Lower Delta Plain Environments Research Articles

Coal petrology, sedimentology and depositional environment of the Parvadeh coals in the Upper Triassic, Tabas Block of Central-East Iran

Peat forming environment strongly influences the economic value of any coal seam and coal-bearing strata. Hence, palaeoenvironmental studies provide important information for coal resource exploration. In this context, detailed studies on selected coals from the Parvadeh Area, Iran, were conducted using sedimentology, coal petrology, X-ray diffraction (XRD), scanning electron microscopy- energy dispersive X-ray analyzer (SEM-EDX), and proximate analysis. The sedimentary facies above and below the coal seams are mainly marine or marine-influenced facies, supporting that the coal-forming mires in the Parvadeh Area developed in a paralic environment, where the base level must be closely related to sea level. Sulfur contents are moderate to high and mark the influence of brackish/marine water, especially during transgression after peat growth in a lower delta plain environment. The peat-forming mires extended on coastal/delta plain lobes. The lower delta plain/coastal plain coals are characterized by lateral continuity and substantial thickness, whereas few coals possibly representing the upper delta plain are thin and more discontinuous. The detrital nature and composition of the numerous partings and the overall high ash yield in the coal seams indicate an active tectonic area with high rates of creation of accommodation space over peat growth. Coal petrology and coal facies analysis exhibits a permanently high water table within a forest swamp and mostly rheotrophic conditions, sometimes with connection to the seawater. According to paleoenvironmental reconstructions, it seems that coal layers may be thicker, with less sulfur (pyrite), but more clastic minerals and partings toward the western part of the area. Although these coal seams presently have low economic potential for the mining operation, partly due to great depth, this humic, high-volatile to medium-volatile bituminous coal may be suitable for exploration of coal bed methane resources.

Organic geochemistry and petrology of the coals in the Laisong Formation, Naga Schuppen Belt, NE India: Inferences on hydrocarbon potential and depositional environment

The Oligocene Laisong Formation in the Naga Schuppen Belt is coal bearing along the Dimapur‐Kohima Road section, Nagaland. The coals are sulphurous and classified as lignite to sub‐bituminous ‘C’ based on various rank parameters. The coals are dominantly composed of vitrinite followed by inertinite and liptinite in order of abundance. The coals have high‐quality Type III kerogens derived mainly from angiosperm plants. The richness and quality of organic matter in the coal is excellent to act as a petroleum source rock. Most of the maturity parameters indicate that maturity ranges from immature to early mature. The biomarker parameters and facies critical maceral associations point to deposition of the coal‐bearing strata in non‐marine/terrestrial depositional environment under moist hydrodynamic conditions, possibly in an upper delta plain environment. However, the high sulphur content of the coal points to deposition in a lower delta plain environment. Comparison with other Palaeogene coal deposits of the same sedimentary basin shows that the Laisong coals contain lesser amount of liptinite macerals and much higher amount of sulphur than these deposits. All these deposits are of similar rank, however, the Laisong coals show marginally low thermal maturity from the viewpoint of hydrocarbon generation.

Lithofacies palaeogeography of the Carboniferous and Permian in the Qinshui Basin, Shanxi Province, China

