The concentration rate can be defined too: it is the number of poles inside the centre vs. A value close to zero characterises a poor concentration, or a wide spread population, yet all around a particular orientation. The rate of concentration might be related to the quality of measurements or to another parameter, e. The angular relationships between the two plans can be determined.
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Get Permissions Abstract Electrical borehole image logs have the potential for the direct interpretation of lithofacies types. The challenge is to create a set of diagnostic criteria with which microresistivity variations can be translated to lithofacies characteristics such as bedding sequences, sedimentary structures, and vertical grain-size successions. Behind-outcrop logging is a method to directly validate the borehole images to real rock.
In this chapter, sediments of the Huesca fluvial fan Miocene, Ebro Basin, Spain are used for the validation of behind-outcrop borehole image logs. The logs were recorded in two m ft -deep wells behind cliff-face outcrops.
In addition to the outcrop control, one well was cored. Borehole image log facies were defined from the vertical color succession and the dipmeter pattern and were correlated with the fluvial facies associations in outcrop. Four fluvial facies associations and corresponding borehole image facies were interpreted: 1 meandering rivers, 2 braided rivers, 3 crevasse deltas, and 4 crevasse splays.
Vertical dipmeter successions were analyzed and yielded directional trends of the fluvial channels. Introduction The exploration focus in mature areas is shifting to small satellite fields surrounding the large hydrocarbon reservoirs, with the aim of optimizing ultimate recovery and postponing the end of field life.
Directional well technology is in place to target the secondary reservoirs from the existing infrastructure. The spatial distribution of the secondary reservoirs is commonly complex and requires detailed static sedimentary architecture models to steer directional drilling.
Borehole image logs have the potential for direct interpretation of sedimentary facies type and depositional trends in such settings and thus are a valuable tool to improve static reservoir architecture modeling.
The logs have the necessary high resolution to visualize the detailed permeability variations in the reservoir sandstones.
Interpretation of sedimentary facies type from the image logs is done by validation with real rock. Direct validation to outcrops has the advantage of a better spatial control of lithofacies distribution than core data. Figure 1. View large Download slide Facies distribution of the Huesca fluvial fan. Fluvial reservoirs take an important function as secondary targets. They are typically referred to as labyrinth-type reservoirs Weber and Van Geuns, with low connectivity between individual reservoir units and complex internal permeability heterogeneity.
Examples of such complex fluvial reservoirs are the Ness Formation wedged in between the large Rannoch-Etive and Tarbert reservoirs offshore Norway Ryseth et al. Luthi et al. Donselaar and Schmidt presented diagnostic criteria for fluvial facies interpretation from borehole image logs. The aim of this chapter is to link borehole image log facies to fluvial facies. The aim is reached by the analysis of the fluvial facies characteristics, such as bedding type, spatial arrangement of cross-lamination, lateral accretion dips, and gradual vertical grain-size successions, which have a high potential to show up in the resistivity and dipmeter patterns of the borehole images.
Data and Methods The borehole image logs were recorded in two m ft -deep, 6. Core analysis was used to ensure that the color ranges in the statically normalized image logs could be directly linked to lithology.
For the dynamic normalization of the image logs, a 0. The sine waves of the dynamically normalized image logs were manually traced and used for the measurement of dip directions and angles.
Sedimentological logs were measured along the 50—m —ft -high cliff-face outcrops in a 7-km2 2. The combination of exhumed depositional topography, in plan, outcrop cross sections, and paleoflow data allowed for the distinction of different fluvial lithofacies and the reconstruction of channel trends.
Of each lithofacies unit, the dip angles and directions of sedimentary structures and the vertical grain-size trends were measured in detail for comparison with the image log-derived dipmeter data. The absence of tectonic deformation allowed for the easy correlation of outcrop and well logs Figure 3. Figure 2. View large Download slide Digital elevation model with an orthoimage of the study area.
Meander belts form ridges in the landscape by differential erosion. The general paleocurrent direction in the meander belts is to the west. The Ebro Basin is the autochthonous part of the foreland basin that formed during the collision of the Iberian and European plates Late Cretaceous to Miocene. The SPFB is underlain by south-moving thrust sheets.
Thrust front deformation in the late Eocene and Oligocene Garrido-Megias, created a 15—km 9. The Huesca fan has a radius of more than 60 km 37 mi Hirst and Nichols, ; Nichols and Hirst, The main paleocurrent directions in the fluvial fan change from due south in the proximal part to southwest and even northwest in the distal part Hirst, The Huesca fan ended in a large permanent lake in the center of the Ebro Basin.
From the late Eocene to the late Miocene, the Ebro Basin was an endorheic basin, and the lake level was the regional base level. Subsidence in the central part of the basin resulted in a thick succession of lacustrine siltstone, claystone, and evaporite deposits. Figure 3. View large Download slide Subsurface-to-outcrop correlation panel. See Figures 2 and 5 for the well and outcrop log locations. Thick red lines: truncation surfaces that mark a major basinward facies shift.
