John C. Horne

Tidally influenced barrier island and estuarine sedimentation in the upper carboniferous of southern West Virginia

Abstract Upper Carboniferous orthoquartzites exposed in the Guyandotte Valley of southern West Virginia are interpreted as barrier island and backbarrier deposits. They overlie fossiliferous marine limestones and shales, and are overlain in turn by fluvio-deltaic subgraywacke sandstones, siltstones and coals. Only the upper one-third of the 180 m thick orthoquartzitic interval is exposed, but these rocks outcrop over a distance of 16 km, and reveal landward transition of the barrier orthoquartzites into backbarrier and alluvial or delta-plain deposits. Within the exposed orthoquartzitic interval are three major orthoquartzite bodies, separated by subordinate siltstones containing smaller sandstone bodies. The major orthoquartzites are 10–25 m thick and are lenticular in transverse section, with a north-south elongation approximately perpendicular to the palaeoslope. These sandstone units are subdivisible into three facies. Facies 1 is most widespread and comprises mainly upward-thinning sets of cross-stratification, with a silty interval occasionally preserved towards the top. Palaeocurrent indicators reflect increasing dispersal from the base of the sequence upward. These deposits are interpreted as originating within laterally migrating tidal inlets, in which ebb currents were dominant. Facies 2 contains low-angle bedding suggestive of wave swash processes, with patterns of minor sedimentary structures similar to those observed in modern ebb tidal deltas. Facies 3 consists of sandstone with a preponderance of landward-directed cross-stratification, and is attributed to a flood tidal delta, flood-channel and washover berm origin. The orthoquartzites merge eastward and southeastward (landward) into inferred estuarine or lagoonal siltstones, but in places they are in lateral contact with fluvio-deltaic subgraywackes. Thus the barrier sediments are thought to have been derived at least in part by the reworking of abandoned delta lobes. Longshore drift created spit extension towards the south and southwest, thereby enclosing lagoons within which accumulated siltstones and sandstones with brackish or marine fossils and a variety of structures suggestive of tidal processes and local subaerial exposure. Washover sands were deposited to landward of the barrier. Both barrier and backbarrier deposits are incised by small tidal channels. Occurrence of washover features in association with small but well-developed tidal deltas suggests moderate to strong wave activity and a mesotidal (2–4 m) range.

Predictive Depositional Models for Eolian Deposits of Leo Member of Minnelusa Formation, Powder River Basin, Eastern Wyoming: ABSTRACT

A study of 980 wells, 24 outcrop sections, and 29 cores was conducted in the Powder River basin of northeastern Wyoming in order to develop predictive depositional models for the Leo sandstones of the middle member of the Minnelusa Formation. Given the limited amount of data available, an approach was devised that relied on synthesis of information from modern analogs with data derived from the ancient to predict regional patterns of deposition. This information was then used to determine genetic relationships between the patterns of regional sedimentation and proven stratigraphic traps. Six Leo oil and gas fields were examined in detail; the Qatar Peninsula and Um Said sabkhas serve as modern analogs. The results of the study show that the Leo member of the Minnelusa and equivalent units of the Powder River basin were deposited as dune sequences within and adjacent to the Lusk embayment, a northward-extending arm of a large epeiric sea that existed southeast of the study area. Situated approximately 15° north of the paleoequator, the study area was the site of accumulation of sands transported from a northerly source by northeasterly trade winds. Accumulation and distribution of these windblown sands in the area surrounding the Lusk embayment were controlled by the local depositional setting, tectonic framework, and a series of minor fluctuations of eustatic sea level. The greatest potential for preservation of these dune deposits occurred during periods of rising base level. End_of_Article - Last_Page 250------------

ABSTRACT: The Aeromagnetic Definition of Wrench Faults and Their Influence on Hydrocarbon Entrapment and Production Fairways

