dominated by salt deposition.If eruption of sea‐floor basalts in the axial rift areas is excluded, volumetrically important volcanism is centred on the present Afar triangle area, and is confined to the Oligocene and early Miocene. The amount of contemporary volcanic débris in the sandstones is consequently not particularly high. Some sandstones in northern Ethiopia, Sudan and Egypt do contain abundant acidic volcanic clasts, but these are derived from the Proterozoic basement and cause less diagenetic reservoir damage than contemporary glassy volcanic ash.Sandstones deposited in freely‐drained alluvial fan settings are characterised by early diagenetic kaolinite, whereas those of sabkha and marginal‐marine settings tend to show relatively early diagenetic chlorite. Those alluvial fan sandstones which were subsequently invaded by reduced pore waters expelled from the basin axis, and those in the basin axis, often developed later diagenetic chlorite.

A Sequence Stratigraphic Interpretation of the Eastern Gippsland Basin

The Latrobe stratigraphic section subcropping the 'Top Coarse Clastics Unconformity' in the eastern Gippsland Basin consists of three depositional sequences of Maastrichtian to Palaeocene age. A Lower Palaeocene sequence illustrates the sedimentological and seismic relationships typical of these sequences. It has been further subdivided into three seismically mappable depositional units. Unit C is a 40 m basal sandstone section characterised seismically by high frequency sigmoidal clinoforms. It is interpreted to represent lowstand fan deposition prior to a marine transgression. Unit B consists of a 50 m argillaceous siltstone overlain by a 70 m coarsening upward sandstone section, characterised seismically by high frequency and high amplitude sigmoidal reflections. It is the product of marine transgression and subsequent highstand progradation. Unit A consists of a thin argillaceous siltstone overlain by a coarsening upwards sandstone deposited during a minor transgression and continued progradation. Unit A also consists of interbedded sands, shales and coals producing parallel, often high amplitude reflections. Deposition of this facies occurred in a coastal plain environment behind the prograding shoreline. The mapping of these seismic facies allow prediction of stratigraphy and reservoir and seal quality in an area of little well control, the deep water part of the Gippsland Basin. Stratigraphic pinchout plays of Unit B shoreface sandstones prograding into offshore siltstones and claystones and Unit C lowstand fan sandstones into Unit B offshore siltstones and claystones are also identified.

The Maureen Field, Block 16/29a, UK North Sea

Abstract The Maureen Oilfield is located on a fault-bounded terrace in Block 16/29a of the UK Sector of the North Sea, at the intersection of the South Viking Graben and the eastern Witch Ground Graben. The field was discovered in late 1972 by the 16/29-1 well, and was confirmed by three further appraisal wells. The reservoir consists of submarine fan sandstones of the Palaeocene Maureen Formation, deposited by sediment gravity flows sourced from the East Shetland Platform. The Palaeocene sandstones, ranging from 140 to 400 ft in thickness, have good reservoir properties, with porosities ranging from 18-25% and permeabilities ranging from 30-3000 md. Hydrocarbons are trapped in a simple domal anticline, elongated NW-SE, which was formed at the Palaeocene level by Eocene/Oligocene-aged movement of underlying Permian salt. The reservoir sequence is sealed by Lista Formation claystones. Geochemical analysis suggests Upper Jurassic Kimmeridge Clay shales have been the source of Maureen hydrocarbons. Estimated recoverable reserves are 210 MMBBL. Twelve production wells have been drilled on the Maureen Field. A further seven water injection wells have been drilled to maintain reservoir pressure.

