Jamie K. Pringle

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.

3D high-resolution digital models of outcrop analogue study sites to constrain reservoir model uncertainty: an example from Alport Castles, Derbyshire, UK

Advances in data capture and computer technology have made possible the collection of 3D high-resolution surface and subsurface digital geological data from outcrop analogues. This paper describes research to obtain the 3D distribution and internal sedimentary architecture of turbidite channels and associated sediments at a study site in the Peak District National Park, Derbyshire, UK. The 1D, 2D and 3D digital datasets included Total Station survey, terrestrial photogrammetry and remote sensing, sedimentary logs and a Ground Penetrating Radar (GPR) dataset. A grid of 2D GPR profiles was acquired behind a cliff outcrop; electromagnetic reflection events correlated with cliff face sedimentary horizons logged by Vertical Radar Profiling. All data were combined into a Digital Solid Model (DSM) dataset of the site within reservoir modelling software. The DSM was analysed to extract 3D architectural geometries for petroleum reservoir models. A deterministic base model was created using all information, along with a suite of heterogeneous turbidite reservoir models, using 1D, 2D or 3D information. The model suite shows significant variation from the deterministic model. Models built from 2D information underestimated connectivity and the continuity of geobodies, but overestimated channel sinuosity. Advantages of using 3D digital outcrop analogue data for 3D reservoir models are detailed.

Time-Lapse Geophysical Investigations over a Simulated Urban Clandestine Grave

Abstract:  A simulated clandestine shallow grave was created within a heterogeneous, made‐ground, urban environment where a clothed, plastic resin, human skeleton, animal products, and physiological saline were placed in anatomically correct positions and re‐covered to ground level. A series of repeat (time‐lapse), near‐surface geophysical surveys were undertaken: (1) prior to burial (to act as control), (2) 1 month, and (3) 3 months post‐burial. A range of different geophysical techniques was employed including: bulk ground resistivity and conductivity, fluxgate gradiometry and high‐frequency ground penetrating radar (GPR), soil magnetic susceptibility, electrical resistivity tomography (ERT), and self potential (SP). Bulk ground resistivity and SP proved optimal for initial grave location whilst ERT profiles and GPR horizontal “time‐slices” showed the best spatial resolutions. Research suggests that in complex urban made‐ground environments, initial resistivity surveys be collected before GPR and ERT follow‐up surveys are collected over the identified geophysical anomalies.

Electrical resistivity survey to search for a recent clandestine burial of a homicide victim, UK

This case report details an electrical resistivity survey to assist the search for a suspected 1-year-old clandestine burial of a murder victim in North Wales in the UK. Conventional search techniques (victim recovery dogs and probing) proved unsuccessful, and with a significant survey area and a high clay content soil precluding GPR as a geophysical search method, a resistivity survey was instead trialled. Ten resistivity grids were collected and site detrended with user-specified, contoured anomalies being generated. The resulting anomalies were compared to anomalies derived from similar-aged, simulated clandestine burial surveys. Seven anomalies with comparative sizes and amplitudes (±3Ω) of the simulated burials were identified within the search area and prioritised for further investigation. The shallowly buried victim was subsequently recovered outside the survey area.

Geophysical Monitoring of Simulated Clandestine Graves Using Electrical and Ground-Penetrating Radar Methods: 0-3 Years After Burial

Abstract:  This study provides forensic search teams with systematic geophysical monitoring data over simulated clandestine graves for comparison to active cases. Simulated “wrapped” and “naked” burials were created. Multigeophysical surveys were collected over a 3‐year monitoring period. Bulk ground resistivity, electrical resistivity imaging, multifrequency ground‐penetrating radar (GPR), and grave and background “soil‐water” conductivity data were collected. Resistivity surveys revealed the naked burial had consistently low‐resistivity anomalies, whereas the wrapped burial had small, varying high‐resistivity anomalies. GPR 110‐ to 900‐MHz frequency surveys showed the wrapped burial could be detected throughout, with the “naked” burial mostly resolved. Two hundred and twenty‐five megahertz frequency GPR data were optimal. “Soil‐water” analyses showed rapidly increasing (year 1), slowly increasing (year 2), and decreasing (year 3) conductivity values. Results suggest resistivity and GPR surveys should be collected if target “wrapping” is unknown, with winter to spring surveys optimal. Resistivity surveys should be collected in clay‐rich soils.

