4D Wheeler diagrams: concept and applications

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1 Downloaded from by guest on March 24, 2014 Geological Society, London, Special Publications Online First 4D Wheeler diagrams: concept and applications Farrukh Qayyum, Paul de Groot, Nanne Hemstra and Octavian Catuneanu Geological Society, London, Special Publications, first published March 24, 2014; doi /SP alerting service Permission request Subscribe How to cite click here to receive free alerts when new articles cite this article click here to seek permission to re-use all or part of this article click here to subscribe to Geological Society, London, Special Publications or the Lyell Collection click here for further information about Online First and how to cite articles Notes The Geological Society of London 2014

2 4D Wheeler diagrams: concept and applications FARRUKH QAYYUM 1 *, PAUL DE GROOT 1, NANNE HEMSTRA 2 & OCTAVIAN CATUNEANU 3 1 dgb Earth Sciences, Nijverheidstraat 11-2, 7511JM, Enschede, The Netherlands 2 dgb Earth Sciences, 1 Sugar Creek Center Boulevard, Suite 935, Sugar Land, TX, 77478, USA 3 Department of Earth and Atmospheric Sciences, University of Alberta, 1 26 Earth Sciences Building, Edmonton, Alberta T6G 2E3, Canada *Corresponding author ( farrukh.qayyum@dgbes.com) Abstract: The conventional Wheeler diagram aids the construction of a spatiotemporal framework of strata. The diagrams are created manually by studying outcrops, wells, or seismic data. For the latter case, automated methods now exist, which support the construction of 2D, as well as 3D Wheeler diagrams. Seismic data contains information in three dimensions, X, Y and Z, where Z is either two-way time or depth. Seismic horizons are correlated surfaces that often follow geological time lines. In this case, a set of interpreted seismic horizons contains information in four dimensions (X, Y, Z, and Geological Time). In the mapping from the structural domain to Wheeler space, information about Z (thickness) is lost. This means that one dimension is missing in the conventional Wheeler diagram. This paper describes a method to add information from the Z dimension to the Wheeler domain. It is done by computing stratigraphic thicknesses per sequence stratigraphic unit and displaying these as colour-coded overlays in the Wheeler domain. Thus displayed, thickness variations help in understanding changes in accommodation, sedimentation rate, and depositional trends. 3D Wheeler displays with colour-coded thickness information are referred to as 4D Wheeler diagrams. In this article, the method is described and applied to a case study from the southern North Sea. Sequence stratigraphy is a stratigraphic interpretation method that aims to subdivide a stratigraphic succession into units that describe changes in stratal-stacking patterns during a full depositional cycle (see the text of Catuneanu 2002 for base level, Catuneanu et al. 2009, 2010, 2011; Csato & Catuneanu 2012; Catuneanu & Zecchin 2013). It essentially deals with four dimensions: the three spatial dimensions (X, Y, Z) and geological time. In an ideal case, absolute geological time (AGT) can be assigned to each interpreted stratigraphic unit. In the absence of AGT, the principle of superposition can be used to interpret units in relative geological time (RGT) and to construct a timestratigraphic model in the form of a Wheeler diagram (Wheeler 1958). This provides a depositional framework of stratigraphic units in which geological time (not space) is the vertical axis; time here may be an RGT scale. Construction of these diagrams has progressively developed through time due to advances in computer technology (for details on the historical developments of Wheeler diagrams, refer to the work of Qayyum et al. 2013; Stark et al. 2013). It is important to mention how these diagrams are routinely prepared, whether manually or in a computer-automated program. An interpreter starts by mapping the top and bottom of stratigraphic units as surfaces in the structural (spatial) domain. These are often important sequence stratigraphic surfaces such as a maximum flooding surface (MFS), maximum regressive surface (MRS), or subaerial unconformity (SU), etc. Such surfaces are considered to represent geological time lines (Vail et al. 1977; Eberli 2000) and each surface is assigned an (arbitrary) RGT value. This concept is illustrated in Figure 1, where the three surfaces (1, 2, and 3) are shown in the structural, as well as in the Wheeler domain (Fig. 1a, b, respectively). The Y-axis of the Wheeler domain is formed by the assigned RGT values, causing the surfaces to be flat in this domain. Note that Figure 1a (the structural domain) is a two-dimensional illustration comprising information from three dimensions, visually: X-axis (distance), Z-axis (depth) and RGT (time lines). The corresponding 2D Wheeler diagram (Fig. 1b) contains only two dimensions: X-axis (distance) and RGT. The quantitative information from the third dimension, that is, Z-axis (depth), is lost in the transformation; only order of succession is preserved. It is not surprising that four stratal dimensions exist in a structural domain. Of these, time and thickness per stratigraphic unit are the most challenging for routine attempts at mapping for any given set of data. All four dimensions appear in the From: Smith, D. G., Bailey, R. J., Burgess, P.M.&Fraser, A. J. (eds) Strata and Time: Probing the Gaps in Our Understanding. Geological Society, London, Special Publications, 404, # The Geological Society of London Publishing disclaimer:

