Click to view article in PDF format.
Resistivity Image Data, Vøring Basin, Offshore Norway*
By
Ian Goodall1, Ian Westlake2, Grete Block Vagle3, Ivar Mundal3, and Matthew Mulcahy4
Search and Discovery article #10049, (2003)
*Adapted for online presentation from Chapter 11, AAPG Methods in Exploration Series, No. 13, with the same title and by the same authors (Goodall, Ian, Ian Westlake, Grete Block Vagle, Ivar Mundal, and Matthew Mulcahy, 2002, Resistivity Image Data, Vøring Basin, Offshore Norway, in Geological Applications of Well Logs, M. Lovell and N. Parkinson, eds.: AAPG Methods in Exploration No. 13, p. 143-159). This publication, copyrighted 2002 by The American Association of Petroleum Geologists, with all rights reserved, is available for sale from the AAPG Bookstore (http://bookstore.aapg.org), where its “address” is: (http://bookstore.aapg.org/detail.html?cat_no=524&ticket=7671547787930892&uniqueid=200310011110).
1Badley Ashton & Associates Ltd., Horncastle, U.K.; currently Goodall Geoscience Ltd., Horncastle, U.K.
2Badley Ashton & Associates Ltd., Horncastle, U.K.
3BP Norge AS, Forus, Norway
4Schlumberger Wireline & Testing, Aberdeen, U.K.
The Luva Discovery well, 6707/10-1, targeted the
Santonian to Campanian Nise Formation on the Nyk High structure in the Vøring
Basin, offshore Norway. It encountered a 156-m gas column in a section 1200 m
thick, dominated by sheet sandstones of excellent quality deposited in a
basin-floor complex. This section comprises stacked sand systems, each as much
as 200 m thick, that have been mapped across the Vøring Basin. They are
separated by hemipelagic "fines blankets" deposited during periods of shutdown
in sand supply. Vertical transmissibility in the sheet sandstones and hence
potential reservoir performance are controlled by the extent of sand-body
amalgamation and the distribution of caps to individual, dewatered high-density
turbidite beds. These caps range in composition from glauconitic,
carbonaceous-rich, and mud-prone/mudclast-rich sandstones to heterolithic
deposits/shales and have markedly different vertical permeabilities ranging from
less than 1 to about 1000 md. These bed caps have nondiagnostic conventional
open-hole log responses, but are identified easily on Formation MicroScanner
(FMS+) images. In addition, the analysis of paleotransport directions
derived from trough cross
-stratified, glauconitic sandstone-bed
caps--interpreted as lower-density deposits associated with waning flow and
identified using the FMS images--provides a mechanism to test regional
paleogeographic interpretations made from 3-D seismic data. The results are not
conclusive, but they highlight uncertainties in the model for sand supply and
dispersal in the Vøring Basin.
+FMS is a registered trademark of Schlumberger Ltd.
|
Recent exploration for petroleum in Norway has concentrated on plays in the Norwegian Sea, north of lat. 66 degrees. Prospects in this area typically are in water deeper than 1200 m; without recent advances in drilling and production technology, these areas could not have been explored. Despite being technically feasible, exploration for petroleum in the Norwegian Sea is expensive. Therefore, to minimize costs, the number of wells drilled must be limited. Sufficient volume of geological data must be acquired early in exploration and field appraisal. A major gas discovery was made in 1997 by the
PL218 license group (BP Norge AS, Norsk Conoco AS, Esso Exploration and
Production Norway AS, and Statoil ASA) in Block 6707/10. This was the
Luva Discovery, which was in 1200 to 1500 m of water. To minimize
long-term costs, a significant amount of data was acquired in the
discovery well, 6707/10-1. This information included Formation
MicroScanner (FMS) data and substantial
LOCATION AND RESERVOIR GEOLOGY OF THE LUVA DISCOVERY The Luva Discovery is in the Vøring Basin near lat. 67 degrees N, long. 7 degrees E, off the coast of mid-Norway (Figure 1a). The field was discovered in 1997 by exploration well 6707/10-1 (Figure 1b); it contains a gas column 156 m thick (Figure 2). The discovery is on the Nyk High, which is a north-to-northeast-trending, three-way dip-closed structure bounded on the northwest by a major fault (Figure 1b). The structure was mapped on 3-D seismic data on an Upper Cretaceous K80 (Campanian) amplitude trough; it contains bright amplitudes and a flat spot, which were targeted successfully by the discovery well. The Vøring Basin is on the mid-Norway passive
margin of the northeastern Atlantic Ocean; it is approximately 400 km
long and 100 km wide. It has a north-northeast-trending
The Luva Discovery is developed in the
sand-prone Nise Formation (Figure
2). The Nise was deposited during the early Santonian to early
Campanian; it can be correlated across the Vøring Basin (Bjørnseth et
al., 1997). Its age corresponds to the units K76 to K80 of BP's genetic
sequence-stratigraphic scheme, which provides a framework for
correlation of these strata in the Shetland Islands, North Viking Graben,
Norwegian Sea, and East Greenland; the framework was used in this paper.
