Channel-fill Sandstones Research Articles

Paradise (and Herrin) lost: Marginal depositional settings of the Herrin and Paradise coals, Western Kentucky coalfield

This is the fourth installment in a series of papers on the Asturian (Westphalian D) disrupted mire margins, termed the “ragged edge” in previous papers, and limestone distributions in the Herrin–Baker coal interval in the Western Kentucky extension of the Illinois Basin. New data, indicating in-situ peat development and marine influence, collected from the first in-mine exposure of this interval are presented. Borehole data from the region are examined in the context of “ragged edge” exposures and a carbonate platform depositional model for this portion of the Illinois Basin is presented. This shows that deposition of the sequence was influenced both by the underlying sediments and by a marine transgression. The former influence is seen in variations in coal and limestone thickness over sandstone-filled channels versus over shale bayfill deposits. The latter is marked by the progressive upwards loss of coal benches (i.e., the bottom bench of both coals is the most extensive and the Herrin coal is more extensive than the overlying Paradise coal) and by marine partings in both coals. Further, the brecciated margins seen in both coal seams are similar to brecciated peats encountered along the Everglades margins of Southwest Florida. Overall coal distributions are similar to both those along the Everglades margins and those along a transect from the Belize coast to Ambergis Caye.

Facies architecture of a net transgressive sandstone reservoir analog: The Cretaceous Hosta Tongue, New Mexico

Net transgressive sandstones form a significant component of many shallow-marine reservoirs, but their shale-poor character commonly masks complex facies architecture and stratigraphy associated with significant permeability variations that impact reservoir drainage patterns and ultimate recovery. In this article, the controls on net transgressive sandstone reservoir architecture are investigated through a detailed analysis of the Cretaceous Hosta Tongue of the Point Lookout Sandstone (informally termed Hosta sandstone in this article) outcrop in New Mexico. Mapping of facies architecture within a series of adjacent canyons has enabled a quantitative three-dimensional reconstruction of key stratigraphic surfaces and sand body distributions from an updip pinch-out to a downdip pinch-out of the net transgressive sandstone complex.The Hosta sandstone contains a complex arrangement of wave- and tide-dominated facies associations arranged in an overall transgressive pattern. Tidal channel-fill sandstones, tidal sheet-form sandstones, and heterolithic tidal-flat and lagoonal deposits comprise the stratigraphy in the updip part of the system. These deposits pass abruptly downdip into wave-dominated shoreface sandstones. The facies composition indicates that the Hosta sandstone represents a wave-dominated barrier shoreline and a tide-dominated back-barrier lagoon. Facies associations are partitioned both vertically and laterally by a hierarchy of transgressive erosion (ravinement) surfaces cut by wave and tidal processes. Reconstructing the geomorphology and spatial organization of these surfaces is critical to understanding sand body distribution and facies architecture at high-resolution (intrareservoir) scale. The exceptional quality of the Hosta Sandstone outcrops has enabled (1) improved understanding of patterns and controls of facies architecture in net transgressive sandstone reservoirs, (2) construction of predictive templates of facies architecture in interwell volumes, and (3) quantification of geobody dimensions and spatial distribution patterns. In combination, these data provide appropriate qualitative and quantitative conditioning for reservoir models.

Geologic factors controlling CO2 storage capacity and permanence: case studies based on experience with heterogeneity in oil and gas reservoirs applied to CO2 storage