Capturing stratigraphic and sedimentological complexity from submarine channel complex outcrops to digital 3D models, Karoo Basin, South Africa

ABSTRACT Submarine slope channel-fills form complicated stratigraphy and lithofacies distributions through repeated phases of erosion and deposition. This provides a challenge to accurate 3D modelling, particularly in representing lithofacies transitions within sand-poor areas. In this paper, traditional (sedimentary logs, palaeocurrent measurements, architectural panels) and non-conventional technologies (Light Detection and Ranging; Ground Penetrating Radar) were integrated to quantitatively describe lithofacies distributions and sedimentary architectures from two large-scale outcrops, one base of slope, high sandstone content system (Unit B) and one from a mid-slope, more mixed lithology system (Unit C), in the Laingsburg Formation, Karoo Basin, South Africa. The workflow described in this study combines digital structural restoration and extrapolation of major stratigraphic surfaces, grouped palaeocurrents and architectural geometries observed at outcrop to create 3D digital models. The model was divided into zones along major stratigraphic discontinuities and populated using lithofacies associations that were adjusted for outcrop rugosity and palaeodispersal direction. Observed channel margin asymmetries, distribution of lithofacies and stacking patterns were all honoured in the digital models. The Unit C slope-channel system differs from many exposed submarine channels due to the low proportion of sandstone present within the infill. Thin-bedded channel margin lithofacies are preserved through the lateral stepping of channels and allow the correlation of stratigraphy from channel axis to margin and on to overbank areas. In the older, sandier Unit B base-of-slope system, the stratigraphic change in stacking pattern, channel aspect ratio, lithofacies of channel-fills and stratigraphic hierarchy were all captured. This research captured the architectural complexity observed at outcrop to generate more realistic models than could be constructed normally using limited subsurface data.

Time-lapse resistivity surveys over simulated clandestine graves

The aim of this study was to develop a better understanding of how electrical resistivity surveys can be used to locate clandestine graves. Resistivity surveys were conducted regularly over three simulated clandestine graves containing a pig cadaver, no cadaver and a pig cadaver wrapped in tarpaulin, respectively. Additionally, soil and groundwater samples were collected from two more simulated graves outside the survey area. The grave containing a pig cadaver was detectable from a low resistivity anomaly in the survey data. Groundwater data suggest that the resistivity anomaly associated with the surveyed pig grave was caused by a localised increase in groundwater conductivity. Wrapping a cadaver was found to initially change the resistivity response of a grave to a high resistivity anomaly. Resistivity surveys did not detect the disturbed soil in the grave that did not contain a cadaver. Although soil samples showed grave soil to be more porous than undisturbed soil, the lack of response from the grave that did not contain a cadaver suggests that disturbed soil was not responsible for the resistivity anomalies observed in this study. Resistivity surveys successfully detected all graves containing cadavers throughout the study, whilst also showing the potential to eliminate the need for mass excavation in a genuine search.

