Iles Formation Research Articles

E&P Notes (June 2021)

E&P Notes (June 2021)

Response of A Coal-Bearing Coastal-Plain Succession To Marine Transgression: Campanian Neslen Formation, Utah, U.S.A.

The process regime of low-gradient coastal plains, delta plains and shorelines can change during transgression. In ancient successions, accurate assessment of the nature of marine influence is needed to produce detailed paleogeographic reconstructions, and to better predict lithological heterogeneity in hydrocarbon reservoirs. The Campanian lower Neslen Formation represents a fluvial-dominated and tide- and wave-influenced coastal-plain and delta-plain succession that accumulated along the margins of the Western Interior Seaway, USA. The succession records the interactions of multiple coeval sedimentary environments that accumulated during a period of relative sea-level rise. A high-resolution data set based on closely spaced study sites employs vertical sedimentary graphical logs and stratigraphic panels for the recognition and correlation of a series of stratal packages. Each package represents the deposits of different paleoenvironments and process regimes within the context of an established regional sequence stratigraphic framework. Down-dip variations in the occurrence of architectural elements within each package demonstrate increasing marine influence as part of the fluvial-to-marine- transition zone. Three marine-influenced packages are recognized. These exhibit evidence for an increase in the intensity of marine processes upwards as part of an overall transgression through the lower Neslen Formation. These marine-influenced packages likely correlate down-dip to flooding surfaces within the time-equivalent Iles Formation. The stratigraphic arrangement of these packages is attributed to minor rises in sea level, the effects of which were initially buffered by the presence of raised peat mires. Post-depositional auto-compaction of these mires resulted in marine incursion over broad areas of the coastal plain. Results demonstrate that autogenic processes modified the process response to overall rise in relative sea level through time. Understanding the complicated interplay of processes in low-gradient, coal-bearing, paralic settings requires analysis of high-resolution stratigraphic data to discern the relative role of autogenic and allogenic controls.

Depositional evolution of a progradational to aggradational, mixed-influenced deltaic succession: Jurassic Tofte and Ile formations, southern Halten Terrace, offshore Norway

Predicting the hydrodynamics, morphology and evolution of ancient deltaic successions requires the evaluation of the three-dimensional depositional process regime based on sedimentary facies analysis. This has been applied to a core-based subsurface facies analysis of a mixed-energy, clastic coastal-deltaic succession in the Lower-to-Middle Jurassic of the Halten Terrace, offshore mid-Norway. Three genetically related successions with a total thickness of 100–300 m and a total duration of 12.5 Myr comprising eight facies associations record two initial progradational phases and a final aggradational phase. The progradational phases (I and II) consist of coarsening upward successions that pass from prodelta and offshore mudstones (FA1), through delta front and mouth bar sandstones (FA2) and into erosionally based fluvial- (FA3) and marine-influenced (FA4) channel fills. The two progradational phases are interpreted as fluvial- and wave-dominated, tide-influenced deltas. The aggradational phase (III) consists of distributary channel fills (FA3 and FA4), tide-dominated channels (FA5), intertidal to subtidal heterolithic fine-grained sandstones (FA6) and coals (FA7). The aggradational phase displays more complex facies relationships and a wider range of environments, including (1) mixed tide- and fluvial-dominated, wave-influenced deltas, (2) non-deltaic shorelines (tidal channels, tidal flats and vegetated swamps), and (3) lower shoreface deposits (FA8). The progradational to aggradational evolution of this coastal succession is represented by an overall upward decrease in grain size, decrease in fluvial influence and increase in tidal influence. This evolution is attributed to an allogenic increase in the rate of accommodation space generation relative to sediment supply due to tectonic activity of the rift basin. In addition, during progradation, there was also an autogenic increase in sediment storage on the coastal plain, resulting in a gradual autoretreat of the depositional system. This is manifested in the subsequent aggradation of the system, when coarse-grained sandstones were trapped in proximal locations, while only finer grained sediment reached the coastline, where it was readily reworked by tidal and wave processes.

