Go With The Flow: Part I Palaeotransport Analysis Using Borehole Images

Part I of an article by Lawrence Bourke and Roddy McGarva to accompany Task's PESGB advert for August 2002.

• Introduction
• Definitions
• Palaeotransport analysis applications
• Resolvability of sedimentary structures with borehole images
• Image description of common key sedimentary structures
• Depositional regimes and bedforms
• Bounding surfaces
• General principles of palaeocurrent analysis from borehole images
• Interpretation limitations
• Concluding remarks
• Acknowledgements
• References

If you would like more information, or to comment on this article, please contact Lawrence or Roddyat Task Geoscience.

Introduction

Palaeotransport analysis is a valuable sedimentological tool in reservoir and basin characterisation. Such interpretations can be derived using borehole images. This document provides an introduction to palaeotransport analysis from borehole images and dipmeter data. It details the applications, methodology and pitfalls associated with such an analysis. This will enable the reader to help plan image tool campaigns, comprehend technical approaches and enable them to get the most from an interpretation report.

Intriguingly, there is very little meaningful literature on the effective use of dipmeter data or borehole images for palaeotransport analysis. This article provides a review of the techniques in borehole image palaeotransport analysis and some guidelines to the interpretation of the results.

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Definitions

A number of terms that are used repeatedly within the text are defined in the table below. For further definitions of terms relating to image and dipmeter interpretation please visit our glossary.

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Resolvability of sedimentary structures with borehole images

Although many sedimentary structures have been described (Allen 1984) only a limited number of these are resolvable with borehole images. The factors controlling the ability to resolve a sedimentary structure include image resolution, feature size and contrast, feature geometry and observers experience. In general, conventional resistivity imaging devices provide better feature resolution than acoustic and density imaging devices. The table below ranks the ability of borehole imaging tools to detect and resolve sedimentary structures.

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Image description of common key sedimentary structures

Analysis of borehole images across a wide range of sedimentary environments reveals a consistent set of commonly recognised sedimentary features. These are described and detailed in the table below. Not every feature listed below will be found in every borehole image, but this list provides the basis of any dip picking scheme in developing an analysis of borehole images.

Sedimentary structure	Borehole image description	Comments	Image Example
Bedding	Within sediments, surfaces traceable around the borehole.	Generic term, used mainly with low resolution imaging tools and/or if no lithological data is available from openhole logs.
Shale bedding	Confident bedding features within mudstone dominated lithologies. Bedding surfaces can be traced around the borehole.	Provides confident data for structural dip determination.
Low-angle bedding within sandstones	Laminations and bedding inclined (= 10º) to the palaeohorizontal.	Problematic interpretation of modes.
Planar cross-bedding	Laminations and bedding strongly inclined (10º-35º) to the principal surface of sedimentation following correction for structural dip.	Good quality palaeocurrent indicator.
Trough cross-bedding	Non-planar, convex down, laminations and bedding inclined (10º-35º) to the principal surface of sedimentation following correction for structural dip.	Good quality palaeocurrent indicator if picked along trough axis, otherwise large data set needed to provide confident palaeotransport direction.
Scours	Non-planar incised bedding feature, usually defines base of bedding unit.	Poor palaeotransport indicators, but may be common in some environments with little else to aid interpretation e.g. turbiditic systems.
Bed contacts & master bedding planes	Bedding and lithology contacts.	Through interbedded sandstone/shale sections (e.g. turbiditic environments) these surfaces are often the only sedimentary structures readily visible.
Bounding surfaces	Bed, coset and succession boundaries.	Key to dividing up a thick composite sandstone body.
Slumps	Deformed bedding, oversteep, irregular, disrupted and non-planar. Well imaged slump noses locally common.	Helps in determination of local palaeoslope. Requires careful picking (non-planarity) and needs large dataset for confident analysis.
Slides and oversteep bedding	Visually anomalous with variable dip magnitude and azimuth. Bedding fabrics are generally discontinuous around the borehole. Usually occurs within argillaceous lithologies.	Helps in determination of local palaeoslope. Non-planar surfaces present.