A variety of structural and stratigraphic factors control geological heterogeneity, inferred to influence both sequestration capacity and effectiveness, as well as seal capacity. Structural heterogeneity factors include faults, folds, and fracture intensity. Stratigraphic heterogeneity is primarily controlled by the geometry of depositional facies and sandbody continuity, which controls permeability structure. The permeability structure, in turn, has implications for CO2 injectivity and near-term migration pathways, whereas the long-term sequestration capacity can be inferred from the production history. Examples of Gulf Coast oil and gas reservoirs with differing styles of stratigraphic heterogeneity demonstrate the impact of facies variability on fluid flow and CO2 sequestration potential. Beach and barrier-island deposits in West Ranch field in southeast Texas are homogeneous and continuous. In contrast, Seeligson and Stratton fields in south Texas, examples of major heterogeneity in fluvial systems, are composed of discontinuous, channel-fill sandstones confined to narrow, sinuous belts. These heterogeneous deposits contain limited compartments for potential CO2 storage, although CO2 sequestration effectiveness may be enhanced by the high number of intraformational shale beds. These field examples demonstrate that areas for CO2 storage can be optimized by assessing sites for enhanced oil and gas recovery in mature hydrocarbon provinces.

The Paleoproterozoic megascopic Stirling biota

AbstractThe 2.0–1.8-billion-year-old Stirling Range Formation in southwestern Australia preserves the deposits of a siliciclastic shoreline formed under the influence of storms, longshore currents, and tidal currents. Sandstones contain a megascopic fossil biota represented by discoidal fossils similar to the EdiacaranAspidellaBillings, 1872, as well as ridge pairs preserved in positive hyporelief on the soles of channel-fill sandstones bounded by mud drapes. The ridges run parallel or nearly parallel for most of their length, meeting in a closed loop at one end and opening with a slight divergence at the opposite end. The ridges are interpreted as casts of sediment-laden mucus strings formed by the movement of multicellular or syncytial organisms along a muddy surface. The taxaMyxomitodes stirlingensisn. igen., n. isp., are introduced for these traces. The Stirling biota was roughly coeval with other presumed multicellular eukaryotes appearing after a long period of profound environmental changes involving a rise in ambient oxygen levels, similar to that which preceded the Cambrian explosion. The failure of multicellular life to diversify during most of the Proterozoic may be due to environmental constraints related to the comparatively low level of oxidation of the world oceans.

The Paleoproterozoic megascopic Stirling biota

AbstractThe 2.0–1.8-billion-year-old Stirling Range Formation in southwestern Australia preserves the deposits of a siliciclastic shoreline formed under the influence of storms, longshore currents, and tidal currents. Sandstones contain a megascopic fossil biota represented by discoidal fossils similar to the Ediacaran Aspidella Billings, 1872, as well as ridge pairs preserved in positive hyporelief on the soles of channel-fill sandstones bounded by mud drapes. The ridges run parallel or nearly parallel for most of their length, meeting in a closed loop at one end and opening with a slight divergence at the opposite end. The ridges are interpreted as casts of sediment-laden mucus strings formed by the movement of multicellular or syncytial organisms along a muddy surface. The taxa Myxomitodes stirlingensis n. igen., n. isp., are introduced for these traces. The Stirling biota was roughly coeval with other presumed multicellular eukaryotes appearing after a long period of profound environmental changes involving a rise in ambient oxygen levels, similar to that which preceded the Cambrian explosion. The failure of multicellular life to diversify during most of the Proterozoic may be due to environmental constraints related to the comparatively low level of oxidation of the world oceans.

Natural fracture distributions in sinuous, channel-fill sandstones of the Cedar Mountain Formation, Utah

A set of regional natural fractures, present in the sandy to conglomeratic, fluvial, channel-fill deposits of the Cedar Mountain Formation (east-central Utah) has a consistent west-northwest strike regardless of the local axial orientations of the sinuous channels. The fracture-producing stresses were not significantly refracted by the mechanical-property contrast between the channel-fill sandstones and the encasing overbank mudstones. In addition, fracture spacing along the sinuous channel axes is relatively constant between one-half and one-third of the bed thickness for both large fractures that cut the full thickness of the channel deposits and for smaller fractures in the thinner, component beds. Fracture spacing was apparently not affected by the variations in stress amplification that commonly result from differently oriented stiff inclusions in a ductile matrix. Therefore, in the absence of other structures, fracture intensity and the orientation of fracture-related maximum horizontal permeability in sinuous elongated reservoirs will be relatively constant regardless of the orientation of the long axis of the reservoir. Whether maximum permeability trends along, oblique, or across such reservoirs, and the relative drainage efficiency of horizontal versus vertical wellbores drilled into them, will vary only with the local trend of the channel axis.

