Shannon Sandstone Research Articles

Sequence stratigraphy of the lower Pierre Shale of the southern Powder River Basin: A ramp margin sequence that terminates Niobrara Formation carbonate deposition

The Upper Cretaceous section of the southern Powder River Basin of the Rocky Mountain region includes the Niobrara Formation, which is one of the most significant source rocks of the Western Interior Cretaceous Seaway, and it is overlain by the lower Pierre Shale interval. The lower Pierre Shale is divided into eight members, which are the progradational highstand deposits of the Gammon Ferruginous Member; lowstand prograding wedge deposits of the Shannon Sandstone, the Unnamed Member, and the Sussex Sandstone; transgressional Ardmore Pedro Bentonite Beds and Sharon Springs members; and highstand deposits of the Mitten Black Shale and Red Bird Silty members. The Shannon and Sussex Sandstone members are known targets for oil production. Based on integrated research of 1490 well logs with two cores, this study demonstrates that the lower Pierre Shale interval is a Type 1 Ramp Margin Sequence. Although previous studies primarily focused on individual sandstone members or parts of the lower Pierre Shale section in relatively limited areas, this study provides an in-depth sequence stratigraphic analysis of the lower Pierre Shale interval. A depositional model was created, which demonstrates that the Shannon Sandstone, Unnamed, and Sussex Sandstone Members were deposited as an encased lowstand prograding wedge between the progradational Gammon Ferruginous Member and the transgressional Ardmore Pedro Bentonite Beds and Sharon Springs members hundreds of miles basinward from the stratigraphically equal Mesaverde lowstand sandstones of the Bighorn Basin. The shift of facies across long distances is explained with a forced regression that has taken place during the deposition of the uppermost Gammon Ferruginous Member and the following deposition of the Shannon and Sussex sandstone members. Rapid eastward progradation of the clastic sediments of the lower Pierre Shale and the following rapid forced regression effectively terminate the deposition of the Niobrara Formation carbonates in the Western Interior Seaway.

The problem of glaucony from the Shannon Sandstone (Campanian, Wyoming)

Terra Nova, 23, 100–107, 2011AbstractThe Shannon Sandstone, one of the most controversial units in the field of stratigraphy, was first interpreted as a shelf sand‐ridge deposit, but subsequent studies have furnished different interpretations, from lowstand‐shoreface to incised‐valley models. Despite a general consensus among previous workers about the significance of glaucony for Shannon interpretation, mineralogical characterization of the green grains has never been undertaken: identification of glaucony has been restricted to visual inspection. Accurate characterization of ‘highly glauconitic’ deposits from the Shannon Sandstone (Powder River Basin of Wyoming, Section W3) reveals that sedimentological interpretation of the greensands relies mostly upon mistaken mineral identity. Glaucony appears as a minor component of the rock and proves to be poorly mature and significantly less abundant (0–6%) than previously inferred (up to 85%). Characterization of the glauconitic minerals, however, provides data that contribute to Shannon interpretation.

ABSTRACT: Comparison of Stratigraphy and Sedimentology of the Shannon Sandstone in Jepson-Holler Draw and Pine Tree Fields, Powder River Basin, Wyoming

ABSTRACT: Comparison of Stratigraphy and Sedimentology of the Shannon Sandstone in Jepson-Holler Draw and Pine Tree Fields, Powder River Basin, Wyoming

ABSTRACT: Reservoir Architecture of the Shannon Sandstone, Hartzog Draw, Wyoming

ABSTRACT: Reservoir Architecture of the Shannon Sandstone, Hartzog Draw, Wyoming

Sequence Stratigraphic and Tectonic Controls on Shannon Incised-Valley Distribution, Hartzog Draw, Wyoming: ABSTRACT

