Lower Cretaceous Muddy Research Articles

Reservoir characterization and geologic modeling of the Lower Cretaceous Muddy Formation at Bell Creek Field, Montana, USA

The Lower Cretaceous Muddy Formation at the Bell Creek Field, Montana, USA, is characterized by a clastic sequence of mixed lithofacies. An integrated workflow was applied to define the reservoir compartments and distribution of petrophysical properties throughout the central and northern areas of the field. The workflow focused on the heterogeneity of the reservoir flow units, diagenetic complexities, and the associated compartments. The compartments play a considerable role in controlling the connectivity and fluid flow through the reservoir. Descriptions of the reservoir properties were used as inputs to a three-dimensional (3-D) geologic model for history matching and prediction of CO2 flooding. History matching of the primary production of the Muddy Formation helped to improve understanding of the distribution of the rock and fluid properties. The simulation model was then used to forecast future CO2 flooding in the study area. Following CO2 injection, the collected production data were utilized to verify the validity of the model by comparing the production forecast with the actual field results. The simulation model successfully predicted the CO2 flood response, pressure, and production data.

Reservoir stratigraphic heterogeneity within the Lower Cretaceous Muddy Sandstone in the Powder River Basin, northeast Wyoming, U.S.A.: Implications for carbon dioxide sequestration

The Muddy Sandstone in the Powder River Basin (PRB), northeast Wyoming, is a promising reservoir for CO 2 sequestration because: (1) existing wells for hydrocarbon production can be used for CO 2 injection when a field is depleted; and (2) data are available to assess the ability and capacity to trap CO 2 . Here, I provide new data and compile published results to: (1) characterize four oil and gas fields (the Amos Draw, Kitty, Hilight, and Sand Dunes fields) in the PRB with respect to lithofacies, sedimentary environment, sandstone composition, sand-body geometry, and porosity and permeability; and (2) assess the controls on reservoir heterogeneity and the CO 2 sequestration potential. Five lithofacies are recognized based on core description and log responses. They are interpreted as offshore, lower–middle wave-dominated shoreface, weathering zone, fluvial incised-valley, and tide-influenced estuarine depositional environments. The Muddy Sandstone contains predominantly quartz, with total quartz higher than 70 percent of the total framework grains. The percentage of feldspar is generally less than 5 percent, except for the Rozet Member in the Amos Draw Field, which is up to 22 percent. Sandstone petrographic examination also shows that the Muddy Sandstone can be divided into four groups based on the relative abundance of pore space, carbonate cement, and matrix. Sandstone with high porosity up to 23 percent is found in the shoreface lithofacies in the Amos Draw and Hilight fields and is also found in the estuarine lithofacies in the Kitty Field. The incised-valley lithofacies is of particularly low porosity due to high matrix content and carbonate cementation. The measured porosity in the sandstone varies between 1 percent and 23 percent, and the permeability is generally less than 10 millidarcys (mD). The variation of porosity is consistent with the observation in thin sections. XRD results show that the pore-filling clay minerals include kaolinite, chlorite, illite, and smectite. Core and well log correlation show that sandstone formed in lower–middle shoreface environments is laterally extensive and of uniform thickness, whereas sandstone of fluvial and estuarine origins is more variable in lateral extent and thickness. Based on examination of lithofacies, sandstone geometry, and thin section petrography, I suggest that the best reservoir interval for CO 2 sequestration in the Amos Draw Field is the lower Rozet Member, in the Sand Dunes and Hilight fields is the Springen Ranch Member, and in the Kitty Field is the Ute Member. Variables examined in this study provide important inputs for calculating CO 2 capacity potential and predicting chemical reactivity after CO 2 injection. Reservoir quality of the Muddy Sandstone is highly heterogeneous, and the complexity may be attributed to a combination of depositional environment, history of relative sea-level change during deposition, and type and extent of diagenetic alteration. The Muddy Sandstone, along with the overlying Mowry Shale, represents one third-order depositional sequence. Diagenetic processes include feldspar and lithic dissolution, secondary clay formation, quartz overgrowth, and four stages of carbonate cementation, which are early dolomite overgrowth, secondary calcite filling, dolomite replacement, and spotty siderite cementation. Carbonate cementation is interpreted as early diagenetic products formed during marine transgressions.