Quartz c-axis orientation patterns in fracture cement as a measure of fracture opening rate and a validation tool for fracture pattern models

We evaluate a published model for crystal growth patterns in quartz cement in sandstone fractures by comparing crystal fracture-spanning predictions to quartz c-axis orientation distributions measured by electron backscatter diffraction (EBSD) of spanning quartz deposits. Samples from eight subvertical opening-mode fractures in four sandstone formations, the Jurassic–Cretaceous Nikanassin Formation, northwestern Alberta Foothills (Canada), Cretaceous Mesaverde Group (USA; Cozzette Sandstone Member of the Iles Formation), Piceance Basin, Colorado (USA), and upper Jurassic–lower Cretaceous Cotton Valley Group (Taylor sandstone) and overlying Travis Peak Formation, east Texas, have similar quartzose composition and grain size but contain fractures with different temperature histories and opening rates based on fluid inclusion assemblages and burial history. Spherical statistical analysis shows that, in agreement with model predictions, bridging crystals have a preferred orientation with c-axis orientations at a high angle to fracture walls. The second form of validation is for spanning potential that depends on the size of cut substrate grains. Using measured cut substrate grain sizes and c-axis orientations of spanning bridges, we calculated the required orientation for the smallest cut grain to span the maximum gap size and the required orientation of the crystal with the least spanning potential to form overgrowths that span across maximum measured gap sizes. We find that within a 10° error all spanning crystals conform to model predictions. Using crystals with the lowest spanning potential based on crystallographic orientation (c-axis parallel to fracture wall) and a temperature range for fracture opening measured from fluid inclusion assemblages, we calculate maximum fracture opening rates that allow crystals to span. These rates are comparable to those derived independently from fracture temperature histories based on burial history and multiple sequential fluid inclusion assemblages. Results support the R. Lander and S. Laubach model, which predicts that for quartz deposited synchronously with fracture opening, spanning potential, or likelihood of quartz deposits that are thick enough to span between fracture walls, depends on temperature history, fracture opening rate, size of opening increments, and size, mineralogy, and crystallographic orientation of substrates in the fracture wall (transected grains). Results suggest that EBSD maps, which can be more rapidly acquired than measurement of tens to hundreds of fluid inclusion assemblages, can provide a useful measure of relative opening rates within populations of quartz-filled fractures formed under sedimentary basin conditions. Such data are useful for evaluating fracture pattern development models.

Stratigraphic Trapping Mechanisms in the Iles Formation, Piceance Basin, Colorado

Compartmentalization within shoreface and deltaic reservoirs is key to trapping gas in the Iles Formation of the southern Piceance Basin. There are two types of reservoirs: 1) compartmentalized gas-saturated Corcoran and Cozzette sandstones, and 2) continuous water-saturated Rollins sandstones. Compartmentalized sandstones are interpreted to have been deposited during a time of relatively low accommodation with relatively low sediment supply while more connected sandstones were deposited during a time of relatively high accommodation and a much higher sediment supply. The compartmentalization is due to 1) location within the overall parasequence stacking pattern; 2) facies variability within the deltaic and wave-dominated deposits; and 3) further partitioning due to the superposition of multiple types of erosion surfaces ultimately related to low accommodation rates. The continuous sandstones are more connected because they were apparently deposited during a period of time when rates of progradation were more regular and overall accommodation and sedimentation were higher.

Anomalous clastic wedge development during the Sevier-Laramide transition, North American Cordilleran foreland basin, USA