The Upper Cretaceous Shannon Sandstone in the Powder River Basin, Wyoming is interpreted as stacked tidal bars infilling northwest-southeast trending incised valleys. The Shannon Sandstone can be subdivided into 3 sequences: Copenhagen Blue, Crimson Red, and Canary Yellow. The Copenhagen Blue sequence boundary is a regional unconformity at the base of the Shannon Sandstone marked by distal tidal-bar deposits resting on offshore mudstones of the Cody. The overlying Crimson Red sequence is the main reservoir interval at Hartzog Draw and is composed of proximal tidal-bar deposits. The highly erosional Canary Yellow sequence boundary forms the trap at Hartzog Draw by juxtaposing the reservoir sandstones of the Crimson Red sequence with the overlying offshore mudstones of the Canary Yellow sequence. The Shannon Sandstone is therefore an erosional remnant of originally more extensive tide-dominated deltas deposited within northwest-southeast trending incised valleys. The orientation and distribution of the Shannon incised valleys is the result of the interplay between tectonics and eustacy. During lowstand incised valley development the orientation of fluvial systems incising the shelf was strongly controlled by pre-existing northwest-southeast trending structural elements in the basin. During the subsequent sea-level rise the shoreline backstepped in a landward direction. The shoreline also gradually rotatedmore » from an east-west orientation during the lowstand to a north-south orientation during the highstand systems tract. Ultimately the highstand shorelines were located over 160 kms landward of the lowstand shorelines and oriented subparallel to the lowstand incised valleys.« less

High-Resolution Sequence Stratigraphic Analysis of the Shannon Sandstone in Wyoming, Using a Template for Regional Correlation

ABSTRACT Recent interpretations of the lower Campanian Shannon Sandstone in the Powder River Basin, Wyoming have suggested that it was deposited in falling stage and lowstand incised shorefaces, rather than in stacked shelf ridges up to 160 km from shore. Despite these detailed studies, there is no published correlation of the outcrop deposits at Salt Creek Anticline with the subsurface deposits in the Hartzog Draw-Heldt Draw (HD-HD) area. A complete high-resolution sequence stratigraphic analysis of the Shannon requires that the outcrops at Salt Creek be correlated with the subsurface fields at HD-HD. There is little core control in the approximately 30 km between these areas, so correlations are based almost entirely on well logs. A template has been constructed in the HD-HD area, where ther is good core and well log control, which embodies data from facies, facies successions, well log signatures, and bounding discontinuities. The template consists of a regressive surface of erosion (RSE) that forms the base of an incised shoreface sandbody, and a transgressive surface of erosion (TSE) that truncates the top of the sandbody and the RSE. Two cross sections have been constructed, and the template has been used to guide correlations between Salt Creek and the HD-HD area. From the cross sections, six progradational events and six transgressions have been defined during Shannon time. The positions of the RSEs and TSEs allow estimates of between a few meters and 60 m for the magnitude of sea level fluctuations that controlled Shannon deposition. The proposed correlations confirm and reinforce the interpretation of the Shannon as a series of incised shoreface sandbodies, separated by transgressive systems tract mudstone deposits.

Shannon Sandstone in Hartzog Draw-Heldt Draw Fields (Cretaceous, Wyoming, USA) Reinterpreted as Lowstand Shoreface Deposits

ABSTRACT The lower Campanian Shannon Sandstone at Hartzog Draw-Heldt Draw (Powder River Basin, Wyoming) is interpreted in the literature as a shelf-ridge complex deposited 160 km from the contemporaneous shoreline. This interpretation poses difficult problems, and does not discuss the significance of bounding discontinuities preserved in the sandbodies. Cross sections show that the Shannon Sandstone is present at several stratigraphic horizons, and is bounded by regionally extensive discontinuities. The presence of these bounding discontinuities suggests that fluctuations in relative sea level were important in controlling geometry of sandbodies on the Shannon shelf. In the Hartzog Draw-Heldt Draw area of Wyoming the base of the Shannon is a planar surface with respect to a lower bentonite marker and is characterized by the abrupt appearance of bioturbated sandstone on top of black basinal mudstone. Siderite pebbles < 1 cm in diameter are common at this contact. In Hartzog Draw the bioturbated sandstone interval has an average thickness of 10 m and is separated from an overlying unit of cross-bedded fine to medium sandstone (up to 20 m thick) by a ubiquitous (50 cm thick) horizon of mud clasts in a cross-bedded, highly glauconitic, medium sandstone. These three units are laterally continuous in Hartzog Draw except where removed by erosion. The top of the Shannon is marked by an abrupt change from the cross-bedded sandstone or bioturbated medium sandstone to laminated black mudstone. Mud clasts, siderite pebbles, chert grains, and glauconite mark this abrupt contact, suggesting erosional transgression. This surface can be traced westward, where in many areas it is a mud-on-mud contact marked by a lag of shell debris. This transgressive surface, which terminates Shannon deposition, is characterized by at least one regionally extensive onlapping marker in the Hartzog Draw-Heldt Draw area. The presence of depositional discontinuities in the Shannon Sandstone is suggested by (1) the sharp basal contact, (2) the lateral continuity of individual facies, (3) the abrupt vertical facies changes and the association of mud clasts, siderite pebbles, and glauconite accompanying these abrupt juxtapositions of facies, (4) the stratigraphic variability, and (5) the termination of Shannon deposition by erosive transgression, marked by onlapping markers. These depositional discontinuities are the bounding discontinuities. These bounding discontinuities have asymmetrical one-sided scour morphologies, with steeper landward margins that flatten basinward. These geometries and the linearity of the overlying sandstone and the troughs defined by these scoured surfaces suggest incised shoref ce profiles truncated by erosive transgression. Thus, the Shannon Sandstone in Hartzog Draw-Heldt Draw is reinterpreted as a lowstand shoreface deposit.

