Underlying Skull Creek Shale Research Articles

Regional Paleotopographic Trends and Production, Muddy Sandstone (Lower Cretaceous), Central and Northern Rocky Mountains (1)

The Muddy Sandstone and equivalent strata have produced more than 1.5 billion bbl of oil-equivalent hydrocarbons. Production is controlled principally by unconformities formed during a relative sea level lowstand. Reservoirs are found in paleohills of older marine sandstones, younger valley fills and associated alluvial plain channel sandstones, and transgressive marine deposits. At least ten paleodrainage basins existed at maximum lowstand. A regional drainage divide formed in southern Wyoming and separated southeast-flowing from northwest-flowing alluvial systems. Local tributaries stripped drainage divides of fine-grained detritus derived from the underlying Skull Creek Shale and older marine sandstones. In contrast, trunk streams carried medium- and coarse grained-sands eroded from eastern, southern, and western provenances of Paleozoic and Mesozoic strata. The best fluvial reservoirs are developed within these coarser grained trunk systems. Reservoir data display little or no correlation between depth and porosity due to secondary dissolution porosity developed at all depths. Valley fill and channel reservoirs have produced at least 359 MMBOE, onlap cycles another 315+ MMBOE, and older marine buried-hill reservoirs more than 268 MMBOE. The best per-field reserves are from marine sandstones. Regional production patterns reflect proximity to mature Skull Creek and Mowry shale source beds and favorable trapping conditions within individual paleodrainages. Future hydrocarbon exploration successes will require drilling to the Muddy Sandstone in deeper basin settings and a better understanding of the role of unconformities and diagenesis in controlling hydrocarbon accumulations.

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

Geologic and Seismic Models, Muddy Sandstone, Lower Cretaceous, Bell Creek-Rocky Point Area, Powder River Basin, Montana and Wyoming: ABSTRACT

Abstract The Muddy Sandstone in the northern Powder River Basin consists of two distinct members (genetic units) that are separated by a widespread subaerial surface of erosion. The older sandstone member, the reservoir rock at the giant Bell Creek field, was deposited in shoreline and associated nearshore marine environments. It is genetically related by intertonguing to the underlying Skull Creek Shale. The younger member is a valley fill deposit of fluvial, estuarine and tidal flat environments. The deposition and distribution of these two genetic units were controlled primarily by relative sea level changes. The Bell Creek sandstone was deposited as a widespread regressive sandstone during a highstand of sea level. A following sea level lowstand caused valley cutting, erosion of all or portions of the Bell Creek sandstone, and paleosoil development causing early diagenesis, especially of kaolinite, siderite and quartz. A rising sea level resulted in valley filling, coastal onlap and a transgressive surface of erosion that is overlain by black marine shale (Nefsy or Shell Creek). Recurrent movement on basement-controlled fault blocks appears to have controlled distribution of the Muddy members, drainage incisement patterns, present structure and heat flow, and possibly petroleum migration. Seismic modeling indicates that it may be possible to recognize seismically the distribution of Muddy reservoir facies that occur in the area. Seismic modeling suggests that the amplitude response of the Muddy is influenced by Muddy facies and thickness changes. The lowest Muddy amplitude observed occurs when the Bell Creek sandstone is prevalent. An increase in the Muddy amplitude is noted when thick valley fill deposits are present. Results from seismic modeling compare well to actual seismic data. Amplitude changes noted in the Muddy reflector in the Bell Creek – Rocky Point area may help explorationists in the future to distinguish between nonprospective and reservoir quality Muddy Sandstone.

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

Unconformities and Facies Relationships of Muddy Sandstone, Northern Powder River Basin, Wyoming and Montana: ABSTRACT

The Muddy Sandstone, although commonly less than 100 ft thick, is one of the most complicated formations in the Powder River basin. Utilizing an integrated sedimentologic and stratigraphic analysis of outcrop, core, and well logs, the Muddy can be divided into six isochronous members. The recognition of two widespread, subaerial unconformities is critical to understanding the complex facies relationships and member-level stratigraphy of the Muddy. Over much of the Powder River basin, the Muddy contains an upward coarsening sequence at its base. At first glance, the sequence appears to be conformable with the underlying Skull Creek Shale. East-west cross sections, however, reveal westward truncation of siltstone and dahllite marker beds in the upper part of the Skull Creek. Rapid transgression of the sea planed off most of the erosional topography on the Skull Creek unconformity, reworking the dahllite concretions and concentrating them as a lag. A regressive, sand-rich strand plain, the Rozet member, was deposited during a subsequent sea level drop. As regression continued, a second unconformity, the Rozet, developed as streams eroded through it and into the underlying Skull Creek Shale, forming deep valleys. The next sea level rise resulted in the deposition of five regionally correlatable members withinmore » the previously eroded valleys and onlapping Rozet hillsides. Layers of volcanic ash frequently fell on the surface, preserved in the valleys as beds of bentonite. Ash that fell on the hillsides was dissolved by rainwater and contributed to growth of interstitial kaolinite within the upper part of the Rozet sand.« less