3 F. QAYYUM ET AL. Fig. 1. The concept of constructing a 2D Wheeler (chronostratigraphic) diagram from the interpreted structural domain (outcrop/seismic data). In the structural domain (a), a stratigraphic succession is interpreted as surfaces. Each surface is assigned an arbitrary value, for example 1, 2, 3 in this figure. The number is assigned according to the principle of superposition and represents RGT. The corresponding Wheeler diagram (b) is constructed by flattening the surfaces. Note that this is unachievable without a scale distortion as originally stated by Wheeler & Maurice (1948). structural domain whenever a thickness map is prepared. However, in the routine automated creation of Wheeler diagrams, this dimension is not transformed and is therefore lost. This issue has not escaped attention before, and an example is the work of Nordlund & Griffiths (1992), which utilized RGT slices ( chronosomes ) over a 2D seismic data set to represent spatiotemporal variations along the RGT slices. Also, Rickett et al. (2008) attempted to plot instantaneous isochronous seismic attribute in a Wheeler diagram to describe its advantage as a measure of relative sedimentation rate. Nonetheless, the thickness per unit was not recognized as a stratal dimension for a 2D/3D Wheeler diagram until the work of Qayyum et al. (2012a). This paper is primarily focused on the representation of thickness as a dimension in a 2D/3D Wheeler diagram. The proposed solution is a colour-coded overlay in a 3D Wheeler diagram to capture information from the missing Z-dimension. In structural interpretation, colour coding the Z-axis itself does not add much value as it yields only structural information. It is more informative to study stratigraphic thickness variations in the Wheeler domain. Thickness is a derived property of the Z-domain that is easily computed as the difference in Z-values between the top and base of the interpreted sequence stratigraphic unit (sequences, systems tracts, or parasequences). This proposed extension into the Z-domain and thickness is described here for specific application to seismic data, with its acknowledged limitations of vertical resolution. Nevertheless, the suggested method has broader applications, for example in outcrop studies, and can be used to add more value to conventional Wheeler diagrams. Methodology HorizonCube In routine seismic interpretation studies, a coarse depositional framework is constructed from a limited number of horizons. The number of horizons is restricted because mapping with conventional interpretation tools is a time-consuming process. To extract detailed stratigraphic information, more horizons are needed, which can only be done practically using a new generation of auto-tracking algorithms. One of these algorithms is the dip-steered auto-tracker of the HorizonCube method (de Groot et al. 2010). The method starts by computing a dip/azimuth volume called a SteeringCube from 2D/3D seismic data. The algorithm generates hundreds of horizons by following the dips of the SteeringCube from a single starting position per horizon. The solution is constrained by a userdefined framework that is, a geologically constrained model consisting of a set of interpreted seismic horizons and faults. The auto-tracked seismic horizons are stored in a volume called an HorizonCube. There are two types of HorizonCube; continuous and truncated. The continuous HorizonCube contains mapped events, each of which extends throughout the entire survey (Fig. 2a). Continuous events can neither cross nor terminate against other events. Events in a continuous HorizonCube tend to become very dense along unconformities and condensed sections. This useful characteristic is exploited in the HorizonCube density attribute (de Groot & Qayyum 2012) and is used to convert a continuous HorizonCube into a truncated HorizonCube. The truncated HorizonCube contains events that terminate against other events when their vertical spacing falls below a user-defined threshold (Fig. 2b). This characteristic is stratigraphically significant as it defines stratal terminations and reveals depositional hiatuses in the Wheeler domain. Because of the latter characteristic, truncated HorizonCubes are preferred to create Wheeler diagrams. Examples of Wheeler diagrams constructed from both types of HorizonCube are shown in Figure 2c, d. It should be noted that the truncation threshold must take account of decreasing resolution with increasing depth.

