GeoScienceWorld
Volume

3D Seismic Technology:

Application to the Exploration of Sedimentary Basins

Edited by Richard J. Davies, Joseph A. Cartwright, Simon A. Stewart, Mark Lappin and John R. Underhill

Abstract

A ‘new age’ of subsurface geological mapping that is just as far ranging in scope as the frontier source geological mapping campaigns of the past two centuries in emerging. It is the direct result of the advent of 2D, and subsequently 3D, seismic data paralleled by advances in seismic acquisition and processing over the past three decades. Subsurface mapping is fuelled by the economic drive to explore and recover hydrocarbons but inevitably it will lead to major conceptual advances in Earth sciences, across a broader range of disciplines than those made during the 2D seismic revolution of the 1970s. Now that 3D seismic data coverage has increased and the technology is widely available we are poised to mine the full intellectual and economic benefits. This book illustrates how 3D seismic technology is being used to understand depositional systems and stratigraphy, structural and igneous geology, in developing and producing from hydrocarbon reservoirs and also what recent technological advances have been made. This technological journey is a fast-moving one where the remaining scientific potential still far exceeds the scope of the advances made thus far. This book explores the breadth of the opportunities that lie ahead as well as the inevitable accompanying challeges.

  1. Page 1
    Abstract

    Three-dimensional (3D) seismic data have had a substantial impact on the successful exploration and production of hydrocarbons. Although most commonly acquired by the oil and gas exploration industry, these data are starting to be used as a research tool in other Earth sciences disciplines. However despite some innovative new directions of academic investigation, most of the examples of how 3D seismic data have increased our understanding of the structure and stratigraphy of sedimentary basins come from the industry that acquired these data. The 3D seismic tool is also making significant inroads into other areas of Earth sciences, such as igneous and structural geology. However, there are pitfalls that parallel these advances: geoscientists need to be multidisciplined and true integrators, and at the same time have an ever-increasing knowledge of geophysical acquisition and processing. Notably the utility of the 3D seismic tool seems to have been overlooked by most of the academic community, and we would submit that academia has yet to take full advantage of this technology as a research tool. We propose that the remaining scientific potential far exceeds the advances made thus far and major opportunities, as well as challenges, lie ahead.

    1. Page 11
      Abstract

      3D seismic data can play a vital role in hydrocarbon exploration and development especially with regard to mitigating risk associated with presence of reservoir, source, and seal facies. Such data can afford direct imaging of depositional elements, which can then be analyzed using seismic stratigraphy and seismic geomorphology to yield predictions of lithologic distribution, insights to compartmentalization. and identification of stratigraphic trapping possibilities. Benefits can be direct, whereby depositional elements at exploration depths can be identified and interpreted, or they can be indirect, whereby shallow-buried depositional systems can be clearly imaged and provide analogues to deeper exploration or development targets. Examples of imaged depositional elements from both shallow and deep sections are presented.

    2. Page 25
      Abstract

      To aid exploration and appraisal of hydrocarbon discoveries in deepwater deposits of the Kutei Basin, a study of analogous sedimentary architectures in Recent deposits of the same basin was undertaken. High quality 3D seismic were used to develop an understanding of the external and internal geometry of slope to basin floor elements in a structured setting. Toe-thrust anticlines and related mud diapirs deflect slope canyons. Over slope-steps, gravity flow deposits are laterally confined with narrow facies belts. In slope mini-basins, flows are less confined resulting in deposition over a broad area. The Recent deposits of a single canyon and associated basin floor system are used to illustrate the deepwater depositional elements. Debrites at the base are followed by a slope channel complex or basin floor fan then a channel-levee complex. Large depocentres occur where gradients are low and the system switches from confined to unconfined. Erosionally confined channels feed basin floor fans at the toe-of-slope, while channels confined by levees feed fans on the ‘distal’ basin floor. Slope channel complexes and basin floor fans are interpreted to be sand prone. From the slope to basin floor these deposits increase in width:thickness ratio and areal extent and apparent lateral connectivity increases while vertical connectivity decreases.

    3. Page 35
      Abstract

      The analysis of 3D seismic data in the near-seafloor interval is a useful speciality in deepwater exploration and production. In addition to the well-established benefits of 3D seismic data, the higher frequency content of near-seafloor data has a variety of applications throughout the life cycle of deepwater plays. These benefits include: (1) depositional process modelling, (2) stratal architectural information for building reservoir models, and (3) drilling hazard assessment.