A stratigraphic section with abundant sandstone and thickness of 1300 m
was predicted in the Nyk High area by an evaluation of 3-D seismic data.
However, the proportion of sandstone could not be confirmed prior to
drilling, because well data were not sufficient. The discovery well,
6707/10-1, penetrated 1200 m of gross reservoir section; net-sandstone
content is high, sandstone is of excellent reservoir quality and is
slightly affected by diagenesis (
The K76-to-K80 section is composed of sandstones and intercalated mud rocks that can be rationalized into three genetically related depositional elements: sheet sandstones, fringe elements, and blankets of hemipelagic fines. Each of these typically is several tens of meters thick; geometries were interpreted from 3-D seismic data (Figure 3a; scale nomenclature as defined by Mutti and Normark, 1987):
· Sheet sandstones. The sheet-sandstone depositional element is dominated by stacked, meter-scale beds of clean, dish- and pipe-dewatered, fine- to medium-grained sandstone interpreted as high-density turbidites. These are erosively amalgamated or, more commonly, are capped by "slurried" bed "tops" rich in carbonaceous material (or "carbonaceous-rich."), mudclast-rich "attached" debris flows ("attached" is used here to describe genetic relationship with underlying sandstones), or flat-laminated heterolithic beds and shales, which record the combination of deposition from low-density turbidity currents and/or hemipelagic fallout. High-density turbidite deposits with low-density, structured, highly glauconitic sandstone caps are also developed, but are not of significant volume. · Fringe elements. The fringe-element depositional units comprise mostly silt- and sand-laminated heterolithic deposits (or "heterolithics"), intercalated with common, decimeter-thick, dewatered high-density turbidite sandstones. Relative to the sheet sandstones, these beds were deposited either in relatively distal settings or in lateral settings. · Blankets of background hemipelagic fines. The blanket-like deposits of hemipelagic fines are dominated by flat- to parallel-laminated mud rocks commonly intercalated with silt- and sand-laminated heterolithic deposits. Respectively, they are interpreted as basinal hemipelagic fallout deposits, interspersed with episodic, low-density turbidites. Extended periods of hemipelagic deposition are indicated by thick strata of highly bioturbated green mud rock that overlie laminated, turbiditic black mud rock.