4 Downloaded from by guest on March 24, D WHEELER DIAGRAMS: CONCEPT AND APPLICATIONS Fig. 2. HorizonCube types and corresponding Wheeler diagrams: (a) continuous HorizonCube the events are continuous over the full distance; (b) truncated HorizonCube events terminate when the vertical spacing falls below a user-defined threshold; (c) Wheeler diagram from the continuous HorizonCube; (d) Wheeler diagram from the truncated HorizonCube. Note that the faults relating to the salt dome are conduits for secondary migration (see the text).

5 F. QAYYUM ET AL. Interpreted thickness volumes The thickness of a stratigraphic unit can be defined in various ways, depending not only on how it is computed but also on the data type. Three common definitions exist: isopach, isochore, and isochron (for details refer to Sheriff 2002). For seismic data, isochron is generally preferred as it measures the vertical thickness in two way travel time, whereas it can interchangeably be used as isochore when measured in depth. As this paper primarily deals with seismic data, which is by definition acquired in time, the isochron term for thickness is preferred. Conventionally, thicknesses of stratigraphic units are presented either in the form of gridded properties or as contour maps. The proposed method presents thickness as a volume that can be covisualized with other information in Wheeler space. The value of a thickness volume was suggested by Lomask et al. (2009), who proposed to compute an instantaneous isochron attribute that can be automatically computed in the Wheeler domain. Our method differs in that we propose to compute thicknesses only between relevant stratigraphic surfaces in the structural domain. An example of our thickness volume is given in Figure 3. It is computed as an isochron between two HorizonCube events that were interpreted as meaningful sequence stratigraphic surfaces. Thickness is defined as a trace attribute. There is only one thickness value per seismic trace segment, where a segment is defined between the two interpreted events. The method thus requires up-front interpretation of top and bottom bounding surfaces of chronostratigraphic units. This interpretation step is performed by interactively adding and removing events from the HorizonCube. This is carried out with an interactive slider that operates simultaneously in the structural domain and in the Wheeler transformed domain. Surfaces are interpreted using sequence stratigraphic principles. If possible, the units are interpreted as systems tracts, but this is not essential for the computation of the thickness volume. The interpretation should be made in a hierarchical manner. Depending on geological settings and data quality, the corresponding thickness volume per sequence stratigraphic unit may contain information of higher order depositional cycles. Case study: North Sea A 3D seismic data set of the Dutch offshore (Fig. 4) is studied. The data cover an area of 380 km 2 in Block F03 in which the F03-FB gas field lies in a deeper (Jurassic) reservoir. Four wells complete the data set available to the study, which aimed to construct 4D Wheeler diagrams and to interpret the sequences within a spatiotemporal framework. For this case study, the Pliocene interval containing siliciclastic sediments is interpreted. These sediments contain shallow gas pockets that are producing in the neighbouring blocks (e.g. F02, A15 blocks of The Netherlands; see also Schroot & Schüttenhelm 2003; Stuart & Huuse 2012). The study aimed to construct an integrated depositional framework of the Pliocene depositional sequences by interpreting the 4D Wheeler diagrams of the area, towards the future identification of further shallow gas pockets. Pliocene depositional sequences The Pliocene interval of the North Sea can be subdivided into various deltaic depositional stages that range from a wave- to a tide-dominated stage Fig. 3. The thickness dz is the seismic two-way time (or depth) difference between top and base of interpreted systems tracts. SU, subaerial unconformity; BSFR, basal surface of forced regression (equivalent to the correlative conformity defined by Posamentier & Vail 1988); CC*, correlative conformity (Hunt & Tucker 1992); MFS, maximum flooding surface; HST, highstand systems tract; FSST, falling stage systems tract; dz, delta Z (systems tract thickness).