Characterization of permeability distributions in six lithofacies from the Helsby and Wilmslow sandstone formations of the Cheshire Basin, UK

Abstract The lithofacies and permeability structure of the Abbey Wood borehole, near Delamere, Cheshire, UK, have been investigated so that permeability distributions can be established for characteristic sandstone lithofacies in the Permo-Triassic sandstone aquifer. The borehole penetrates the Helsby Sandstone and Wilmslow Sandstone formations and six major facies (coarse- and fine-grained fluvial channel fill sandstones, sandy sabkha, sand sheet and dune aeolian sandstones, and a massive sandstone facies) and one minor lithofacies, a mudstone facies, have been identified. The permeability distributions for each lithofacies show subtle systematic differences. The two fluvial sandstone facies and the massive sandstone facies have systematically lower permeabilities than the aeolian sandstones, and within the aeolian sandstones the sand sheet and dune sandstones appear to be systematically more permeable than the sabkha facies. Statistical tests on the permeability distributions of each facies indicate that the distributions are statistically distinct and so support the lithofacies classification.

The Archean deep-marine environment: turbidite architecture of the Burwash Formation, Slave Province, Northwest Territories

The Slave Province is an Archean craton that contains an unusually large proportion of preserved sedimentary rock, including thick turbidite successions. The Burwash Formation is exposed in the southern Slave Province between the Anton and Sleepy Dragon basement massifs. At the base of the succession, volcanics and clastic metasedimentary rocks of the Raquette Lake Formation record initiation of the basin in a rifted arc environment. These are overlain by thin black slates representing a transgression, followed by well over 5 km of Burwash Formation metamorphosed turbiditic sandstones and slates interspersed with thin felsic tuff layers. Lateral correlation within the formation is possible using airphotos and recognizable tuff units. Burwash Formation sandstones include thinly bedded units displaying Bouma sequences and thicker bedded units with scour-and-fill structures and stratification bands, characteristic of dense sediment gravity flows. The sedimentary rocks are organized in architectural elements that include channel-fill sandstones and conglomerates, muddy levees, interchannel sandstones resembling high-amplitude reflection packages (HARPs) described from modern fans, and possible depositional lobes. The overall sedimentary architecture was probably controlled by events in the tectonically active source area or areas. The Archean turbidites resemble their Phanerozoic and modern analogues, although they show less voluminous levees, and are generally less organized, than large modern passive-margin fans, which probably have no equivalents in the Archean.

Three-dimensional imaging of a deep marine channel-levee/overbank sandstone behind outcrop with EMI and GPR

Deepwater channel-levee/overbank sandstones are important sources of oil and gas, but their internal stratigraphy is complex and not well understood. One means to better characterize this complex stratigraphy is through the study of outcrops, such as the Dad Sandstone member of the Lewis shale (Figure 1) in Carbon County, Wyoming, U.S. (Pyles and Slatt, 2000). Figure 1. (top) The outcrop of the Dad Sandstone member of the Lewis Shale. A series of channel-fill sandstones dip toward the lower right (west). The lowermost sandstone is the subject of this study. This sandstone trends along the outcrop face toward the lower right, but is juxtaposed against shale toward the west, at the same stratigraphic level. The GPR line on the lower left of the figure (modified from Young et al., 2003) shows the base of the channel-fill sandstone (red line) where it intersects the ground surface. Northwest of the base of the sandstone, at equivalent radar times, the lithology is shale. The lower right figure shows the EMI volume in plan view, with sandstone in yellow and shale in gray; note the bend in the sandstone at the location where the channel base intersects the ground surface. Channel-levee/overbank systems are a result of multiple sediment gravity flows. The channel itself is formed from aggradation of levees over long periods of time. As sediment flows within the channel, fine particles are expelled over the channel edges, thus forming the levees. Coarser particles not deposited on the levee are transported beyond the channel to form frontal splays or lobes. Sediment may slump off the levee walls onto the channel floor during this time interval. Channels are later backfilled with sand and/or mud as transport energy diminishes, probably during an early rise in sea level. The materials that fill the channel can be configured in different arrangements …

