David Hodgetts

Virtual outcrop models of petroleum reservoir analogues: A review of the current state-of-the-art

A subsurface reservoir model is a computer based representation of petrophysical parameters such a porosity, permeability, fluid saturation, etc. Given that direct measurement of these parameters is limited to a few wells it is necessary to extrapolate their distribution. As geology is a first order control on petrophysics, it follows that an understanding of facies and their distribution is central to predicting reservoir quality and architecture. The majority of reservoir modelling systems used for the subsurface are based on correlation of seismically-derived surfaces to define reservoir zones. Well data are then used to define further, sub-seismic scale horizons and determine the zone properties which are represented in grid cells. Understanding the distribution of both sub-seismic surfaces and potential heterogeneous geology between them remains a significant challenge. Furthermore as the typical grid cell size is c. 50-200 m2 it is challenging to incorporate small-scale heterogeneities. It is critical, therefore, to use realistic values for both key stratigraphic horizons and internal facies distributions. Depositional facies is a fundamental control on petrophysics. However, facies scale heterogeneities are not resolvable using current seismic methods, and well data provide little or no data on 3D geometries beyond the well bore. Studies of modern sedimentary events can give some indication of the link between depositional processes and facies distribution (e.g., Kenyon et al., 1995); however preserved depositional architecture is also strongly controlled by changes in accommodation through time (Jervey, 1988). Laboratory-based experiments (e.g., Kneller & Buckee, 2000) and process-based modelling (e.g. Aigner et al., 1989; Peakall et al., 2000) further illustrate the link between depositional mechanism and facies architecture. However, such models are typically on a scale that is far smaller than the typical field and are more applicable to upscaling studies (Nordhal et al., 2005; Ringrose et al., 2005). Outcrop studies have long been employed as a mechanism of studying analogues and understanding petroleum fields (Collinson, 1970; Glennie, 1970; Breed & Grow, 1979). Once the type of depositional system and the accommodation history of a hydrocarbon field are derived from subsurface data, appropriate outcrop analogue(s) can then be identified (e.g. Alexander, 1993). Suitable analogues are those that are geologically comparable to the system that is being studied and also have excellent 3D outcrop exposure over an area that is large enough to capture the scale of heterogeneity required (Clark & Pickering, 1996). Outcrop analogue studies are thus a key way of improving understanding of reservoir facies architecture, geometry, and facies distributions. Outcrop analogue studies have been undertaken both qualitatively and more recently quantitatively. Traditional quantitative studies (e.g., Dreyer et al., 1993; Chapin et al., 1994; Bryant & Flint, 1993; Clark & Pickering, 1996; Reynolds, 1999) have been focused on the collection of outcrop data to populate inter-well reservoir model areas by stochastic, object-based methods (Floris & Peersmann, 2002). However, it can be difficult to extract usable data from traditional outcrop studies, especially when it needs to be integrated with petroleum engineering databases or to be visualized in 3D. Furthermore, outcrops which represent a topographic cut through solid geology are 2D and while rare examples show multiple sections through the solid geology with different orientations, geological expertise is still required to fully understand and interpret the 3D nature of the bodies. Such work may also need geostatistical data manipulation to overcome outcrop orientation and size issues (Geehan & Underwood, 1993; Vissa & Chessa, 2000) but ideally the data should be reconstructed in 3D. Accurate 3D reconstruction is the only way that parameters such as channel sinuosity, connectivity, and continuity of target sandbodies in 3D may be defined. Such parameters are a key control on hydrocarbon production, including sweep efficiency (Pringle et al., 2004a; Larue & Friedmann, 2005). Software for representing geology in 3D is routinely used to model subsurface reservoirs. This paper will show how recent digital data capture technique advances aids the interpreting reservoir geologist by obtaining accurate and quantitative outcrop analogue datasets to aid and perhaps modify his reservoir model.

Estimating mass properties of dinosaurs using laser imaging and 3D computer modelling.