6 4D WHEELER DIAGRAMS: CONCEPT AND APPLICATIONS Fig. 4. Location map of F3 Block, offshore, The Netherlands. (Qayyum et al. 2012b). The succession developed during the enlargement of a large-scale fluviodeltaic drainage system (Eridanos delta) that dominated northwestern Europe during the late Cenozoic (Overeem et al. 2001). According to Rohrman et al. (1995), this drainage system started during the Oligocene period while the Scandinavian shield was being uplifted, resulting in the development of a siliciclastic delta system. The uplift rate increased during the late Miocene (Sales 1992) and again in the early Pliocene (Ghazi 1992; Jordt et al. 1995). Because of late Miocene uplift, high sediment influx filled the northern offshore regions of the Dutch sector (see Fig. 1 in Overeem et al. 2001). The increasing sediment load resulted in a differential load throughout the region. Consequently, the buried Permian Zechstein salt started moving in the region and several localized unconformities, underlain by salt domes, were formed within the Pliocene interval (Qayyum 2008). These unconformities were often sub-aerially exposed in the southern North Sea, and the exposure caused the erosion of the topset beds of some clinoforms. Biostratigraphic dating of the northern part of the study area was earlier established by Kuhlmann et al. (2006). Further information on the biostratigraphy is taken from well reports (Jansen & Gervais 1997, 1999). In this study, three depositional sequences are identified and correlated with well data (Fig. 5a). The interpretation is made using a four systems tract sequence stratigraphic framework (Hunt & Tucker 1992) and described as Depositional Model IV by Catuneanu (2002). The identification of the sequence stratigraphic surfaces and corresponding systems tracts is based on co-visualization of the structural, as well as the Wheeler domain in conjunction with well data. A summary of the defining criteria of the interpreted systems tracts expected in the Hunt and Tucker model is given below (for more details on defining criteria and associated surfaces, please refer to Catuneanu 2006). Falling-stage systems tract (FSST). This is a prograding, downstepping, and offlapping systems tract. It may also show detached clinoforms from the previous shoreface. During the formation of this systems tract, the SU expands in a basinward direction; this is recognizable in Figure 5. Lowstand systems tract (LST). During the formation of this systems tract, progradation continues, but the system also starts aggrading. Topset beds may also form and may onlap the SU. The top of this systems tract is defined by a conformable MRS, which may onlap the SU in an up-dip direction. This is evident in Figure 5a. Transgressive systems tract (TST). In this systems tract, strata mainly show backstepping patterns, with onlap onto the MRS clinoform ( healingphase wedge ). The top of this systems tract is defined at an MFS. Highstand systems tract (HST). In this systems tract, strata show progradation with aggradation of topset beds, and fluvial onlap. The top of this systems tract is a composite surface defined by the SU and a basal surface of forced regression (BSFR), underlying the FSST (Fig. 3). Applying these criteria, the systems tracts are interpreted and sequences are defined in Figure 5. The lowest, Sequence 1, is comprised of TST, HST, and FSST, which is the product of forced regression. The intermediate, Sequence 2, is a dominantly regressive sequence that consists of LST, HST, and FSST. Note that TST is not annotated in this sequence as it falls below seismic resolution. Sequence 3 contains an LST, TST, and HST. The TST of Sequence 3 has mainly formed a healingphase wedge with a distinct transgressive lag (TL in Fig. 5a). The detailed interpretation of these