Lower Campanian Shannon Sandstone (Hartzog Draw): A Template for Regional Correlation: ABSTRACT

Lower Campanian Shannon Sandstone (Hartzog Draw): A Template for Regional Correlation: ABSTRACT

Shannon Sandstone in Wyoming: A Shelf-ridge Complex Reinterpreted as Lowstand Shoreface Deposits

ABSTRACT The Shannon Sandstone (Campanian) of Wyoming was formerly interpreted as two stacked shelf-ridge complexes. Sand was believed to have been transported from a time-equivalent shoreline 110-150 km to the west and reworked or molded into ridges at the depositional site. We show that this time-equivalent shoreline section at Lucerne, Wyoming, consists of not one shoreface sandbody, but two superimposed. They are both storm-dominated, and the lower one terminates in beach facies capped by root traces. There must have been a rise in relative sea level of at least 14 m to make accommodation space for the second shoreface sandbody. In the Salt Creek area and the adjacent subsurface Teapot Dome, there are two sandier-upward facies successions. In the lower succession there are abrupt vertical acies contacts between offshore bioturbated sandstones, thicker hummocky cross-stratified sandstones, and coarser cross-bedded sandstones. There is also evidence that the cross-bedded sandstones rest erosively on underlying facies. The trace fauna in the sandstones includes Macaronichnus segregatis, Rosselia socialis, and Ophiomorpha. The M. segregatis suggests a foreshore or upper-shoreface depositional environment, and R. socialis indicates a lower to middle shoreface; neither are characteristic of a shelf-ridge complex. The abrupt and probably erosive facies contacts, along with the trace fauna, suggest that the cross-bedded sandstones in this succession represent a shoreface deposit that formed during a stage of actively falling relative sea level. Overlyi g muddy bioturbated sandstones indicate that shoreface deposition was terminated by a transgression. The upper sandier-upward succession contains facies and trace fauna similar to those of the lower succession, and is also interpreted as a prograding shoreface. Rather than the sand moving 110-150 km from the shoreline and then being molded into shelf-ridge complexes at Salt Creek, we suggest that the shoreline moved into the Salt Creek area as the result of a fall in relative sea level. It was transgressed and moved back westward; a second fall of relative sea level moved the shoreline back to Salt Creek, and a final transgression terminated Shannon deposition.

Upper Cretaceous Shannon Sandstone Reservoirs, Powder River Basin, Wyoming: Evidence for Organic Acid Diagenesis? (1)

Comparison of the petrology of shallow and deep oil reservoirs in the Upper Cretaceous Shannon Sandstone Beds of the Steele Member of the Cody Shale strongly suggests that organic acids have had a more significant impact on the diagenetic alteration of aluminosilicate grains and carbonate cements in the deep reservoirs than in the shallow reservoirs. Vitrinite reflectance and Rock-Eval measurements, as well as the time-temperature index and kinetic modeling, indicate that deep reservoirs have been subjected to maximum temperatures of approximately 110-120 degrees C, whereas shallow reservoirs have reached only 75 degrees C. Only the deep reservoirs, therefore, have reached higher temperatures and have been (and some still are) within the zone (80-120 degrees C) of maximum organic acid production. Burial history reconstruction and paragenetic relations show that oil migration into Shannon reservoirs occurred in the middle to late Tertiary. In shallow reservoirs, detrital grains exhibit minor dissolution, sparse and small overgrowths, and secondary porosity created by dissolution of early calcite cement. However, deeper sandstones are characterized by extensive dissolution of detrital K-feldspar and detrital glauconite grains, and precipitation of abundant, large quartz and feldspar overgrowths. Quartz overgrowths commonly have crystallographically controlled etch pits. Throughout the Shannon and Steele, dissolution of glauconite and degradation of kerogen were probably aided by clay mineral/organic catalysis, which caused simultaneous reduction of iron and oxidation of kerogen. This process resulted in release of ferrous iron and organic acids and was promoted in the deep reservoirs by higher formation temperatures acco nting for more extensive dissolution of aluminosilicate grains. At the temperatures of deep Shannon reservoirs, alkalinity was buffered by organic acid anions so that iron released from glauconite precipitated as chlorite and abundant, multistage ferroan carbonate overgrowths. Carbonic acid produced from the dissolution of early calcite cement, decarboxylation of organic matter, and influx of meteoric water after Laramide uplift produced additional dissolution of cements and grains. Dissolution by organic acids and complexing by organic acid anions, however, best explain the intensity of diagenesis and absence of dissolution products in secondary pores and on etched surfaces of framework grains in deep reservoirs.