GPR and bulk ground resistivity surveys in graveyards: Locating unmarked burials in contrasting soil types

With graveyards and cemeteries globally being increasingly designated as full, there is a growing need to identify unmarked burial positions to find burial space or exhume and re-inter if necessary. In some countries, for example the U.S. and U.K., burial sites are not usually re-used; however, most graveyard and cemetery records do not have maps of positions. One non-invasive detection method is near-surface geophysics, but there has been a lack of research to-date on optimal methods and/or equipment configuration. This paper presents three case studies in contrasting burial environments, soil types, burial styles and ages in the U.K. Geophysical survey results reveal unmarked burials could be effectively identified from these case studies that were not uniform or predicted using 225 MHz frequency antennae GPR 2D 0.5 m spaced profiles. Bulk ground electrical surveys, rarely used for unmarked burials, revealed 1 m probe spacings were optimal compared to 0.5 m, with datasets needing 3D detrending to reveal burial positions. Results were variable depending upon soil type; in very coarse soils GPR was optimal; whereas resistivity was optimal in clay-rich soils and both were optimal in sandy and black earth soils. Archaeological excavations revealed unmarked burials, extra/missing individuals from parish records and a variety of burial styles from isolated, brick-lined, to vertically stacked individuals. Study results, evidence unmarked burial targets were significantly different from clandestine burials of murder victims which are used as analogues.

Comparisons of magnetic and electrical resistivity surveys over simulated clandestine graves in contrasting burial environments

Determining the effectiveness and limitations of near-surface, non-invasive geophysical techniques is imperative when attempting to locate clandestine burials. Unlike in archaeology, there has been limited forensic research with regard to optimum methodologies, with most emphasis to date being on metal detectors and ground-penetrating radar. However, these techniques may not be suitable in certain soil types (e.g., conductive or highly magnetic) or for certain non-metallic targets. Therefore, in this study, magnetic and electrical resistivity detection techniques have been utilized over different aged (0.25–1 year) simulated clandestine burials with no buried metal, in contrasting depositional environments. These environments included semi-rural, urban, woodland and a parkland medieval grave site acting as an archaeological analogue. The magnetic surveys showed mixed success in detecting clandestine burials. Elevated magnetic gradient readings, with respect to background values, were observed over very shallow burials, whereas deeper burials displayed a reduction in gradient and/or no associated magnetic anomalies. Magnetic anomalies were observed over surface-burials and validated by simple 2D forward modelling. Magnetic anomalies were also observed in the control data set. Electrical resistivity surveys produced anomalies over all the simulated burial positions, including surface burials but did not produce anomalies at the archaeological analogue site. Laboratory analysis of fluid retrieved from simulated graves showed an overall increase in iron levels over a year post-burial, which may account for the observed magnetic anomaly variation. There was also a corresponding increase in grave ‘fluid’ conductivity, which was interpreted to be the cause of the observed low resistivity anomalies. This research suggests that, as a technique for locating clandestine burials, bulk ground resistivity is more successful than the tested magnetic methods. Moreover, magnetic techniques are more effective when used as part of a multi-technique study over rural and semi-rural sites that are relatively low in magnetic and electrical ‘noise’. These results have important implications for the use of geophysical techniques when searching for clandestine burials. We emphasize that local depositional environment, soil type, likely style of burial and search area size should all be considered when choosing forensic geophysical detection techniques. We also provide evidence to show that geophysical data can assist in locating a primary deposition site even when no physical evidence remains.

Search protocols for hidden forensic objects beneath floors and within walls

The burial of objects (human remains, explosives, weapons) below or behind concrete, brick, plaster or tiling may be associated with serious crime and are difficult locations to search. These are quite common forensic search scenarios but little has been published on them to-date. Most documented discoveries are accidental or from suspect/witness testimony. The problem in locating such hidden objects means a random or chance-based approach is not advisable. A preliminary strategy is presented here, based on previous studies, augmented by primary research where new technology or applications are required. This blend allows a rudimentary search workflow, from remote desktop study, to non-destructive investigation through to recommendations as to how the above may inform excavation, demonstrated here with a case study from a homicide investigation. Published case studies on the search for human remains demonstrate the problems encountered when trying to find and recover sealed-in and sealed-over locations. Established methods include desktop study, photography, geophysics and search dogs: these are integrated with new technology (LiDAR and laser scanning; photographic rectification; close-quarter aerial imagery; ground-penetrating radar on walls and gamma-ray/neutron activation radiography) to propose this possible search strategy.