7 Downloaded from by guest on March 24, 2014 F. QAYYUM ET AL. Fig. 5. (a) Seismic transect through two wells with overlain systems tracts interpretation of the Pliocene deltaic target interval. (b) Automated Wheeler diagram of the studied interval. The Y-axis of the diagram represents relative geological time. The colour-coded lines are the truncated HorizonCube events, and the colours represent thickness per systems tract as illustrated in Figure 2. Note that Sequence 1 shows a low rate of sedimentation in the basinward direction. In seismic Wheeler diagrams, such condensed sections show up as hiatuses (truncated HorizonCube) or stretched sections (continuous HorizonCube) as illustrated in Figure 2. TL, transgressive lag; BSFR, basal surface of forced regression; MFS, maximum flooding surface; MRS, maximum regressive surface; SU, sub-aerial unconformity; CC, correlative conformity.

8 4D WHEELER DIAGRAMS: CONCEPT AND APPLICATIONS systems tracts was previously discussed by Qayyum et al. (2012b). It was interpreted that the depositional sequences are of third order ( 2.8 Ma) and they cover the widespread area of the F3 block and its neighbourhood. The Pliocene delta prograded at a higher rate during the early phases (Sequence 1 and Sequence 2) compared to the later phases (Sequence 3). This is evident on the Wheeler diagram discussed in the following section. Depositional sequences in 4D Wheeler domain The first automated Wheeler diagram is presented as a section view (Fig. 5b). It shows distinct changes between periods of aggradation, progradation, and retrogradation. Erosional hiatuses are also evident, for example, during the FSST of Sequence 1. This gap is interpreted as an SU and its correlative conformity (CC) is placed at the top of the FSST. The corresponding sequence boundary (SB 1) is defined by a composite surface (SU plus CC). In the same diagram, the thickness of the systems tracts is overlain and colour coded over the flattened HorizonCube events. In this manner, it is now possible to interpret how net accommodation space was filled in conjunction with depositional trends observed in the Wheeler diagram. Figure 6 shows a fence view of a 4D Wheeler diagram with two vertical sections and an horizon slice. The vertical sections are colour coded with the systems tract thickness data while the horizontal slice shows colour blended Spectral Decomposition attributes. The display shows spatiotemporal depositional shifts from normal regressive units to forced regressive units. Each sequence (Sequence 1, Sequence 2, and Sequence 3) shows its own spatiotemporal relationship. Sequence 1 progrades more into the basin with increasing thickness basinward compared to the others. Sequence 2 is dominated by normal regressive units that lie in the middle of the area. Sequence 3 contains a normal regressive (LST; see also Fig. 5b) unit. It underlies a transgressive healing-phase wedge (TST) that is defined by thin intervals in the landward direction. In Figure 6, thickness variations and seismic geomorphology around the lowest MFS of the delta are also exposed. Several marine channels (NE SW) are observed that formed due to the gradient established by movement of the Zechstein salt (blue arrows, Fig. 6). In addition, NW SE oriented sand ridges are observed that formed during the marine transgression. The present day Dutch offshore can be used as a possible analogue to explain their origin (see Fig. 1 of Walgreen et al. 2002). The average height of these ridges ranges Fig. 6. 4D Wheeler diagram for the Pliocene interval of the study area. The three dimensional space (X, Y, and RGT) is filled with information from the 4th dimension, systems tracts thickness in this case. The bottom slice is a colour-blended spectral decomposition attribute slice for a particular HorizonCube event (MFS, maximum flooding surface). Along the surface, several geomorphological features are identifiable NE to SW direction flowing deep water channels (blue arrows) and NW SE oriented elongated features that are interpreted as sand ridges, which are analogous to present day North Sea sand ridges (for details, refer to Walgreen et al. 2002).