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Depositional regimes and bedforms

It is the study of depositional sedimentary structures that allow us to complete a palaeotransport analysis from borehole images. The sedimentary structures employed are those that arise out of an interaction of flow, transport and bedform (Leeder 1999). The key point is that the geometry and orientation of the sedimentary bedforms allow interpretations to be made about the formative flow. This occurs across a range of scales allowing derivation of spatial information from a ripple to a systems tract scale. Three major geological regimes can be envisaged for the formation of sedimentary bedforms: unidirectional water flows, oscillatory water flows (and combined flows) and atmospheric flows. Each situation has a particular type, scale, range, stacking, orientation and dimension of sedimentary structures. These are detailed in the table below (after Leeder 1999).

The variation in the style of sedimentary structures as a function of depositional regime implies that palaeotransport analysis must go hand-in-hand with an understanding of the depositional conditions, either from previous information or from an environmental interpretation of the succession.

The bulk of palaeotransport information is derived from cross-bedding resulting from the down-flow migration of straight-crested and sinuous crested dune bedforms. Computer modelling, however, has shown (Rubin, 1987) that migrating dunes can display complicated internal structures, and that the orientation of cross-bedded surfaces may not simply relate to directions of bedform migration, a useful warning when interpreting such.

A further key feature in the understanding of sedimentary structures is that bedforms can stack on top of one another in a hierarchical manner, e.g. ripples covering dunes, themselves stacked into cosets and packed into macroforms. The bounding surfaces between the bedforms are an important component of any palaeotransport analysis, and although often the most difficult to identify and classify from borehole images, provide invaluable geometrical data.

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Bounding surfaces

Bounding surfaces enclose a hierarchy of architectural units. Interpretation approaches using the concept of bounding surfaces within cross-bedded successions has a long history applied across a range of depositional environments. Brookfield (1977) applied this approach to aeolian deposits. Its application matured with Allen (1983) and Miall's (1985) approach in fluvial environments in classifying a hierarchy of bounding surfaces. It has been applied to paralic sand bodies (Allen 1980, Harris, 1988) and even turbiditic systems (Ghosh and Lowe, 1995).

As an example, the Miall classification of bounding surfaces in a fluvial context is summarised in the table below. While this bedding hierarchy classification has proven to be very useful in the study of fluviatile sediments, the approach can also be helpful in the classification of many shallow marine and aeolian sequences.

Because of the resolution limitations of borehole image logs it is not always possible to follow these field-based hierarchical classifications in defining all the bounding surfaces within a given section. These limitations particularly apply to acoustic borehole image logs, which lack the resolution and dynamic measurement range of the microresistivity borehole images. However, the choice of drilling mud is usually made for other borehole considerations and the use of oil-based mud can often preclude the use of microresistivity borehole imaging devices. At the other end of the scale, sixth order bounding surfaces may be difficult to identify within a single well bore.

The underlying principle of using a bounding-surface based dip classification is a sound approach to segregating dip types into meaningful directional dip categories in, often complex, sedimentary sequences.

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General principles of palaeocurrent analysis from borehole images

There are a number of key features that make borehole images ideally suited for palaeotransport analysis. These are:

• Features on borehole images can be identified and classified as sedimentary structures.
• Features can be oriented consistently through a succession.
• A sedimentological succession from a borehole is geometrically comparable to an equivalent that can be studied at outcrop and within core.
• Structures can be corrected for structural tilt. This may not be possible if rotation around a vertical axis has occurred.

A generic approach to make an effective interpretation of directional bedding features in a palaeotransport analysis from borehole images will make use of the following, recognition, orientation, correction, knowledgably checking and scrutinising and interpreting data (ROCKS).

Recognition of lithologies and features

Borehole image interpretation involves the integration of openhole log data, together with any hard calibration data such as core or sidewall cores, in the context of a conceptual depositional model. The general methodology is out with the scope of this article and is discussed in Bourke 1991. The interpretation steps involve:

1. Subdivision of the sequence into its constituent lithologies using image and openhole log data.
2. Recognition of the various fluid phases present using openhole logs.
3. Subdivision of lithologies into lithofacies based on the recognition from borehole images of sedimentary structures and textures.
4. Succession of lithofacies may then be broken down to define depositional sequences.
5. The borehole images can be analysed to record the orientation of bedforms, which should be classified into specific bedding categories relevant to their depositional setting. Only when the depositional context of individual beds or depositional units has been defined, should palaeotransport inferences be drawn from the bedding data.

Orientation of features

Modern computer systems allow the visualisation and analysis of oriented borehole images. Such images can be interpreted in the same manner as a core, with the additional benefit of interactive picking of features on-screen leading to a high confidence dip data set. Different features are picked using a classification scheme suitable for the depositional environment, with resulting tadpoles and sine curves usually colour coded by type of sedimentary structure.

