Interpretation of Channelized Architecture Using Three Dimensional PHOTO REALISTIC Models

Interpretation of channelized architecture using three dimensional PHOTO REALISTIC models, PENNSYLVANIAN DEEPWATER DEPOSITS AT BIG ROCK QUARRY, ARKANSAS

Abstract

Mapping the geological details and interpreting 3three-Ddimensional geometryies in a highly heterogeneous outcrop such as the exposure at Big Rock Quarry has been a continuous challenge especially due to the fact that at this specific location location largehigh vertical cliffs the steepness of the cliff faces makes access to most of the rocks difficult for direct geological observations and sampling. Previous interpretations of facies architecture were derived from gamma-ray profiles, a core and observations and measurements made on two-dimensional photomosaics. In this study field observations integrated with a three-dimensional photorealistic model of the outcrop with assigned lithologies effectively helped in reconstruction of submarine channel architecture.This paper represents the first attempt of 3three-Ddimensional interpretation of the geometry and facies pattern of the Jackfork nested-channel complex deposited at the base-of-slope.

Examination of the three-dimensional photo realistic model of the outcrop with assigned lithologies allowed extraction of accurate 3-D qualitative (lithology, contacts), as well as, quantitative (bed, channel width and thicknessdimensions) geometric information. This facilitated interpretation and reconstruction of the submarine channel complex architecture making possible correlations of strata exposed on the two distinct sides of the quarry.

Three dimensional photorealistic mapping techniques have been recently developed as effective tools for detailed outcrop studies, but still their potential has not been fully exploited. This study makes effective use of the quantitative information incorporated in three-dimensional virtual outcrops and provides new tools for geologic mapping and interpretation.

The three-dimensional model of sedimentary bodies allowed capturing the three dimensional spatial distribution of lithological units, which is fundamentally important for understanding the internal architecture of erosional and depositional features, in this case channelized features. Most of the exposed vertically and laterally stacked channels channels are large, have aggradational with well defined axial regions composed of high relief basal erosional surfaces overlain by well developed intraformationalmatrix-supported conglomerate/ breccia which grades upward into amalgamated sandstones. or thin-bedded sandstone interlayered with shale. The thickness of the sandstone decreases toward the southeastern end of the quarry where more shale is present. The channel infill here consists of thin-bedded sandstones interlayered with shale which overlain the breccia. The upper part of the quarry is made up of smaller, lateral migrating channels.

Significant channel width/thickness variation can be recognized at outcrop scale. The 348 identified channels are characterized by a relatively low aspect ratio (width/thickness ratio runs from 4:1 to 3239:1) with channel dimensions ranging from 3025 m to 265314 m wide and 2 m to 2424 m deep.

Compared to previous attempts of reconstruction our 3-D virtual model is more realistic and because the model has accurate real dimensions it can be used to calibrate simulation of processes in deep water environments. The three-dimensional model of sedimentary bodies allowed capturing the three dimensional spatial distribution of lithological units, which is fundamentally important for understanding the internal architecture of erosional and depositional features, in this case channelized features. Compared to previous reconstructions our 3-D virtual model is more realistic and because the model has accurate real dimensions it can be used to calibrate simulation of processes in deep water environments.

INTRODUCTION

Understanding the depositional processes and lateral geometry of deep-water base-of-the slope systems is important for predicting the extent and internal architecture of these reservoirs (Bouma et al., 1995; Slatt et al., 2000).

Adequately documentation of reservoir properties and flow behavior at reservoir scale Deep-sea sediments have received considerable interest both for research purposes and economic reasons due to large hydrocarbon accumulations associated with turbidite deposits (Bouma, 2000).

Ideally, characterization of hydrocarbon reservoirs and flow behavior requiress information about heterogeneity at a submetersub meter scale in three dimensions. However, it is difficult to include realistic distributions of shale and sandstone bodies in reservoir models because of the wide spacing of wells and limited vertical resolution of seismic surveys. One solution is to characterize shale-sandstone distribution using data from large, continuous three-dimensional outcrops outcrop exposures (Coleman et al., 2000; Slatt, 2000).). Accurate mapping of the architecture of these reservoirs requires interpretation of erosional and depositional geometries preserved in outcrops.

Channelized facies of the Pennsylvanian Jackfork sandstone exposed at Big Rock Quarry in Arkansas have long been controversial. Originally thought to be fluvial in origin (Taff, 1902) they are now widely agreed to be submarine, although there is still debate about what kind of processes generated them (debris flow, slumps, high vs. low density turbidity flows) and what submarine settings they should be assigned to (upper canyon slope vs. base-of-slope). Also the geometry of these deposits was difficult to interpret due to high variability of bed thicknesses and lateral discontinuities in three dimensions.