Alluvial Architecture and Fluvial Cycles in Quaternary Deposits in a Continental Interior Basin, E Hungary

The thickness of the studied Quaternary alluvial complex, locatedin the eastern part of the Pannonian Basin System, can exceed500 m. Based on subsurface facies analysis the following large-scaledepositional elements were identified: channel-fill deposits, point bardeposits, alluvial fan (sandy sheet-flood) deposits, floodplain andfloodbasin deposits, and thinner sandy–silty beds. They are classifiedinto four types of facies associations, showing a characteristicstacking pattern on the logs. Facies zonation and basin-scale faciesmapping of the overall Quaternary sedimentary succession shows thatin several areas dominated by stacked, multistorey sandy channel fillsediments, pre-existing superimposed channel belts can be presumed.In the central and deepest part of the basin muddy floodbasin (distalfloodplain and wetland) sediments dominate. Between these the largestarea represents the floodplain where single channel fill sands areinterbedded in the alluvial plain muds. In the eastern part of the basinthe well-logs highlight the distal part of an alluvial fan where sandysheet-flood deposits alternate with floodplain sediments.The recognized facies associations show a vertical pattern, i.e.they form a 40–100 m thick fining-upward fluvial cycle. The mostcharacteristic and even ideal cycle can be observed in the channelbelts and in the proximal floodplain zone. Here the basal memberof the cycle is made up of multistorey channel fill beds cut into theunderlying floodplain deposits. This is overlain by an alternatingsandy–muddy succession of channel fill and floodplain depositsforming the intermediate member. The upper member is composedof silty–clayey floodplain deposits with occasional very thin, discretesilty–sandy bodies.These three members form a fining upward sedimentary cycleinterpreted as representing low-, increasing- and high-accumulationspace deposits, respectively. As the basal multistorey channelfill sandstone facies association generally proved to lie above anextensive erosional surface which can be correlated regionally in thebasin, allocyclic controls can be assumed. In some parts of the basinthe cycle is not complete as the incised channels can be single, so thelow-accumulation space deposits can be missing and the high accumulation space deposits, i.e. the aggrading floodplain sediments, can be truncated.On a regional scale, six regionally extensive cycles were differentiatedabove each other. Although these cycles were allocyclicthe question of whether they were tectonically or climatically drivenremains open. However, the fact that six of them have been identified,suggests that they represent the large-scale 400 ka Milankovitchcycles during the Quaternary. The tectonic overprint is apparent in thethickness and internal architecture of the individual cycles.

Basin-floor fans of the Central Tertiary Basin, Spitsbergen: relationship of basin-floor sand-bodies to prograding clinoforms in a structurally active basin

Abstract Lower Eocene shelf-slope clinoforms are exposed in 1 × 10 km mountainside outcrops in the Central Tertiary Basin, Spitsbergen. Where clinoforms are sand-prone they include a deepwater sand complex. Submarine fans represent an early, basin-floor aggradational phase of clinoform growth, whereas later growth of the same clinoform involves a phase of shelf-margin accretion. Individual fans, within stacked series, can be distinguished when traced towards the slope, where a thickening wedge of mudstones separates successive fan bodies. The sand-prone parts of basin-floor fans are some 15–60 m thick and extend into the basin by up to 10–12 km. The lower levels of any fan consist of ripple- to parallel-laminated thin-bedded turbidites interbedded with some thick-bedded turbidites. This association changes irregularly upwards to a succession dominated by thick beds that are structureless and parallel-laminated. The thin-bedded facies are interpreted as turbidite sheets that formed as channel-mouth sandy lobes, sandy levees and crevasse splays. The erosively based, thick-bedded facies are interpreted as constructional channel-fill sandstones. The shallow channels fed sheet-complexes both laterally and distally. The apparent short basinward extent and longitudinal palaeocurrents for the youngest fans suggest that downslope sediment transport became longitudinally deflected by anticlinal topography once sediment reached the basin floor.