9 F. QAYYUM ET AL. from 10 to 15 m; they contain porous sands sealed by shales underlying the MFS of Sequence 1. Similar features contain shallow gas in the neighbouring blocks (e.g. Block A; see also Stuart & Huuse 2012). The FSST of Sequence 1 forms a detached lower shoreface facies belt prograding into the basin. It contains porous sands (15% or higher) that are sealed by interbedded shales of the same systems tract. Thus, it forms a potential stratigraphic trap in the middle of the study area. The marked stratigraphic hiatus (Fig. 6) underlain by a salt dome (the dome in Fig. 2a) is referred to as a fault leak trap in the classification of Connolly et al. (2008), allowing secondary migration into the Tertiary system. Such a migration has also filled some parts of the FSST with gas, and the sand bodies appear as localized bright anomalies in the F3 Block. Discussion 4D representation of a stratigraphic unit Advances in seismic technology have helped the construction of Wheeler diagrams in three dimensions, thus allowing depositional systems to be studied in both the space and geological time domains. A semi-automated thickness volume that can be used as a colour-coded overlay in the Wheeler diagram is the systems tracts attribute created after sequence stratigraphic interpretation. Such an attribute overlay in the 3D Wheeler domain allows for the study of the four dimensions of the stratigraphic unit. Extending to other data sets While the proposed method was established specifically for seismic data, the underlying concept may also be extended to data sets from, for example, outcrop or well cross-sections. One may colour code the thickness of a sequence stratigraphic unit along the time lines defined on such data sets. Outcrop data, however, are at best pseudo-threedimensional, and it is only with truly 3D data sets that the full advantages of the method can be realised. Meeting different interpretation objectives It may also be noted that the stratal thicknesses from seismic data can be computed in various ways for different interpretation objectives. For example, seismic frequency contents are considered to contain bed thickness information. The frequency contents are decomposed into frequency volumes using a spectral decomposition approach and are interpreted to compute net thickness from strata (Partyka et al. 1999). The 4D Wheeler diagram of F3 Block (Fig. 6, horizontal slice) shows a horizontal slice a horizon slice. It contains three isofrequency responses low, mid, and high that are co-rendered using an RGB (Red ¼ 20 Hz, Green ¼ 40 Hz, and Blue ¼ 60 Hz) colour blending technique. Red regions are interpreted as thicker regions compared to the blue regions (thin/tuning effects). Not only are there relative thickness variations, but the geomorphological features are also evident around the MFS of Sequence 1. This suggests that the thickness attribute is useful for highlighting geomorphological features that can then be interpreted within the same spatiotemporal context provided by the Wheeler diagram. Both stratigraphically and from an exploration point of view, the missing time associated with a surface is also of significant interest, and this information can be derived (immediately in relative terms) from the 3D Wheeler truncated Horizon- Cube. A translation from relative time to absolute time will then be necessary using well data, as discussed by Hull & Griffiths (2002) and Nordlund & Griffiths (1993). Limitations of the 4D Wheeler diagram As well as the merits of 4D Wheeler diagrams, there are also interpretation limitations that should be addressed. The fundamental concept of Vail s seismic sequence stratigraphy method is that seismic reflectors follow geological time lines and this has been corroborated by other workers. For example, Eberli (2000), who studied the Bahama Banks, confirmed that seismic reflectors indeed follow geological time lines in this case. However, the assertion is not always valid (e.g. in the case of fluid-related seismic anomalies) and a seismic sequence stratigraphic framework is also necessarily affected by seismic resolution. A general rule of thumb is that the deeper the interpretations are, the lower the sequence stratigraphic resolution. For instance, first order depositional sequences interpreted in a deeply buried interval (e.g. at 5 km depth) could be equivalent to third order sequences interpreted at shallower depths in the same data set. The stratal boundaries in such cases cannot be precise and therefore may not correspond to the actual time lines defined by subsurface geological mapping. The tracking algorithm used to generate the example discussed in this article does not track reflection amplitudes but instead tracks the dip field. As dip is derived from seismic reflection energy, the tracked horizons do follow seismic reflection patterns. As such and within the limitations described above, the tracked horizons are therefore geological time lines.