Petroleum system and production characteristics of the Muddy (J) Sandstone (Lower Cretaceous) Wattenberg continuous gas field, Denver basin, Colorado

Wattenberg field is a continuous-type gas accumulation. Estimated ultimate recovery from current wells is 1.27 tcf of gas from the Lower Cretaceous Muddy (J) Sandstone. Mean gas resources that have the potential to be added to these reserves in the next 30 yr are 1.09 tcf; this will be primarily through infill drilling to recover a greater percentage of gas in place and to drain areas that are isolated because of geologic compartmentalization. Greatest gas production from the Muddy (J) Sandstone in Wattenberg field occurs (1) from within the most permeable and thickest intervals of Fort Collins Member delta-front and nearshore-marine sandstones, (2) to a lesser extent from the Horsetooth Member valley-fill channel sandstones, (3) in association with a large thermal anomaly that is delineated by measured temperatures in wells and by vitrinite reflectance contours of 0.9% and greater, (4) in proximity to the bounding Mowry, Graneros, and Skull Creek shales that are the hydrocarbon source rocks and reservoir seals, and (5) between the Lafayette and Longmont right-lateral wrench fault zones (WFZs) with secondary faults that act as conduits in areas of the field. The axis of greatest gas production is north 25 to 35 degrees northeast, which parallels the basin axis. Recurrent movement along five right-lateral WFZs that crosscut Wattenberg field shifted the Denver basin axis to the northeast and influenced depositional and erosional patterns of the reservoir and seal intervals. Levels of thermal maturity within the Wattenberg field are anomalously high compared to other areas of the Denver basin. The Wattenberg field thermal anomaly may be due to upward movement of fluids along faults associated with probable igneous intrusions. Areas of anomalous high heat flow within the field correlate with an increased and variable gas-oil ratio.

ABSTRACT: Incised Valley Reservoirs And Enhanced Exploration Potential In The Lower Cretaceous Muddy Sandstone, Northern Denver Basin

ABSTRACT: Incised Valley Reservoirs And Enhanced Exploration Potential In The Lower Cretaceous Muddy Sandstone, Northern Denver Basin

Abstract: Facies architecture and sequence stratigraphy of the Lower Cretaceous Muddy Formation in the southeastern Bighorn Basin, Wyoming

Abstract: Facies architecture and sequence stratigraphy of the Lower Cretaceous Muddy Formation in the southeastern Bighorn Basin, Wyoming

Revised Stratigraphy, Depositional Systems, and Hydrocarbon Exploration Potential for the Lower Cretaceous Muddy Sandstone, Northern Denver Basin

The reservoir sandstones of the Lower Cretaceous Muddy Sandstone, the major hydrocarbon-producing reservoir in the northern Denver basin, are restructured into six informal members (MS-1 to MS-6) based on sedimentary structures, ichnofacies, geophysical logs, bounding discontinuities, and sediment distribution patterns. Bounding discontinuities defining allostratigraphic units are used to assign these six informal members to three major depositional systems that may be used to redefine hydrocarbon reservoirs, leading to new exploration targets in this mature basin.

Hydrodynamic Flow in Lower Cretaceous Muddy Sandstone, Bell Creek Field, Montana

Hydrodynamic Flow in Lower Cretaceous Muddy Sandstone, Bell Creek Field, Montana

Stratigraphy of the Lower Cretaceous Muddy Formation and the Drainage Pattern of Its Paleovalleys, Wind River Basin, Wyoming

Valley-fill deposits of the Lower Cretaceous Muddy Sandstone contain significant stratigraphically trapped hydrocarbon accumulations and are an important exploration target in Wyoming's Bighorn, Powder River, and eastern Wind River basins. An integrated subsurface and outcrop study of the Muddy Sandstone in the western Wind River basin was conducted in order to determine (1) if Muddy paleovalleys and are present there, (2) if potential for stratigraphic traps exists, and (3) if source rocks of sufficient quality and maturity are able to charge the Muddy reservoirs. The results of the study reveal that the Muddy Sandstone (up to 100 ft thick) contains a series of unconformity bounded progradational shoreface deposits, separated by a valley-fill sequence (up to 60 ft thick). In general, the Muddy Formation is divided into three informal zones: (1) a lower marine zone, containing at least two progradational shoreface highstand parasequences; (2) a middle zone, which consists of a lowstand fluvial/estuarine valley-fill sequence (8-10 mi wide); and (3) an upper marine zone, containing at least three landward-stepping highstand shoreface parasequences. In the southwestern Wind River Basin, the Muddy paleovalley trend is north-northwest, parallel to the present-day structural trend of the Wind River thrust and its parasitic anticlines. However, insteadmore » of continuing into the potentially prospective northwestern Wind River basin, the paleovalley makes an abrupt 90{degree} turn westward, a few miles north of Ft. Washakie. A detailed understanding of how Muddy paleovalley drainage reacts to basement faults could become a key predictive tool for future Muddy exploration in the northern Rocky Mountains.« less