      Detailed mapping of well-imaged 3D seismic intervals in the near-seafloor interval is providing new insights to deepwater depositional processes and architectures. Depositional patterns are more confidently identified in near-seafloor settings, enabling the investigation of testable relationships between stratal stacking patterns, gradient changes and accommodation. These relationships as well as spatial and geometric information from these data are useful for building and constraining reservoir models, linking key observations from subsurface data at prospective levels with fine-scale outcrop analogue data. In particular, near-seafloor 3D data can image surfaces related to episodes of aggradation, starvation, bypass, and/or erosion that are typically hard to recognize or map at exploration depths, but are critical in controlling reservoir bed-length and connectivity in three dimensions. Near-seafloor 3D seismic data can supplement or even replace traditional 2D-based site surveys for assessing potential drilling hazards. Although usually lower in vertical resolution than 2D site survey data, 3D data have the distinct advantage of better imaging of 3D geometric bodies, providing insight into complex stratal stacking patterns, and allowing data volume manipulation and perspective.

    4. Page 45
      Abstract

      Recently acquired 2D seismic data shot over the western Niger Delta have enabled a pre-delta rift framework to be delineated inshore of a transform fault dominated continental margin which lies beneath the later, delta sediment apron. The delta apron has been deformed by toe-of-slope thrusting where faults have climbed from a detachment surface at or near the top of the over-pressured Akata Formation mudstones. The overlying mixed clastic succession of the Agbada Formation has been faulted by a broadly oceanward stepping series of NW-SE trending thrusts climbing from this detachment level. The principal thrusts have been offset by NE-SW trending transfer zones, the positions of which have been inherited from trends within a pre-delta rift framework that underlies part of the western delta slope.

      3D seismic data partly covering the 2D grid show turbidite channel complexes at numerous stratigraphic levels within the Agbada Formation and clustered in particular areas of the slope. Commonly, submarine channels can be seen to have cut through the relief caused by folding at the positions of intersection with transfer fault zones. These data show the relationship between structure and channel formation and highlight the importance of transfer fault zones in localizing channel systems on the lower slope. Nevertheless, the 2D seismic data has provided an explanation for the location of the transfer zones within the toe-thrust belt in the form of an underlying structural framework, and both data types have contributed to the understanding of controls on reservoir distribution in an area where the principal sand delivery systems are perpendicular to the main structural trend.

    5. Page 53
      Abstract

      First returns from 3D exploration surveys have been utilized to display seafloor morphology of the Faroe-Shetland Channel between the UK and the Faroes. The image combines 32 datasets creating a regional perspective of Quaternary sedimentary processes. Geomorphic information is of significance for sea bed geohazard evaluation, environmental studies and as an analogy for former sedimentary environments. The image covers more than 25000 km2 extending from the shelf (water depth ∼ 120 m) to the basin floor (water depth up to ∼ 1600 m).

      On any margin knowledge of the sea bed morphology is essential for understanding the environmental setting and for safe operations in deepwater. Under favourable circumstances, the sea bed can be picked from 3D exploration seismic surveys in a similar manner to any other horizon to provide detailed images of the seafloor, thereby negating the need for dedicated sea bed surveys. Combining first returns from several surveys creates a regional perspective, essential when considering importance of features e.g. the rarity of a certain seafloor environment or the presence of a potential landslide upslope from an operations area.

      The Faroe-Shetland Channel displays a wide range of sea bed features including, sediment waves, contourite deposits, polygonal cracking, landslides, debris flows, turbidity current channels, glacial moraines and iceberg ploughmarks. Resolving the spatial aspects of these features greatly assists the interpretation of shallow profile data for geohazard and environmental studies and provides a backdrop onto which biologists, oceanographers, sedimentologists and engineers can overlay their data sets and thus their interpretations.

    6. Page 63
      Abstract

      We have combined 3D mapping of key reflectors with seismic profiles to describe the geometry and anatomy of contourite drifts formed by deep waters passing through the oceanic gateway of the Faroe-Shetland Channel. The West Shetland Drift complex is characterized by sheeted-mounded drift units, and upslope migrating sediment waves accreting over an early Pliocene unconformity. The basin section is constructed by a series of asymmetric depositional units of early Pliocene-Pleistocene age, interlayered by three mega-debrite sequences that extend into the basin. The Pliocene drift surface display an enhanced topography of bifurcating moat-channels that tend to branch out in a southwest direction. Along the lower slope a succession of upslope migrating sediment waves has accumulated from the Pliocene drift topography. These features extend to the present sea bed at water depths of 700-1000 m where they appear as a series of linear, bifurcating ridges. The high accumulation rates of the West Shetland Drift since the early Pliocene transition and the formation of upslope migrating sediment waves is related to a sustained flow of Norwegian Sea deep waters and cross-slope transport of fine-grained sediments from the NW European shelf.