Black arrows: fining upward trends. Thick conglomerate sheets formed by confined braided streams occupy the apical zone. South of the apex, braided river deposits formed multistory gravelly sandstone beds that amalgamated to extensive sheets with associated thick, extensive overbank siltstone deposits. Toward the fan fringe, the sandstone-to-siltstone proportion decreases and the sandstone beds are thinner and finer grained. The sandstone beds amalgamated to sheets or formed elongate ribbons Hirst, The fluvial deposits at the fan termination dominantly consist of siltstone and claystone, with interbedded thin, ribbon, and sheet-shaped siltstone and very fine sandstone bodies formed by shallow, narrow meandering streams and associated unconfined terminal lobes Fisher et al.
In time, the Huesca fan expanded to the Ebro Lake and produced a general coarsening-upward sedimentary succession. These affected the overall fan progradation, which resulted in a punctuated progradational succession Figure 3. Figure 4. View large Download slide Outcrop photograph and sedimentological log of a coarse-grained meandering river sandstone body with trough cross-bedding. The sets of trough cross-bedding become smaller toward the top. Thin shale layers mark the upper part of the point bar.
The sandstone body is in stratigraphic interval B and is connected to the lowermost sandstone in outcrop log 2 Figure 3. The sandstone forms part of an elongated belt of amalgamated sandstone bodies see Figure 5. See Figure 3 for the legend from Donselaar and Schmidt, Reprinted with permission from Wiley-Blackwell Publishing Ltd. Fluvial Facies Associations The study area is located approximately 45 km 28 mi from the fan entrance in the Ebro Basin and 15 km 9. The last phase of fan deposition is exposed here as a 50—m —ft -thick siltstone and claystone succession with fluvial sandstone bodies Figure 3.
The siltstone and claystone are ochre to yellow in outcrop. The siltstone is well consolidated in core and outcrop, and may be intensively bioturbated by vertical and horizontal burrows and rootlets. Distinct paleosol layers, which are pinkish to brick red, have a high concentration of rootlets. Paleosol lithology is fine siltstone to claystone. The claystone is structureless or has parallel lamination. The siltstone and claystone are interpreted as floodplain deposits.
Sandstone is a lithic wacke and very fine to very coarse grained but most commonly very fine to medium grained. Granular sandstone occurs locally; the granules are intraformational claystone and siltstone. Quartz and mica grains and quartzite clasts are the most common constituents. The mica causes the relatively high gamma-ray readings of the sandstone Figure 3.
In outcrop, the sandstone beds stand out as steep rock faces. This is because of late-diagenetic superficial calcite cementation.
In the core, the sandstones are commonly poorly consolidated. The sandstone was deposited by a variety of fluvial streams. On the basis of the lithofacies characteristics, four types of fluvial facies associations are distinguished: 1 meandering river sandstone-siltstone, 2 braided river sandstone, 3 crevasse delta sandstone, and 4 crevasse splay sandstone to siltstone.
Meandering River Sandstone-Siltstone Within the meandering river lithofacies association, a coarse-grained and fine-grained facies type is distinguished.
The lower parts of the coarse-grained sandstone bodies consist of very coarse- to fine-grained sandstone and have frequent internal reactivation surfaces with granule and pebble stringers.
The grain size is fining up to fine- or very fine-grained sandstone at the top. By contrast, the bases of the finegrained sandstone bodies consist of a medium-grained to fine-grained sandstone matrix with intraformational siltstone clasts. The upper parts are heterolithic and consist of thin siltstone and very fine-grained sandstone beds.
They have sharp erosional lower surfaces. Large intraformational siltstone and eroded paleosol claystone clasts up to 4 cm 1. Trough cross-bedding set height up to 40 cm [ The upper surface of the sandstone bodies is undulating, with gently inclined surfaces that updip grade into flat surfaces covered with wave ripples.
Siltstone drapes are limited to the uppermost part of the inclined surfaces. Purple mottling and small, vertical burrows are common in the upper part of the sandstone bodies. The average fluvial transport direction was to the southwest Figure 5 but has a wide scatter to the south and northwest. The upward-fining grain-size succession, in combination with the change from trough cross-bedding to inclined accretion surfaces, and the wide scatter of transport directions are the typical expressions of meandering river deposits.
The coarse grain size in combination with the large sets of trough cross-bedding, the gentle dip of the accretion surface, and the relative scarcity of siltstones suggests a high-energy sediment transport in the rivers.
They have sharp erosional, flat, or slightly undulating bases and irregular tops. The lower part of each sandstone body has small sets of trough cross-bedding. The upper parts of the sandstone bodies are heterolithic and consist of steeply inclined beds of siltstone and fine- to very fine-grained sandstone that may reach down to two-thirds of the sandstone body thickness Figure 6.
Dipmeter and Borehole Image Log Technology
Recent breakthroughs in acquisition, tool design, and modeling software provide real-time subsurface images of incredible detail, from the drill bit straight to a workstation. Associated interpretation workflows offer the high level of detail that is needed to make operational decision and to increase the predictability of subsurface models. Many exploration and production companies have acquired a wealth of dipmeter and image log data. The data are readily available and provide, for example the orientation of fractures and fluvial channels in space. Further applications of borehole imaging technology include matrix and fracture characterization, pore-type partitioning, geosteering, and in-situ stress determination.