Stratigraphy and Hydrocarbon Potential of Muddy Sandstone, Wind River Basin, Wyoming: ABSTRACT

The Lower Cretaceous (late Albian) Muddy Sandstone in the Wind River basin and adjacent areas rests unconformably upon the Thermopolis Shale and is disconformably overlain by and interfingers with the lower part of the Shell Creek Shale. The Muddy Sandstone is divided into five informal members: the Lazy B, Rozet, lower Recluse, upper Recluse, and Cyclone members. These members, defined as chronostratigraphic units bounded by bentonite beds, transgressive disconformities, and/or unconformities, consist of asymmetrical, coarsening and thickening-upward progradational deltaic/strand-plain and bay-fill deposits and/or asymmetrical, fining and thinning-upward aggradational valley-fill deposits. The Lazy B and Rozet members are marine progradational strand-plain deposits bounded by unconformities. They occur as discontinuous, erosional remnants in the western and northwestern parts of the study area. Following Rozet deposition, a major sea level fall resulted in the development of a headward-eroding valley system. During a subsequent sea level rise these valleys aggraded with fluvial/estuarine deposits of the Grieve member. Eventually the study area was blanketed by landward-stepping, progradational bay-fill, nearshore, and offshore marine deposits of the lower Recluse, upper Recluse, and Cyclone members. In addition to localized production from anticlinal or fault-bounded structural closure, the most significant oil and gas production is from updip pinch-outs ofmore » sand-rich valley fill of the Grieve member in the southeastern Wind River basin. Extensive outcrop and core analysis reveals these sand-rich paleovalleys, which range up to 10 mi wide and 100 ft thick, contain numerous scour-and-fill sequences composed of either fluvial bed load, fluvial suspended load, or abandoned channel deposits. Individual scour-and-fill sequences range up to 300 ft wide and up to 50 ft thick.« less

Sequence Stratigraphy and Onlap Relationships of a Stratigraphic Trap, Lower Cretaceous Muddy Sandstone, Hilight Field, Powder River Basin, Wyoming: ABSTRACT

In Hilight field more than 74 million bbl of oil and 244 bcf of gas have been produced from thin fluvial and shallow marine sandstones of the Lower Cretaceous Muddy Sandstone. In this area the Muddy was deposited during a late Albian sea level rise and onlaps a lowstand subaerial surface of erosion including a dendritic valley system incised into the underlying Skull Creek Shale. Transgressive valley-fill deposits of the Muddy Sandstone in the Hilight field area consist of four regionally correlative, time-bounded, back-stepping, progradational units. The retrogradational nature of these units indicates they belong in a transgressive systems tract rather than a lowstand wedge, as is often implied for valley-fill sequences. Each unit is capped by a regionally extensive marine flooding surface. The contact between units is placed at the surface representing maximum flooding or at the correlative horizon landward of the maximum transgression. Two of these contacts include thin bentonite beds which substantiate the time-stratigraphic interpretation. Petroleum is trapped in each unit by the onlap of reservoir sandstone facies against the lowstand surface of erosion on the impermeable Skull Creek Shale. Top seals are formed by mudstones at the base of overlying units or by the Shell Creekmore » Shale, which overlies the Muddy Sandstone above a regionally extensive marine flooding surface. Reservoir heterogeneity is explained by lateral facies changes within each unit. The lateral and vertical distribution of reservoir-quality sandstone in the Muddy sequence in this area can be used as a model to predict occurrences in similar sequences elsewhere.« less

Structural Control on Paleovalley Development, Muddy Sandstone, Powder River Basin, Wyoming: ABSTRACT