Hydrocarbon plays in the northern North Sea

Abstract The hydrocarbon finds of the Norwegian and British sectors of the North Sea, north of 56°, can be grouped into six discrete plays. The key to all the plays is the presence of organic-rich Upper Jurassic shales (source rocks) and a rift system of the same age. The rifting provided the structures (the traps) and post-rift cooling caused the subsidence necessary for hydrocarbon generation. The pre-Jurassic Play is of least importance. The Lower-Middle Jurassic Play, with about 40% of the resources, results from pre-rift uplift in the south providing clastic input for a delta system in the north, the thick sandstone reservoirs of which were preserved during the subsequent rifting. In late Jurassic times graben formation by rift collapse was accompanied by erosion of marginal uplifts, resulting in thick sand sequences, which interfinger with graben shales containing the source rocks. This ideal relationship provides the Upper Jurassic Play which contains 30% of the resources. The Lower Cretaceous Play occurs in submarine fan sandstones and is of minor importance. Chalk deposition in a tectonically unstable environment, with subsequent rapid burial beneath Tertiary muds and clays, is responsible for the Chalk Play. Although geographically limited, it is prolific and accounts for nearly 10% of the hydrocarbon resources. Geologically youngest is the Paleogene Play. Uplift of the Orkney-Shetland Platform in early Tertiary times was a consequence of sea-floor spreading in the North Atlantic. Great quantities of sand derived from this uplift spilled eastwards into the northern North Sea, blanketing the western flank of the Tertiary basin and extending axially far south along the Central Graben. These sands have captured vertically migrating hydrocarbons and have allowed extensive lateral migration. Although important volumes of oil and gas, about 20% of the total resource, are trapped within these sands, large volumes have probably been lost to the basin via their outcrop in the west.

WAXINGS AND WANINGS IN STRATIGRAPHY, PLAY CONCEPTS AND PROSPECTIVITY IN THE CANNING BASIN

The prospectivity of the Canning Basin is by no means exhausted. Furthermore, low product prices will offer those with the will and the wherewithal some relatively low cost opportunities to drill seismically well- defined, selected plays in selected acreage. There may never be a better time to invest in the Canning Basin.The Canning Basin rock record includes at least 16 distinct regional episodes of onlapping, quiet- water conditions that transgressed higher energy reservoir- type facies. These vertical successions often constitute correctable seismic sequences and represent apt horizons at which to map prospective, shale- capped trap configurations. All of these 16+ top- sealed reservoir levels are associated with oil and/or gas shows in some part of the Canning Basin. Indeed, the majority of Canning Basin wildcats are associated with reports of petroleum shows.There are seven separate petroleum discoveries (four developed) in the Canning Basin. These span seven different formations and three distinctive trap- types: draped bioherms, anticlinal culminations and tilted horsts. While the overall historical ratio of discoveries to wildcats is low (~1:19), the most successful joint venture in the Canning Basin can claim a 1 in 5.3 rate of discovery leading to development since its first wildcat (Blina 1) in 1980.The most effective oil source rocks in the Canning Basin are thought to be Arenigian to Llanvirnian (Ordovician) marine shales, the Givetian to Frasnian (Devonian) Gogo Formation and the Late Devonian to Early Carboniferous Fairfield Group, in particular the Tournaisian Laurel Formation.The most consistently permeable reservoirs that are most frequently in favourable juxtaposition to source rocks and seals are the Permo- Carboniferous glacigenic quartzose sandstones of the Grant Group. Most other Palaeozoic reservoirs that are judged to have adequate top seals are less regularly porous. All significant porosity in carbonates in the Canning Basin is apparently diagenetic and irregularly distributed. Those carbonates most likely to be permeable are leached and/or dolomitised and/or fractured. Regressive carbonates, carbonates interfingering with permeable siliciclastics, carbonates adjacent to major faults, and carbonates that either lie above or are cut by unconformities are those apparently most frequently dolomitised. Fenestrate (especially algal) and oolitic fabrics provide excellent habitats for high levels of secondary dolomite and subsequently leached porosity. The Nita, Mellinjerie (lowermost Pillara), uppermost Nullara and Yellow Drum Formations are those units that most frequently exhibit these characteristics in the Canning Basin.Reefs, salt domes and anticlines have enticed, and will probably continue to attract, explorers to the Canning Basin. Traps including (1) intra- Grant Group palaeo- monadnocks, (2) Carribuddy salt pillows and salt evacuation- related turtle- backs, (3) low- stand submarine fan sandstone complexes in the Frasnian Gogo Formation and (4) tilted horsts at Ordovician levels are additional recognised play types that warrant continued interest and will probably be further explored, if product prices permit.