Shannon Sandstone Depositional Model: Sand Ridge Dynamics on the Campanian Western Interior Shelf

The Campanian Shannon Sandstone Member of the Cody Shale forms the reservoir for several significant oil fields in the western Powder River Basin of Wyoming. The extensive data set available for this economically important unit makes it one of the best known ancient analogs to the sand ridges of modern shelf seas. The Shannon Sandstone was deposited as linear ridges by intermittent, along-shelf, southerly storm flows in a transgressive middle- to outer-shelf environment of the Cretaceous Interior Seaway. Comparison of Shannon sandstone bodies with modern sand ridges shows that they were formed in response to the kind of flow field observed over the modern systems. The long axes of these bodies are aligned northwest-southeast to north-northwest--south-southeast. The sandstone bodies are asymmetric in transverse section: northeastern flanks are shorter and steeper than the gentle accretion surfaces on the southwestern flanks. Paleocurrent direction is southerly; hence, the sandstone bodies are aligned obliquely with respect to the prevailing transport direction, as required by hydrodynamic theory. The four basic lithofacies types recognized in these sand ridge deposits are cross-bedded sandstone; thin-bedded sandstone: bioturbated, silty sandstone; and lenticular-bedded to bioturbated siltstone. The cross-bedded lithofacies has tabular, trough, glauconitic trough cross-bedded and bioturbated variants. This lithofacies grades down-section and down-current into a thin-bedded lithofacies comprising flaser- to wavy-bedded sandstone and low-angle, laminated sandstone. This thin-bedded lithofacies overlies a widespread, massively bioturbated sandstone lithofacies which in turn rests disconformably on a siltstone lithofacies. The distribution of lithofacies within a Shannon sandstone body is most readily explained in terms of the velocity distribution over a sand ridge, and textural modification by the distortion of the regional flow field over the growing sand ridge. The up-current flank underwent erosion, and deposition occurred on the crest and down-current side, so that the sand ridge grew and migrated in a southwestward direction. Each sand ridge served as a fractionation mill in which successive environments on the transport pathway across the ridge extracted successively finer grades of sand from the transported load.

Relation of Sedimentologic Features and Reservoir Quality in a Shelf Ridge Sandstone Deposit--Comparison of Outcrop and Subsurface Properties: ABSTRACT

The High Energy Ridge Margin (HERM) facies of the Shannon Sandstone in the Salt Creek anticline area of the Powder River basin, Wyoming, is comprised of fine to medium-grained glauconitic sandstone (82%), which is cross-bedded, rippled, and burrowed, and shale and limonitic clay (18%). Histograms of log permeability-to-air values measured from 214 1-in. diameter cores drilled from the face of an outcrop of the HERM facies of the lower Shannon indicate the existence of two major permeability populations. These two permeability populations can be related to cross-bedded sandstone (higher permeabilities), and noncross-bedded, rippled, and burrowed sandstone with clay laminae (lower permeabilities). Examination of the HERM facies in upper Shannon cores from Teapot Dome oil field, located 5 mi from the studied outcrop at a depth of 300 ft, indicate similar sedimentologic features and similar permeability populations related to cross-bedded and noncross-bedded subfacies within the HERM facies. Comparisons of permeability and porosity histograms, cumulative distribution functions, correlation lengths, and natural logarithm of permeability versus porosity plots also indicate a strong similarity in HERM reservoir properties between outcrop and Teapot Dome field. The similarities of outcrop and Teapot Dome permeabilities and porosities, which are from the same geologic facies but from differentmore » depositional episodes (upper and lower Shannon), suggest that rocks deposited under similar depositional processes within a given deposystem have similar reservoir properties.« less