Previous

Excellent cliff faces exposed at Big Rock Quarry were interpreted as proximal deep-water fans within slope channels canyons (Jordan et al., 1993) or at the base-of-slope (Bouma and Cook, 1994), but geometry of these deposits was difficult to be interpreted due to highly variability of bed thicknesses and lateral discontinuities in three dimensions.

The upper Jackfork strata that crop out in the main face of the quarry have previously been the focus of several studies into facies architecture and individual bed geometries. Ffacies interpretation was derived from gamma-ray profiles (Jordan et al., 1993), a core drilled about 15 m behind the outcrop face (Link and Stone, 1986) and from direct geological observations where the outcrop was easily accessible. Measurements on large photographic prints (Cook, 1993; Bouma and Cook, 1994) facilitated statistical characterization of width and thickness dimensions, as well as, reconstruction of facies architecture. The three loggamma profiless taken at this site illustrated potential problems in well log correlations of laterally discontinuous strata. When compareding the information from well logs and cores it was obvious that some of the thinner beds could not be identified on the gamma-ray profiles since they are often beneathlow the resolution of the logging tools. Cores provide the necessary resolution for distinguishing these features, but unfortunately there is only one core taken at this site. Also two-dimensional photomosaic interpretations are not effective tools in correlation of strata with a complicated three-dimensional geometry.

Understanding of turbiditic systems has improved with the use of high-resolution shallow 2-D and 3-D seismic data, but the detailed internal architecture of these reservoirs below seismic resolution remains uncertain. Seismic modeling of Big Rock Quarry outcrop indicated that internal sand-body architecture and geometry may not be clearly imageable with conventional seismic profiling (Coleman et al., 2000)) mostly due to the cemented nature of the sediments, providing little or no acoustic impedance contrast between sedimentary intervals.

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Due to its horse shoe shape Big Rock Quarry has a horse shoe shape and iis a good candidate for three-dimensional studies imaging and interpretations. This study uses 3-D photorealistic methodology to understand the three-dimensional architecture of the channelized features. Facies architecture and dimensions of channel sandstones are highly anisotropic (width/length/depth). As a consequence it is critical to know how outcrops are oriented within depositional strike/dip to make meaningful quantitative analysis of sand body dimensions. Conventional outcrop analog are hampered by the 2-D nature of data sets and the inability to overcome or moreover to take advantage of parallax. Compared to previous two-dimensional outcrop photomosaics, the three-dimensional virtual model adds more valuable information for outcrop interpretation (Bhattacharya et al., 2002; Aiken et al., 2004).

In this study we use innovative 3-D methods to generate accurate dimensional data that represent true dip/strike orientation. These dimensions are compared to those derived from more traditional 2-D methods that have been used to build reservoir analog data bases. The study of bed thickness distributions of various facies can help to identify important reservoir facies within the fill of stacked channels at the base of slope where massive, thick sandstone is interbedded with thinner shales.Using a combination of real-time kinematic global positioning system (RTK-GPS), and laser scanners we were able to capture 3-D terrain data of the outcrop in global coordinates with centimeter accuracy. Oblique close-in photography acquired from the ground was integrated with terrain data and converted into a 3-D digital photorealistic model of the outcrop. Examination of the virtual model of the outcrop allowed extraction of accurate 3-D qualitative (lithology, contacts), as well as, quantitative (bed width and thickness) geometric information. This facilitated interpretation and reconstruction of bed architecture making possible further correlations of strata exposed on distinct sides of the quarry. Our interpretation is based on mapping of three-dimesional facies distributions and collection of new paleocurrent data. Compared to previous two-dimensional outcrop photomosaics, the three-dimensional virtual model adds more valuable information for outcrop interpretation (Bhattacharya et al., 2002).

GEOLOGIC SETTING

Big Rock Quarry is located in the southeastern part of the Ouachita Mountains along the north bank of the Arkansas River in North Little Rock, Arkansas (Fig. 1). The cliff faces of the quarry expose a twohree-dimensional view of the lower part of the upper Jackfork Group (Jordan et al., 1993). The exposure is oriented at different angleshas a horse-shoe shape and is up to 60 m high and almost 1000 1250 m long.