A Sand-Rich Submarine Fan in the Lower Mesozoic Mineral King Caldera Complex, Sierra Nevada, California

ABSTRACT A turbidite sequence in the lower Mesozoic Mineral King roof pendant (southern Sierra Nevada, California) is interpreted to be a small submarine fan, informally named the Bullfrog fan, which formed in a continental arc setting. Abundant sand-sized sediment was supplied to the fan by rapid downslope reworking of rhyolitic pyroclastic debris from a silicic volcanic source that lay to the southeast in an area now occupied by younger plutons. The Bullfrog fan accumulated in a segment of an epicontinental silicic volcanic are in which caldera subsidence structures have been identified. The Bullfrog fan is a 480-m-thick progradational sequence. Bed thickness, average grain size, and abundance of channelized mass-flow deposits all increase toward the southeast where the most proximal part of the fan and the inferred source area have been intruded by a large, Cretaceous granitic pluton. The Bullfrog fan prograded onto starved basin-plain sediments consisting of very thin bedded and laminated silt and pyritic mud. Fan-fringe and lobe-fringe deposits compose only the basal tenth of the section, resulting in abrupt lateral pinch-out of fan sandstones into starved basin-plain deposits. The overlying outer-fan depositional lobes compose over one-third of the section, have high sandstone to shale ratios, and commonly lack fine-grained interlobe deposits, suggesting that depo itionally the lobes laterally interfinger and are vertically stacked. These are overlain by midfan sandstones with abundant channel deposits that form over half of the Bullfrog fan. The midfan deposits consist of a lower sequence of lobe-apical channel cycles and an upper sequence of braided channel cycles. Each lobe-apical channel cycle is characterized by an unchannelized upward coarsening and thickening sequence overlain by a channelized upward-fining and thinning sequence. Channelized deposits form about 40 percent of the lobe-apical channel cycles, and each cycle records the extension of a channel over the depositional lobe that it fed. Pebbly sandstones characterize both the lobe-apical channels and the braided-channel cycles, and interchannel material is rare. Pebbly mudstone debr s-flow deposits are locally interstratified with the braided-channel deposits and overlie the midfan in a nearly continuous sheet less than 8 m thick. The debris flows locally pass upward into laminated and rippled shallow-marine siltstones, coquinites, limestones, and cross-bedded sandstones bearing Late Triassic megafossils, although faulting has obscured this contact in most areas. Nonetheless, the shallow-marine sedimentary rocks apparently directly overlie midfan deposits with no inner-fan or slope deposits intervening. Without sufficient clay-sized sediment, highly mobile turbidity currents capable of depositing basin-plain, fan-fringe, lobe-fringe, and interchannel turbidites apparently could not develop. Instead, high-density turbidity currents and fluidized flows deposited sandand pebble-sized pyroclastic debris on smooth and channeled lobes that filled a relatively shallow basin within a submarine caldera complex.

Oil and Gas Production from Submarine Fans of Los Angeles Basin: ABSTRACT

The Los Angeles basin is a small but relatively deep Neogene basin located in the most northeast portion of the southern California continental borderland. It was formed along a transform margin during the early to middle Miocene, as were numerous basins within the continental borderland south of the Transverse Ranges. A series of prograding submarine fans filled most of the basin during the middle to upper Miocene, Pliocene, and Pleistocene. The morphology of these fans appears to fit most basin-floor fan models but these models were greatly modified by paleobathymetry. The sediment transport mechanism was primarily turbidity flows from submarine canyons but other mass sediment transport such as debris flows, fluidized sediment flow, grain flow, etc was also significant. Three primary, often coalescing, submarine fans have been recognized: the Tarzana fan in the northwestern Los Angeles basin, the San Gabriel fan in the central and southern parts, and the Santa Ana fan in the eastern part of the basin. Most of the oil produced in the Los Angeles basin comes from submarine fan sandstones. Much of the future oil to be found will be from the same fans or heretofore unrecognized submarine fans. Structural geologic concepts will be of equal importance when exploring for these subtle traps. Total cumulative production to 1982 has been 7.3 billion bbl of oil and 5.8 tcf of gas. Per unit volume of sediment, the Los Angeles basin is one of the richest in the world. It was a silled basin within the oxygen-minimum oceanographic zone during the upper Miocene and Pliocene. A combination of rich biogenic sedimentation along with rapid burial by coarse to fine clastics and relatively high paleoheat flow provided almost perfect conditions for the generation and migration of oil and gas. Structural deformation was intermittent throughout the Neogene but was most intense and culminated in the late Pleistocene to Recent. The multistoried oil sands with different crude oil types suggest most of the oil is indigenous to the formation in which it is found. End_of_Article - Last_Page 520------------