Stratigraphic and Structural Analysis of Shannon Sandstone, Teapot Dome Field: Implications for Secondary and Tertiary Oil Recovery: ABSTRACT

The Department of Energy and Lawrence-Allison Associates have initiated three enhanced oil recovery (EOR) pilot projects in the last 4 years in the Shannon Sandstone at the Teapot Dome field. Performance of these pilot projects has generally been poor. As a result, a reevaluation of the geology for the entire field and for the pilot areas was conducted in an attempt to explain the pilots' performances. Based on core descriptions, conceptual reservoir flow patterns, and post-fireflood coring, only the bar-margin facies was found amenable to fluid displacement processes for oil recovery. This results in 25 million bbl of oil originally in place vs 180 million bbl of oil originally in place as a target for EOR. Stratigraphy alone does not explain the observed production patterns and simulation of the reservoir. Faulting and fracturing are extensive. Based on faults mapped in the in-situ pilot area, 12 to 16 northeast-trending normal faults per mile can be projected. Fracture orientations were obtained by mapping calcareous streaks in the upper and lower Shannon. These orientations confirm directions of premature fluid breakthroughs observed in the pilot projects and in the old East Teapot waterflood. Water resistivities and total dissolved solids measurements, water-cut maps, and dailymore » oil production maps suggest that some faults are partial to total barriers to fluid flow across the field.« less

Relationship Between Wireline Log Response and Quantitative Pore Geometry Data in Shannon Sandstone, Hartzog Draw Field, Wyoming: ABSTRACT

Relationship Between Wireline Log Response and Quantitative Pore Geometry Data in Shannon Sandstone, Hartzog Draw Field, Wyoming: ABSTRACT

Petrography, Diagenesis, and Facies Controls on Porosity in Shannon Sandstone, Hartzog Draw Field, Wyoming

On the basis of lithology and sedimentary structures, six sedimentary facies (A-F) were distinguished in cores of Shannon Sandstone in Hartzog Draw field, Campbell County, Wyoming. Core porosities and permeabilities are highest in facies A (average 15% and 16 md), moderately high in facies B (average 13% and 11 md), low in facies C (average 11.3% and 3.5 md), and very low in facies D and E (average 9-10% and 1-2 md). Dispersed detrital clay is a primary control on porosity, but in cross-bedded sandstones of facies A and B, the main productive interval, dispersed detrital clay is sparse and variations in porosity are due mainly to diagenesis. Microporosity does not contribute to permeability. It is low in facies A and B, high in facies C-E, and is associated mainly with detrital clay. As microporosity increases, water saturation increases. The paragenetic sequence inferred is: (1) siderite cementation; (2) formation of a brown, grain-coating clay, possibly illite/smectite; (3) quartz cementation; (4) calcite and dolomite cementation; (5) dissolution of carbonate cements and some framework grains; (6) a second generation of quartz cementation along with some chlorite and interlayered illite/smectite formation; and (7) oil emplacement.

Geological Factors Influencing Reservoir Performance of the Hartzog Draw Field, Wyoming