A subaerial unconformity within the Lower Cretaceous Muddy Sandstone in the Powder River basin developed during a late Albian sea level lowstand and resulted in a markedly rectangular drainage pattern. Numerous right-angle bends and perpendicular confluences of Muddy paleovalleys are believed to reflect syndepositional movement on basement faults and dissolution of salts in the Goose Egg Formation. A detailed subsurface analysis of geophysical logs from closely spaced wells reveals that up to 30 ft of vertical displacement occurred along northwest- and northeast-trending faults prior to and during the development of the subaerial unconformity. An analysis of a high-resolution magnetic survey (NewMag) of the Powder River basin reveals that numerous paleovalleys parallel the boundaries, or basement shear zones, between basement blocks. Small, irregularly shaped, thin intervals of the Permian Goose Egg Formation, which resemble karst topography, also occur along these northwest- and northeast-trending basement faults beneath Muddy paleovalleys. An arcuate Muddy paleovalley located in the northern Powder River basin parallels contours of isopach and trend surface maps of the Goose Egg Formation. These relationships suggest that the location and orientation of Muddy paleovalleys were controlled by a combination of movement along northwest- and northeast-trending faults and syntectonic dissolution of salt withinmore » the Goose Egg Formation. Simultaneous dissolution of Goose Egg salts and headward erosion of Muddy paleovalleys along this conjugate fault pattern also indicate that the Powder River basin was influenced by wrench fault tectonics during the late Albian.« less

Sea Level and Paleotectonic Controls on Distribution of Reservoir Sandstone of Lower Cretaceous Muddy Sandstone, Hilight Field, Powder River Basin, Wyoming: ABSTRACT

To date, over 74 million bbl of oil have been produced from stratigraphic traps at Hilight field. Production is primarily from thin but stratigraphically complex fluvial and shallow marine sandstone of the Lower Cretaceous Muddy Sandstone. The deposition and preservation of these reservoirs were controlled by the interplay between sea level and tectonics. The Muddy Sandstone in Hilight field was deposited during a late Albian sea level rise. It onlaps an erosional surface, developed during the preceding sea level drop, including a dendritic valley system cut deeply into the underlying Skull Creek Shale. In this area, the Muddy consists of four members that are bounded by transgressive disconformities. These members were deposited during stillstands in the overall rise of sea level. The lower two members consist of fluvial and fluvial-estuarine deposits which fill the valley system; the upper two members consist of fluvial-deltaic and barrier island deposits. Three northeast-trending lineaments transect Hilight field. These lineaments are interpreted to represent basement faults that experienced recurrent movement during Muddy deposition. Relative structural downdrop controlled the orientation of drainages that cut the Hilight valley system. Recurrent movement provided structural and topographic lows within which relatively thick fluvial-deltaic and barrier island sandstones were depositedmore » and preserved. Thinner sequences were deposited and subsequently eroded on adjacent structural and topographic highs.« less

Valley-Fill Sequences and Onlap Geometries, Lower Cretaceous Muddy Sandstone, Kitty Field, Powder River Basin, Wyoming: ABSTRACT

The Muddy Sandstone at Kitty field is a valley-fill sequence that records a late Albian sea level rise and accompanying transgression. The valley was cut during a preceding sea level lowstand. Stratal geometries and facies successions within the valley fill demonstrate the history of transgression was not gradual and progressive. Rather, the valley fill comprises a series of discrete, time-bounded, depositional units which onlap the erosional surface. Five time-bounded depositional units were defined by facies successions and were used to define onlap geometries. Facies successions within individual units record progressive shoaling. Capping each succession, there may be a planar disconformity, a thin bioturbated interval, or the deepest water facies of the next depositional event. Thus, the termination of each depositional event is marked by an episode of rapid deepening. At a single geographic location, stratal successions within older depositional units represent more landward facies than those within younger units. Therefore, the onlap geometry of the valley-fill sequence consists of a landward-stepping arrangement of depositional units. The primary reservoirs within the valley-fill sequence, at Kitty field, are laterally coalesced, channel-belt sandstones at the base and barrier island sandstones at the top. Reservoir sandstones of lesser quality occur within the intermediate estuarinemore » facies. The stacking pattern, developed by onlap of the units, results in multiple pay zones within mid-valley reaches. The boundaries of each depositional unit define a high-resolution, chronostratigraphic correlation of valley-fill strata, a correlation corroborated by bentonites. This correlation method gives an accurate description of the internal geometry of valley-fill strata and, therefore, provides a basis for understanding the process of transgressive onlap.« less

Relationship Between Illite/Smectite Diagenesis and Hydrocarbon Generation in Lower Cretaceous Mowry and Skull Creek Shales of The Northern Rocky Mountain Area