    7. Page 73
      Abstract

      Interpretation of 3D and 2D seismic data in the Faroe-Shetland Basin (FSB) has revealed the important role that structurally controlled bathymetry had in controlling sedimentary dispersal during Early Cenozoic thermal subsidence. The Flett Ridge was a major NE-SW structural high during some of the Palaeogene, actively growing and influencing adjacent sedimentary systems. During the Palaeogene this area of the FSB was a key entry point for siliciclastic sediment with a major deltaic system prograding towards the NW during the Middle Eocene. Prior to delta development, the Flett Ridge was onlapped during the Late Palaeocene and subsequently blanketed and drowned in Early Eocene times. Major periods of fluvial incision cutting up to 100 m into the Middle Eocene strata are identified and a variety of channel networks with differing trends documented. Broad channels or valleys of earliest Middle Eocene age inherited the palaeotopography created by the Flett Ridge, whereas subsequent later Middle Eocene meandering channels trend perpendicular to the shelf edge and traverse the Flett Ridge structure. Seismic amplitude maps suggest that a complex and variable channelized drainage system developed across the coastal plain and delta top in the Middle Eocene. These channels influenced sediment supply creating an area of bypass to the more distal fan systems preserved at the base of slope. Later faulting on the ridge crest may also have affected the channel network pattern.

    8. Page 83
      Abstract

      The near top Oligocene unconformity is a major sequence boundary in the eastern North Sea Basin. It is characterized by erosional scarps below the boundary and a pronounced basinward shift in onlap above. The shift in onlap has previously been interpreted as caused by a major fall in sea level. Detailed 3D seismic analysis of a 20 by 20 km area at and basinward of the uppermost Oligocene clinoform breakpoint reveals that the erosional scarps were caused by undercutting of steep clinoforms by contour-parallel currents and resulting mass wasting whilst the lowermost onlap package consists of a contour-parallel drift deposited as the erosive currents waned. The 3D seismic analysis corroborates a recent analysis based on regional 2D seismic data, which found that the erosional scarps and the geometry of the onlap sequence were indicative of a major shift in sediment input directions and not necessarily associated with any change of sea level. The paper thus demonstrates the utility of local 3D seismic analyses as a form of 'ground truthing' regional basin analyses based on widely spaced 2D seismic grids.

    9. Page 91
      Abstract

      This paper documents the complex three-dimensional geometry of the Messinian Unconformity in the Tarraco concession area on the Ebro Shelf, based on the Tortuga 3D seismic survey. A detailed map of the Messinian Unconformity Surface has been constructed in the survey area, and shows a dendritic drainage pattern with valleys that are around 1 km wide, and 6-7 km in length. The overall morphology of the surface is strongly reminiscent of badlands topography, from semi-arid zone erosion of sandstone-shale sequences. The depth of incision of the valleys into the pre-Messinian clastic sequences is around 400 m. The results obtained confirm previous models of sub-aerial exposure creating erosion of the pre-Messinian shelf and slope sequences that generated the spectacular Messinian Unconformity.

    1. Page 101
      Abstract

      3D visualizations of modern, high-resolution seismic data have provided valuable insights into the finite geometries and spatial extent of extensional fault systems, but their evolution in time is poorly understood. Scaled 3D analogue models of rift basin evolution provide kinematic templates for understanding the 4D evolution of extensional fault systems. This paper reviews the development of extensional fault systems in analogue models of orthogonal, oblique and offset rifts. In orthogonal and oblique models, stretching above a zone of ductile deformation at the base of the model initially produced segmented rift border faults whose orientations were strongly controlled by the underlying baseplate configuration. In contrast, the intra-rift faults generally initiated at high angles to the extension direction. With increased extension both the rift border faults and the intra-rift faults propagated along strike, first producing segmented fault systems separated by relay ramps, which, with increased extension, became breached as fault linkage occurred. Kinks in the fault traces indicate linkage points. Within the models, asymmetric intra-rift sub-basins were formed where the extensional fault arrays had a dominant dip polarity. Intra-basin accommodation zones, separating individual sub-basins along the rift axis, were formed by interlocking oppositely dipping fault systems. Offset oblique rift models, formed above a zone of ductile stretching with basement offsets, generated intra-basin accommodation zones whose orientation was controlled by the underlying basement fabric. The results of the analogue models can be directly compared with fault systems in the Northern Ethiopian rift system, with the accommodation zones in the Gulf of Suez, Egypt, with extensional fault arrays in Canyonlands, Utah, and with rift fault systems in the Gulf of Thailand and the southern North Sea.