New, high-resolution, regional correlation and isopach maps provide evidence that (1) Laramide-style deformation began as early as ca. 77 Ma in central Utah, and (2) rapid (208 km m.y. –1 ) and extensive (400 km) progradation of a clastic wedge was facilitated by reduced subsidence during the transition from Sevier- to Laramide-style deformation. This study defi nes three Campanian, alluvialto-marine clastic wedges that traversed 200– 400 km eastward across the Utah-Colorado segment of the Cordilleran foreland basin. Wedges A and C are thicker successions with rising-trajectory shoreline stacking patterns (Blackhawk Formation and Lower Castlegate Sandstone, Bluecastle Tongue and Rollins Sandstone) that refl ect relatively slow overall progradation (50–81 km m.y. –1 ) of narrow (10–20 km wide), wave-dominated shorelines. In contrast, wedge B consists of lower-volume successions with a fl at to falling shoreline stacking pattern (Middle Castlegate Sandstone, Sego Sandstone, Neslen Formation, Corcoran and Cozzette Members of the Iles Formation) that suggests rapid progradation (~208 km m.y. –1 ) of embayed (60–80 km wide), mixed-energy (wave- and tide-infl uenced ) shore lines. Wedges A and C prograded 200–250 km in more than ~3 m.y., whereas wedge B prograded 340–400 km in ~2 m.y. (~170 km m.y. –1 ). The anomalously extensive wedge B is unique in the Utah-Colorado segment because of its long extent, rapid progradation rate, dominance of tidally infl uenced facies, long-transit transgressions/regressions, and low-accommodation, nested sequence architecture. Stratigraphic relationships indicate development of wedge B coeval with both Sevier- and Laramide-style deformation in Utah. Assuming a constant sediment supply, the extensive (>300–400 km) “sheet-like” amalgamated wedge (wedge B) may have been caused by (1) reduced subsidence driven by a fl exural interference pattern, whereby two short-wavelength (±200 km) fl exural profi les are superimposed during the uplift of both basement-cored and thin-skinned thrust belt, (2) an increase in, or eastward migration of, dynamic subsidence during tectonic transitions associated with slab fl attening or rollback (i.e., long-wavelength fl exure), or (3) reduced subsidence due to short-wavelength fl exural interference augmented by the reduction, or migration, of long-wavelength fl

Single-Trip TAML Level-4 Multilateral Junction Reduces Well Costs in Norwegian Sea Subsea Well - Case History

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 123935, ’New Single-Trip, TAML Level-4, Multilateral-Junction Technology Reduces Well Costs in Norwegian Sea Subsea Well - A Case History,’ by Olle Balstad, Statoil, and Mark Glaser, SPE, and Tage Heng, Weatherford, prepared for the 2009 SPE Offshore Europe Oil & Gas Conference & Exhibition, Aberdeen, 8-11 September. Installation of a single-trip Technology Advancement for Multilaterals (TAML) Level-4 multilateral (ML) junction was completed in a subsea gas-injection well. The well was drilled to support oil production from the Tilje and Ile formations in the Smørbukk Sør field in the Norwegian Sea, approximately 193 km offshore in 300-m water depth. After injecting for 10 years, the well will be converted to a gas producer from the same formations. Introduction The åsgard license is a subsea development incorporating three fields, Smørbukk, Smørbukk Sør, and Midgard. This area of the Norwegian Sea experiences very unpredictable and extreme weather conditions. Winds can change from dead calm to full gale in a period of less than 1 hour, and the accompanying sea conditions present major challenges to both equipment and personnel. Prospect development is well advanced, with only a few slots remaining in the subsea templates for new-well development. New wells are more cost effective if an ML approach is used that maximizes the number of drainage points in the reservoirs. The Smørbukk Sør field has four templates—three for production wells and one for injection, with only one available slot (R-3). Studies of injection patterns and requirements concluded that additional injection was required into both the Ile and Tilje formations to optimize production patterns. Another complication was that both zones would require sand control. An ML well was planned with the objectives of providing the needed gas injection and subsequent conversion to a gas producer after 10 years. To achieve these goals, a TAML Level-4 ML completion was planned. Injection into the well would be controlled by the use of a newly developed downhole-instrumentation and -control system, and as a result, it was deemed that no subsequent access to the main bore would be required after completion. The decision was made to construct the ML junction with a single-trip hollow-whipstock system that can withstand high temperatures (static downhole temperature anticipated to be 140°C) and can reduce the number of trips into the well in the hostile operating environment prevailing in the area. A single-trip hollow-whipstock design was chosen for installation in the Smørbukk Sør R-3 well. Drilling and Completion The drilling and completion of Well R-3 were performed from a semisubmersible rig. The well was drilled and cased without incident. Main-Bore Completion. A detailed procedure was written to communicate all stages of the completion process clearly including risk analysis, contingencies, comprehensive pretesting of assemblies, and other details to provide a smooth transition between phases, a seamless operation, and clear quality, health, safety, and environmental guidelines. Following a thorough cleanout of the main bore to TD and the running of the requisite logs, the main bore was completed with a 5½-in.-sand-screen section. Because of the required total length of the section, the decision was made to run the sand screen in two sections because the need to reach the correct depth with the top liner hanger was critical.