The Qinshui Basin in the southeastern Shanxi Province is an important area for coalbed methane (CBM) exploration and production in China, and recent exploration has revealed the presence of other unconventional types of gases such as shale gas and tight sandstone gas. The reservoirs for these unconventional types of gas in this basin are mainly the coals, mudstones, and sandstones of the Carboniferous and Permian; the reservoir thicknesses are controlled by the depositional environments and palaeogeography. This paper presents the results of sedimentological investigations based on data from outcrop and borehole sections, and basin-wide palaeogeographical maps of each formation were reconstructed on the basis of the contours of a variety of lithological parameters. The palaeogeographic units include the depositional environments of the fluvial channel, flood basin (lake), upper delta plain, lower delta plain, delta front, lagoon, tidal flat, barrier bar, and carbonate platform.The Benxi and Taiyuan Formations are composed mainly of limestones, bauxitic mudstones, siltstones, silty mudstones, sandstones, and economically exploitable coal seams, which were formed in delta, tidal flat, lagoon, and carbonate platform environments. The Shanxi Formation consists of sandstones, siltstones, mudstones, and coals; during the deposition of the formation, the northern part of the Qinshui Basin was occupied mainly by an upper delta plain environment, while the central and southern parts were mainly occupied by a lower delta plain environment and the southeastern part by a delta front environment. Thick coal zones occur in the central and southern parts, where the main depositional environment was a lower delta plain. The thick coal zones of the Taiyuan Formation evidently occur in the sandstone-rich belts, located mainly in the lower delta plain environment in the northern part of the basin and the barrier bar environments in the southeastern part of the basin. In contrast, the thick coal zones of the Shanxi Formation extend over the mudstone-rich belts, located in the areas of the lower delta plain environments of the central and southern parts of the Basin.The Xiashihezi, Shangshihezi, and Shiqianfeng Formations consist mainly of red mudstones with thick-interbedded sandstones. During the deposition of these formations, most areas of the basin were occupied by a fluvial channel, resulting in palaeogeographic units that include fluvial channel zones and flood basins. The fluvial channel deposits consist mainly of relatively-thick sandstones, which could have potential for exploration of tight sandstone gas.

Paleoenvironments and paleogeography of the Lower and lower Middle Jurassic coal measures in the Turpan-Hami oil-prone coal basin, northwestern China

Detailed facies and paleogeographical analyses of extensive borehole and outcrop data from Lower and lower Middle Jurassic oil-prone coal-bearing sequences in the Turpan-Hami oil-producing basin of northwest China have led to the reconstruction of a basinwide depositional model. A total of 20 distinct lithofacies have been identified and grouped into braided fluvial plain, meandering fluvial plain, braided delta, meandering channel delta, and lacustrine depositional systems. Coal-forming swamps occur in each depositional system, but the preferred sites of accumulation are interdelta bay and lower delta-plain environments in the braided delta and meandering channel depositional systems, and it is in these sites that major oil-prone source rocks are located. A series of age-specific, basinwide, paleogeographical maps have been constructed leading to a depositional model for the basin. Results indicate that the basin experienced cyclic flooding to produce swamps and lakes, and that these characterize the deposition of the Lower and lower Middle Jurassic coal measures. Analyses indicate that both the Bogda and Harlik Mountains were uplifted prior to the Early Jurassic, and a lake separated the two mountain regions during the Early and early Middle Jurassic. From these results, it is interpreted that major oil-prone coal sequences are to be found in the western part of the Taibei depression of the basin, and thus, the full economic potential of the basin has yet to be fully realized.

The Trent Gas Field: correlation and reservoir quality within a complex Carboniferous stratigraphy

The Trent Field gas accumulation in the UKCS (United Kingdom Continental Shelf) Southern Gas Basin is reservoired in Carboniferous age sediments. It is contained within a NW–SE trending base Permian structural feature that straddles Blocks 43/24a, 43/23 and 43/25. The proven reserves within the core area lie solely within the 43/24a block. Field appraisal and development encountered three main areas of complexity: structure, reservoir continuity and reservoir property variations. The 20 km long base Permian feature is controlled by NW–SE trending post-Rotliegendes reverse faults which produced a positive inversion. Antithetic faults to these reverse faults also occur within the structure. A second WSW–ENE normal fault trend comparmentalizes the field, and is also thought to have influenced sedimentation during Carboniferous times. Similar fault-influenced deposition is documented at equivalent times in the UK onshore. Ultimately this structural complexity controls the Carboniferous subcrop, which is seen to increase in age from late to early Westphalian A, in a southeasterly direction, although dramatic changes can occur on a more local scale in some fault compartments. The Trent Field accumulation is contained within an interval ranging in age from Marsdenian to early Westphalian A. It encompasses a number of facies associations in delta top and lower delta plain environments, which vary rapidly both vertically and laterally. Reservoir facies include incised valley-fills, transgressive reworked sandstones and distributary channels, which vary markedly in their lateral extent. Reservoir quality of these facies ranges from very poor to excellent and is controlled primarily by the depositional environment in which grain size and sediment maturity are the most important factors.