    2. Page 117
      Abstract

      A high-quality 3D seismic survey, located in the northwest Porcupine Basin (Irish Atlantic Margin), has been used to investigate the geometry and origin of pervasively developed and complexly distributed linked extensional fault arrays, present within Late Cretaceous and Early Tertiary sequences. The faults show a downwards transition from relatively simple, planar fault segment geometries (~N-S-trending) within younger Early Eocene sand-dominated clastic sequences, into complex conjugate arrays in the underlying older Early Eocene to Late Cretaceous shale-dominated sequences. Rectilinear to polygonal structural configurations are developed at the deeper levels. Most of the fault array ultimately terminates downwards into the Late Cretaceous, where structural accommodation may have taken place by localized or more regional bedding plane slip and/or by volume changes resulting from compaction of fine-grained sequences. Locally, reactivated Jurassic syn-rift extensional faults are locally seen to link upwards into the shallow fault array and appear to have controlled both the intensity and facing direction of the shallower faults on a km scale. The seismic data also clearly show that early upslope-throwing faults are cross-cut by later, downslope-throwing faults. Such geometries are comparable to those formed in sandbox models where gravitational collapse of a tilted sequence is the dominant process controlling fault development. Overall, the fault array geometries seen in the Cretaceous and lower Tertiary successions in this area are interpreted to have resulted from gravitational collapse processes during basin subsidence and sediment compaction, and where the main deformation mechanism was non-rigid block rotation. Differential compaction of Cretaceous and lower Tertiary sediments over pre-Cretaceous rift topography and selective reactivation of the Jurassic fault array are also considered important influences on the resultant fault distribution in 3D.

    3. Page 133
      Abstract

      We investigate fault growth and linkage during development of a rafted terrain in the Lower Congo Basin, offshore Angola. Miocene thin-skinned extension has led to the development of isolated raft blocks separated by a graben filled with syn-deformational strata. Angular unconformities together with thinning and onlapping of intra-raft strata onto salt bodies suggest that thick salt was mobile during thin-skinned extension. 3D fault array geometries and displacement patterns record the subsequent deformation history of the graben during further thin-skinned extension. The mode of thin-skinned extension has important consequences for the Neogene turbidite hydrocarbon play associated with the rafted province of the Lower Congo Basin. The presence of thick mobile salt will influence pre-salt source rock maturation and the development of pre-salt/post-salt hydrocarbon migration windows.

    4. Page 143
      Abstract

      3D seismic data from the Nankai margin provide detailed imagery documenting the onset of deformation at an active sediment-dominated accretionary prism, including a previously unmapped network of normal faults. The Nankai margin off southwest Japan is characterized by active subduction, seismogenesis, and a large accretionary prism with fold-and-thrust belt structure. Imbricate thrusting is the dominant structural style of the outer 20 km of the prism. This structural domain develops at the prism toe, where an incipient imbricate thrust displays significant along-strike variability in dip, offset, and development of hangingwall anticlines.

      Compressional deformation is preceded by normal faulting that initiates seaward of the trench axis. Seismic data in this area reveal a complex, intersecting pattern of normal faults within the incoming hemipelagic sediments. Underlying the faulted section is a high-amplitude reflector interpreted as representing oceanic basement. This reflector contains elongate horsts and grabens oriented perpendicular to the margin interpreted as relict spreading centre fabric.

      Analysis of the orientation of normal faults within the Shikoku basin sequence shows a correlation between fault geometry and basement structure. This faulting is notably similar to layer-bound compaction faults, documented in the North Sea and elsewhere, attributed to both hydrofracturing and volumetric contraction of fine-grained sediments. Mapped normal faults may thus be the result of a combination of differential compaction of sediments above irregular, dipping oceanic basement and compactional dewatering seaward of the toe of the accretionary prism.

    5. Page 149
      Abstract

      The remarkable spatial resolution of 3D seismic data is particularly important in the study of salt structures, where changes in the distribution of accommodation space resulting from salt withdrawal can be mapped and related to the evolution of the individual salt structures. The Salt Dome Province of the Danish Central Graben provides an interesting example of this approach. Three adjacent salt structures (Skjold, Dan and Kraka) exhibit very different geometries, and their evolution has been the subject of some debate. The present study suggests that the Dan structure is composed almost entirely of Zechstein salt, which, facilitated by de-coupled extension during the Mid-Late Jurassic, has been intruded into and along weak planes of Triassic salt, resulting in an overall domal, circular outline. Skjold is located to the NW of Dan. It consists of Zechstein salt and forms a mature salt stock, which terminates within Upper Cretaceous strata. Skjold evolved from a linear NW-SE trending salt wall and became a point source structure during the Early Cretaceous, in common with the many other Central Graben diapers and coincident with the commencement of thermal subsidence. By contrast, Kraka, a NW-SE trending pillow structure located to the south of the Dan-Skjold alignment never became diapiric. Gravitational downbuilding of the mature Skjold diapir during rapid Cenozoic deposition was punctuated by rejuvenated (active) growth induced by regional compression, most significantly during the Mid-Miocene. This event affected also the Dan and Kraka structures, which otherwise experienced very limited growth during the Cenozoic.