Permian–Triassic sedimentology of Jameson Land, East Greenland: incised submarine channels in an anoxic basin

The presence of deeply erosive, conglomeratic sandstone-filled channels around the Permian–Triassic (P–Tr) boundary in East Greenland has been widely cited as evidence for a major, late Permian eustatic lowstand. Facies analysis of such channels and their associated strata in fact suggests that they record part of an intrinsic style of basinal deposition (that persisted into Early Triassic time). Their origin can be explained without the need to invoke eustatic sea-level fall. Active faulting and hinterland elevation is envisaged to have generated powerful sediment-gravity flows that were capable of submarine erosion of fine-grained, unconsolidated strata. Stacked, turbidite-sandstone-filled lenses also occur beneath fan delta sandbodies and probably record infill of channels or chutes cut into the subaqueous slopes of such deltas. Shale successions developed between the incised channel locations record complete P–Tr transitions in which a rapid onset of anoxic deposition is seen in latest Permian time. This is the local manifestation of a global anoxic event. Unusual aspects of the Lower Triassic sequence in Greenland include the presence of well-oxygenated intervals, stromatolite bioherms and red algae.

Abstract: Coal Geology And Underground-Mine Degasification Applied To Horizontal Drilling For Coal-Bed Methane

In eastern Oklahoma’s Arkoma Basin in I998 at least three energy companies drilled coalbed methane wells 700-2,000 feet deep that eventually penetrated 700-800 feet horizontally into the 4-6-foot-thick Hartshorne coal, but also drilled through shale, mudstone, and interlaminated shale and sandstone. Those well segments in shale and mudstone may collapse, leading to well abandonment. Obviously coal will not be penetrated if the bit drifts into strata overlying or underlying the coalbed. Coal may be missing if the bit intersects a normal fault, a thick noncoal parting, or a channel-fill sandstone. and to hold down costs. Also data should be tabulated and maps constructed showing net coalbed thickness, cleat frequency and orientation, coalbed structural contours, faults and secondary coal fractures, cleat-filling minerals, coal-rank isocarbs, inherent moisture, vitrinite reflectance, and lithology of strata overlying or underlying the coalbed. Most of these items, in addition to the laws of gas movement, affect or control the permeability and porosity of the coalbed methane reservoir and the flow of gas to the well. company into the 4-ft-thick Hartshorne coal in an underground inine in Oklahoina in the middle 1970’s, removing great quantities of 97% methane gas without hydraulic fracturing. to drain gas from coal beds in underground mines, should be applied to maximize success in coalbed methane drilling, production, and profit. Coal geology studies, including coal characterization before drilling begins, should help in lease selection Twenty-three horizontal, openhole, experimental boreholes, 300-2,200 feet long, were drilled by a coal Therefore, detailed geological evaluation, combined with information from the history of horizontal drilling