Depositional environments and coal petrography of the Permian Karoo Sequence: Witbank Coalfield, South Africa

A sedimentological and coal-petrographic study of the Permo-Carboniferous Karoo Sequence in the Witbank Coalfield, South Africa, has revealed that coal seams formed in fluvial environments are thick and laterally continuous, and have higher inertinite and mineral matter contents than coals formed in lower delta plain environments. These latter coals are thin and laterally discontinuous. Twe depositional sequences from Witbank Coalfield have been interpreted in terms of depositional environments, and related to coal thickness, distribution and composition. The first depositional sequence comprises a sequence of diamictites, conglomerates and sandstones deposited by glacial and glaciofluvial processes. These deposits are overlain by the No. 1 and No. 2 coal seams. Contemporaneous braided fluvial sedimentation during peat accumulation resulted in the splitting of the No. 1 and No. 2 seams into subseams. The second depositional sequence comprises two upward-coarsening deltaic sequences, capped by the No. 3 and No. 4 seams respectively. The No. 3 seam accumulated in a lower delta plain setting, while the No. 4 seam accumulated in an upper delta plain environment. The deposition of fine-grained sediment within an embayment and later, fluvial sedimentation during the accumulation of the No. 4 seam has caused splitting of the No. 4 seam into the No. 4 Upper and No. 4 Upper A subseams. Coal seams associated with glaciofluvial and upper delta plain sedimentation, such as the No. 1, No. 2 and No. 4 coal seams, are thick, laterally extensive and generally have high inertinite and low vitrinite contents. The No. 3 seam which accumulated in a lower delta plain setting is thin, laterally discontinuous, and has a high vitrinite and low inertinite content.

Depostional [sic] Environments of the Rock Springs Formation, Southwest Flank of the Rock Springs Uplift, Wyoming

A record at an ancient delta system Is preserved in outcrops along the flanks of the Rock Springs Uplift, southwestern Wyoming. The deltaic Upper Cretaceous Rock Springs Formation of the Mesaverde Group was deposited during early Campanian time near the end of the regressive phase of the Niobrara cyclothem. On the southwest end of the Uplift, part of the delta system is exposed near the seaward edge of a series of transgressive/regressive sequences, which consist of intertonguing prodelta, delta-front, and delta-plain deposits. Eight major delta-front sandstones are vertically stacked and laterally continuous throughout the main study area, intertonguing with prodefta deposits of the Blair Formation and Black Butte and Coulson Shale Tongues bf the Rock Springs Formation. Delta-front sandstones consist mainly of interdistributary shoreface deposits composed of sand redistributed from point sources by longshore currents. Preservation of distributary-channel and mouth-bar deposits is rare due to intense wave reworking. The shorelines were modified by large-scale storm events. Distributary-channel, crevasse-splay, overbank, crevasse-subdelta, interdistributary-bay, marsh, swamp, flood-tidal delta, and washover deposits accumulated behind delta-front sandstones, mostly in lower delta-plain environments. In this lower delta-plain setting, peat was deposited in freshwater forest swamps as isolated pods and linear belts behind delta-front sands. The resulting coal beds are thin, pinching and splitting to the southeast of the study area. In contrast, in more landward settings north of the study area, non-marine sequences contain thicker and more continuous coal beds that accumulated in the lower parts of upper delta-plain environments.