    6. Page 165
      Abstract

      By integrating 3D and 2D seismic interpretation with structural restorations we have reconstructed the evolution of a complex, composite stepped counter-regional salt system in the West Delta/South Pass (WDSP) area of the northern Gulf of Mexico. Biostratigraphically calibrated well data allow the last 10 Ma of the evolution of the salt system to be divided into six stages: (1) sea-floor extrusion of isolated salt tongues fed from the Jurassic Louann salt through northward dipping feeders prior to 7.5 Ma; (2) amalgamation of the salt tongues to form a salt-tongue canopy between 7.5 and 6.4 Ma; (3) counter-regional evacuation of the salt-tongue canopy as a result of enhanced sediment loading due to progradation of the shelf margin between 6.4 and 5.0Ma; (4) evacuation of salt into a series of salt walls linking salt domes between 5.00 and 2.55 Ma; (5) evacuation of the salt walls to form counter-regional fault welds between 1.95 and 0.5 Ma; and (6) final evacuation of most of the salt from deeper levels leaving a series of isolated salt domes connected by counter-regional fault welds. The counter-regional evacuation of the WDSP salt systems illustrates the value and limitations of published 2D models for allochthonous salt, and the reconstructed evolution yields insights into the complex interactions between salt deformation and sedimentation. The results also suggest that the WDSP salt systems significantly affected sediment transport pathways, trap geometries and possibly late stage petroleum migration across evacuating salt welds.

    7. Page 177
      Abstract

      We present an analysis of a unique 3D survey that allows us to relate the deep structure of the crystalline crust to the shallow structure of the overlying, potentially hydrocarbon-rich sedimentary basins. The survey is located over the Gjallar Ridge, Mid-Norway, and extends from a Moho-level reflector at around 15 km depth to polygonal faulting and diapiric structures at or near the seabed. 3D visualization techniques using seismic workstations and the Cave immersive environment have been used to illustrate the geometries of these features. The deep reflector is correlated with the top of a deep, high-density, high-velocity body that is interpreted to indicate the presence of magmatic underplating and is intimately related to localized uplift of the Gjallar Ridge. Abundant high-amplitude reflectors in the deep Cretaceous sections of the survey are interpreted as sills emplaced during the Palaeocene magmatic event and are therefore interpreted to be coeval with the magmatic underplate. In contrast, the shallow parts of the survey have numerous gas-charged mud diapirs and an extensive network of polygonal faults extending to the seabed. Study of such very deep or very shallow features is not standard industry practice. However, the intention here is to demonstrate that, by utilizing the full volume of 3D seismic data, it is not only of scientific interest but also results in a greater understanding of the tectonic history of a hydrocarbon prospect.

    8. Page 187
      Abstract

      At the shallowest point of the Faroe-Shetland Channel, between the Faroe Islands and the Shetland Isles, the sea bed is deformed into a series of major scarps and hollows. The cuspate scarps, or ‘Judd Falls’, are up to 15 km in length and are over 200 m high. Interpretation of 3D seismic data and high resolution 2D seismic data shows that the scarps are part of a larger series of structures that are partly buried. A second series of buried asymmetric hollows has been mapped 50 km to the northwest. Both sets of hollows are interpreted to have a deep-water erosional origin, postulated to be associated with the initiation of the high-energy bottom currents of the south-flowing Northern Component Water from the Norwegian-Greenland Sea into the North Atlantic. Present-day measurements presented here show that deep-water current velocity can peak at over 0.8ms−1. Both erosional complexes are positioned directly above Tertiary inversion structures, and this study has identified two periods of compressional deformation, latest Ypresian and late Lutetian, in addition to previously documented phases. Compression in the area has been linked to changes in the interaction between the Mid-Atlantic Ridge and the Iceland mantle plume. Enhanced plume activity also concentrated deep-water flow in the Faroe-Shetland Channel by physically impeding deep-water currents elsewhere. Where enhanced deep-water flow encountered the partial barriers of the inversion structures, accelerated turbulent erosional currents carved the scarps into the sea bed.

    9. Page 199
      Abstract

      We use 3D seismic data in a novel way to describe the three-dimensional geometry of a number of igneous bodies intruded into the upper crust as well as to define and classify sill junction relationships. Igneous intrusions were emplaced into Upper Cretaceous and Palaeocene sediments of the Faroe-Shetland Basin during the Early Palaeogene and in many cases they adopt remarkable saucer- or trough-shaped geometries that are 2-8 km in diameter and have a vertical relief of several hundred metres. Individual intrusions are interlinked and form highly interconnected sill complexes. Three geometrically distinctive classes of sill junctions are defined and illustrated with examples from seismic data. Each class implies a specific evolutionary sequence of events and these are discussed for each of the classes of junction. The class of junction often changes along the line of junction with one class evolving in space to another. This has significant implications for spatial reconstruction of sill complexes based on two-dimensional outcrop and this is illustrated with reference to an example from a 3D seismic dataset.