Abstract Structurally and stratigraphically entrapped hydrocarbons, as well as the trends of reservoir facies and hydrocarbon migration pathways, appear to be strongly influenced by the wrench fault systems present in basins throughout the Rockies and elsewhere. At least three types of basement structural features influence production: 1) basement structural highs; 2) relatively short basement faults, which mostly define the margins of basement structural highs, and 3) regional crosscutting wrench faults, which define and create major structural and compositional discontinuities. These three structural feature types are most readily interpreted from patterns seen on the Second Vertical Derivative and SUNMAG displays, as well as structures defined by line profile analysis. Integration of these data with log, facies, hydrocarbon show, and production information indicate that motion along wrench faults is instrumental in controlling where entrapment of hydrocarbons takes place. Production bears a direct and obvious relationship to either the juncture of basement structural highs, with the cross-cutting wrench faults that can be interpreted from discontinuity of patterns present on the aeromagnetic displays, or to certain structural features orthogonal to these wrench faults. Essentially, all fields occur on the tops or immediate flanks of mapped basement structures, an indication that even subtle structures at basement level are important in the stratigraphic entrapment of hydrocarbons. In basins such as the Williston, Big Horn, Powder River, Piceance, Uinta, and Greater Green River, fields are located on mostly northeast trending wrench faults, or on orthogonal structures limited by these cross-cutting wrench faults. Major fields such as Little Knife, Billings Nose, Fryburg, the Dickinson-Eland Wausortian Mound Fields, Oregon Basin, Cottonwood Creek, Jonah, Highlight, and the Rulison-Parachute-Grand Valley fields bear a distinct relationship to these major, but mostly subtly defined faults. Production commonly ends abruptly or changes trend at wrench fault discontinuities. Consequently, these faults appear to be of critical importance in controlling not only structural development, but also the updip productive limit of many stratigraphic entrapments, whether being caused by diagenetic pore throat entrapment, or a change in facies. In either case, these relationships are an indication that regional wrench faults were active during deposition or influenced diagenetic fluid movement through the reservoir system at a later time.

Abstract: Transgressive Valley Fill Sequences: Implications to Sequence Stratigraphy and Petroleum Exploration

ABSTRACT Large amounts of hydrocarbons have been discovered in association with valley fill deposits. The lateral and vertical distribution of siliciclastic rock types in transgressive valley fill deposits is governed by the stratigraphic sequence. The sequence stratigraphy is controlled primarily by the interaction of subsidence, eustacy, and volume of sediments. During periods when eustatic sea level decline exceeds the rate of subsidence, much or all of a basin may become exposed, and a lowstand surface of erosion will develop forming a stratigraphic sequence boundary. In deeper basins such as the Gulf Coast Basin, the lowstand erosion surfaces are confined to the shelves at the margins of the basin. When the sedimentation rate is less than the sum of the rate of subsidence and eustatic sea level change, a transgressive sequence develops over the lowstand surface with deeper water facies overlying shallower water and/or nonmarine facies. Reservoir potential siliciclastics accumulate in valley fill, estuarine, and wave reworked transgressive deposits. Differences in the spectrum of transgressive deposits can be attributed to variations in the rate of eustatic sea level rise, subsidence, and sediment supply. Rapid rates of subsidence and/or eustatic sea level rise relative to sediment supply result in an abrupt landward shift in facies and valleys filled primarily with estuarine muds. Examples include the Opeche formation of the Powder River Basin and the lower Wilcox and Yegua formations of the Texas Gulf Coast. The valley fill mudstones in these units trap hydrocarbons in laterally adjacent facies. Slower rates of relative sea level rise, or increased sediment supply, result in valley fill sequences with thicker basal fluvial deposits overlain by estuarine to marine sands and muds. Petroleum is produced from fluvial-estuarine reservoirs within this type of valley fill in the Morrow Formation in southeast Colorado and southwest Kansas and the Doig Formation of western Alberta. Complex valley fill sequences consisting of multiple backstepping progradational parasequences occur in association with varying rates of eustatic sea level rise or subsidence relative to sediment supply. Examples from the Muddy Sandstone of the Powder River Basin illustrate such complexities. Petroleum has been produced from fluvial, bayhead delta, and barrier island facies of the Muddy Sandstone valley fills. End_of_Record - Last_Page 303-------

Tidal-Inlet Fills as Hydrocarbon Reservoirs: Example from Halfway Formation, Alberta: ABSTRACT