Subsurface Lignite Occurrence in Wilcox Group, Northeast Louisiana and Northwest Mississippi: ABSTRACT

An investigation of lignite occurrence in the Wilcox Group of northeast Louisiana and northwest Mississippi revealed high lignite concentrations associated with a highly constructive elongated delta in the lowermost Wilcox Group. Bar-finger sandstone and distributary channel sandstone facies of an elongated delta lobe were recognized trending north-south through the central portion of the study area by sandstone percent maps, net sandstone isopach maps, and characteristic SP curves on electric well logs. Lignite isopleth maps identified principal areas of lignite occurrence adjacent to the elongate delta. Fewer, but thicker seams were found in the northern regions, where a maximum of four seams >= 5 ft thick were identified. To the south, a greater concentration of thi ner seams occur. The distribution is attributed to a change in the position on the deltaic plain. Numerous, thin lignite beds in the southern region are indicative of lower delta-plain environments, where bifurcating distributaries, crevasse splays, and marine inundation inhibited thicker peat development. The presence of thicker, but fewer lignites to the north supports a transitional to upper delta-plain environment. Mapping of individual sandstone beds revealed dip-oriented, bifurcating, fanlike geometries indicative of a lower to transitional delta plain. Capping lignite seams are blanket type, having areal extents of up to several hundred square miles and thicknesses ranging from 2 to 20 ft. Thicker seam development occurs along the delta-lobe margins, extending landward into adjacent interdistributary basins. The large extent of the blanket seams is in part attributed to (1) lignite beds capping channel-fill deposits, and (2) distributary channel and point-bar sandstones directly overlying lignite beds, with no truncation evident. Cross sections illustrate subsidence of the lignite beneath the overlying channel deposits. High-quality lignite originates in transitional to upper delta-plain environments. Optimal areas for high-quality lignites in the Wilcox Group should be the extreme northeastern and northwestern portions of the study area, and farther north into central Tensas and Franklin Parishes, where thicker, transitional to upper delta-plain paleoenvironments should exist. Minimum subsurface depth to the highly lignitic, lowermost Wilcox Group ranges from approximately 5,000 ft (1,510 m) in the northwestern region to 7,000 ft (2,114 m) along the extreme southern boundary. End_of_Article - Last_Page 1429------------

DEFOSITIONAL FRAMEWORK AND DIAGENESIS OF THE LATE PERMIAN GAS RESERVOIRS OF THE BONAPARTE BASIN

Late Permian sandstones form the reservoir of the Tern and Petrel gas fields in the offshore Bonaparte Basin. The producing reservoirs of the Petrel field were deposited in various environments associated with a major northwesterly trending deltaic system. The producing sands in the Tern field were deposited in the shoreface environment of a barrier-bar system.The reservoir quality of the sands is controlled by the diagenesis, which is facies dependent. In the Petrel field, sandstones deposited in the upper delta plain and along the shoreline are clean, medium-to coarse-grained and highly quartzose but have very low porosity and permeability due to extensive quartz diagenesis. However, sands deposited in delta front and lower delta plain environments are medium to fine grained, argillaceous and have fair to good reservoir potential. In these sands, the dispersed clays formed coats and rims on quartz grains during early diagenesis and inhibited quartz overgrowth. In the Tern field, sands of the upper shoreface have poor reservoir quality due to early calcite cementation. However, finer-grained sandstones of the lower shoreface facies have good reservoir quality. The porosity in these sands is mainly primary and preserved due to low carbonate and high clay content. The processes of quartz and calcite cementation which drastically reduced the reservoir quality of the coarse-grained sands occurred early and were influenced by the texture of the sands and probably also by the chemical character of the formation waters.