    10. Page 209
      Abstract

      This paper describes some of the results from a 3D seismic-based analysis of the mechanics of igneous sill emplacement in sedimentary basins. Detailed 3D interpretation of igneous intrusions flanking the Corona Ridge in the Faroe-Shetland Basin has led to the discovery of a sill (the Corona Sill) with a previously unrecognized morphology. Two potential feeder sources have been interpreted for the broadly rectangular intrusion, of which the surveyed portion measures approximately 15 by 4 km. The Corona Sill has a linear NW margin and two lobate protuberances along the SE side. Arcuate ridges that radiate from a central point at the intersection of the lobes cover the imaged surface of the sill. The ridges have wavelengths of 220-350m and amplitudes in the range 25-50m.

      The ridged morphology on the surface of the Corona Sill has not previously been described from any seismic or outcrop-based study of igneous sills. The ridges are interpreted to have formed as a direct result of the propagation mechanism, and are thought to have been influenced by the viscosity of the magma, host sediment and the depth of intrusion, which is likely to have been within 400 m of the sediment-water interface. It is suggested that during very shallow intrusion of viscous magma into soft, waterlogged sediments, magma is able to spread, creating a geometry similar to that expected for a high viscosity lava flow.

      Ridges were formed by compression of a more rigid outer layer of magma in the sill, retarded by the solidifying sill front. Forward movement of the surface layer is likely to be caused by viscous drag from within the sill body. The ridges on the sill top surface are a kinematic indicator for the flow direction of the magma, enabling identification of the feeder zone, which displays a clear link to an underlying sill. It is demonstrated that 3D seismic data has significant untapped potential for the study of magma transport and intrusive processes in the upper crust.

    1. Page 219
      Abstract

      Examples from the Carboniferous of the UK Southern North Sea are used to illustrate the application of visualization tools to increase the effectiveness of 3D seismic interpretation. Maintaining the seismic data to the fore throughout the interpretation workflow and decision-making process is vital for well planning in field development and trap validation in prospect evaluation. Scaling of the seismic into the ‘true-depth’ domain and the automatic generation and visualization of layers using isochores derived from well data are the key elements in this interpretation workflow. Together these allow full integration of the seismic with various well data and interpreted elements. The main example is from Murdoch K, one of ConocoPhillips’s most recent UK discoveries, part of a pre-Permian extensional graben inverted during the Early Tertiary tectonic phase. Other examples from the McAdam Field and an exploration prospect are used to highlight additional aspects of the method.

    2. Page 227
      Abstract

      Advances in 3D visualization and volume detection have greatly improved our ability to find stratigraphic targets in areas of complex structure. In this paper we apply volume sculpting, seismic facies analysis and prediction of rock properties from seismic data to resolve a potential reservoir channel from a highly faulted setting in offshore Indonesia. These techniques use computational power to search for patterns in the data. These patterns may be presented to the interpreter or may be used to transform seismic and well data into estimates of rock properties. Such techniques should improve development efficiency in areas where complex structure has previously precluded the use of these techniques.

    3. Page 235
      Abstract

      A case study is described that illustrates a complete reservoir property prediction workflow, from petrophysical analysis, through rock physics, impedance inversion, and on to interpretation and final drilling locations. This study is from onshore Algeria, where the hydrocarbons are found in several clastic reservoirs of varying ages and properties. The controls on the presence of both reservoir and hydrocarbon location are not straightforward, therefore reducing the risk of drilling locations has significant value. This study concentrated on the Triassic TAG-I formation, which forms the major reservoir in the study area. The prediction method used was an impedance based deterministic approach, using relationships based on rock physics, although the interpretation method includes fuzzy set classifications to take into account the knon-uniqueness inherent in any seismic attribute. The petrophysical work ensured that the analysis of each well reconstructed ‘virgin-zone’ conditions. Rock physics models were then used to predict shear wave velocities. Acoustic impedance (AI), shear impedance (SI), and elastic impedance (EI) profiles were derived. Shear impedance was the best pure lithology indicator, with acoustic impedance showing a good relationship to porosity and elastic impedance most sensitive to fluid content. As the near angle seismic data were too noisy, gradient/intercept analysis was impossible, so shear reflectivity and consequently SI could not be derived. Although AI showed some lithological discrimination between sands and shales, it was not sufficient to be used as a single discriminator, so interpreted horizons were used to separate reservoir and non-reservoir intervals. The far-angle data were inverted to EI and this was analysed using a fuzzy logic approach. This method produces a classification volume and 3D body tracking was then used to find the best drilling targets. Generally, the analysis correctly predicted the results of the wells, both discoveries and non-discoveries. However, some of the discoveries were not predicted, which appears to be where the classification was not correctly calibrated. Work is now underway to improve the accuracy of the prediction process.