AbstractThe percentage of expandable layers in illite/smectite (I/S) mixed-layer clay decreases with increasing temperature and depth in a section through marine Cretaceous shales in the Champlin 1 Hartley Federal well in the Powder River basin, Wyoming. This systematic change in I/S expandability is evidence that low-expandable I/S in Cretaceous shales of the northern Rocky Mountain area reflects, at least in part, thermal alteration during burial diagenesis. In eastern Montana and western North Dakota where I/S in the Lower Cretaceous Mowry and Skull Creek source rocks is diagenetically unaltered, only trace amounts of hydrocarbons have been found in the Lower Cretaceous and other Cretaceous sandstones. Elsewhere in the northern Rocky Mountain-Great Plains region, hydrocarbons in the Lower Cretaceous Muddy Sandstone and its equivalents occur within or immediately adjacent to areas in which I/S clay in the Mowry and Skull Creek shales has been diagenetically altered during burial. Altered I/S and thermally mature organic matter, as defined by Rock-Eval pyrolysis values, coexist in these source rocks. Both may be used as maturation indicators in the search for Cretaceous-source hydrocarbons in the northern Rocky Mountain area.

Deepest Production in Rocky Mountain Province: Madden Anticline, Fremont County, Wyoming: ABSTRACT

The Monsanto Oil Company 1-5 Bighorn well was drilled at a crestal location on the 6-mi (9.6-km) wide by 20-mi (32.2-km) long Madden anticline. It penetrated all the previously undrilled pre-Cody rocks on the structure before reaching a total depth of 24,877 ft (7,582 m), or 64 ft (20 m) into precambrian granitic rocks, where the bottom-hole temperature was recorded at 425/sup 0/F (218/sup 0/C). The well was completed August 1985 through perforations from 23,758 ft (7285 m) in the lower part of the Mississippian Madison Formation. Gas flowed at 20 MMCFD (566,000 m/sup 3//d) with a flowing tubing pressure of 5800 psi (40 MPa) and a shut-in tubing pressure of 6700 psi (46 MPa). The porosity logs indicated 70 net ft (21.3 m) of porosity, with an average porosity of 8% and a maximum of 14%. The gas composition is 74% methane, 14% CO/sub 2/, 10% H/sub 2/S, and 2% miscellaneous inerts. The productive interval in this well deepened the Rocky Mountain province depth-of-production record by 3400 ft (1034 m). Mechanical logs, gas shows, and drilling parameters indicated additional potentially commercial accumulations of gas in the Upper Cretaceous Frontier sands, the Lower Cretaceous Muddy and Cloverly sands, the Triassicmore » Crow Mountain Sandstone, the Permian Park City Formation, and the Pennsylvanian Tensleep Sandstone. The Frontier through Crow Mountain interval was significantly overpressured and required 17 lb/gal 92037 kg/m/sup 3/) of drilling fluid to maintain balance while drilling. Pressure regressed to a normal gradient near the top of the Paleozoic rocks at approximately 22,700 ft (6919 m). As many as 20 additional wells may be needed to develop the previously untested pre-Cody reservoirs on the Madden anticline.« less

Recognition of Paleoenvironments in Muddy Sandstone of Buffalo Creek Area, Sheridan County, Wyoming: ABSTRACT

In 1983, Hershey Oil Company drilled the 1-10 Buffalo Creek well in the NW 1/4, NE 1/4, Sec. 10, T56N, R78W, Sheridan County, Wyoming. The primary target was a Paleozoic reservoir. The Lower Cretaceous Muddy Sandstone was penetrated and tested a minor amount of oil, which prompted a more detailed examination of the Muddy Sandstone. Exploration techniques used for the Muddy Sandstone included a series of sequential isolith maps based on data from the few widely spaced wells in the area. The top of the subjacent Skull Creek Shale and the bottom of a superjacent bentonite marker were used as boundaries. This method revealed the apparent transgressive nature of the remnant Muddy Sandstone. No cores were taken over the Muddy interval, so all petrographic and lithologic data were obtained from cuttings and open-hole logs. A paleoenvironmental study was done using the High Resolution Dipmeter Tool. The data were processed to obtain detailed Cluster and Geodip plots. The dipmeter information defined a remnant northeast-southwest-trending, transgressive barrier bar overlying a tidal-channel sequence oriented northwest-southeast. The length of the tidal channel would be relatively short, but due to channel migration, could be nearly as wide as the barrier bar is long.