Identifying Point of Failure and Repairing Damaged Sand Screens

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 122153, ’Identifying Point of Failure and Repairing Damaged Sand Screens in Gravel Packed Wells. A Case History From the Heidrun Field,’ by Per Einar Svela, SPE, Oyvind Gustavsen, Hilde Songstad Solli, SPE, and Oystein Brandal, StatoilHydro, originally prepared for the 2009 SPE European Formation Damage Conference, Scheveningen, The Netherlands, 27-29 May. The full-length paper describes a method used on several wells in the Heidrun field to mitigate failures in gravel-packed screens. The method described includes how to identify sand production, the preferred way to run the well after sand production has been identified, and how to perform data acquisition to determine the point of failure. The data acquisition described includes gravel-pack logging, injection logging, and saturation logging. Heidrun Field The Heidrun field is located on Haltenbanken in the Norwegian Sea. The sea depth in the area is 350 m. The field has been developed with a floating concrete tension-leg platform installed over a subsea template with 56 well slots. The northern part of the field is developed with five subsea templates. Oil and gas production started in October 1995. The reservoir consists of sandstones in the Garn, Ile, Tilje, and Aare formations of the Early and Middle Jurassic. The reservoir is heavily faulted. The Garn and Ile formations have good reservoir quality, while the Tilje and Aare formations are more complex. Most of the formations produced in the Heidrun field have a moderate-to-low sand strength and are prone to sand production. Several approaches to sand control have been tried, and the current base-case solution is openhole gravel-packed screens. The recovery strategy for the field is pressure maintenance using water and gas injection. Sand Management Sand production has both a production and a health, safety, and environmental (HSE) impact. Massive sand production can result in unwanted discharge into the environment during jetting of separators because oil coats the sand being jetted out. With regard to the safety aspects, gas velocity has to be considered. A high gas velocity means high erosion risk during sand production. Wells are divided into three categories, high-, medium-, and low-erosion risk, depending on the gas rate of the well.

Sedimentology and unexpected pressure decline: the HP/HT Kristin Field

Abstract Since production began in the HP/HT Kristin Field off mid-Norway, reservoir pressure in each of the three mid to late Jurassic reservoir units (the Garn, Ile and Tofte formations) has declined significantly more rapidly than was initially predicted. In the Garn Formation, the Tofte Formation and to some extent also the Ile Formation, this has occurred at least partly because an unusual distribution of reservoir properties led to bias in the four-well appraisal dataset and this in turn resulted in an overestimation of reservoir properties. Of particular importance to this bias was the fact that very good but unrepresentative reservoir properties were encountered in all three reservoir zones in the discovery well located in the centre of the field. These, it is now realized, are not even typical of most of the central part of the field but are, instead, restricted within one, small, anomalous area. Study of cores and thin sections indicates that in each reservoir unit this directly reflects a concentration of more energetic depositional facies in the area while less energetic facies are present on three sides. This pattern was not predictable from the original dataset and seems to have arisen because there was structural control upon facies positioning during accumulation of the reservoir section. This influenced the distribution of cleaner, coarser grained, more proximal depositional facies and, ultimately, reservoir quality distribution and pressure development. What is interesting about Kristin Field is that the structural influence upon sedimentation is observed within the footwall stratigraphy of a major relay structure where the primary provenance direction was on the hanging-wall side. This pattern is the reverse of what is normally reported in tectono-stratigraphic studies.

Extracting sub-bandwidth detail from 3D amplitude data: An example from the Mesaverde Group, Piceance Basin, Colorado, U.S.A.