These relatively small-scale depositional elements are the building blocks of the reservoir. They are organized into stacked, large-scale systems as thick as 200 m, each of which has a diagnostic, bow-shaped gamma-ray log motif that is the record of progradation, aggradation, and abandonment of a turbidite sheet-sand system (Figure 3a). There is no evidence of significant erosion or of bed-scale synsedimentary deformation that would be associated with the steeper depositional gradients and instability of a slope environment (e.g., slump deposits). This lack of evidence implies that the sheet sandstones and associated fringe elements were deposited in a basin-floor setting (Figure 3b). The scale of this basin-floor sheet-sand complex is difficult to quantify. The complex has been mapped beyond the PL218 license by use of seismic data; it is considered to be extremely extensive, potentially several hundred kilometers long, and several tens of kilometers wide. High-density turbidity currents were the dominant transportational process that delivered sand to the basin floor (Lowe, 1982). Rapid collapse and/or sustained deposition by hindered settling of flows (Kneller and Branney, 1995) resulted in bed-scale dewatering fabrics. Flow segregation produced various kinds of bed caps, which range from carbonaceous-rich sandstones to "attached" mud-prone debris flows. Decimeters to meters thick, mud-prone debris-flow deposits also are commonly intercalated with mud rocks, in the fringe elements and in background hemipelagic "fines blankets." Whereas these could be interpreted as run-out deposits of "attached" debris flows that were beyond the limits of sand deposition, they are more likely to be the record of entirely separate flow events, not linked to deposition of high-density turbidites. These debris-flow deposits may be products of slope- or canyon-wall failure up-depositional-dip to the main sand supply, or they may have been sourced in a transverse slope setting. Substantially increased amalgamation of
sandstone beds toward the center of each bow-shaped log response
reflects a period of maximal sediment input and is characterized by a
smoother gamma-ray motif, which is evidence of the absence of bed-cap
fines (Figure
3b). The sheet-sandstone systems are increasingly mud-prone toward
their lower and upper margins. They are separated by "fines
blankets"--each several tens of meters thick--of sandstone and mud rock
intercalated on the scale of centimeters. The "fines blankets" were
deposited by low-density turbidity currents and background-suspension
settling that marked shutdowns in sand supply. The scale and
distribution of these "fines blankets" form the principal, large-scale
layering of reservoirs in the K76-to-K80 section. However, the presence
or absence and distribution of thin heterolithic/shale caps of
high-density turbidite beds and the extent of sand-body amalgamation are
likely to determine the performance of each dynamic reservoir layer.
This compound factor controls the vertical transmissibility of the
petroleum-bearing sandstones (Figure
4). Therefore, an accurate model of the vertical distribution and
lateral extent of nonreservoir heterolithic/shale caps is critical to
understanding the performance of petroleum-bearing
Current understanding of the depositional
setting and sand-body architecture of the K76-to-K80 sandstones in well
6707/10-1 is based largely on detailed evaluation of a 177-m-thick,
continuously cored section from the uppermost, gas-bearing part of the
K80 sand body. Models based on this core were extrapolated to the
uncored underlying
Data from Cores and Formation MicroScanner Core recovered from the uppermost, gas-saturated part of the K80 sand body has been rationalized into a nongenetic core-lithotype scheme, in which each gravity-flow event has been described in detail, on a bed-by-bed basis. For example (Figure 4a):
· Volumetrically dominant sandstones are classified in terms of texture (grain size of sand and detrital-clay content) and internal structure, with emphasis on the intensity, type, and distribution of dewatering fabrics. The dewatering fabrics potentially affect permeability; for example, a consolidation-laminated bed probably has "horizontal" pathways of permeability, whereas a pipe/ sheet-structured bed is likely to have "vertical" pathways. · In contrast, detailed examination of the composition, color, and internal structure of intercalated mud-prone fines facilitates discrimination among turbiditic, debris-flow, and hemipelagic components, which is captured by a simple lithologic classification into heterolithic deposits, mud-prone conglomerates, and mud rocks, respectively.
This approach provides detailed data that are integrated with an extensive conventional and a special core-analysis data set. These are the basis for the depositional model and, more important, for the petrophysical rock typing that can be input into the Luva Discovery reservoir model. These core-based data can clearly be scaled up to capture the distribution, origin, and geometries of the gravity-flow deposits, at a scale that accurately and consistently describes key, bed-scale heterogeneities in the K76-to-K80 reservoir. However, these heterogeneities have poorly diagnostic conventional open-hole log responses; therefore, they can be identified and described accurately only in the extensive, uncored section that underlies the gas column, by use of high-resolution FMS data. The need to identify these heterogeneities was the rationale behind classification of FMS data into a genetic-image facies scheme (Figure 4).