Stratigraphy and Structure of Clinton Section in Wayne County, Ohio: ABSTRACT

The Clinton section in Wayne County represents a clastic wedge of the Lower Silurian Albian Group. It was deposited on the distal flank of the Appalachian basin as a result of the Taconic orogeny. The section extends vertically from the Queenston unconformity to the base of the Packer shell (Brassfield Limestone). It is primarily composed of sandstone, shale, limestone, and dolostone. Within this stratigraphic section, the sandstone facies is an important hydrocarbon reservoir rock. The sandstone facies pinches out laterally along a north-south depositional limit that is roughly located along the western boundary of Wayne County. The study relied primarily on geophysical log data. However, drill cuttings were used to correlate lithologies to specific curve assemblages on the geophysical logs. This helped define lithologies to logs in areas where drill cuttings were lacking. Eight cross sections--four north-south and four east-west--show the complexity of the intertonguing clastic and carbonate deposits resulting from the progradation and shifting of deltaic and nearshore marine environments. The cross sections also show that the driller's terms of stray, red (1st), and white (2nd) Clinton sandstone are arbitrary units and are not temporally equivalent or correlative from well to well over great distances. Stratigraphic interpretations indicate that the deposition of the Clinton section began with a marine transgression from the northwest across a subaerially exposed coastal plain that was deposited as the distal end of the Ordovician Queenston delta. This transgression reworked the upper Queenston sediments and redeposited them as calcareous silts and sands. Renewed uplift of the Taconic highlands caused a clastic influx into the areas and a relative regression of the sea. These sediments were deposited in prodeltaic and lower delta-plain environments. Some of the sediments were reworked offshore by ocean currents and wave action and redeposited as offshore bars. As the influx of Taconic sediment ended, muds were deposited, which were eventually transgressed by a carbonate-rich sea. A imestone unit, the Packer Shell, was then deposited ending the Alexandrian Epoch. The structural setting is one of homoclinal dip to the southeast, with localized basement-controlled, minor folds normal to the basinal axis. Small-scale faulting can also be seen on structure maps of the base of the Packer Shell and the top of the Queenston. Local structural highs and faults affect oil and gas production in this mainly stratigraphic trap. Local structure commonly segregates the oil and gas in the same reservoir body. End_of_Article - Last_Page 1930------------

Types of Tight Gas Reservoirs, San Juan Basin, New Mexico and Colorado: ABSTRACT

The San Juan basin of New Mexico and Colorado is an asymmetrical basin formed during the Late Cretaceous and early Tertiary. The structurally deepest part of the basin is near the Colorado-New Mexico border. Presently, the only applications for the incentive price for gas produced from tight sandstones involve reservoirs of Cretaceous age. These reservoir rocks were deposited in open shelf to lower delta-plain environments associated with the Western Interior Cretaceous sea which transgressed and regressed the area in a general southwest-northeast direction. Coastal-barrier sandstones are significant gas producers and typically exhibit a strong marine influence. These sandstones are represented by long, linear northwest-southeast reservoir trends. The tight gas applications for the San Juan basin, to date, concern reservoirs which can be placed into a spectrum with two end-member types. One type is characterized by continuation of mappable sand trends which extend from economic producing areas. Even though reservoirs of this type generally look similar to those in economic wells, they require production testing to determine their economic potential. This unpredictability is caused by the presence or absence of natural fractures. The other type of tight gas reservoir is related to the thinning of the good reservoir section or the increase in content of silt and clay-size material within the productive interval. This type of reservoir looks poor on well logs because of interbedding of siltstones and shales with thin, good reservo r sandstones and/or the presence of considerable silt and clay in the sandstones themselves. End_of_Article - Last_Page 520------------

Application of Depositional Modeling to Coal Exploration, Green River Basin, Southwest Wyoming: ABSTRACT