Epiclastic sedimentation and stratigraphy in the North Spirit Lake and Rainy Lake areas: A comparison

In the North Spirit Lake greenstone belt within the Sachigo geological subprovince epiclastic metasediments interlayered with mafic and intermediate to felsic metavolcanics unconformably to disconformably overlie a sequence of ultramafic to felsic metavolcanics and quartz-magnetite iron-formation. Polymictic conglomerate of mixed volcanic and plutonic provenance overlies the unconformity. In the northern part of the belt it is followed by cross-bedded sandstone, mudstone and marble: alluvial fan, braided fluvial, and lacustrine deposits. In the southern part of the belt the basal conglomerate is overlain by channelled submarine fan deposits: inverse to normally graded and normally graded conglomerate; sandstone showing dish structure, Bouma cycles, and flame structures; mudstone and quartz-magnetite iron-formation. The submarine fan deposits are separated from the alluvial fan deposits by an area of non-deposition. In the Wabigoon geological subprovince near Rainy Lake polymictic conglomerate of volcanic provenance unconformably overlies volcanic rocks, a gabbro-anorthosite body, and a granitic subvolcanic intrusion. This alluvial fan deposit is followed upwards by braided river deposits: parallel laminated or low-angle cross-stratified sandstone and trough cross-bedded sandstone. Siltstone and magnetite iron-formation associated with the sandstone may be lacustrine deposits. To the south in the adjacent Quetico subprovince are medium-grade submarine fan sandstones. They are separated from the alluvial fan deposits to the north by a fault. Within the context of present tectonic concepts the North Spirit Lake area is comparable to a marginal basin whereas the Rainy Lake area shows characteristics more compatible with an arc-continent interface.

Early Porosity and Permeability Reduction in Deep-Sea Fan Sandstone and Shale By Authigenic Smectite and Carbonate Cement: ABSTRACT

A preliminary study of DSDP cores from deep-sea fan deposits off southern California and Baja California indicates that sandstone turbidites and interbedded shales can be rapidly and extensively altered at shallow burial depths. Their porosity and permeability are reduced or totally occluded by the early (commonly pre-compaction) formation of authigenic smectite and carbonate cements. As a result, many turbidite sands become tightly cemented at burial depths shallower than 400 m and thus lose their potential as petroleum reservoirs. End_Page 735------------------------------ Authigenic smectite, recently reported to occur over large areas of the deep sea, forms in fan deposits as young as 0.4 m.y. at temperatures less than 10°C and has been found at burial depths less than 10 m. Scanning electron microscopy shows that crystallization of authigenic smectite is related to the progressive downhole dissolution of biogenic silica and volcanic glass. Sandstone cemented with only a few percent smectite retains high porosities (25 to 30%) but permeabilities are greatly reduced (< 100 md). Ancient sandstone turbidites commonly have their original interparticle pore space filled with carbonate cement. Textural evidence (few grain contacts, low grain density) indicates that, as in the DSDP cores, much carbonate cement in ancient sandstone turbidites is the product of an early pre-compaction cementation. At greater depths, these relatively uncompacted calcite-supported sandstones may once again become prospective petroleum reservoirs as decarbonatization generates secondary porosity. End_of_Article - Last_Page 736------------

Devonian Sediments of East Greenland IV. The Western Sequence, Kap Kolthoff Supergroup of the Western Areas.