Interpretation of Formation MicroScanner Data Evaluation of the extensive FMS data set acquired in the K76-to-K80 section was undertaken using Schlumberger GeoFrame software for borehole-image interpretation, installed on a Sun Sparc workstation. The first stage of evaluation was to calibrate the recovered core to the processed FMS data, to ensure that the image responses on which interpretations were based were understood fully and were verified by the rock data. Then FMS data were evaluated by the following approach:
· Lithologies were determined by using an extensive suite of conventional open-hole logs. This involved analysis of the gamma-ray, neutron-porosity, density, and sonic logs. Caliper curves of the General Purpose Inclinometry Tool (part of the FMS tool string) were also used to evaluate image quality. · Specific-image facies designations were made by detailed, bed-by-bed analysis of the textures/fabrics resolved by the static- and dynamic-normalized FMS images. To aid in the observation of features, these images were viewed at different scales, in 2-D and 3-D modes, and with varying degrees of image enhancement. When image facies classifications were made, care was taken to ensure that artifacts of drilling and of data acquisition and processing were not misidentified as sedimentary features.
·
Bedding surfaces were identified and handpicked by using
the borehole-image-interpretation software; the surfaces were classified
according to their origins. These attribute-coded bedding-surface
orientation data formed the basis for evaluation of both
In the absence of core control, the genetic-image facies used to describe the petroleum-bearing sheet sandstones are an accurate means to record the heterogeneity of permeability in the K76-to-K80 reservoir. These deposits are composed of stacked, meter-scale beds of dewatered, high-density turbidite sandstone. The resolution of dewatering fabrics is variable. Dewatering dishes and consolidation laminae commonly are resolved by impersistent, slightly discordant nonplanar features, although locally these and all pipe structures are not resolved, because their resistivity contrast is not sufficient to be detected by the FMS tool. Five types of sheet sandstone are identified, each with reservoir properties very different from properties of the others. They are distinguished easily on FMS images (and indeed in core) by the presence or absence, the compositions, and the origins of bed caps (Figures 4b, 5, and 6):
· Amalgamated sheet sandstones (rSa). Meter-scale beds of dewatered high-density turbidite sandstone compose the amalgamated sheet sandstones (Figure 5a). Bed caps are absent, either because they were not deposited or were eroded. Vertical permeabilities of these sandstones typically are more than 1000 md; therefore, the sandstones have high reservoir potential. They are characterized by blocky, low-gamma-ray motifs. FMS images resolve indistinct beds bounded by flat-lying, planar, and low-angle surfaces of erosion.
·
Sheet sandstones with · Sheet sandstones with carbonaceous-sandstone-rich caps (rSbm). These dewatered sandstones have prominent, upward-fining caps of gray sandstone that is rich in carbonaceous material (Figure 5b). Typically, vertical permeabilities are less than 100 md. The gamma-ray log motif is serrated and nondiagnostic but recognizable on FMS images by a highly layered, bedded fabric. In the gas zone, carbonaceous-sandstone-rich bed caps are resolved by thin, typically unstructured to weakly structured, low-resistivity layers that bound beds (Figure 5b). (Note: The resistivity response of these caps is fluid-zone dependent.) · Sheet sandstones with "attached" debris-flow caps (rSd). The uppermost parts of these sheet sandstones are unorganized, slurried, mudclast-rich sandstones as thick as 50 cm (Figure 6a). Typically, the vertical permeability of these beds is less than 10 md. However, the bed tops are likely to be less extensive laterally than the blanketing background fines, because there is high potential for erosive scouring of beds by turbidity currents in proximal/axial locations relative to the 6707-1 well location and because of an offset sand-body architecture. The offset architecture is related to compensation stacking of individual turbidite beds in the sand systems. These caps are recorded as gamma-ray spikes; therefore, they are easily misidentified on open-hole logs. However, they have diagnostic, chaotic FMS images with characteristic, steeply dipping internal surfaces that resolve concentrations of large mudclasts (Figure 6a). · Sheet sandstones with shale/heterolithic caps (rSbs). In these beds of massive/dewatered sandstone, the uppermost sets of strata are background hemipelagic shale and low-density turbidite heterolithic deposits as thick as 20 cm that potentially have large areal extent. Permeability of these caps typically is less than 1 md; therefore, they are important heterogeneities in the K76-to-K80 reservoir. These sandstones have nondiagnostic, serrate gamma-ray motifs but can be identified easily on FMS images by thin but well-defined, highly structured, relatively low-resistivity layers (Figure 6b; light blue lines). The resistivity response of these caps is fluid-zone dependent.