Mamm Creek Field produces from sandstones of the Mesaverde Group that are part of a pervasive gas accumulation in the deeper part of the Piceance Basin. Most production is from discontinuous fluvial sands in the Williams Fork Formation, but marine sands in the Corcoran, Cozzette, and Rollins members of the Iles Formation and middle and upper sands of the Williams Fork Formation also contribute. Coals are present in the lower third of the Williams Fork, and are believed to be the primary source of gas for the Williams Fork production. The sandstones of the Mesaverde have permeabilities in the 1–100 microdarcy range and have average porosities of about 9%. Abundant natural fracturing has been documented in core and image logs, and is critical to providing permeability for commercial production. Detailed mapping of all features that may control the distribution of both reservoir gas and fractures is critical to effective development of the resources at Mamm Creek. The application of advanced seismic analysis...

Evolution of provenance in the NE Atlantic rift: The Early–Middle Jurassic succession in the Heidrun Field, Halten Terrace, offshore Mid-Norway

An integrated provenance study using provenance-sensitive heavy mineral ratios, mineral chemistry and U/Pb dating of detrital zircons has revealed significant changes in sediment provenance during deposition of the Early to Middle Jurassic succession in the Heidrun Field, offshore Mid-Norway. The variations result from the interplay of two source regions, one of which was located on the Norwegian landmass and the other on the conjugate East Greenland margin. Sediment sourced from central East Greenland is distinguished by high garnet:zircon, high rutile:zircon, low chrome spinel:zircon, garnet assemblages rich in low-Ca, high-Mg varieties, and zircon populations that include an Archean group, a diverse range of Early–Middle Proterozoic grains, and an Early Paleozoic group. These features indicate derivation from a high-grade (granulite facies) metasedimentary terrain together with Archean basement and Early Paleozoic granitoids. Norwegian-sourced sandstones differ by having lower garnet:zircon and rutile:zircon, variable chrome spinel:zircon, garnet assemblages scarce in low-Ca, high-Mg varieties, and zircon populations that lack an Archean group. Derivation from the Caledonian Nappe Domain, comprising metasediments (predominantly at amphibolite facies), ophiolites and Early Paleozoic granitoids, is indicated. Initially, during deposition of the non-marine lower part of the Åre Formation (Hettangian–Sinemurian), sediment was fed from the west, but in the upper (tidally influenced) part of the Åre Formation (Sinemurian–Pliensbachian), Norwegian-sourced material appears. Greenland-derived material disappears in the subsequent Tilje Formation (Pliensbachian), with the Tilje and subsequent Ror and Ile Formations (Toarcian–Aalenian) being predominantly sourced from the east. The regional regression at the base of the Garn Formation (Bajocian) was accompanied by a switch in provenance, with Greenland-derived material replacing sediment sourced from Norway. Variations in mineralogy offer a framework for correlation on both local and sub-regional scales.