Body mass reconstructions of extinct vertebrates are most robust when complete to near-complete skeletons allow the reconstruction of either physical or digital models. Digital models are most efficient in terms of time and cost, and provide the facility to infinitely modify model properties non-destructively, such that sensitivity analyses can be conducted to quantify the effect of the many unknown parameters involved in reconstructions of extinct animals. In this study we use laser scanning (LiDAR) and computer modelling methods to create a range of 3D mass models of five specimens of non-avian dinosaur; two near-complete specimens of Tyrannosaurus rex, the most complete specimens of Acrocanthosaurus atokensis and Strutiomimum sedens, and a near-complete skeleton of a sub-adult Edmontosaurus annectens. LiDAR scanning allows a full mounted skeleton to be imaged resulting in a detailed 3D model in which each bone retains its spatial position and articulation. This provides a high resolution skeletal framework around which the body cavity and internal organs such as lungs and air sacs can be reconstructed. This has allowed calculation of body segment masses, centres of mass and moments or inertia for each animal. However, any soft tissue reconstruction of an extinct taxon inevitably represents a best estimate model with an unknown level of accuracy. We have therefore conducted an extensive sensitivity analysis in which the volumes of body segments and respiratory organs were varied in an attempt to constrain the likely maximum plausible range of mass parameters for each animal. Our results provide wide ranges in actual mass and inertial values, emphasizing the high level of uncertainty inevitable in such reconstructions. However, our sensitivity analysis consistently places the centre of mass well below and in front of hip joint in each animal, regardless of the chosen combination of body and respiratory structure volumes. These results emphasize that future biomechanical assessments of extinct taxa should be preceded by a detailed investigation of the plausible range of mass properties, in which sensitivity analyses are used to identify a suite of possible values to be tested as inputs in analytical models.

High-resolution LiDAR and photogrammetric survey of the Fumanya dinosaur tracksites (Catalonia): implications for the conservation and interpretation of geological heritage sites

Increasing political and social awareness of the importance of protecting the geological heritage is compelling geoscientists to consider new methods for reconciling conservation and exploration of their research sites. Terrestrial Light Detection And Range (LiDAR) imaging is an accurate method of collecting 3D spatial data that has so far been under-utilized in the geological sciences. This aim of this paper is to assess the value of integrated LiDAR and photogrammetric imaging as a tool for synchronizing scientific exploration with conservation of geological heritage sites.   Fumanya (Catalonia) is one of the most important Cretaceous tracksites in Europe, but the nature of exposure of the track-bearing surface has hindered quantitative documentation of the ichnites. Using integrated Light Detection And Range (LiDAR) imaging and photogrammetry it has been possible to construct high-resolution Digital Outcrop Models (DOM) of the tracksites. Photo-textured DOMs are a powerful visualization tool and function as fully 3D interactive databases that preserve information about the site that would otherwise be lost to erosion. LiDAR-derived DOMs have the potential to contribute profoundly to future geoconservation projects, particularly as a tool for documenting and monitoring heritage sites and promoting education and tourism. LiDAR scanning also provides sufficient resolution to perform robust quantitative analysis of dinosaur tracks.

Three-dimensional geological models from outcrop data using digital data collection techniques: an example from the Tanqua Karoo depocentre, South Africa

Abstract Recent technological advances have made the collection of digital geological data from outcrops a realistic and efficient proposition. The world-class exposures of Permian basin-floor turbidite fans of the Tanqua depocentre, Karoo Basin, South Africa have been the focus of one such study. These outcrops are faulted at a subseismic scale (displacements of up to 40 m), with continuous exposures of up to 40 km in depositional dip and 20 km strike directions. Digital data collection has been undertaken using a variety of methods: differential global-positioning systems (DGPS) mapping, surveying using laser total station and laser rangefinders, ground- and helicopter-based digital photography and photogrammetry, and digital sedimentary outcrop logging as well as geophysical data from boreholes. These data have then been integrated into several 3-D geological models of the study area, built using a subsurface reservoir-modelling system. The integrated dataset provides insights into the stratigraphic evolution of a deep-water fan complex by allowing true 3-D analysis and interpretation of data collected in the field. The improved understanding of these deep-water fan systems will improve existing models of offshore analogues by enhancing understanding of geometries and trends not resolvable from existing offshore data and by identifying potential problematic areas for fluid flow. Initial results from the application of this approach have been successfully applied to the conditioning of stochastic geological models of a subsurface deep-water reservoir from the North Sea.