Thick and laterally extensive blankets of
background hemipelagic fines separate the K76-to-K80 sand bodies. They
commonly contain intercalated packages of turbiditic mud rocks and
centimeter-scale, silt-prone, low-density turbidite beds, both of which
can be identified easily on FMS imagery. These nonreservoir
· Laminated mud rocks (rMl). Basinal hemipelagic fallout deposits make up the laminated mud rocks. They have wide neutron-density separations and are characterized on FMS images by a weakly defined flat/parallel lamination on the scale of centimeters. · Mottled mud rocks (rMm). The mottled mud rocks are turbiditic. They have wide neutron-density separations similar to those of laminated mud rocks, but they are characterized by diagnostic unstructured/mottled FMS images.
·
Laminated heterolithic deposits (rHl).
These turbiditic packages probably are the
lateral/distal deposits of sand-prone depositional systems; hence, they
indicate the proximity of sand bodies near to but not penetrated by the
borehole. Therefore, accurate characterization of the mud-prone
In the 156-m-thick gas column, the resistivity signature of both the static- and the dynamic-normalized FMS images is controlled mostly by the highly resistive gas that saturates the sandstones (Figure 8). FMS images in the sheet-sandstone elements are typically of high resistivity, having bright yellow to white static-normalized signatures, in which the gas response masks most geologic data. Therefore, as compared to water-saturated sandstones, it is much more difficult to distinguish different genetic elements in gas-saturated sandstones.
· Regular, meter-scale bedding is visible on the images of sheet sandstones with high-resistivity, high-permeability, carbonaceous-sandstone-rich caps, whether the rocks are gas-saturated (Figure 8a1) or water-saturated (Figure 8a3). However, thickness and distribution of these finer-grained, typically unstructured, more compacted, resistive bed caps are resolved clearly only in the absence of gas saturation. · Low-permeability shale/heterolithic caps can be recognized easily in water-saturated sandstones because they are highly compacted and contain less saline fluid than the sandstones; hence, they are more resistive (Figure 8b3). Only their strongly layered fabric permits their identification in the presence of gas (Figure 8b1). This is because both the gas-saturated sandstones and compacted/cemented shale/heterolithic caps have similar, relatively high resistivities. · Dewatering fabrics--including dish structures and consolidation laminae--are features of high-density turbidite deposits and therefore are observed in cores, but they are not visible in FMS images of gas-saturated sandstones (Figure 8b2). In contrast, they are resolved clearly as impersistent, discordant, bed-parallel features in water-saturated sandstones (Figure 8b4).
In addition, in the gas column in sandstones with either high- or low-permeability caps, static-normalized resistivity commonly is reduced significantly in strata immediately above bed bases (Figure 8a2; notice the duller orange static-normalized FMS images). This pattern is considered to have resulted from slumping of high-density drilling-mud filtrate in strata above the less-permeable bed caps; it highlights the effect that bed caps potentially have on fluid flow in the reservoir.
To develop a realistic exploration model of K76-to-K80 sandstones in the Vøring Basin, a crucial step is to model their sources, paleotransport directions, and distributions. Longitudinal turbidity currents in elongate basins are a well-established concept (Hsu, 1977), yet in limited subsurface data sets, evidence pertaining to sediment sources and paleotransport directions can be equivocal.