Summary The Hartzog Draw field, a major sandstone oil reservoir in the Powder River basin of Wyoming, was discovered in 1975 and unitized in 1980. The field is under waterflood and is being considered for CO2 miscible flooding. To aid in evaluating enhanced recovery, the working interest owners approved a comprehensive geological-engineering study using data from existing wells and new infill wells. Geologic facies identified in the study provided the stratigraphic basis for mapping reservoir flow units. These flow units more precisely describe variations in rock properties that control fluid flow. Within the flow units, bedding characteristics of the reservoir sandstones cause permeability anisotropy. Mineralogy and diagenesis affect permeability, porosity, water saturation, and sensitivity to injection fluids. The reservoir description resulting from this study provides insight into reservoir performance and forms the provides insight into reservoir performance and forms the framework for reservoir engineering studies to predict water flood and CO2-flood recovery. Introduction An accurate reservoir description-the internal, three-dimensional variation of reservoir rock properties-is essential to effective reservoir management. During primary production, areal variation of properties such as primary production, areal variation of properties such as permeability, porosity, thickness, and sand continuity permeability, porosity, thickness, and sand continuity influence both oil recovery and its distribution in the field. During secondary and enhanced production, vertical reservoir heterogeneity is a predominant factor affecting oil recovery. Knowledge of reservoir geology is particularly important when reservoir simulation models particularly important when reservoir simulation models are used to evaluate and predict reservoir performance. This paper presents the results of a study conducted to improve the reservoir description of the Hartzog Draw field, a large sandstone reservoir in the Powder River basin of northeastern Wyoming (Fig. 1). The development, reservoir properties, production history, and unitization of the field were described by Hunt and Hearn, and are summarized here. The field is approximately 22 miles [35.4 km] long and from 1 to 4 miles [ 1.6 to 6.4 km] wide. Production is from the Shannon sandstone at 9,400 ft [2865 m]. Maximum sand thickness is 60 ft [18.3 m]; average thickness is 20 ft [6.1 m]. Average porosity is about 12%, and average air permeability is about 12 md. The reservoir contains a black oil with an original bubble point pressure of 1,550 psig [10.7 MPa] and solution GOR of 292 scf/STB [52.6 std m /stock-tank m ]. The volumetric initial oil in place (IOIP) was 350 million STB 155.7 × 10 stock-tank m ]. Early engineering studies recognized that calculated IOIP and predicted oil recovery were subject to uncertainties in reservoir properties and geology. properties and geology. Field development began in 1975. Drilling of the original wells, on 160-acre [64.8-ha] spacing, was completed in 1979. During primary production, reservoir pressure dropped from the initial value of 5,000 psig pressure dropped from the initial value of 5,000 psig [34.5 MPa] to about 1.000 psig [6.9 MPa] average pressure. Cumulative production through 1981 was pressure. Cumulative production through 1981 was 32X 10 STB [5.1 × 10 stock-tank m ]. The effects of areal variation of reservoir properties became evident during the primary production period in Hartzog Draw. There was a significant difference in reservoir performance between the east and west sides of the field. Fig. 2 is a contour map of cumulative oil production through 1980 per wellbore porosity-foot. Such production through 1980 per wellbore porosity-foot. Such production maps must be interpreted with caution, since production maps must be interpreted with caution, since the performance of an individual well is related to production practices (such as well completion and artificial production practices (such as well completion and artificial lift methods) in addition to reservoir properties. However, a consistent trend is evident in Fig. 2. On the average, wells east of the long axis of the field were much better performers than wells west of the axis. It will be shown that this trend is related to the reservoir geology. The field was unitized in 1980 for secondary recovery, and development of a water flood system was begun in 1981. Water flooding started on a 640-acre [2.6× 10 -M ] inverted nine-spot pattern (Fig. 3) with plans to convert later to a 320-acre [1.3×10 M ] five-spot. There was concern that water flooding on such wide spacing would not be optimum for this reservoir. JPT p. 1335

Possible Tectonic Influence on and Facies Distribution of Shannon Ridge Sandstones, Wyoming: ABSTRACT

In the Powder River basin of Wyoming, Upper Cretaceous Shannon Sandstone shelf sand ridges are formed along the crest of a broad, subtle, southwest to northeast-trending paleoarch. During Shannon deposition, relief on the arch was great enough to alter shelf energies and cause sand ridges to develop within a predominantly silty shale interval. Possible recurrent movement in the Salt Creek anticline created a paleohigh which strongly localized development of thick sand ridge complexes in the Shannon Sandstone. During Shannon deposition, relief on the paleohigh apparently was strong enough to cause ridges to build laterally as well as vertically. Shannon ridge complexes at Salt Creek are more oblate, bigger, thicker, and more closely spaced than most central Powder River basin ridges. Also, there are two vertically stacked ridge systems developed within the Shannon Sandstone. While the lower ridge system is coeval with the Shannon ridge system in the central basin, the upper ridge system is only developed locally and, we believe, is related to active growth on the paleohigh during Shannon deposition. At no time, however, did he paleohigh cause ridges to be subaerially exposed. Eleven Shannon shelf ridge and ridge-associated facies were defined in outcrops on the Salt Creek anticline. Vertical and lateral changes in facies are relatively abrupt where observed in closely spaced outcrop sections and, in general, facies are stacked in coarsening-upward sequences with central bar facies commonly immediately overlying interbar sandstone facies. Porous and permeable potential reservoir facies include: central bar facies, a clean, cross-bedded sandstone; bar margin facies (Type 1), a highly glauconitic, cross-bedded sandstone containing abundant shale and limonite (after siderite) rip-up clasts and lenses; and bar margin facies (Type 2), a cross-bedded to rippled sandstone. These facies were formed by sediment transported and deposited in the form of medium to larg -scale troughs and sand waves on and across the tops of ridges by moderate to high-energy shelf currents. End_of_Article - Last_Page 1358------------