Coalbed methane resource potential and current prospects in Pennsylvania

Coalbed methane gas content analyses from exploratory coal cores and existing data indicate that gas content generally increases with increasing depth and rank. The coal beds studied are from the Main Bituminous field of Pennsylvania (which currently contains 24 coalbed methane pools) and the Northern and Southern Anthracite coal fields. They range from the Middle Pennsylvanian Allegheny Group to the Late Pennsylvanian–Early Permian Dunkard Group. Previous US Bureau of Mines studies revealed gas contents from 0.4 to 13.8 cm 3/g at depths of 99 to 432 m for the bituminous coal beds of the Allegheny Group. More recent core data from the Allegheny Group yielded gas contents from 2.2 to 8.9 cm 3/g at depths from 167 to 387 m. In the Anthracite region of eastern Pennsylvania, the little data that are available show that gas content is anomalously high or low. Gas yields from test holes in eastern Pennsylvania are low with or without artificial stimulation mainly due to the lack of a good cleat system. Overall estimates of coalbed methane resources indicate there may be 1.7 Tm 3 (61 Tcf) of gas-in-place contained in the Northern Appalachian coal basin. The amount of technically recoverable coalbed methane resources is projected by the US Geological Survey National Oil and Gas Resource Assessment Team [US Geological Survey National Oil and Gas Resource Assessment Team, 1996. 1995 National assessment of United States oil and gas resources—results, methodology, and supporting data, US Geological Survey Digital Data Series DDS-30, CD-ROM, Denver, CO, 80 pp.] and Lyons [Lyons, P.C., 1997. Central-northern Appalachian coalbed methane flow grows. Oil and Gas Journal 95 (27) 76–79] at 0.3 Tm 3 (11.48 Tcf). This includes portions of Pennsylvania, Ohio, West Virginia, and a small part of Maryland. Consequently, a mapping investigation was conducted to evaluate the regional geology of the bituminous coal-bearing intervals in southwestern Pennsylvania and its influence on coalbed methane potential. Phase I of this study involved the entire Pennsylvanian coal-bearing interval of southwestern Pennsylvania. Phase II focused on a stratigraphic delineation and evaluation of Allegheny Group coal beds and associated sandstones. Several prospective coal beds and associated facies relationships with channel–fill sandstones were determined. Possible non-coal scenarios for coalbed methane include erosional contacts between coal beds and overlying channel–fill sandstones and areas of stacked channel–fill sandstones. Repetitive sequences of coal accumulation are stacked, commonly with shale interburden, and are also potential coalbed methane targets. Additional Pennsylvania Geological Survey drilling/coalbed methane sampling occurred in Armstrong, Beaver, Cambria, Greene, Lawrence, Somerset, and Washington Counties. Raw coalbed methane desorption data tables/graphical displays of gas contents versus depth, thickness, and time, and average composition and heating values from coal beds of the Allegheny Group to the Dunkard Group are available at the Pennsylvania Geological Survey. Further information on cross-sections, isopleth maps, isopach maps, raw drillhole data, and ownership issues can also be obtained from the same source.

The stratigraphy and depositional context of a temporary exposure in the Anstruther Formation (Strathclyde Group), Anstruther Wester, Fife

Synopsis A temporary exposure in the Anstruther Formation (Strathclyde Group: Viséan, Asbian) behind the now-demolished Anstruther Wester swimming pool uncovered 13 m of strata within a cyclical sedimentary succession. A total of 68 m of this succession were logged from an arbitrary 10 m below the base of the Johnny Dow’s Pulpit sandstone to 5.2 m above the Billow Ness Marine Band. The measured section includes two complete and two incompletely measured cycles representing variations on a primitive ‘Yoredale’ pattern, in which the transgressive marine shale–carbonate shelf facies is largely suppressed in favour of upward-coarsening progradational prodelta and delta front, and aggradational delta plain facies. Each cycle commenced in a non-marine to marginal marine coastal environment in which shales, thin dolostones (some oncolitic), and ‘musselband’ shales and dolostones were deposited (the mussels are non-marine bivalves). Shales were succeeded by progradational channelized delta front facies, and aggradational delta plain facies also including channel-fill sandstones, typically with pedogenic tops. Variations in cycle thickness relate partly to fluctuations in the supply of sand-grade sediment, and partly to contemporary subsidence, possibly related to tectonic processes. Comparisons are made with the late Brigantian Lower Limestone Formation at St Monans, which together with the Anstruther Formation forms part of a transgressive megasequence initiated late in the Devonian with continental, alluvial, redbeds.