Extensive 3-D seismic mapping of the K76-to-K80 section has been undertaken over the PL218 license block area. The 3-D seismic data are characterized by regularly spaced peaks that correlate with high gamma-ray spikes in the limited well data set. The fact has been demonstrated that these high-amplitude gamma-ray peaks are signals from background hemipelagic "fines blankets," deposited during phases of shutdown in sand supply. Therefore, they provide a means to identify and map individual turbidite-sandstone systems throughout the Vøring Basin. The correlations undertaken have been confirmed by biostratigraphic data. Seismic-amplitude
The extensive FMS data set acquired in the Luva
Discovery well, 6707/10-1, provides a means to assess independently the
paleotransport directions of the K76-to-K80 sandstones in the Nyk High
area of the Vøring Basin. To determine the paleotransport directions
accurately, first the influence of present-day
Determination of
The most reliable means to determine
· Nonreservoir mud rocks include both diffusely laminated and unstructured background hemipelagic and turbiditic shales (image facies rMl and rMm). With the exception of uncommon bedding surfaces that were deformed by compaction around siderite concretions--which have been excluded from the calculations--these bedding surfaces were most probably deposited flat-lying; they are in thick packages that probably were not deformed by differential compaction above and below sand bodies.
·
Nonreservoir heterolithics, the low-density turbidite
sandstones intercalated with background hemipelagic shales (image facies
rHl) in "fines blankets" and fringe elements, are also likely to have
been deposited flat-lying. However, these deposits probably are less
reliable than nonreservoir mud rocks as indicators of
In addition, absence of slump structures and of
any evidence of significant erosion in massive/dewatered sheet
sandstones with carbonaceous-sandstone caps or shale/heterolithic caps
leads to the inference that these sandstones were deposited on a
relatively flat basin floor. This conclusion is supported by evaluation
of 3-D seismic data, which demonstrates that the sand bodies are
laterally extensive, with sheeted geometries that can be mapped across
the PL218 license. These sheeted, high-density turbidite sandstones
probably have slightly uneven flow tops that resulted from the
"freezing" of "attached" debris flows; therefore, they are characterized
by a significant degree of azimuth dispersion. Nevertheless, they can be
used to test calculations of Calculations of
Interpretation of Paleotransport Directions High-density sheet sandstones with trough
The interpreted paleotransport directions are broadly northward; i.e., indicating that the sediment-entry point was to the south of the Luva Discovery (Figure 7c). In detail, bed-form dip directions are eastward at the base of the K80, northeastward at the top of the K80, and they show evidence of dispersion, including apparent reversals in direction of flow in the K80 level (Figure 7c). The observed variations in bed-form orientation are considered to be a consequence of two factors.
· Regional, basin-scale, sediment supply and dispersal interacted with ephemeral, localized sourcing of sandstones. This interaction is predicted by the depositional model of these deposits, and it is consistent with the thin, unattached debris flows and localized glauconitic sandstones, the sources of which may have been from an adjacent shelf (Figure 3b). · Turbidity currents were deflected around localized features of high relief that resulted from compensation stacking of adjacent, sheetlike sand bodies. This may explain the apparent reversals in paleotransport direction, which are observed at several stratigraphic levels in the K76-to-K80 section (Figure 7c).
The broad northeasterly to easterly paleotransport directions implied by FMS data contradict interpretations made from 3-D seismic data. They imply that the source of these basin-floor sediment-gravity flows was south of the Gjallar Ridge. If this inference is correct, the models for sand supply and dispersal in the Vøring Basin must be reassessed.
The conflicting evidence of sediment-transport
directions drawn from borehole-image interpretation and from seismic
mapping have not been resolved. Both data sets permit alternative
interpretations. The thicknesses and stratal relationships observed in
seismic imagery could reflect simply the filling of topography of a
preexistent basin, independent of sediment-transport directions.
Alternatively, although the observed southwesterly pinchout of the
uppermost K80 sand body (Figure
9a) may provide evidence of paleotransport directions to the south,
this sand body simply may have filled a depositional low. Similarly, the
sediment-transport directions inferred from borehole-image data merit
low confidence. This possibility was considered: The northerly dipping,
structured bed forms were deposited by turbidity currents that were
reflected, and hence reversed, by a major However, interpretations of a paleoflow to the north are supported by the fact that background-hemipelagic mud rocks both below and above the K76-to-K80 reservoir section--i.e., separated by approximately 1000 m of sandstones--have identical, low-angle (2 degrees to 3 degrees) northerly sedimentary dip. These mud rocks, inclined at low angles, are inferred to have been deposited on a paleoslope; the observed paleoflow directions could have been downslope, or transverse or oblique to such a slope. This line of reasoning leads to the hypothesis that sediment dispersion in the Vøring Basin was controlled by syndepositional intrabasinal tectonism that generated local paleoslopes.
