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April/May 2003 |
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Technical
Focus
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Glossary
Fig.
1. Likelihood of borehole wall features being resolved from borehole images.
Fig.
2. Schematic showing where borehole images straddle the scale gap between
seismic and core resolution.
Fig.
3. Flow diagram summary of the analysis of borehole images after Bourke
(1992).
Fig.
4. Generating a graphical description of borehole images.
Fig.
5. Typical deep marine resedimented facies successions, after Stow (1985).
Fig.
6. Examples of some borehole images of typical deep water fabrics.
Fig.
7. Schematic channel complex end point
Fig.
8. Photo of stacked and amalgamated channel sequence overlying a contorted
infill and by-pass sequence. Ainsa channel complex, photo courtesy of
John Millington, Shell.
Fig.
9. Schematic representation of slope apron deposition, after Surlyk (1978).
Fig.
10. Channel mouth fan, Arro Sandstone, French Pyrenees, after Millington
and Clark (1995).
Fig.
11. Example of a channel mouth fan sequence of the Aberystwyth Grits,
UK. Note the upwards thickening sandstone packages. Photo courtesy of
Dr John Millington, Shell Expro.
Fig.
12. Photo of overbank channel deposits courtesy of Dr John Millington,
Shell Expro. John for scale.
Fig.
13. Photo of basin-filling sand rich sheet system composed of laterally
extensive unconfined sheet sands in the Annot Sandstone, SE France. Photo
courtesy of Dr Vince Hilton. |
Finding
Yourself In Deep Water (Part 1) Introduction Turbiditic systems display considerable variation as a result of eustatic changes, basin tectonics, syn-sedimentary deformation, and the rate, type and source of sediment supply (Reading and Richards, 1994). These variables act together to limit the application of one all-encompassing model that could allow the prediction of reservoir type and architecture. This lack of an all-embracing model requires accurate identification of facies, facies associations, internal stacking patterns and spatial information for the derivation of an appropriate depositional model. Suffice to say that identifying prize targets in deep water is a serious financial commitment, consequently a good understanding of turbiditic sedimentary systems means risk can be reduced. Borehole images play an important role in providing such information, but require experienced interpreters to recognise such details. Borehole images bridge the resolution gap between seismic and core resolution data, providing key sedimentary and sub-seismic structural information along the entire well-path. Wireline images provide high resolution image data capable of supporting detailed sedimentological and structural analysis. LWD (logging while drilling) image acquisition techniques can also provide important data to aid in the interpretation of reservoir characteristics for the development of a representative reservoir model. This article outlines the interpretation reliability of the tools involved, presents a summary of the key interpretation methodologies, and also considers some more advanced applications of data derived from deep water sedimentary systems using borehole image and dipmeter data. The
use of wireline borehole images for the generation of detailed sedimentological
analysis from turbidites is well documented. The highest resolution image
logs for this purpose are the microresistivity image logs, but these logs
only operate in conductive, water based mud systems (WBM). While widely
used in the industry, two key factors have limited the widespread usefulness
of microresistivity images: There are lower resolution alternatives available for OBM borehole imaging, but these have attendant reductions in feature and textural resolution. However, in late 2000, new high resolution microresistivity image logs designed to operate in oil based mud were introduced. As a result, it is now possible to derive a sedimentological analysis, reliably, in all drilling mud environments. Lower vertical resolution image logs are also acquired from some LWD systems. These include resistivity images of the RAB/GVR* and azimuthal density images such as the AND*, ASLD plus† and APLS elite† tools. Such image logs are playing an increasingly key roll in reservoir navigation. As these images are predominantly acquired in highly deviated or horizontal wells, they offer a new analytical perspective on sand bodies that often outweighs their low-resolution reputation. Evolution
of Image Tools and Their Uses Imaging technologies were enhanced by subsequent introduction of advanced microresistivity tools such as the FMI* in the early 1990s, the EMI† and finally the STAR† tool in the late 1990s. These high-tech generation tools offered much greater borehole coverage and hence a sound basis for high resolution sedimentological interpretation from borehole image logs. During this period, numerous papers outlined methodologies for image interpretation (Adams, 1990; Bourke, 1992; Lofts et al., 1997). During the 1980s, there was a dramatic increase in the usage of oil based mud systems and today a high proportion of wells are drilled, world wide, using OBM. Unfortunately, microresistivity images did not provide satisfactory images in non-conductive mud systems and so alternative imaging technologies have been developed. The most successful alternative imaging techniques (to microresistivity imaging tools) are the acoustic imaging tools. Acoustic, or Televiewer, image logs have been around since the 1950s, but it was again during the 1980s when tools such as the CBIL and UBI* were introduced to the marketplace. These tools had adaptations to limit the standoff between the transducer/receiver so that the acoustic path between tool and borehole wall is minimised, thereby limiting the relatively large signal attenuation effect of the mud system itself. Other similar acoustic tools include the CAST† and the AST**. Further technical efforts were made to tackle OBM imaging by