HP/HT Gas-Condensate Well Testing for the Shell Onyx SW Prospect

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 108665, "HP/HT Gas- Condensate Well Testing for Shell's Onyx SW Prospect," by Arild Fossa, Expro; Derek MacKenzie, SPE, Norske Shell; Odd Steinveg and Eric Henderson, Expro; and Bart van den Bosch, SPE, Halliburton/Shell, prepared for the 2007 Offshore Europe, Aberdeen, 4–7 September. Complex data gathering occurs over a shorter time with much stricter environmental and safety constraints than ever before. Coupled with increased needs for more-accurate reservoir data for prospect evaluation, emphasis is placed on upfront planning and improved technical performance together with extensive use of advanced fluid-data-gathering methodologies. The latest technologies in testing gas-condensate wells were used on this job. Experiences were used later as the basis for other gas-condensate prospects, including those in the Russian sector of the Barents Sea. Introduction Onyx Southwest (SW) exploration Well 6406/9–1 was the second well drilled under License PL 255 offshore Norway. The first well, Well 6406/5–1 in the Tott East prospect, was drilled in 2002 to intersect sands of the Middle-to-Lower Jurassic Garn, Ile, and Tofte formations. Well 6406/9–1, was planned as a vertical exploration well for a possible high-pressure/high-temperature (HP/HT) regime. Although the HP/HT conditions were marginal, the test was designed and executed as if under full HP/HT conditions. Maximum surface pressure with reservoir fluids to surface was considered to be less than 690 bar. Bottomhole static temperatures were anticipated to be in excess of 150°C. The main objective was to investigate the stacked Ile, Ror/Tofte, and Tilje formations by conducting a multizone drillstem test (DST), with each interval being tested separately. Testing of two zones with a third was planned as a contingency to cover formations of interest. Results from well logging narrowed the selection to a two-zone DST. Test Objectives For the Onyx SW exploration-well formations, the primary test objectives for each reservoir were to assess the productivity and to obtain representative fluid samples. Obtaining reservoir pressure and temperature; determining skin and permeability-thickness product, kh; and identifying possible reservoir boundaries and heterogeneities for the formations were secondary objectives of the well tests. These secondary objectives would aid safety and equipment selection for future wells, as well as provide the starting point for further reservoir analysis. The kh and skin could not be attained satisfactorily for HP/HT wells in any other way because alternative methods have poor performance under such well conditions. The kh and skin parameters were selected to further aid analytical reservoir-simulation work and to enable improved use of the productivity data. Detection of possible reservoir boundaries and heterogeneities was included as a secondary target for planning a future field-drainage strategy. To achieve the test objectives, a set of special requirements was established to obtain the desired data quality. These requirements were downhole shut-in, testing the well with seawater as the packer fluid, and use of downhole memory gauges and single-phase sampling.

Ichnofabrics and sedimentary facies of a tide-dominated delta: Jurassic Ile Formation of Kristin Field, Haltenbanken, Offshore Mid-Norway

Abstract Tide-dominated deltas are poorly known from the stratigraphic record and are notoriously complex, owing to the wide spectrum of facies encountered and their spatial/temporal variability. The tide-dominated deltaic palaeoenvironment combines the ecological harshness of brackish-water settings with complex tidal channel/tidal-flat type facies architecture on the delta top, in association with more classic deltaic facies-stacking patterns. The Ile Formation is interpreted herein as a tide-dominated delta deposited in a microtidal setting. Its palaeoenvironments are interpreted based on a combination of ichnology, ichnofabric analysis and sedimentology. Ichnofabric stacking patterns are used to elucidate the internal architecture of the notoriously problematic aggradational multi-storey tidal channel units. The tide-dominated deltas of the Ile Formation have a distinctive ichnological signature that may be used to characterize tide-dominated deltas. In comparison to typical river-dominated deltas the Skolithos ichnofacies is less well developed and ichnodiversity is lower than expected in wave-dominated deltas. The ichnofabric model presented has potential to be used, with modification, in other tide-dominated deltaic settings.

Regulation of biliary secretion by extracellular nucleotides

AbstractBile formation is an essential function of the liver and depends upon complimentary actions of hepatocytes and intrahepatic bile duct cells, or cholangiocytes. Cholangiocytes contribute importantly to the volume and composition of bile secreted by the liver through absorption and secretion of fluid and electrolytes. Biophysical measurements suggest that increases in Cl– permeability at the apical membrane of cholangiocytes represent an important cellular site for regulation of secretion. Recent evidence suggests that extracellular ATP may serve as a signal modifying cholangiocyte secretion through the regulation of membrane Cl– permeability. Through activation of P2Y2 receptors in the apical (luminal) membrane of cholangiocytes, ATP functions as a local autocrine/paracrine regulatory factor that modulates biliary secretion. Consequently, the purpose of this review is to describe the cellular mechanisms thought to contribute to ATP release and review the evidence supporting a role for ATP in the regulation of cholangiocyte Cl– secretion and bile formation. Characterization of the factors that regulate local nucleotide concentrations and purinergic signaling may represent therapeutic targets for the pharmacologic modulation of bile composition and flow. Drug Dev. Res. 52:232–238, 2001. © 2001 Wiley‐Liss, Inc.