In study area Jackfork Group is divided in to the lower Jackfork (Irons Fork Mountain Fm.) and upper Jackfork (Brushy Knob Fm.). The Jackfork Group was dated as Pennsylvanian (Morrowan) (Fig. 2) on the base of correlative units on the shelf and is Pennsylvanian (Morrowan) in age (Fig. 2). and it represents the low stand system tract, time equivalent of a major unconformity on the shelf (Coleman, 2000) (Fig. 2).

Sediments that crop out at this quarry were interpreted as fan channel deposits (Stone and McFarland, 1981), stacked channelized packages of an inner (upper) fan valley (Moiola and Shanmungan, 1983), channel-fill and levee deposits in a submarine canyon or upper part of a submarine fan channel system (Link and Stone, 1986; Link and Roberts, 1986) and slope canyon fill generated by retrogressive slope failure (Jordan et al., 1993; Slatt et al., 1997).

Bouma and Cook, 1994 and Bouma et al., 1995 considered these sediments as part of a submarine channel complex likely deposited at the base of slope based on the limited development of levees and overflow deposits and on the stacked pattern of the channels. The presence in the southeastern part of the quarry of small, scoured depressions eroded in sandstone bodies and filled with mudstone and sandstone-clast debris flows or drapes of laminated mudstone suggest that scour and successive slope failures have cut out many originally more continuous units (Jordan et al., 1993).

Fig.1 Location map. Big Rock Quarry Outcrop Belt

T Coleman et al. (2000)his estimated that this channel complex is at least 9.6 km wide and 16 to 24 km long. and Iit appears to pinch out about 4 km north of the quarry. At least 14 channels are exposed here, some with relief exceeding 3.6 m. Most of the channels have flow indicators oriented west-southwest (Coleman et al., 2000).

Shanmungan and Moiola, 1997 suggested that Jackfork sandstones were not deposited by high-density turbidity currents and are predominantly of sandy debris flow origin because traction-generated sedimentary structures are not present and the matrix content in sandstone is high. However this view received much critique (Slatt et al., 1997; Lowe, 1997; Coleman ColemanJr., 1997; Bouma et al., 1997; D’Agostino and Jordan, 1997). The major fill of the base-of-slope channels consist of somewhat clayey sandstone layers because the high-density currents that transported this fill may have not been able to move their very fine grained material in suspension toward their upper part and tail (Bouma et al., 20020).

Facies architecture of the Jackfork channel complex at Big Rock Quarry

The channel complex infill consists mostly of amalgamated sand-rich individual channels. These massive sandstones comprise a significant proportion of the total channel fill and their overall thickness decreases in an easterly direction where more shale is present. This and the fact that the basal upper Jackfork sandstone is thicker at Big Rock than in exposures in the northwest and southeast suggest proximity to the axis of the sandy submarine canyon fill (Jordan et al., 1993).

Jackfork channel complex consist of 38 erosive-based channels that stack up in a nested pattern. The lower two thirds of the quarry are made up of large, sandstone-rich aggradational channels while the upper part consists mostly of small, laterally migrating channels. The larger, aggradational channels have well defined axial regions and display massive fill with tabular geometry in the central part of the exposure while toward the southeastern end the channels display a layered fill with convergent geometry. Mud clasts dispersed in a sandy matrix are common in the channel axis. Sometimes the basal erosional surface is directly overlain by highly amalgamated sandstones.

Smaller channels dominate the upper part of the quarry. Some of them display lateral accretion surfaces. Sand lenses at the channel margin incline in the direction of channel migration; most of the time northward. Lateral accretion packages are characterized by interbedded high-concentration turbidites deposited by suspension (massive sandstones) and mud-clast conglomerates deposited by traction as bed load (Abreu et al., 2003). The formation of the sinuous channels at the top of the aggradational channels can be explained by a reduction in the volume of the turbidity currents that preceded the overall abandonment of this part of the turbidite system.

In the upper part of the quarry there are also present some intervals with thin bedded sandstone and shale (mud-clast conglomerates) which might represent remnants of laterally equivalent levees of the channels. The levees are absent at the base of the channel complex.

Main ffacies associations of the Jackfork channel complex at Big Rock Quarry

Jackfork channel complex consist of 34 erosive-based channels that stack up in a nested pattern. The lower two thirds of the quarry are made up of sandstone-rich aggradational channels while the upper one third consists of laterally migrating channels. The formation of the sinuous channels at the top of the aggradational channels can be explained by a reduction in the volume of the turbidity currents that preceded the overall abandonment of this part of the turbidite system. Levees are absent at the base of the channel complex, but remnants of bedded levee-upper overbank deposits can be found up in the section. However the lateral extension of the levees is limited. They only formed when the canyon morphology was almost filled.