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.

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

Introduction

Palaeotransport applications
ExplorationDevelopment/Production
  • Sediment dispersal determination.
  • Regional paleoslope.
  • Palaeogeographical reconstructions.
  • Orient sediment geometries and lateral extent.
  • Depositional interpretation.
  • Geometry and inter-relationship of facies and depositional units.
  • Orient fluid flow pathways.
  • Orient flow barriers - reservoir anisotropy.
  • Preferred cementation pathways.
  • Input to reservoir simulations.
  • Bed orientation stats for stochastic modelling.
  • Orient cores for goniometry studies.

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.

 

Sedimentary structureBorehole image description
BedA bed is a unit of strata thicker than 1 cm defined by variations in grain size, grain sorting, lithology, mineralogy, colour, resistivity and sonic contrast that has been deposited under essentially the same conditions.
BeddingInternal structure and fabrics of a bed.
BedformsForms and shapes that develop on beds of sediment during the transport of sediment grains, (Leeder, 1999) and they can range in scale from sub mm to hundreds of metres in thickness. They include constructive and erosional forms.
Bounding surfacesBedding planes that lie between beds, sets, cosets and larger scaled composite sedimentary packets and bodies. They allow sedimentary bodies to be broken down into a hierarchy of units. The nature and orientation of bounding surfaces aid the analysis of successions.
Borehole imageDown hole device that provides fine-detailed maps of a borehole wall.
ChannelAn elongate expression of negative relief produced by confined flows, usually representing a major and long term pathway for sediment transport (after Mutti and Normark, 1987).
ClinoformThe sloping depositional surface of a major morphological feature such as a continental slope, delta front or shoreface; modern sandy shorefaces display concave-up seaward dipping profiles (from Hampson, 2000).
CosetsVertically stacked and unbroken sequence of sets of similar size, shape, attitude and lithology (after Allen 1984).
Cross-bedding (stratification)Single layer comprising internal laminations inclined to the principal surface of sedimentation (Reineck and Singh, 1980).
LaminationStrata thinner than 1 cm defined by variations in grain size, grain sorting, lithology, mineralogy or colour, resistivity and sonic contrast that has been deposited under essentially the same conditions.
MacroformArchitectural element related to long-term morphological product such as point bars, sand flats and draas. Often employed with reference to fluvial systems.
PalaeoslopeLocal to large scale
Palaeotransport analysisPalaeotransport (also referred to as either; sediment dispersal or palaeocurrent analysis) is the measurement and interpretation of sedimentary features, in particular bedforms, to reconstruct the transport direction of the depositing sedimentary system.
Sedimentary structuresRicci Lucchi (1995) defines sedimentary structures as "... objects and forms that are produced by sedimentary processes and are preserved in rocks." It is the study of particular sedimentary structures that allow us to complete a palaeotransport analysis.
SetSingle lithologically consistent cross-stratified sediment packet bounded by bedding planes.
Trough cross-beddingLarge bedform arising from the migration of three-dimensional (sinuous crested) dune that generates convex down non-planar troughs.

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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.

 

Structures usually resolved on imagesStructures sometimes resolved on images Structures rarely resolved on images

Bedding
Cross-bedding
Lamination
Scours
Bounding surfaces
Slumps and slides
Oversteep bedding
Bed contacts
Desiccation cracks (large)
Injected sandstones
Heterolithic bedding
Mudstone clasts
Large sandstone clasts
Current drag on cross-bedding
Carbonate concretions
Herring bone cross-stratification
Lags

Ripples, climbing ripple lamination
Dewatering fabrics, e.g. pipes
Dish and pillars
Imbrication
Individual fossils (large)
Bioturbation fabric
Stratification in rudites
Evaporites, (not well imaged with resistivity devices)
Load casts
Convolute laminations
Root traces
Sand volcanoes
Hummocky cross-stratification
Antidunes
Mud drapes
Tepee structures
Grading
Slide scars

Flutes
Gutter marks
Colour mottling
Grain size variations
Sole marks
Tool marks
Obstacle marks
Grooves
Trace fossils
Adhesion ripples and warts
Pitted surfaces, rain prints
Rain drop impressions
Primary current lineation
Interference ripples
Dropstones
Rill marks
Borings
Geopetal fabrics

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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. bedding
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. low angle bedding
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. planar cross-bedding
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. trough cross-bedding
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. scours
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. bed contacts
Bounding surfaces Bed, coset and succession boundaries. Key to dividing up a thick composite sandstone body. bounding surfaces
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. slumps
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. slides and oversteep bedding

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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).

 

Depositional regime Typical bedforms Comments
Unidirectional water flows Current ripples
Plane beds
Linear (2D) dunes
Lunate/linguoid (3D) dunes
Compound bedforms
Ripples are not usually recognisable from images. The cross-bedding of dunes is the dominant useful oriented feature from such regimes. Can occur in a range of depositional environments.
Oscillatory water flows (and combined flows) Wave ripples
Combined wave and current flows, including HCS
Tidal bedforms including channels, dunes, bars & sand waves.
Ripples are not usually recognisable from images. HCS has been recognised and interpreted from images. The cross-bedding of dunes is the dominant useful oriented feature from such regimes. Such flows usually restricted to paralic, shallow marine and lake systems.
Atmospheric flows Wind ripples
Planar sheetsDunes, in particular transverse (2D & 3D) and longitudinal types
Parabolic dunes
Draas (composite bedforms)
Highly variable bedform morphologies usually present. Two end members of transverse and longitudinal bedforms occur along with star dunes. Bedforms readily stack on top of each other at range of scales. Cross-stratification may comprise grain fall, grain flow and ripple lamination.

 

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.

 

Classification of bounding surfaces
1st orderCross bedding set bounding surfaces, with little or no erosion apparent between set boundaries.
2nd orderCoset bounding surfaces, which indicate changes in flow conditions or change in flow direction, but no time break.
3rd orderThese surfaces indicate stage changes, or reactivation, but no significant change in sedimentary style.
4th orderThese represent the upper bounding surfaces of macroforms and may be recognised by changes in the lithofacies assemblages above and below.
5th orderThose bounding major sandsheets, such as broad channels and channel-fill complexes. They may have basal lags.
6th orderSixth order surfaces define groups of channels or palaeovalleys.

 

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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  • Recognition
  • Orientation
  • Correction
  • Knowledgably checking
  • Scrutinising and interpreting

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).

Interpretation limitations
  • Limitations of borehole imaging tools.
  • The scale of the features resolved and whether they can be adequately measured at the borehole scale.
  • Whether directionally significant bedding elements are present in the study sequence.
  • Collection and classification of a representative sample of the key directional features.
  • Understanding bedding surfaces in the context of a meaningful classification scheme.
  • Interpretation in the context of a relevant depositional model.
  • Understanding the present structural dip and whether basin depositional and structural development have been linked.
  • Adequate recognition of depositional dip in low angle of low depositional slope sediments.

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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Interpretation checklist
  • Perform feature recognition from images.
  • Measure accretionary surface dips and low angle surfaces.
  • Reconcile these dips in the context of a hierarchical bounding surface classification, where appropriate.
  • Relate these surfaces and features to a depositional model.
  • Perform structural dip recognition and removal.
  • Reconcile this dip data to the depositional model.
  • Make palaeotransport inferences based on the above.

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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In next month's issue
  • The basic tools of palaeotransport analysis: stereoplot, rose diagram, azimuth vector plot etc.
  • Palaeotransport characteristics for specific depositional environments.
  • Techniques for identifying low angle sediments.

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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