Comparison of constrained sparse spike and stochastic inversion for porosity prediction at Kristin Field

Kristin Field is a recently discovered gas and condensate field offshore Norway (See Figure 1 of Dolberg et al.). Reservoir sandstones are tidally influenced, laterally continuous shallow marine deposits of Jurassic age (Figure 1). A diagenetic overprint during progressive burial to depths of 5 km creates a patchwork of reservoir quality making 3-D mapping of these properties challenging. At the start of reservoir modeling, two exploration wells had penetrated the reservoir. One main focus was to predict lateral variations in reservoir quality (porosity and permeability). During the project, two additional appraisal wells were drilled and used for reservoir characterization refinement. Four potential reservoirs exist at Kristin Field (Figure 1). Only the Garn and Ile Formations are proven to be hydrocarbon-bearing. For this study, porosity prediction techniques are applied only to the Ile Formation. Total thickness of Ile is close to 50 ms TWT (80–100 m), with a depth of approximately 4.1 s (4600 m).

Porosity prediction from seismic inversion, Lavrans Field, Halten Terrace, Norway

Several authors have changed employment since this paper was written. Those authors and their new locations are: David Dolberg, Mobil Oil Canada, Calgary, Alberta, Canada; Jan Helgesen, CGG Norge, Hovik, Norway; Tore Hakon Hanssen, Fortum Petroleum, Oslo, Norway; Ingrid Magnus, Norsk Hydro ASA, Bergen, Norway; Girish Saigal, Norsk Hydro ASA, Oslo, Norway; and Bengt K. Pedersen, Norwegian University of Science and Technology, Trondheim, Norway. Lavrans Field lies offshore Norway on the western extreme of Halten Terrace, 15 km south of Smorbukk Field (Figure 1). Exploration in this area has been extremely active, yielding two large gas and condensate discoveries within Petroleum License 199—Lavrans Field to the east and Kristin Field to the west. Combined reserves are approximately 1200 million barrels oil equivalent of gas and condensate. Unique conditions have come together to provide hydrocarbons and preserved porosity at depths greater than 5 km. Figure 1. Lavrans and Kristin fields relative to Smor-bukk. Field outlines (rose) are drawn on the map of the Base Cretaceous Unconformity. View is to the north. Hydrocarbon-bearing sandstones at Lavrans have a thickness of 600 m. These reservoirs consist of shallow-marine deposits of Jurassic age. Although the facies can be laterally extensive, the overprint of diagenesis makes prediction of reservoir quality difficult. Seismic inversion provides insight to porosity variations away from limited well control. Thus, seismic inversion can be a valuable tool for reservoir characterization prior to field development. This study developed out of a need to better understand the significance of seismic amplitude variations over the crest of Lavrans Field (Figure 2). Hypotheses examined to explain amplitude variations were fluid effects, porosity, pressure changes, or processing and illumination artifacts. Figure 2. Cross-line 3185 is a dip line through well 6406/2-2. Seismic anomalies are seen below the Top Ile Formation. Understanding the meaning of these amplitudes was a primary motivation …

Abstract: Discrimination Between Incised Valley and Distributary-Channel Complexes in a Wave-Dominated Deltaic Setting: Upper Cretaceous Iles Formation (Mesaverde Group), Sand Wash Basin, Northwestern Colorado.

Abstract: Discrimination Between Incised Valley and Distributary-Channel Complexes in a Wave-Dominated Deltaic Setting: Upper Cretaceous Iles Formation (Mesaverde Group), Sand Wash Basin, Northwestern Colorado.

Asphaltene-rich residues in sandstone reservoirs of Haltenbanken province, mid-Norwegian continental shelf