The principal conclusion of this evaluation is that high-resolution resistivity images provide valuable data that can be used to improve geological models of petroleum reservoirs in basin-floor sediment-gravity-flow deposits. Detailed evaluation of FMS data from a K76-to-K80 sand-rich turbidite complex, the Luva Discovery well, integrated with data from cores and open-hole logs, resulted in three secondary conclusions.
· Where core control is absent, FMS data and data from open-hole logs are a means to accurately identify and quantify the distribution of bed-scale disparities in permeability. Heterogeneities in dewatered high-density turbidite beds range from amalgamated sandstones with high vertical permeability through glauconitic, carbonaceous, and mudclast-rich bed tops deposited by waning turbidity flows, to caps of background-hemipelagic shale and low-density turbidite heterolithic deposits with low vertical permeability. These three sheet sandstone bed types have very similar open-hole log responses, but they are distinguished easily by analysis of resistivity-image data. · High gas saturations have a significant effect on resolution of geologic features by resistivity images. This makes identification of key bed-scale heterogeneities more difficult, because gas-saturated sandstones and compacted bed caps have similar high resistivities. However, identification is possible. In addition, relatively low resistivities commonly are observed in sandstones immediately above the low-permeability bed caps. This is attributed to slumping of high-density drilling-mud filtrate and reduction of resistivity.
·
Orientations of low-density, trough
The authors extend their thanks to BP Norge AS and the PL218 license partners Norsk Conoco AS, Esso Exploration and Production Norway AS, and Statoil ASA for giving permission to publish this work. Dr. Peter Haughton provided invaluable input for this paper. Badley Ashton & Associates Ltd. and, in particular, Gerry Hyde, who drafted the figures, are thanked for their assistance in preparation of this paper.
Bjørnseth, H.M., S.M. Grant, E.K. Hansen, J.R. Hossack,
G.D. Roberts, and M. Thompson, 1997, Hsu, K., 1977, Studies of Ventura Field, California, I: Facies geometry and genesis of Lower Pliocene turbidites: AAPG Bulletin, v. 61, p. 137-168. Karl, H.A., D.A. Cacchione, and P.R. Carlson, 1986, Internal wave currents as a mechanism to account for large sand waves in Navarinsky Canyon head, Bering Sea: Journal of Sedimentary Petrology, v. 56, p. 706-714. Kneller, B.C., and M.J. Branney, 1995, Sustained high-density turbidity currents and the deposition of thick massive sands: Sedimentology, v. 42, p. 607-616. Lowe, D.R., 1982, Sediment gravity flows: II. Depositional models with special reference to the deposits of high-density turbidity currents: Journal of Sedimentary Petrology, v. 52, p. 279-297. Lundin, E.R., and A.G. Dore, 1997, A tectonic model for the Norwegian passive margin with implications for the NE Atlantic: Early Cretaceous to break-up: Journal of the Geological Society (London), v. 154, p. 545-550. Mutti, E., and W.R. Normark, 1987, Comparing examples of modern and ancient turbidite systems: Problems and concepts, in J.K. Leggett and G.G. Zuffa, eds., Marine clastic sedimentology: Concepts and case studies: London, Graham and Trotman, p. 1-38. Shanmugam, G., T.D. Spalding, and D.H. Rofheart, 1993, Process sedimentology and reservoir quality of deep-marine bottom-current reworked sands (sandy contourites): An example from the Gulf of Mexico: AAPG Bulletin, v. 77, p. 1241-1259.
Walker, I.M.,
K.A. Berry, J.R. Bruce, I. Bystøl, and J.H. Snow, 1997, |