Shannon Sandstone, Powder River Basin: Hydrodynamic Control of Sand Body Geometry and Facies Sequences in Western Interior Cretaceous Seaway: ABSTRACT

The Campanian Shannon Sandstone Member of the Cody Shale forms the reservoir for several significant oil fields in the western Powder River basin of Wyoming. Linear Shannon sand bodies were deposited on the muddy shelf of the Cretaceous Interior Seaway up to 200 km (125 mi) east of the paleoshoreline. The sandstone bodies are asymmetric in transverse section: northeastern flanks are shorter and steeper than the gentle accretion surfaces on the southwest flanks. The long axes of these bodies are aligned northwest-southeast to north-northwest-south-southeast. Paleocurrent direction is generally southerly, hence the sandstone bodies are aligned at a small angle to the prevailing transport direction, as required by hydrodynamic theory. Three major facies types are recognized in cores, in subsurface wireline logs, and in outcrop. In a typical coarsening-upward vertical sequence, a large-scale cross-bedded facies overlies a thin-bedded facies which in turn overlies a bioturbated facies. These facies are genetically related lateral equivalents. The large-scale cross-bedded facies lies on the upcurrent flank and crest of the sandstone body, and grades down the down-current flank into a thin-bedded facies, which grades in turn into bioturbated facies. The cross-bedded facies is typified by current-rippled surfaces, and by trough to tabular cross-bed sets. The sandstone is glauconitic, very fine to medium-grained, and commonly contains sideritic and shale intraclasts. The thin-bedded facies consists of sand and shale in flaser-, wavy-, and lenticular-bedded associations which may exhibit significant burrowing. Thin-bedded sandstones are generally very fine to fine-grained with a significant silt and clay component. The bioturbated facies is a shaly fine sandstone to siltstone interpreted as a distal thin-bedded facies extensive y reworked by burrowing. The Shannon appears to have been deposited by intermittent storm flows in an outer shelf environment where the water was deep enough for sharp-crested rather than hummocky megaripples to develop. Numerical modeling of geostrophic circulation due to wind stress forcing of Campanian shelf waters demonstrates a good correlation between measured paleotransport indicators and model results. The time-averaged bottom circulation induced by a typical mid-latitude storm is southerly. However, a significant onshore or offshore component of bottom flow may be present due to interaction of storm setup and resultant geostrophic currents with a varying coastal and bathymetric configuration. In this model, storm currents decelerating across the crests of subtle topographic highs on the shelf surface would deposit preferentially the coarser fraction of their transported load, so that the aggrading sea floor would become enriched in sand. When a portion of the Campanian shelf End_Page 1338------------------------------ attained a critical sand content, large-scale linear sand bodies were triggered, which were dynamically analogous to the sand ridges of modern storm-dominated shelves. These features tended to migrate southward across the aggrading shelf surface by means of the accumulation of successive facies packages on their down-current slopes. End_of_Article - Last_Page 1339------------

No Title Provided: ABSTRACT

Late Cretaceous marine deposition in the western interior of the United States occurred in an epicontinental seaway elongate in a north-south direction. In central Montana, the western side of the seaway was characterized by a broad, tectonically active shelf. In eastern Montana and the western Dakotas, an actively subsiding basin was located in the central part of the seaway. In western and central South Dakota, the eastern side of the seaway was a more stable west-sloping ramp. Distinctive facies belts in the Eagle Sandstone and equivalent rocks are found in each of these tectonic settings, and some specific tectonic features have expression in the facies patterns. However, paleotectonism was even more important than suggested by these regional patterns. Selected study reas End_Page 1355------------------------------ show that subtle tectonic features have influenced deposition within each of the regional facies belts. On the western shelf, coastal sandstones of the Eagle-Sandstone near the Bearpaw Mountains show facies and isopach variations which are controlled by linear features visible on satellite images; the linear features generally trend north-south and east-west. Inner shelf sandstones of the Eagle thin and pass laterally northeastward into marine siltstones and shales across the Cat Creek fault zone near Winnett, Montana. Sandstone lenses in the lower Eagle, which are interpreted to be sand ridges, prograde south and west at approximately right angles to the fault zone. Farther east on the outer margin of the western shelf, areas of sand ridge fields in the Shannon Sandstone Member of the Gammon Shale are delimited by northeast and northwest linear features observed on satellite images nea the northern Black Hills. Within the basin, thick areas of Gammon Shale are delimited by northeast and northwest lineaments interpreted from Landsat linear features. On the eastern ramp, noncalcareous shales of the Gammon Member of the Pierre Shale thin and intertongue eastward with chalks in the upper part of the Niobrara Formation. This facies change occurs across linear features visible on Landsat images in western South Dakota. To the east at the inner margin of the ramp, the degree of erosion on the unconformity between the Niobrara Formation and the overlying Pierre Shale changes systematically across northeast-trending Landsat linear features observed near the Missouri River in central South Dakota. Based on these studies, we interpret the stratigraphic variations to be the expression of paleotectonism on discrete basement blocks bounded by fault zones which are observed on Landsat images as linear features. On the western shelf, elevated blocks controlled the sites of the winnowing and deposition of sandstones. Within the basins, subsiding basement blocks were filled by deposition of shales. These basin blocks acted as sediment sinks which inhibited the eastward dispersal of terrigenous materials from the west. On the eastern ramp, chalks were deposited and locally eroded on slightly elevated blocks which were relatively free of terrigenous material. Paleotectonism, therefore, influenced deposition not only on the active western shelf and in the basin, but also on the more stabl eastern ramp. End_of_Article - Last_Page 1356------------