    4. Page 249
      Abstract

      In this paper we demonstrate the benefits of 3D seismic visualization techniques for fault interpretation where the structural geology is complex and the seismic data quality is often poor. Production from the Njord field is strongly influenced by a complex pattern of segmented and linked extensional faults. The current interpretation of the well test data and production history is that the faults form barriers to fluid flow, reducing oil production, and limiting effective gas injection and pressure support. Drilling results demonstrate that geometrical uncertainties remain in the seismic interpretation. An additional problem is that currently available commercial reservoir modelling technology cannot easily handle a very complex fault pattern, making simulation grid construction difficult. Accurate well placement and production forecasting requires that fault geometries and properties are suitably represented in the reservoir simulation model. 3D visualization of depth-scaled volumes and depth-converted interpretations helped to decide how to best simplify the fault geometry for simulation, and compare automatically generated geological model components against seismic interpretations and data. The reservoir simulation model runs resulted in the identification of a number of well targets. 3D visualization techniques were then used to predict faults and structures that the proposed well trajectories may intersect.

    5. Page 263
      Abstract

      Post-depositional remobilization and injection of sand can significantly change the geometry of deepwater clastic reservoirs. Features associated with these processes are particularly well developed in the lower Paleogene of the South Viking Graben of the UK and Norwegian North Sea. Seismic scale sandstone intrusions can be grouped in two classes. Class 1 comprises low-angle (20-40 degrees) tabular sandstone intrusions emanating from steep-sided in situ sand bodies within the Balder Formation. The intrusions may be 5-30+m thick and crosscut 120-250+m of compacted stratigraphic section. They terminate at an unconformity at the top of the Frigg interval where they may have extruded onto the palaeo-seafloor. Class 2 comprises conical sandstone intrusions that emanate some 50-300+m upward from distinct apexes located 400-700+m above the nearest depositional sand body. The conical intrusions may have been sourced from underlying sand bodies by clastic blow out pipes. Both types of intrusions seem to adopt their particular geometry independently of (but occasionally exploiting) polygonal faults within the encasing mudstones. Sandstone intrusions may be highly porous and permeable and are thus important both as reservoirs and as plumbing within thick mudstone sequences.

    6. Page 279
      Abstract

      Examples from exploration acreage and field developments across the glaciated Northwest European Continental Shelf and Slope demonstrate the usefulness of conventional three-dimensional (3D) seismic data to spatially image geological features. Compared to previous grid-based two-dimensional (2D) seismic this allows fundamentally more confident identification, mapping and prediction of geotechnical conditions which is important to ensure safe, efficient engineering and drilling operations. Whilst of immense benefit, the paper argues that the 3D seismic data often do not meet the full expectations, particularly in terms of critical vertical resolution and accurate depth prediction requirements. To illustrate the limitations, direct comparison is made between conventionally acquired 3D and HiRes 2D seismic data. Whilst industry funding to support innovative HiRes 3D seismic acquisition remains sparse, much can be achieved by the careful integration and interpretative calibration of the 3D and HiRes 2D seismic datasets. Three field development case studies illustrate this. Short offset trace correction and reprocessing of the 3D seismic data followed by limited, target specific HiRes 2D seismic, calibrated where possible with drilling or other geological data, is an optimal cost-effective approach.

    7. Page 297
      Abstract

      4D or time-lapse seismic data has become a business of usual tool for reservoir management in high-cost environments for oil production such as the West of Shetlands. 4D seismic can be used to visualize fluid movement in a qualitative/semi-quantitative sense to visualize field behaviour, predict well performance and predict reservoir pressure. Whilst the data may be easily visualized using 3D visualization technology the challenges for the future include the need to obtain quantitative predictions through better quality seismic data.

    1. Page 303
      Abstract

      Unexpected incidents leading to lost time when the rig is on location cause unplanned cost to the hydrocarbon industry of over one billion dollars annually. Processing and interpretation of 3D seismic data usually focuses on reservoir levels. But from a drillers perspective, geological features of the overburden are often more significant than those at reservoir level, since over 90% of the well is typically spent drilling the overburden, coping with a wider variety of challenges than those associated with the reservoir itself. 3D seismic data defines overburden tectonostratigraphy, the framework of a geological model that can be used in well planning to reduce geological uncertainty, surprises and expense along the whole well track. Many technologies applied in reservoir modelling are equally valid in defining overburden features relevant to well planning. The overburden 3D volume can be populated with key parameters for well design, such as pore pressure and geomechanical attributes, though the complexity of the model will often be restricted by well cost and perception of drilling risk. The role of 3D seismic data in forming the tectonostratigraphic framework of multi-attribute, kilometre-scale Earth models, is illustrated here by a number of examples where model sophistication has been scaled to match project requirements. Overburden Earth models also provide a framework where several ‘academic’ research themes, for instance 3D fault geometry, can be put into a commercial context. Construction of overburden models for well planning has also highlighted a number of future geological research areas that could have a significant impact on drilling performance. Some of these, such as hydraulic properties of fault systems, are highlighted here.