Ephemeral-Fluvial Deposits: Integrated Outcrop and Simulation Studies Reveal Complexity

Ephemeral-fluvial sandstones form the main reservoirs in many oil and gas fields. Production histories and development difficulties suggest these deposits are more complex than is commonly supposed in the petroleum industry. There is inadequate published information to account for this observation. Relevant sedimentological literature on such deposits is qualitative and unsuitable for detailed reservoir characterization studies; therefore, integrated sedimentological, petrophysical, and fluid-flow simulation studies have been conducted on outcrop analogs in the southwestern United States. Our key objectives were to define flow units and to find areas of sensitivity inherent both in the sediments and in the analytical and statistical procedures used to describe and model s ch systems. We report results of integrated studies on the Lower Jurassic Kayenta Formation in southeastern Utah. The Kayenta Formation is a relatively proximal, sand-rich succession dominated by channel-fill sandstones. Sedimentological analysis identified eight main genetic units. In contrast, reservoir simulations, based on probe-permeameter data collected at outcrop, show that six of the main genetic units have uniform flow behavior. Intraformational mudclast conglomerates are the dominant permeability barriers, both because of the clasts themselves and because they act as sites of preferential cementation. Stochastic permeability models conditioned to the outcrop data produce results little different from models based on averaged values for each facies. The most sensitive feature of the mode s concerns the assumptions made about the mudclast conglomerates. The model results show that only by integrating analytical approaches can we understand the full character of an outcrop analog in such a way that we can use it predictively.

Subsurface Definition of the Allegheny Group Coalbed Methane Prospect Interval in Southwestern Pennsylvania and New Gas Content Results: ABSTRACT

A preliminary reconnaissance of coalbed methane gas content data from exploratory coal cores and pre-existing data implies that the greater the depth and rank, the greater the total and cumulative gas content. The coal seams studied, ranging in age from the Pennsylvanian-Permian Dunkard Group to the Middle Pennsylvanian Allegheny Group, are from the Main Bituminous Field and two of the anthracite fields. Consequently, the Pennsylvania Geological Survey and the West Virginia Geological and Economic Survey conducted a mapping investigation to evaluate the regional geology of the coal-bearing intervals and its influence on coalbed methane potential. Phase I of this study involved the entire Pennsylvanian coal-bearing interval; Phase II focused on a stratigraphic delineation and evaluation of Allegheny coalbeds and associated sandstones. A variety of cross sections and isopach maps show several prospective coalbeds and facies relationships with channel-fill sandstones. This suggests that some of these sandstones may be traps for coalbed methane. Often overlooked in reservoir characterization is the quality of a coal seam. Coal rank, grade, and type influence the reserves and production of coalbed methane; the higher the rank, the greater adsorptive capacity of the coal. The integration of coal quality with other critical tools of exploration maymore » increase the success rate of finding {open_quotes}sweet spots.{close_quotes} Additional Pennsylvania Geological Survey drilling occurred in Beaver, Lawrence, Somerset, and Washington counties. Gas contents were graphically displayed against depth, thickness, and time for a variety of samples from 21 coal seams; average gas composition and Btu values were determined for selected samples.« less

Depositional Controls on Reservoir Properties in a Braid-Delta Sandstone, Tirrawarra Oil Field, South Australia