Data from over 1,400 coal exploration drill holes, 21 measured sections, and 90 deep mine maps, in conjunction with cursory examination of oil and gas logs and seismic sections, have been used to reconstruct the depositional settings of the Rock Springs Formation in the Green River basin. From examination of approximately 20 coal seams in the Rock End_Page 593------------------------------ Springs Formation, a depositional model was developed to account for areas of variable thickness in coal accumulation. Coals within the formation developed along lower delta plain, upper delta plain-fluvial or on abandoned deltaic lobes and are referred to as Type A, Type B or Type C coals, respectively. Figure Depositional regression represented by extensive sheet sandstones are inferred to be delta-front deposits which reflect the cuspate to arcuate geometry of wave-dominated delta deposits. Widespread coal deposits up to 22 ft (6.7 m) thick that occur on top of the deltaic sandstones extend for up to 15 mi (25 km) along depositional dip and 36 mi (58 km) along depositional strike. They accumulated in lower delta plain environments as Type A coal seams. Thick coal seams that were deposited in upper delta plain-fluvial environments are less than 20 mi (32 km) in length and are more variable in thickness (1 to 17 ft, 0.3 to 5.2 m). They are referred to as Type B coal seams. Persistent but thin coals, less than 25 mi (40 km) in length and 1 to 8 ft (0.3 to 2.4 m) thick, that occur on top of d lta plain-fluvial deposits and that are overlain by sheet sandstones are inferred to represent peat accumulation during delta lobe abandonment and are referred to as Type C coal seams. Coal seam discontinuities, represented by areas of reduced coal thickness or by wedges of sediment producing multiple benches or rider coals, are caused by sediment influx from distributary channels, fluvial channels, and splays. Analysis of the geometries and spatial distributions of coal seams is used to develop a detailed geologic model that can serve as a predictive tool for future coal exploration in this region and in other basins with similar depositional settings. End_of_Article - Last_Page 594------------

Evidence for Deltaic Environment of Deposition for Aguja Formation (Upper Cretaceous), Southwest Texas: ABSTRACT

The Aguja Formation consists of approximately 650 ft (200 m) of claystones, calcareous concretions, sandstones, ironstone concretions, humate-bearing shales, and seams of humate material, in an area 8 mi (13 km) northwest of Big Bend National Park, Texas. Three informal members of the Aguja Formation are based on the varied lithology. The lower member is gradational with the underlying Pen Formation and is composed of approximately 200 ft (60 m) of yellow and light to dark-brown massively bedded claystones, lenticular beds of calcareous concretions, and thin to massive-bedded and cross-bedded channel sandstones. The lenticular beds of calcareous concretions and cross-bedded channel sandstones occur near the top of this member. Massive claystones dominate the lower member and contain a wide variety of forams, and a restricted fauna of gastropods and pelecypods. The relation of claystone to sandstone, within the lower member, reflects a coarsening-upward sequence and suggests a gradational change from a prodeltaic to a lower delta-plain nvironment. The middle member of the Aguja Formation is gradational with the underlying lower member. The middle member is composed of approximately 300 ft (90 m) of lenticular, thin to massive-bedded, and cross-bedded sandstones, dark-gray massive claystones, and interbedded claystones and sandstones. Lenticular beds of ironstone and ironstone concretions, humate-bearing shales, and seams of humate material also are present in this member. The wide variety of gastropods and pelecypods within the channel sandstones and vertebrate and wood remains in the massive claystones in the middle member suggest both marine- and brackish-water conditions typical of a delta-plain environment. The upper member of the Aguja Formation unconformably overlies the middle member and is composed of approximately 150 ft (45 m) of cross-bedded and massive-bedded sandstones, lenticular beds of limestone-pebble conglomerate, and minor amounts of claystone. Marine fossils are associated with some lenticular sandstones and include cephalopods, pelecypods, and gastropods. Vertebrate remains and petrified wood are present throughout the upper member. The upper member probably reflects a prograding upper delta-plain environment. Marine fossils associated with the lenticular sandstones may represent intermittent destructive phases during progradation. The Aguja Formation was deposited within a deltaic environment as is indicated by the geometry of the claystones, channel sandstones, and humate-bearing units. The lower member represents a prodeltaic to lower delta-plain environment. The source area for the Aguja was probably on the west, northwest, and southwest as is suggested by paleocurrent indicators in the channel deposits. End_of_Article - Last_Page 764------------