A 2 km succession of upper Middle and Upper Devonian fluvial conglomerates and sandstones outcrops along a north-south distance of 160 km, and forms the western sequence of the Devonian outcrop area of East Greenland. The thickness of the conglomerates varies considerably along the western margin, and this appears to represent the growth of distinct coarse grained fans. These coarse fluvial deposits range up to 1000 m in thickness. Breccias are locally present on the basal unconformity, sometimes including clasts up to 15 m long, that appear to have slipped and rolled from nearby outcrops of basement (pre-Devonian) sediment. Although some of the breccias were certainly deposited from water, others may have accumulated from debris flows. A number of ravines and valleys eroded in the basement, are preserved with plugs of breccia or conglomerate. Thin limestone and siltstone units occur, also close to the unconformity, and are thought to have formed in lake hollows between the highgradient alluvial deposits and the basement topography. Most of the outcrops of the area are of rather similar sandstone sequences. These have been analysed, particularly using our multivariate analytical technique, described in number 1 of this volume. Palaeocurrents and lithology show the presence of a number of distinct sandstone bodies, hundreds of metres thick, and kilometres in lateral extent. These distinctive bodies were deposited by broadly eastward flowing river fan systems at the margin of the Devonian outcrop area. A southward flowing (axial) river system deposited sands further east, in the centre of the basin. The fan sandstone bodies are characterised by cycles, 20 to 100 m thick. In the central parts of the fans, these cycles consist of an upwards increase in the proportion of flat bedding to cross-bedding, although the medium sand grade is relatively constant. The edges of the fan bodies are marked by very-fine sand grade accumulations, interpreted as the fills of hollows between fans, or between a fan and the basement topography. Five of these fan sandstone bodies are green, and one is red and we suggest that this reflects different hydrological conditions.

Sargassum vegetation growing in the sea around Tsuyazaki, North Kyushu, Japan : Yoshida, T., T. Sawada, and M. Higaki, 1963. Pacif. Sci. 17 (2): 135–144

This composite standard establishes a scaled biostratigraphic sequence that incorporates all the fossil occurrences usually reported in paleontological studies of the North Sea Paleogene. All relevant taxonomic groups are represented, including benthic and planktic foraminifera, siliceous microfossils (diatoms and radiolarians), calcareous nannoplankton and palynomorphs (dinoflagellate cysts and sporomorphs). The North Sea Composite Standard was initiated in graphic correlation to a general Cenozoic composite standard with essentially complete representation of the geological record on a worldwide basis. Following this initial chronostratigraphic calibration, additional graphic correlations of area wells were done to develop a separate North Sea Composite Standard which precisely catalogues the relatively limited ranges and overall endemism of the intrabasinal biostratigraphy. The resulting database has extensive well control in the Central and South Viking grabens and provides definitive stratigraphic coverage, in that wells formally designated as lithostratigraphic types and references are included.In application, the composite standard provides a basis for consistent stratigraphic evaluations unencumbered by the incongruities of the many consultant-specific zonal schemes. The graphic correlation results clearly indicate that the Paleogene sediments accumulated in a punctuated series of regional pulses. Major time-stratigraphic breaks are defined by data terraces in relation to the composite standard scale. The length of each horizontal terrace spans an interval of composite standard units (time) that is not represented by a significant rock record in the comparison well. Geologically, horizontal terraces are related to missing section (i.e., faults and/or unconformities) or periods of sediment starvation. In the Paleogene of the North Sea, most of the terraces appear to represent periods of widespread sediment starvation because: they usually occur within a shale unit that is time-correlative throughout the North Sea; deep-marine (bathyal) conditions are indicated by associated benthic foraminifera assemblages in the sediments above and below; most are marked by a distinctive fossil assemblage (e.g., unusually high abundances of radiolarians, planktic foraminifera and calcareous nannofossils); they are confirmed by appropriate seismic stratal geometries.The terraces occur in a time-succession, each correlating in terms of composite standard units from well to well throughout the study area. Considered in the broader context of related geological and seismic data, the graphic correlation terraces appear to be marking major hiatuses resulting from widespread sediment starvation. Therefore, the stratigraphie subdivision afforded by the graphic correlation terraces may be used to define major depositional sequences within the Paleocene-Eocene.In general, the hiatuses are situated between rapidly deposited sections (i.e., steep lines of correlation) that contain major subsea fan sandstones (lowstand systems tract). The nomenclature introduced in this study for these depositional sequences emphasizes the relationship to established lithostratigraphic units based on analysis of the type and reference wells. However, the past regional application of lithostratigraphic terms does not always respect the boundaries between the biostratigraphically defined depositional sequences. Based on electric log criteria alone, genetic packages are difficult to resolve regionally due to the repetitive lithological character of the Paleogene deposits and the pronounced lenticularity of sandstone units. Consequently, the Paleogene Composite Standard has an essential role to play in guiding sequence stratigraphie interpretations.