10 4D WHEELER DIAGRAMS: CONCEPT AND APPLICATIONS As the methodology is based on seismic data, noise in the seismic data may affect the interpretation and the quality of the Wheeler diagrams. An isochron is a measure of vertical thickness and it remains valid as stratigraphic thickness if the strata are not structurally folded or dipping. If so, the isochron thickness remains apparent and no longer equates to the true stratigraphic thickness. In an ideal case, stratigraphic thickness must be measured by restoring the seismically driven Wheeler volumes. A secondary solution is that a measured isochron volume per unit will require adjustments by applying dip-azimuth corrections. This pitfall should always be considered if quantitative interpretation is meant to be made in a dipping interval on seismic data. On seismic data, the thickness variations could be a cause of frequency/velocity changes. In interpreting seismically derived thickness variations, it is therefore generally preferable to work with seismic data in depth. In the example discussed in this article the vertical axis of the seismic data is in two-way time. In this setting, it is assumed that the lateral velocity heterogeneities are low and that the thickness mainly varies due to depositional processes. Conclusions Stratigraphic units are four dimensional objects, whose dimensions are X, Y, and Z in space, and geological time. However, only three dimensions (X, Y, and Geological Time) are routinely illustrated in the conventional Wheeler diagram. The remaining dimension the thickness of a sequence stratigraphic unit (sequence, systems tract, or parasequence) is represented in our method using colour, as a colour coded line for 2D sections, as an horizon slice, or as a 3D volume. Extending an existing 3D Wheeler diagram to four dimensions thus helps to illustrate the full 4D nature of a stratigraphic unit. Such a diagram can help in explaining the accommodation cycle and preserved thicknesses within a defined spatiotemporal framework. While an automated 3D Wheeler diagram helps in the construction of chronostratigraphic charts in 3D for a particular part of the basin (or an oil/ gas field) and has powerful commercial applications, the extension of Wheeler diagrams to 4D improves our understanding of stratal architecture, as thickness has previously been ignored. Existing Wheeler/chronostratigraphic charts can be revised and enhanced using the proposed method, the result being a better understanding of the subsurface and the establishment of an improved sequence stratigraphic framework. We acknowledge the government of The Netherlands for releasing the data set online under creative common licence ( TNO ( is hereby acknowledged for maintaining and providing the well data and reports to the public. Research support to O. Catuneanu was provided by the University of Alberta and the Natural Sciences and Engineering Research Council of Canada. The proposed method is developed in the SSIS Consortium, which aims to develop and improve a sequence stratigraphic interpretation system (OpendTect SSIS) and the underlying HorizonCube technology. We are also grateful to C. Griffiths (CSIRO Petroleum), A. Davies (Neftex), and the editors of this volume for their constructive thoughts and detailed review. References Catuneanu, O Sequence stratigraphy of clastic systems: concepts, merits, and pitfalls. Journal of African Earth Sciences, 35, Catuneanu, O Principles of Sequence Stratigraphy. Developments in Sedimentology. Elsevier, Amsterdam, The Netherlands, 58. Catuneanu, O.& Zecchin, M High-resolution sequence stratigraphy of clastic shelves II: controls on sequence development. Journal of Marine and Petroleum Geology, 39, Catuneanu, O., Abreu, V.et al Towards the standardization of sequence stratigraphy. 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