Thin zones of asphaltene-rich residue are present near the crests of two adjacent oil- and gas-filled structures. In one structure, the Smørbukk Sør Field, multiple thin (5–50 cm) dark coloured zones are observed within the lower 20 m of the 88 m thick Garn Formation. These dark coloured zones tend to occur immediately above stylolitized mica-clay laminations and become gradually lighter upwards, apparently reflecting gradual upward decrease in residue abundance. In the other structure, the Smørbukk Field, the lowermost 1 m of the formation is dark coloured and the sandstone gradually becomes lighter in colour upwards for a distance of about 3 m. Similar, but more diffuse, residues are also present throughout the oil zone in two other fields nearby. Organic geochemical analyses show that the residue-rich (dark coloured) zones differ from the sorrounding lighter coloured sandstone mainly in having a higher content of asphaltenes, whereas the total content of extractable organic matter is often similar. Thus the darker rocks might be regarded more as zones of conspicuous oil staining due to asphaltene deposition, rather than as small tar mats. The residues typically appear to have accumulated under the influence of gravity at points where the downward flow was hindered by a lithological discontinuity. Present day zonation (upward-decreasing gas to oil ratio) in the oil column of the Smørbukk Sør reservoir is consistent with a model of asphaltene precipitation in the lower part of the reservoir in response to upward leakage of gas from an underlying gas condensate reservoir in the Ile Formation. This is also consistent with the available organic geochemical data. In thin section, the asphaltene-rich material is seen to preferentially coat feldspar and clay but not quartz surfaces, thus illustrating why sandstone wettability should be expected to depend on both mineralogy and the asphaltene content of the petroleum involved.

Increase in fidelity of rat liver Ile-tRNA formation by both spermine and the aminoacyl-tRNA synthetase complex

To examine the polyamine effects on the fidelity at the aminoacylation level and the physiological significance of the existence of the aminoacyl-tRNA synthetase complex (ARSC) in animal cells, a single-chain Ile-tRNA synthetase (IRSS) was isolated from the complex by treatment with trypsin. Ile-tRNA formation by IRSS was strongly stimulated by spermine, similar to the results with ARSC. Two misacylations (Val-tRNA Ile and Ile-tRNA i Met formation) by IRSS were measured. The error frequency was higher in Ile-tRNA i Met formation (tRNA misacylation) than in Val-tRNA Ile formation (amino acid misacylation). Spermine did not influence significantly Ile-tRNA i Met formation, but it stimulated Val-tRNA Ile formation by IRSS. Accordingly, spermine decreased the error frequency of tRNA misacylation, but not amino acid misacylation. These results suggest that the conformational changes of individual tRNA by spermine differ from each other, meaning that spermine influences the interaction between individual tRNA and aminoacyl-tRNA synthetase variously. When the aminoacylations of tRNA Ile from rat liver, yeast, and Escherichia coli were compared with ARSC and IRSS, the relative speed of Ile-tRNA formation with tRNA Ile from other species was faster with IRSS than with ARSC. This indicates that ARSC can recognize tRNA Ile from the same species more specifically than IRSS. These results show that both spermine and ARSC are involved in the increase of fidelity of rat liver Ile-tRNA formation.

Development of Laramide-Style Subsidence Trends During Late Cretaceous Foreland Basin Deposition, Northwest Colorado: ABSTRACT

Isopach maps of Upper Cretaceous rocks in the Laramide Piceance basin of northwest Colorado have revealed that Laramide-style subsidence trends developed long before the end of Cretaceous foreland basin deposition in this area. Foreland basin deposits in northwest Colorado can be subdivided into three major unit: (1) the Cenomanian through lower upper Campanian marine Mancos Shale, (2) the upper Campanian marginal marine Iles Formation, and (3) the upper Campanian through Maestrichtian largely nonmarine Williams Fork Formation. Isopach maps of the Mancos Shale show west- and northwest-thickening trends similar to regional isopach trends of foreland basin deposits. In contrast, isopach maps of the Iles interval reveal a change to east- and northeast-thickening trends identical to those found in the Laramide Piceance basin. However, the Iles interval continues to thicken over the White River uplift, which forms the eastern boundary of the Laramide Piceance basin. This shift occurred near the end of Baculites asperiformis faunal zone (78 Ma), or considerably before the White River uplift began to rise in Paleocene time. Shoreline trends for the Iles interval were not noticeably influenced by these variations in subsidence; they typically run northeast-southwest, nearly perpendicular to isopachs. Thickening trends of the Williams Fork interval aremore » similar to those of the Iles. An unconformity occurs at the top of the Williams Fork, however, and it was previously thought that early Laramide uplift and beveling increased toward the west, producing this thickening trend. Significant beveling may have occurred, but the westward thinnins of the Williams Fork may also be depositionally controlled.« less