Integration of digital outcrop models (DOMs) and high resolution sedimentology – workflow and implications for geological modelling: Oukaimeden Sandstone Formation, High Atlas (Morocco)

ABSTRACT Outcrop analogue studies provide key information for reservoir modelling which is difficult to obtain from traditional subsurface datasets (i.e. seismic data, well data). Terrestrial laser scanners or LiDAR (light detection and ranging), combined with digital photography, provide a new technique to create high resolution 3D digital outcrop models (DOMs). These DOMs generate exhaustive information which is used to build more realistic three-dimensional facies-based geocellular models and populate the different model zones. This paper documents the use of an extensive dataset, which combines high resolution traditional field data and DOMs. We provide an accurate description of the workflow followed in the geocellular modelling of a fluvial-dominated continental formation. Geocellular facies association and connectivity models are well constrained by outcrop observations and a number of different techniques are used in the quality control of the final model. The study also qualitatively discusses the uncertainties identified during the workflow and proposes methods to reduce them. The workflow and results shown in this paper can be applied in similar analogue systems in order to help improve model building for subsurface reservoirs.

Synthetic seismic modelling of a large-scale geological cross-section from the Book Cliffs, Utah, USA

Synthetic seismic sections have been generated from a 125 km long, depositional dip orientated cross-section compiled from outcrop data from the Book Cliffs of Eastern Utah, USA. These data are used to define the earth filter for a synthetic seismic section. An outcrop-based dataset has been used in preference to well data (though well data have been incorporated into the study section) because of the high confidence in the lateral continuity of data. The regional cross-section is modelled at 20, 50 and 100 Hz seismic source frequencies. As resolution increases, more laterally discontinuous reflections relating to facies terminations are imaged. Particularly important is the increase in internal reflections within each of the stratigraphic members (particularly the Sunnyside Member). These low angle clinoform events are imaged in low-resolution data but constructively interfere to form a lower number of more continuous looking events.

Comparison of digital outcrop and conventional data collection approaches for the characterization of naturally fractured reservoir analogues

Abstract In this study, fracture systems developed within faulted, high-porosity sandstones in the decommissioned mines of Alderley Edge, Cheshire, UK are characterized using lidar (Light Detection And Ranging)-based analysis. The geometry of the mine workings prove to be conducive to the extraction of fracture attributes, whilst providing a degree of exposure of a notable Triassic-aged reservoir outcrop analogue (Helsby Sandstone Formation) not afforded at the surface. To test the fidelity of the approach, fracture statistics generated from lidar-derived digital outcrop models are compared to an equivalent dataset collected using conventional manual surveys, with digital outcrop and manually acquired fracture attributes used to populate discrete fracture network models. These are upscaled to provide equivalent porous medium properties, enabling the impact of uncertainties introduced into fracture modelling workflows by lidar-based techniques to be assessed. Whilst broadly comparable to fracture attributes obtained using manual surveys, the systematic underrepresentation of fracture properties is observed within lidar-derived dataset, resulting in the underestimation of fracture network flow capacity. The study results suggest that, whilst enhancing data acquisition rates and coverage of exposure surfaces, the use of digital discontinuity analysis may introduce additional biases into fracture datasets, increasing the level of uncertainty within resultant modelled networks.