Lithologic Variations and Diagenesis of Lower Cretaceous Muddy Formation in Northern Powder River Basin, Wyoming: ABSTRACT

Lithologic Variations and Diagenesis of Lower Cretaceous Muddy Formation in Northern Powder River Basin, Wyoming: ABSTRACT

Stratigraphy, Sea Level Fluctuations, and Depositional History of Lower Cretaceous Muddy Sandtone, Northern Powder River Basin, Wyoming: ABSTRACT

The Muddy Sandstone may be the most complicated Cretaceous formation in the Powder River basin. Although generally less than 100 ft thick, it contains a wide variety of facies. Two unconformities, one of which is largely intraformational, are associated with the Muddy. It has long been known that Muddy deposition was preceded by a lowering of sea level in the Western Interior. Their study indicates that there were, in fact, two cycles of lowering/rising of sea level. The first lowering and the subsequent rise were abrupt; they were accompanied by only minor deposition of sandy sediments. The second lowering of sea level was slower. Sand provided by longshore drift from a southwestern source accumulated as an extensive strand-plain system. As sea level continued to lower, rivers cut downward locally through the old strand plain and into the Skull Creek Shale. The deepest Muddy valleys formed at this time. The following transgression was not continuous but comprised at least five pulses of abrupt rise separated by periods of stability. During periods of abrupt rise, valleys flooded to form estuaries; during still-stands, valleys were largely filled with estuarine, bayhead-deltaic, and fluvial sediments. Barrier islands and associated tidal inlets formed near the mouthsmore » of the estuaries. All of the sandy facies of the Muddy - fluvial, strand plain, barrier island, and bayhead delta, in decreasing order of importance - serve locally as reservoirs for oil and gas.« less

Types of Stratigraphic Traps in Lower Cretaceous Muddy Formation, Northern Powder River Basin, Wyoming: ABSTRACT

Stratigraphic traps account for most of the oil produced from the Muddy Sandstone in the northern Powder River basin. Two categories of traps exist. The first trap type is the result of lateral and vertical facies changes. Reservoir facies include tidal channels, point bars, bayhead deltas, barrier islands, and strand-plain sandstones; trapping facies include bay-fill and estuarine sediments, mud-filled tidal channels, and flood-plain deposits. The second of the two categories of traps results from an unconformity that juxtaposes permeable and impermeable sediments of quite different ages. Structural and diagenetic factors often modify and locally enhance reservoir quality within both categories of stratigraphic traps. The various types of traps are demonstrated by studies of six field areas: (1) barrier-island sandstones, sealed updip by back-barrier shales, produce at Ute and Kitty fields; (2) tidal channels produce at Collums and Kitty fields; (3) bayhead deltas, encased in estuarine sediments, form traps at Oedekoven and Kitty fields; (4) fluvial point-bar sandstones form traps at Oedekoven, Store, and Kitty fields; (5) unconformity-related traps exist where Muddy fluvial valley-fill sediments lap out against impermeable valley walls of Skull Creek Shale on the updip side at Store, Oedekoven, and Kitty fields; and (6) the clay-rich weathered zone,more » directly beneath an intraformational unconformity, forms the seal to the reservoirs at Amos Draw field.« less

Three Ancient Montana Fluvial Systems: Pennsylvanian Tyler, Lower Cretaceous Muddy, and Upper Cretaceous Eagle--Their Reservoir and Source Rock Distribution: ABSTRACT

The importance of using Holocene geology as a model in mapping reservoir and source rock distribution is demonstrated in three Montana river-related systems: alluvial valley, barrier bar, and distributary channel-prodelta. The Pennsylvanian Tyler Formation was deposited by a westward-flowing meandering-stream system controlled by an east-west-trending rift valley, and surrounded by backswamp deposits. It is underlain by its probable hydrocarbon source, the marine Mississippian Heath shale and limestone, and overlain locally by the lagoonal Pennsylvanian Bear Gulch Limestone. To date, about 90 million bbl of recoverable oil have been found in Tyler sands. The oil-producing Lower Cretaceous Muddy sandstones in the northern Power River basin are considered to be barrier bars, encased in organic-rich shales, which are most probably the source rock. The Upper Cretaceous Eagle Sandstone in north-central Montana is a distributary channel system, similar to that of the modern Mississippi, which dumped highly carbonaceous materials into an organic-rich delta system. The Eagle now contains possibly enormous amounts of biogenic methane. By using Galveston Island and the modern Mississippi delta as models, in conjunction with employing electric log shapes and porosity logs, it is possible to map ancient fluvial patterns in the study areas. One can then predict the location of possible hydrocarbon accumulations in porous and permeable sand bodies, along with their encasing hydrocarbon source rocks. End_of_Article - Last_Page 866------------