Correction for structural tilt

Palaeocurrent analysis requires borehole data to be corrected for any structural dip present. Structural dip is derived from the thickest available mudrocks bracketing the sequences containing the traction current features, with the general assumption that these beds were originally deposited as flat lying sediments. Removal of structural dip is completed using a stereonet and by computation. Originally steep planar bedding features (e.g. cross-bedding) are less affected by errors in structural dip removal than those deposited near horizontally. Incorrect dip removal can introduce large errors in azimuths for those beds deposited at low angle of repose.

Areas under the influence of faulting and fracturing need particular care in their interpretation and in some cases it is advisable to ignore bedding data drawn from strongly deformed strata, e.g. the drag zone of a fault.

Low angle sedimentary dips (typically 5º–15º) are critical to defining the large scale sandbody geometry in reservoirs. Low angle sedimentary dips is associated with laterally extensive morphology, may be expressed over the scale of inter-well distances (00s of metres) rather than within the scale of individual sedimentary units (decimetres).

Knowledgably checking results after dip removal

After dip removal it is necessary to re-examine the dip data set and reclassify overly steep, shallow or incorrectly identified surfaces. Removal of structural dip will inevitably result in changes in relative steepness of bedding surfaces, and some will need reclassification to other types of sedimentary dip. At this point, residual shale bedding (post structural dip correction) should be checked for the accuracy of the dip removal; if correct, residual dip should be 0º.

Scrutinising and interpreting the data

FMI derived description and feature categorised dip data. The complex nature of dip azimuth directions between individual units can only be explained in the context of a braid bar model.

Derived sedimentary dip data should be examined at scales appropriate to the type of depositional setting. This is dependant on the scale of the sediment body being considered (e.g. single fluvial channel fill). Data is scrutinised by way of cores, images, tadpoles, and interactive stereonets; rose plots, azimuth and strike histograms; stick plots and vector azimuth plots. Key features that need to be examined before a interpretation can be made are: number of azimuth modes, mean azimuth directions, dispersion within and between modes, angular relationship between bounding surfaces, relationship of cross-bedding directions and bounding surfaces, strike modes, determination of local palaeoslopes and stacking geometries. Borehole images allow the vertical arrangement of bedding surfaces to be understood, extending the dimension of the usual spatial analytical techniques.

The orientation of bounding surfaces relative to that of contained cross-bedding is assessed to determine the geometry of any macroform present. Particular care needs to be taken with coupling the orientation of low-lying bounding surfaces with any associated cross-bedding. Common practise is to identify the bedding hierarchy; examine how cross-beds relate to their immediate bounding surfaces, and how packets of sediment relate to each other. If bounding surfaces and cross-bedding are not aligned, then simple models of sediment organisation have to be abandoned, and more complex schemes introduced.

Examples of the azimuth modes of some bedding styles in relation to flow direction.

Resulting data can aid in environmental interpretations and much has been made of the different styles of azimuthal modes and dispersion. Selley 1972 summarised some general azimuthal rose diagram for a number of generalised depositional settings. The accompanying figure illustrates that azimuthal patterns can appear quite similar between sedimentary structures in several different depositional settings. However, the actual flow direction or axial orientation of migrating bedforms can be very different in specific depositional settings.

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Concluding remarks

A structured approach to borehole image interpretation is essential to ensure the development of a meaningful palaeotransport interpretation result. The lithologies should be identified using open hole logs and images. Lithofacies can then be characterised from images and reconciled to an appropriate depositional model so that bedding orientations can be measured and assigned to the appropriate dip categories, consistent with the model. Where possible, a hierarchy of sedimentary surfaces should be established, this may require some re iteration of dip classifications. Appropriate structural dip for the study sequence should be established from thick associated mud rocks and rotated from the sedimentary dip dataset. It should now be possible to make meaningful deductions about palaeotransport direction in the context of a depositional model when approached in this manner. This stepped approach to bedform reconstruction is essential to develop a representative dip dataset for palaeotransport analysis.

Significant improvements to 3D seismic resolution have emerged over the last decade together with the development of seismic sedimentological interpretation methodology. These resolution improvements have allowed quite sophisticated sedimentological analysis to be conducted directly from seismic data. However, much of the specific reservoir fabric information concerning; sand body orientation, sand connectivity and preferential fluid pathway recognition together with a plethora of key defining sedimentary information is not resolvable from seismic datasets. Borehole images, together with core data can put the ‘flesh on the bones’ of a reservoir model.