adaptations from dipmeter technology. The OBDT* is a four arm, single button-per-arm dipmeter which uses an induction measurement principle. The Hexdip is a six arm, single button-per-arm conventional resistivity tool which has an OBM kit which includes modified buttons in the form of knifes to cut through the non-conductive filter cake. Similar adaptations have been applied by a number of logging companies. These adaptive dipmeter technologies have truly excellent results in some situations. Indeed, oil based mud dipmeters are often chosen in preference to televiewers, particularly where formations are poorly consolidated and sands are prone to thick filter cake development. However, OBM dipmeters can be very sensitive to borehole wall condition, filter cake and hole deviation, and at the end of the day can sometimes provide a very low-density dataset. Last, but not least, is an important group of drill pipe conveyed LWD image logs. Horizontal and highly deviated directional wells offer new perspectives on the well-based study of deep marine sand bodies (McGarva et al., 1999). These well orientations provide bedding information along the length of sand bodies. While traditional wireline image logs are most frequently acquired from substantially vertical or high angle wells and have a sampling bias for the tops, bases, internal lamina and vertical dimensions of sand bodies, horizontal wells offer a sample of the sides, margins and gradational lateral changes of geological bodies. Key interpretative image information includes: features, texture and lithological recognition. Figure 1 summarises some of the, often subtle, sedimentary details which can be recognised from image data. What images lack in feature resolution (compared to core), they gain by providing long sections of accurately oriented data that can be classified to specific bedding types. Significantly, a borehole image provides geological data from the whole reservoir interval in the study well. Borehole images infill a resolution gap between core and seismic scale data (Figure 2). Image interpretation requires an integrated approach to data interpretation involving the full wireline dataset, local core control and the use of local geological knowledge to constrain possible feature interpretations (Bourke, 1992). Sound image quality control procedures have been developed, and are a key element in any image interpretation, so that non-geological artefacts can be identified and eliminated from the interpretation (Bourke, 1989; Lofts and Bourke, 1999). Image
Quality and Interpretability Each type of image measurement approach has its advantages and disadvantages. It is generally true to say that microresistivity images provide the highest resolution measurements (in WBM only). Following on from this statement, acoustic televiewer logs (in perfect conditions) have the next highest resolution, followed by LWD images and, finally, or similarly, dipmeter tools. The key weakness of the acoustic televiewer is its limited dynamic measurement range, which is two orders of magnitude less than that of microresistivity images. The televiewer also generates an image of the borehole wall surface. If this surface has a coating of filter cake, or is heavily tooled, then the borehole wall geology can be entirely absent from the resultant image. The new generation OBM imaging tools (OBMI*, EARTH Imager†) now slot into this hierarchy somewhere between the microresistivity tools and the borehole televiewer logs, however, worldwide such datasets are still limited. All tools work better in some geological or borehole circumstances than others, and in our experience the following key factors affect the usefulness of an image log: •
Vertical and lateral resolution Interpretation
Approach The interpreter would concentrate on the details of the reservoir sequence, to extract accurate, feature-specific dip data. The images, together with the other open hole logs, would be used to establish and define lithologies. Finally, this lithological information would be recorded, together with observed textural and sedimentary features seen on the images to derive a suite of image lithofacies that can be detailed into a high-resolution sedimentological description log (Figure 4). A key element of sedimentary feature recognition is to constrain the possible interpretation of features seen on images by using local geological knowledge. Local knowledge is the understanding of the local geology of an area based on regional and stratigraphic information, geological literature, cores and neighbouring field knowledge. Deep
Water Systems The sandstone, or reservoir flow units, of a deep marine reservoir sequence typically include several of the sandstone facies successions summarised in Figure 5. Such deposits often have subtle sandstone fabrics displaying little internal contrast. Microresistivity images are generally very good at defining even very subtle dish structure fabrics. The acoustic televiewer tends to poorly resolve internal detail in such amalgamated bedding units due to a lack of dynamic measurement range. In high porosity reservoirs, filter cake will further mask internal detail. However, the televiewer will see some internal sand fabrics when there is a strong contrast arising from differential cementation or fabric roughness on the borehole wall. Televiewers will commonly see the nature of bed boundaries and thereby tend to provide clear definition in highly inter-bedded units. Fortunately with the televiewer, where turbidite systems are muddy, bedding contrast can be satisfactory where consolidation is moderate to good. Dipmeters will reveal some of the internal fabric differences between flow units; however LWD images will not always register such details. Examples of some imaged deep marine sedimentary fabrics are presented in Figure 6. These images help to demonstrate that borehole images are generally capable of showing gross, and commonly very fine, sedimentary detail on the borehole wall. Let us now briefly consider five common deep marine depositional environments in the context of an integrated borehole image interpretation. These are: •