Unconformity-Related Oil Entrapment in Muddy Sandstone: Comparison of Kitty and Amos Draw Fields, Powder River Basin, Wyoming: ABSTRACT

It has long been known that an unconformity plays a critical role in trapping oil in the Muddy Sandstone in the Powder River basin, but opinions have varied as to exactly where in the section that unconformity is located. Their work indicates that there are, in fact, two unconformities associated with the Muddy in the northern part of the basin. The older of these occurs on top of the Skull Creek Shale, whereas the younger is largely intraformational. In places, the younger unconformity has truncated the older one. It is the younger unconformity that is responsible for creating favorable settings for stratigraphic entrapment of oil. Two types of unconformity-related oil traps result from fluvial downcutting into and through the strand-plain sandstones of the oldest member of the Muddy during a major sea level drop. In cases where the unconformity cuts through the Muddy into the underlying Skull Creek Shale, permeable valley-fill sediments, deposited during the Muddy transgression, are juxtaposed against the impermeable Skull Creek Shale along the valley walls. Where valleys are oriented roughly perpendicular to regional structure, as at Kitty field, the updip portion of the valley wall can form a permeability barrier to the fluvial reservoir sandstones ofmore » the adjacent valley fill. In cases where the unconformity is intraformational, such as at Amos Draw field, early diagenetic clay, associated with the weathered horizon directly beneath the unconformity, can create a seal on top of the strand-plain sandstones of the oldest member of the Muddy.« less

Depositional Environment and Petroleum Geology of Muddy Formation (Lower Cretaceous) in Parts of Converse and Niobrara Counties, Wyoming: ABSTRACT

The Lower Cretaceous Muddy formation in northeastern Converse and northwestern Niobrara Counties, Wyoming, can be subdivided into informal lower, middle, and upper mappable units. The lower unit, predominantly sandstone and siltstone, is interpreted to be a subtidal channel-fill sequence. The channels were incised into the underlying Skull Creek Shale. The middle unit consists of mudstones and sandstones deposited on a broad intertidal flat. The upper unit is composed almost entirely of marine mudstones. The occasional sandstones that are found in the upper unit are thought to have been deposited as offshore bars in an inner to middle-shelf position. The lower and middle units of the Muddy developed during a marine regression that began at the end of Skull Creek deposition. The upper unit reflects transgressive conditions that continued into the time of Mowry deposition. Oil has been produced from the Muddy formation at several fields in the study area; Steinle Ranch field is the most significant. End_of_Article - Last_Page 931------------

Tectonic Influence on Sedimentation of Lower Cretaceous Strata, East-Central Powder River Basin, Wyoming: ABSTRACT

Recurrent movement of basement fault blocks in the east-central Powder River basin has controlled the distribution of porous and permeable reservoir facies within Lower Cretaceous strata. Subsurface isopach data for Lower Cretaceous time-stratigraphic intervals show repetitive thickness variations for both marine (Skull Creek and Mowry shales) and nonmarine (Inyan Kara and Newcastle/Muddy sandstones) units. Thickness patterns seem to be controlled by recurrent Early Cretaceous structural movement. Paleostructures ranging in width from 2 to 10 mi (3.3 to 16.6 km) and in length from 10 to 30 mi (16.6 to 50 km) trend northeast, northwest and north, and include segments of the Black Hills and Fanny Peak monoclines, which bound the west flank of the Black Hills uplift. Early Cretaceous paleostructures seem to control the distribution of Newcastle valleys which are incised into the underlying Skull Creek Shale and drain southwest and northwest (corresponding to the Clareton, Hilight, Osage, Fiddler Creek, and Rozet fields). Alluvial plain valley-fill deposits in Newcastle Formation outcrop show abrupt facies and thickness changes which coincide with evidence of structural control (e.g., drape folds, faults, sandstone dikes, geomorphic lineaments, and increased igneous activity). A depositional model, incorporating tectonic and sea-level adjustments, illustrates that Newcastle channel incisement and valley fill are generally restricted to topographic and structural low (graben) areas. This model has been confirmed by detailed analysis of seismic data. /P> A model for tectonic influence on sedimentation aids in petroleum exploration by helping to predict facies distribution and fluid flow. End_of_Article - Last_Page 567------------