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Acknowledgements

This document has been largely drawn from the collective course materials and presentations on the subject of borehole image interpretation by Stuart Buck, David Lawrence and the authors.

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References - Selected Bibliography

ALLEN, J. R. L. 1980. Sand waves: a model of origin and internal structures. Sedimentary Geology, 26, 281-328.

ALLEN, J. R. L. 1983. Studies in fluviatile sedimentation: bars, bar complexes and sand sheets (low sinuosity braided streams) in the Brownstones (L. Devonian) Welsh Borders. Sedimentary Geology, 33, 237-293.

ALLEN, J. R. L. 1984. Sedimentary Structures Their Character and Physical Basis. Developments in Sedimentology, 50. Elsevier, Amsterdam p593 and 663.

BOURKE L.T. 1991. Sedimentological borehole image analysis in clastic rocks - a systematic approach to interpretation. Geological Applications of Wireline Logs II. (HURST, A., GRIFFITHS, C.M., & WORTHINGTON, P.F. (eds.). Special Publication of the Geological Society of London, 65,

BROOKFIELD, M. E. 1977. The origin of bounding surfaces in ancient aeolian sandstones. Sedimentology, 24, 303-332.

DUECK R. N. and PAAUWE, E. F. W. 1994. The use of borehole imaging techniques in the exploration for stratigraphic traps: an example from the Middle Devonian Gilwood channels in North-Central Alberta. Bulletin Canadian Petroleum Geology, 42, 137-154.

GHOSH, B. AND LOWE, D. R. 1993. The architecture of deep-water channel complexes, Cretaceous Venado Sandstone Member, Sacramento Valley, California. In: Advances in the Sedimentary Geology of the Greater Valley Group, Sacramento Valley, California (GRAHAM, S. A. and LOWE, D. R. (eds.) pp51-65. Field Trip Guide book Sec. Econ. Paleont. Miner, Tulsa, Pacific Section.

HAMPSON, G. J. 2000. Discontinuity surfaces, clinoforms, and facies architecture in a wave-dominated, shoreface-shelf parasequence. Journal of Sedimentary Research, 70, 325-340.

HARRIS P. T. 1988. Large-scale bedfrorms as indicators of mutually evasive sand transport and the sequential filling of wide-mouthed estuaries. Sedimentary Geology, 57, 273-298.

JONES C. M. 1977. The effects of varying discharge regimes on bed form sedimentary structures in modern rivers. Geology, 5, 567-570.

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LEEDER, M. 1999. Sedimentology and Sedimentary Basins. Blackwell, Oxford, p592.

LE ROUX J. P. 1992. Determining the channel sinuosity of ancient fluvial systems from palaeocurrent data. Journal of Sedimentary Petrology, 62, 283-291.

MIALL, A. D. 1974. Paleocurrent analysis of alluvial sediments, a discussion of directional variance and vector magnitude. Journal of Sedimentary Petrology, 44, 1174-1185.

MIALL, A. D. 2000. The Geology of Fluvial Deposits. Springer, Berlin, p582.

MUWAIS, W. and SMITH, D.G. 1990. Types of channel fills interpreted from dipmeter logs in the McMurray Formation, northeast Alberta. Bulletin Canadian Petroleum Geology, 38, 53-63.

READING H. G. (ed.) 1978. Sedimentary Environments: Processes, Facies and Stratigraphy. First edition. Blackwell, Oxford, p557.

READING H. G. (ed.) 1996. Sedimentary Environments: Processes, Facies and Stratigraphy. Third edition. Blackwell, Oxford, p688.

REINECK H. E. and SINGH, I. B. 1980. Depositional Sedimentary Environments. Second edition. Springer-Verlag, Berlin, p549.

RUBIN D. M. 1987. Cross-bedding, Bedforms, and Palaeocurrents. Concepts in Geology, 1. Sec. Econ. Paleont. Miner. Tulsa.

SELLEY R. C. 1967. A classification of palaeocurrent models. Journal of Geology, 76, 99-110.

WILLIAMS P. F. and RUST, B. R. 1969. The sedimentology of a braided river. Journal of Sedimentary Petrology, 39, 649-679.

WOODCOCK N.H. 1976. Ludlow series slumps and turbidites and the form of the Montgomery Trough, Powys, Wales. Proceedings of the Geological Association, 87, 169-182.

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