Reservoir Management of the Hartzog Draw Field

Summary The Hartzog Draw field is a recently discovered major oil reservoir in northeastern Wyoming. Initial reservoir performance indicated that the field soon would be depleted below the bubble-point pressure. To evaluate secondary recovery possibilities, the working interest owners, cooperated in an extensive reservoir study. Although there were some unique problems. both in evaluating reservoir data and in obtaining a unitization agreement, this effort led to the formation of the Hartzog Draw Unit within 5 years of field discovery. Secondary recovery by waterflooding has begun, and enhanced recovery possibilities are being evaluated. Introduction The Hartzog Draw field, discovered in 1975, is in the Powder River basin of northeastern Wyoming, (Fig. 1). There are 177 wells in the field, producing on 160-acre (647 x 10 3-M2) spacing from the Shannon sand at 9,400 ft (2865 m). Initial oil in place (IOIP) was 350 MMSTB (55.6x 10 6 stock-tank M 3). As the field was developed, pressure measurements revealed that reservoir pressure was dropping rapidly from the initial value of 5,000 psig (34.5 MPa). Reservoir fluid analysis indicated that the oil was an undersaturated black oil with a bubble point of 1,550 psig (10.7 MPa). Pressure reduction significantly below the bubble point would lower ultimate recovery if the field eventually were waterflooded. Prudent reservoir management required that the field be unitized. An operators committee was formed, and under the direction of various subcommittees an extensive reservoir analysis was conducted. This included geological correlation and mapping, reservoir fluid and rock analysis, pressure transient testing, and relative permeability tests. Reservoir simulation studies were conducted to predict remaining primary recovery and secondary recovery for various proposed methods. These efforts resulted in a recommendation to the working interest owners to unitize and begin secondary recovery immediately by waterflooding. The field was unitized in mid-1980, and waterflood facilities currently are being installed. A study of enhanced recovery possibilities by CO, injection is being conducted also. The objectives of this paper are (1) to present the geology. performance history, and reservoir properties of the Hartzog Draw field, (2) to describe the unitization process, and (3) to discuss the current status of field development and plans for enhanced recovery. Field Geology The Hartzog Draw field is a northwest-to-southeast trending, stratigraphically trapped hydrocarbon reservoir. The field is approximately 22 miles (35.4 km) long and from 1 to 4 miles (1.6 to 6.4 km) wide. Oil and gas production is from the Shannon sandstone member of the Cody shale of Upper Cretaceous age. The reservoir sands range from less than 5 ft (1.5 m) to more than 60 ft (18.3 m) thick (Fig. 2), and lie within a 20- to 90-ft (6- to 27-m) sequence of inter-bedded sand, silt, and shale. The Hartzog Draw discovery well is shown in Fig. 3 as a type log for the field. It is believed that the Shannon sand of Hartzog Draw was deposited in an offshore marine shelf setting. The sand originally was spread over the shelf as a blanket deposit. The reservoir was formed by southward-flowing marine currents, which reworked and redistributed the sand in an elongate bar-type deposit. These marine currents varied in strength and energy, as indicated by the sedimentary structures, which range from ripples to large scale trough cross-beds within the reservoir. The grain size of the sand ranges from fine to medium, with an average clay content of about 20%. JPT P. 1575^