When placed within the context of their depositional setting and compared to modern analogs, the proven hydrocarbon reservoirs of the Middle Triassic Halfway Formation provide models for future exploration of similar sequences in the stratigraphic record. The Halfway sandstones accumulated along the northeastern margin of the Triassic seaway. Barrier-strandplain deposits amassed along a depositional embayment in western Alberta downdrift of an area of sediment influx into the basin in eastern British Columbia. Along the Halfway coastline, porous deposits accumulated either in the wave-reworked upper shoreface-foreshore zone of barrier islands or in tidal-inlet areas. The barrier-island reservoir sandstones are thin (less than 5 m) and elongate with depositional strike, whereas the tidal-inlet deposits are thick (up to 20 m) with abundant shell-hash lag conglomerates and elongate with depositional dip. Similar to modern coastal configurations, the frequency and thickness of tidal-inlet sequences increase toward the center of the depositional embayment because of tidal amplification. There, more conduits through the barriers were necessary to exchange the larger volumes of water during a tidal cycle. Most of the significant Halfway hydrocarbon reservoirs have been inlet-fill sequences. An excellent example is the Wembley field. Positioned near the center of the Halfway depositional embayment, this field contains 37.5 million bbl of oil. A majority of the reserves are found in inlet deposits. Porosities and permeabilities have been significantly enhanced by secondary solution of the shell lags. Other Halfway inlet reservoirs exist along depositional strike. They are most abundant near the axis of the embayed shorelines where tides were amplified. Future hydrocarbon exploration along other embayed coasts should emphasize the locations and abundance of inlet-fill deposits. End_of_Article - Last_Page 267------------

Facies Control on Oil Production From Upper Member of Permo-Pennsylvanian Minnelusa Formation, Powder River Basin, Wyoming: ABSTRACT

Lateral and vertical facies variations within the predominantly eolian upper member of the Minnelusa Formation control both the regional reservoir distribution and the localization of oil-producing trends. Sands sourced by northeasterly trade winds were deposited in a land area bounded on the west by the Lusk embayment, which was a shallow, restricted extension of the Permo-Pennsylvanian sea. This embayment was present throughout Minnelusa deposition, and was located in the western portion of the present-day Powder River basin. Another extension of the epeiric sea, located in western South Dakota, formed the eastern boundary of the land area. In the northern part of this area, an inland sand-sea developed; in the southern part, the sand supply was less and isolated barchan dunes migrated over a coastal sabkha. Dune sandstones are bounded laterally by predominantly sandy interdune deposits in the north and by coastal interdune deposits, including sandstone, dolomite, and anhydrite, in th south. Major marine transgressions deposited laterally extensive dolomites that separate the dune sandstones. Interdune deposits constitute permeability barriers adjacent to dune sandstones. The dune sandstones, which can be of excellent reservoir quality, were subjected to early cementation by anhydrite. Later dissolution of the anhydrite cement, facilitated by good to excellent sorting and possibly enhanced by hydrocarbon migration, led to development of significant secondary porosity. Interdune sandstones are less well sorted and so did not develop good secondary porosity. Interdune carbonates and evaporites have virtually no permeability. The coastal interdune deposits in the southern part of the region, therefore, form more effective lateral permeability barriers than do the sand-dominated interdune deposits in the north. End_of_Article - Last_Page 852------------

Fan-Delta Hydrocarbon Reservoirs: Example from Utikuma-Nipisi Fields, Alberta: ABSTRACT