LiDAR-based digital outcrops for sedimentological analysis: workflows and techniques

Abstract Recent developments in workflows and techniques for the integration and analysis of terrestrial LiDAR (Light Detection And Ranging) and conventional outcrop datasets are demonstrated through three case studies. The first study shows the power of three-dimensional (3D) data visualization, in association with an innovative surface-modelling technique, for establishing large-scale 3D stratigraphical frameworks. The second presents an approach to derive reliable geometrical data on sediment-body geometries, whereas the third presents a new technique to quantify the proportions, distributions and variability of sedimentary facies directly from outcrop. In combination, these techniques provide essential conditioning data for geocellular and stochastic facies modelling. Built upon robust, reproducible and quantitative data, the resultant models combine realistic 3D geological architectures with sufficient quantities of reliable numerical data required for stable statistical analysis and establishing uncertainty. Together this new information provides detailed understanding and quantification of the 3D complexity of the sedimentary systems in question, thus offering insights of value for predicting the subsurface anatomy of analogous petroleum systems. As such, use of LiDAR, when combined with conventional field geology, offers a powerful tool for quantitative outcrop analysis, tightly constraining 3D structural and stratigraphical interpretations, and effectively increasing the statistical significance of outcrop analogues for reservoir characterization.

An integrated study of Permo-Triassic basins along the North Atlantic passive margin: implication for future exploration

Permo-Triassic rift basins offer important hydrocarbon targets along the Atlantic margins. Their fill is dominated by continental red beds, comprising braided fluvial, alluvial fan, aeolian, floodplain and lacustrine facies. These relatively lightly explored basins span both the Atlantic and Tethyan domains and developed above a complex basement with inherited structural fabrics. Sparse data in offshore regions constrain understanding of depositional geometries and sedimentary architecture, further impeded by their deep burial beneath younger strata, combined with the effects of later deformation during continental breakup. This paper provides results from a multidisciplinary analysis of basins along the Atlantic margin. Regional seismic and well data, combined with geochemical provenance analysis from the European North Atlantic margins, are integrated with detailed outcrop studies in Morocco and Nova Scotia. The research provides new insights into regional basin tectonostratigraphic evolution, sediment fill, and reservoir distribution, architecture and quality at a range of scales. Regional seismic profiles, supported by key well data, indicate the presence of post-orogenic collapse basins, focused narrow rifts and low-magnitude multiple extensional depocentres. Significantly, Permo-Triassic basin geometries are different and more varied than the overlying Jurassic and younger basins. Provenance analysis using Pb isotopic composition of detrital K-feldspar yields new and robust controls on the sediment dispersal patterns of Triassic sandstones in the NE Atlantic margin. The evolving sedimentary architecture is characterized by detailed sedimentological studies of key outcrops of age equivalent Permian-Triassic rifts in Morocco and Nova Scotia. The interplay of tectonics and climate is observed to influence sedimentation, which has significant implications for reservoir distribution in analogue basins. New digital outcrop techniques are providing improved reservoir models, and identification of key marker horizons and sequence boundaries offers a potential subsurface correlation tool. Future work will address source and seal distribution within the potentially petroliferous basins.

Virtual fieldtrips for petroleum geoscientists

Abstract Significant advances in geosciences data acquisition, visualization and analysis now allow highly detailed outcrop models to be constructed for a range of petroleum industry purposes. From a given field locality, a virtual outcrop is created from a centimetre-scale digital elevation model and colour photographs with geological information overlaid as appropriate. In a visualization environment, these datasets can be viewed sequentially to simulate undertaking a fieldtrip. These virtual fieldtrips allow geoscientists to improve and expand the traditional fieldwork experience in a number of ways, ranging from planning and health and safety considerations for management, to providing live supplemental technical content on a mobile device to the fieldtrip participant. The fieldtrips are easily archived and content can be reviewed in the office to provide analogue information during technical work. Examples of virtual fieldtrips are provided on the DVD that accompanies this volume.