Hilight Muddy Field--Lower Cretaceous Chenier Plain Deposits in Powder River Basin, Wyoming: ABSTRACT

Thin sandstones and shales of the Muddy Formation produce large amounts of oil at Hilight field. The Muddy section is generally tight, and the best reservoir sandstones have effective porosity of 17% and average permeability of only 115 md. High production rates are caused primarily by extensive fractures, and the reservoir will yield an ultimate recovery of more than 80 million bbl of oil. Sedimentary structures and petrographic analyses show that Muddy sands were deposited in littoral marine, lagoonal, and fluvial environments. Porous sandstones average 10 ft and rarely attain 20 ft in thickness. Lower Muddy sandstones are fluvial, whereas upper Muddy sandstones are mostly littoral or lagoonal in origin. Fluvial sandstones are associated with shales and siltstones that are highly carbonaceous and were deposited in poorly drained marshes. Lagoonal sandstones End_Page 603------------------------------ and marsh deposits also occur lateral to the littoral sandstones. The presence of thin, littoral marine sandstones with widespread marsh sediments suggests a depositional environment similar to the modern chenier plain of South Louisiana. The entire Muddy sequence is transgressive toward the east and southeast over a low-relief topography developed on the underlying Skull Creek Shale. End_of_Article - Last_Page 604------------

Stratigraphy and Exploration of Lower Cretaceous Muddy Formation, Northern Powder River Basin, Wyoming and Montana: ABSTRACT

The Lower Cretaceous Muddy Formation in the Northern Powder River Basin of Wyoming and Montana was deposited during a marine transgression across a stream-dissected surface of the underlying Skull Creek Shale. The transgression occurred over most of the area, but was limited by a prograding delta on the northeast which supplied most of the sand for the area. The Muddy Formation is divided into 2 units--lower and upper. The lower Muddy was restricted to a system of dendritic channels which were incised into the Skull Creek Shale during a period of emergence. The sands were supplied from the delta on the northeast and transported south by longshore currents. They were deposited principally in a transitional marine and estuarine environment, and are comprised of fine-grained moderately sorted, partly clay-filled quartzose sands. By the time of deposition of the upper Muddy unit the incised depressions in the Skull Creek topography had been filled largely and the upper Muddy sands were deposited in a complex marine shoreline environment which resulted in offshore bars, barrier islands, beaches, and tidal deposits. End_Page 542------------------------------ Several shoreline trends are recognizable in the upper Muddy. They are progressively younger eastward and reflect the overall west to east transgression. These trends were controlled by the remnant Skull Creek topography and changing conditions of sediment supply. Production from the Muddy Formation is principally stratigraphic, however, in places structure is important in localization of accumulations. Lower Muddy production is restricted to updip channel boundaries and is localized by structural noses and updip channel reentrants. Upper Muddy production is controlled chiefly by porosity development and lateral facies changes. Exploration for Muddy sandstone reservoirs is accomplished best by the use of an isopach of the total Muddy Formation. This map shows the configuration of the Skull Creek channels and therefore the distribution of the lower Muddy sandstone bodies. It is also helpful in predicting the orientation of the upper Muddy shoreline trends where they were related to remnant Skull Creek highs and by showing an increased Muddy thickness due to sand buildups in non-channel areas. As the sand geometry is complex, abrupt stratigraphic changes are common. Electric log maps combined with zonal sandstone isopachs provide a means of visualizing these abrupt changes in sand geometry and also aid in the interpretation of depositional environments. Exploration must be focused on the location of primary stratigraphic traps which have not been altered strongly by later structural movements. The ubiquitous presence of clay-filled porosity has eliminated large areas as nonproductive. It is believed that the clay fill largely is diagenetic and occurred subsequent to primary oil accumulation. The lower percentage of clay fill in the oil-filled primary traps suggests that the presence of the oil inhibited clay diagenesis. In the last 3 years nearly 3,000 wells have been drilled in the study and search for Muddy oil. Each year new fields of significant size have been discovered. Abrupt stratigraphic changes require detailed stratigraphic work and, most important of all, courage to use the drill as an exploration tool. End_of_Article - Last_Page 543------------