    2. Page 311
      Abstract

      CO2 produced at the Sleipner field is being injected into the Utsira Sand, a major saline aquifer. Time-lapse seismic data acquired in 1999, with 2.35 million tonnes of CO2 in the reservoir, image the CO2, plume as a number of bright sub-horizontal reflections. These are interpreted as tuned responses from thin (< 8 m thick) layers of CO2 trapped beneath intra-reservoir shales. A prominent vertical ‘chimney’ of CO2 appears to be the principal feeder of these layers in the upper part of the reservoir. Amplitude-thickness scaling for each layer, followed by a layer summation, indicates that roughly 80% of the total injected CO2 is concentrated in the layers. The remainder is interpreted to occupy the feeder ‘chimneys’ and dispersed clouds between the layers. A prominent velocity pushdown is evident beneath the CO2 accumulations. Velocity estimation using the Gassmann relationships suggests that the observed pushdown cannot readily be explained by CO2 present only at high saturations in the thin layers; a minor proportion of low saturation CO2 is also required. This is consistent with the layer volume summation, but significant uncertainty remains.

    3. Page 321
      Abstract

      There is a need for quantitative models which predict the structural and thermal evolution of sedimentary basins and margins in three dimensions. Although many different, two-dimensional algorithms exist, most of them are forward models which assume that rifting is instantaneous. We outline a three-dimensional optimization strategy which calculates spatial and temporal variations in strain rate. This approach is a generalization of an existing two-dimensional inversion algorithm which already tackles three issues of interest to the hydrocarbon industry. First, the residual misfit between observed and predicted basin geometries allows competing structural and stratigraphic interpretations to be objectively tested. Secondly, the animated evolution of basin and margins can be produced using the strain rate tensor. Thirdly, spatial and temporal variations of strain rate control basal heatflow, which in turn constrains the temperature and maturation histories of the sedimentary pile. Here, we present a small selection of two-dimensional results and show how our three-dimensional formulation is a logical extension of earlier work. A three-dimensional algorithm is under development.

    4. Page 333
      Abstract

      Certain seismic objects, like faults and gas chimneys, are often difficult to delineate using conventional attribute analysis. Many attributes contain useful information about the target object but each new attribute provides a new and different view of the data. The challenge is to find the optimal attribute for a specific interpretation. In this paper the optimal attribute is found with a pattern recognition approach based on multi-dimensional/multi-attributes and neural network modelling. Multi-dimensional attributes, as opposed to point attributes, can provide the spatial information on the seismic objects. The role of the neural network is to classify the input attributes into two or more output classes. Neural networks are trained on seismic attributes extracted at representative example locations that are manually picked by a seismic interpreter. This approach is a form of supervised learning in which the network learns to recognize certain seismic responses associated with the identified target objects. Application of the trained network yields an ‘object probability’ cube for the target object. Essentially, the neural network can target any seismic or geological feature requiring detailed analysis. In this paper the method is described and examples are shown of gas chimneys, faults, salt domes and 4D anomalies. Some interpretation aspects are discussed.

    5. Page 339
      Abstract

      Traditional methods for building fault models are time-consuming when applied to a complex fault network or where many faults exist since the workflows typically rely on manual intervention at several stages. Structural detail is often simplified to reduce cycle times and consequently, the workflow favours large-scale and simplistic fault systems. There is generally no integrated assessment of kinematic information that would be useful in guiding fault interpretation. A new methodology for constructing a complex fault network with small offset is presented. The method recognizes that interpretation of large numbers of interconnected low displacement faults, is most efficiently done using map based interpretations. A novel semi-automated skeletonization algorithm is used to extract fault traces from horizon maps providing a polyline data set for subsequent use in 3D surface creation. Displacement information is derived automatically during or after the skeletonization providing kinematic information for guiding further interpretation. The new method is validated against manual interpretations of fault geometry and displacement before application to a region of the Central North Sea exposing polygonal faults. The new technique allows for the first time, a rapid and accurate appraisal of complex near-seismic scale fault geometry and displacement from interpretations of 3D seismic data across a large survey area.

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