Because large amounts of hydrocarbons have been found in reservoirs of deltaic origin, deltas have been extensively studied in both the modern and rock record. Internal morphologies and geometries of reservoir-potential deposits within most types of deltas are today reasonably well understood. Fan deltas and the geometries of their sandstone reservoir bodies are exceptions. To provide a better understanding of fan-delta reservoirs, 700 wells and 365 cores from the Utikuma-Nipisi fields of north-central Alberta were studied in detail. The Utikuma-Nipisi fields, which contain 751.4 million bbl of oil reserves in fan deltas, produce from structural-stratigraphic traps in Middle Devonian Gilwood sandstones of the Watt Mountain Formation. These sediments are part of the clastic apron that surrounded the Peace River arch, a positive granitic terrane that had relief of more than 2,300 ft during Middle Devonian time. In the Utikuma-Nipisi area, arkosic sediments were transported from the Peace River arch by ephemeral braided streams and deposited as a fan delta at the margin of the Elk Point Sea. In upper reaches of the delta, porosities and permeabilities in the coarse alluvial fan-braided stream portions have been occluded by fine-grained sieve deposits. Seaward of the delta front, prodelta sediments act as fine-grained permeability barriers. Only in the delta front have significant reservoir-potential porous deposits accumulated. These primary intergranular porosities and permeabilities are attributable to sorting and reworking by fluvial processes as well as wave and tidal energies in the depositional basin. Discontinuities in these delta-front reservoirs were the result of delta-lobe switches Results of this analysis suggest hydrocarbon exploration in fan deltas should target delta-front depositional settings. End_of_Article - Last_Page 267------------

Depositional and Exploration Models for Cretaceous Lower Mannville Fluvial Sandstones of South-Central Alberta: ABSTRACT

Production of hydrocarbons from fluvial strata of the lower Mannville Formation in the Taber-Milk River area of south-central Alberta occurs primarily from combination structural-stratigraphic traps situated on subtle north-northwest trending anticlinal features. Lower Mannville sediments were deposited in a north-trending valley that formed when sea level lowered and shorelines receded to the edge of the continent during the Late Jurassic and Early Cretaceous. The river that cut this valley shifted eastward in response to rising of the Cordilleran highlands, producing a west-facing escarpment. We regard this escarpment as a southward extension of the Fox Creek Escarpment of west-central Alberta. In latest Neocomian or earliest Aptian time, the river system began to aggra e as a result of southward transgression of the Boreal sea. The basal aggradational valley fill, the Sunburst Sandstone, is generally the coarsest, best sorted, and texturally most mature of the sandstones in the Mannville Group. Stratigraphic traps in the area are the result of: (1) updip pinch-out of the Sunburst Sandstone against the north-trending Fox Creek Escarpment (e.g., Horsefly Lake field); (2) general eastward-thinning of the Sunburst Sandstone within tributary valleys east of the Fox Creek Escarpment (e.g., Chin Coulee field); and (3) updip interruption of blanket fluvial sandstone units by clay-filled, abandoned reaches of the river system that deposited the lower Mannville sandstones (e.g., Taber field). A logical exploration strategy both in the Taber-Milk River area and i areas to the north and south would be to pursue the trends of the Fox Creek Escarpment and its tributary valleys. End_of_Article - Last_Page 543------------

Variation in Depositional Systems Along Shoreline Embayments--Modern and Ancient Examples: ABSTRACT

The stratigraphic record of strandline depositional environments shows a systematic change along shoreline embayments in response to changes in the ratio of wave-energy flux to tidal-energy flux. Waves diminish in size and tidal ranges increase from the entrances to the heads of such embayments. A depositional model for shoreline embayments emphasizing sand bodies shows the following: embayment entrance--wave-dominated deltas, microtidal barriers, abundant washover fans, and flood-tidal deltas in lagoons; mid-zone--mixed-energy deltas, mesotidal barriers, numerous inlets, back-barrier tidal-channel sands; and embayment head--tide-dominated deltas, offshore tidal sand ridges, no barriers, extensive marsh/tidal flat systems. Two ancient shoreline embayments, along the Carboniferous shoreline of the southern Appalachians and the Late Cretaceous shoreline of Wyoming and Colorado, illustrate the model. Both examples illustrate a change in sand-body geometry from micro-tidal, wave-dominated barriers at the entrances to mesotidal, inlet-dominated barriers farther inside the embayments. Thus, subsurface exploration for sand bodies containing economic deposits should focus on strandline-parallel sands with lagoonward building washovers and flood-tidal deltas at embayment entrances, and strand-perpendicular tidal sands at embayment heads. Exploration in the mid-zones of the embayments would be the most difficult, because of the complexity brought about by the migration of tidal inlets at the shoreline and tidal channels in the back-barrier area. End_of_Article - Last_Page 580------------

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------------