The Tirrawarra Sandstone contains 146 million bbl of oil in Tirrawarra field in the Cooper basin of South Australia. We used core, well logs, and petrophysical data to construct a depositional-facies-based flow-unit model of the reservoir, which describes rock properties and hydrocarbon saturations in three dimensions. Using the model to calculate volumes and residency of original and remaining oil in place, we identified an additional 36 million bbl of oil in place and improved understanding of past production patterns. The Tirrawarra Sandstone reservoir was deposited in a Carboniferous-Permian proglacial intracratonic setting and is composed of lacustrine and fluvial facies assemblages. The stratigraphic framework of these nonmarine facies is defined by distinctive stacking patterns and erosional unconformities. Mudstone dominated zones that are analogous to marine maximum flooding surfaces bound the reservoir. At its base a progradational lacustrine-delta system, composed of lenticular mud-clast-rich sandstones enclosed in mudstone, is truncated by an unconformity. Sandstones in these lower deltaic facies lost most of their porosity by mechanical compaction of ductile grains. Above the unconformity a braid-delta system, dominated by aggradational bed-load channel-fill sandstones, forms the core of he reservoir. Sediment reworking by channel migration and locally by shore-zone processes created quartz-rich, multilateral sandstones, which retained the highest porosity and permeability of all the reservoir facies and contained most of the original oil in place. The braid delta is erosionally truncated and overlain by a more proximal braid-plain system. Braided-channel sandstones, however, are overlain by lenticular meandering-channel sandstones, which in turn grade upward into widespread mudstones and coals. Thus, this uppermost part of the reservoir displays a retrogradational stacking pattern and upward-decreasing reservoir quality. On the basis of the systematic arrangement of facies within this stratigraphic framework, as well as facies-related differences in composition and texture, we identified reservoir flow units that have characteristic petrophysical properties. Our results demonstrate that depositional variables are the primary controls on reservoir quality and productivity in the Tirrawarra Sandstone.

Preservation of Primary Porosity in the Neogene Clastic Reservoirs of the Surma Basin, Bangladesh: ABSTRACT

The Surma Basin is a Tertiary sub-basin within the greater Bengal Basin, in N.E. Bangladesh. The Neogene sequence ({approximately}17 km thick) contains the producing hydrocarbon reservoirs with proven gas reserves. These sediments are alternating coarse and fine clastics, representing a complex interfingering of deltaic and marine subenvironments, with the former dominating. The principal reservoir facies are distributary channel-fill sandstones in a lower delta-plain setting. Kailashtila, Beanibazar and Rashidpur, located in anticlinal structures, are major hydrocarbon-producing fields in the E. Surma Basin. Petrographic analysis shows that primary intergranular porosity mainly controls the reservoir quality of these Neogene sands, which occur at a depth of {approximately}3000 m. Most samples show primary pores with about 20% porosity and permeabilities of about 200 mD. The preservation of a higher proportion of primary pores in fine to medium grained sandstones is a result of (1) moderate compaction resulting from overpressuring caused by a higher rate of subsidence and sedimentation, (2) weak cementation, and (3) a general lack of deleterious clays and the presence of some grain-rimming chlorites. The general absence of long and sutured grain contacts also supports these observations. Some of the existing literature suggests that secondary pores are dominant in the Neogene sandymore » reservoirs of the Bengal Basin; however, they contribute little ({approximately}2%) to the total porosity in the Surma Basin.« less

Lowstand slope channels of the Itilli succession (Maastrichtian-Lower Paleocene), Nuussuaq, West Greenland

The Maastrichtian-Lower Paleocene Itilli slope succession of Nuussuaq, West Greenland consists of an at least 2.5 km thick turbidite succession, composed of mudstone, thinly interbedded sandstone and mudstone, chaotic beds, amalgamated sandstone and giant-scale cross-bedded sandstone. The amalgamated and giant-scale cross-bedded sandstones were deposited in 1–2 km wide slope channels draining basin-margin source areas. The chaotic beds invariably underlie undisturbed channel fill sandstones suggesting that initially channels were excavated largely by retrogressive slumping of unstable slope mudstones. Retrogressive slumping was followed by intense channel excavation with continued scouring of channel base and walls before deposition of channel fills from high-density turbidity currents. It is suggested that destabilisation of the slope mudstones was triggered by drops in relative sea-level, associated with increased sediment deposition on the slope. Sequence stratigraphic concepts have been applied to the Itilli slope succession. The chaotic beds are related to the forced regressive wedge systems tract. The sequence boundary is placed at the erosional unconformity separating the chaotic beds from the overlying channel sandstone turbidites and it represents the lowest point of relative sea-level fall. The channel fill represents the lowstand prograding wedge systems tract and the transgressive systems tract. Poorly developed highstand systems tract deposits include thinly interbedded sandstones and mudstones and are probably also represented in the redeposited material in the chaotic beds.