Submarine canyon and channels Submarine canyons and channels. Channel sediments range from mudstones to gravel rich sandstones deposited in low to high sinuosity channel systems, respectively. Sand rich channel deposits generally form broad lobate sand-bodies with channelised elements. These channel elements can stack to form channel complexes and, with variable heterogeneity, lead to compartmentalisation across a range of reservoir scales. In addition, our experience of mud-rich channel systems has identified that local channel margin failure, which is commonly difficult to recognise, provides an additional factor in reducing reservoir performance. Along with lithology and facies recognition, borehole images can reveal such localised variations in sand body contiguity, and also isolate thin-bed barriers that are commonly unrecognised by other methods. Sand-rich channel complexes can comprise very thick, amalgamated, stacked sandstone beds. These thick-bedded sandstone units commonly appear quite massive or mottled in borehole images and lack internal sedi-mentary detail. Figure 6 shows typical image examples. Down-cutting and backfilling can occur repeatedly in such channel sequences (Figures 7 and 8). Where visible on images, such sands commonly show oversteepening or slumping that can be a local indicator of palaeoslope. Conglomeratic deposits are generally quite obvious on image logs. Slope aprons. The slope processes of deposition, erosion and bypass dominate slope aprons and, with remobilisation (Figure 9), commonly lead to the development of poorly organised and chaotic facies successions. These sediments accumulate in local failure-generated topographic lows and include thin bedded to massive amalgamated and sheeted turbidites, along with muddy to sandy debris flows, slumps and slides. They are commonly disturbed by later slope failures. Reservoir development risks include high reservoir heterogeneity and limited bed continuity. Resistivity imaging logs are ideal for the analysis of such sections, readily allowing facies identification and adding geometrical understanding to the reservoir architecture and assessment of lateral bed continuity. Some of these fabrics are summarised in Figure 6. Channel mouth fans. Such sediments typically include medium to thin-bedded turbidites consisting of mixed beds of sandstone, siltstone and mudstone organised into lobes and sheets. The proximal portions of these deposits are likely to be associated closely with lower slope processes such as slumping and debris flows (Figures 8 and 12). The deposits of debris flows and slumps are typically thin and may not be resolved by borehole images. However, images and dipmeters are most useful in thin-bedded sequences for identifying the vertical organisation and stacking of lithofacies assemblages. This can be carried out across a range of scales from which sedimentary dip data can be used to interpret accretion directions for individual lobes and sheets. Field studies have shown that the lateral association of lithofacies within channel mouth fans can be more complex and variable than simple models would predict, and that such successions can be strongly affected by small-scale faulting. Channel overbank. Overbank or levee deposits are most thickly developed adjacent to submarine channels. They typically comprise thin-bedded sandstones, siltstones and mudstones deposited by turbidity flows spilling over from the channel, and can grow to form a very significant proportion of a channel system volume (Figure 12). Individual sandstone beds are thin but widespread, and diminish in thickness away from the channel. The reservoir potential of these thins beds is often underestimated. Most borehole image and dipmeter logs are successful at capturing the essential bedding character of overbank deposits, commonly allowing packeting and shingling to be recognised. Conventional openhole logs may not indicate the presence of thin beds, whereas borehole images allow detection to a centimetre scale, or better. Gas charged levees commonly provide striking resistivity images, arising from the conductivity contrast between hydrocarbon-filled thin sandstone beds and interbedded mudstones, allowing ready identification and quantification of extra pay. Basin floor sheets. These successions are sand rich sediments deposited on the basin floor away from the slope (Figure 13). They typically comprise massive and medium grained turbidites that are often amalgamated, forming sheet and mound-shaped sandbodies. Deposition from unconfined flows on the basin floor is an important factor in their development. These sand bodies are very sharp based, and upward termination can be sharp or gradational with upward decaying trends. The upward shaling and fining patterns could be misinterpreted as channels. Basin floor sheets are sometimes overlain by channelised slope successions. It is the close proximity of sheets and channels, with their widely different reservoir geometries, that can lead to difficulties in reservoir description. Criteria derived from borehole images and dipmeters can facilitate discrimination between decaying sheets and slope channels. These criteria include facies identification, bed-stacking trends, deformation fabrics, degree of erosive down cutting, patterns of compactional drape and numerical thinning rates calculated between wells. Conclusions Image
logs will, in most situations, provide key sedimentary detail to enhance
understanding of the reservoir and associated sequences. High-resolution
images are particularly useful as a source for sedimentary descriptive
data from deep marine sedimentary systems. However, a systematic approach
to interpretation is essential. Part II examines sediment dispersal interpretation and thin bed analysis using images. The important contribution of LWD logs in deep water sediment are reviewed and emerging reservoir modelling applications are considered. |
