Pau Arbués

Three-dimensional reconstruction of geological surfaces: An example of growth strata and turbidite systems from the Ainsa basin (Pyrenees, Spain)

The external and internal geometry of four turbidite systems outcropping around the Buil syncline (Ainsa basin, Spanish Pyrenees) has been reconstructed with reservoir-scale resolution in three dimensions (3-D). The irregular geometry of the syncline and the resolution required for the reconstruction cannot be resolved with cross sections. Therefore, reconstruction has been carried out with a new methodology that applies a 3-D dip-domain geometrical model and 3-D restoration techniques to achieve reservoir-scale resolution in kilometric-scale reconstructions. This methodology is aimed at resolving 3-D geometries in folded areas and regions with variable thickness stratigraphy.The 3-D reconstruction of the Buil syncline reveals the synsedimentary growth of an intrabasinal anticline and the foreland lithospheric flexure associated with tectonic loading north of the Ainsa basin.

Supervised identification and reconstruction of near-planar geological surfaces from terrestrial laser scanning

Terrestrial laser scanning is an effective method for digitally capturing outcrops, enabling them to be visualized, analyzed, and revisited in an office environment without the limitations of fieldwork (such as time constraints, weather conditions, outcrop accessibility, repeatability, and poor resolution of measurements). It is common practice in geological interpretation of digital outcrops to use visual identification and manual digitization of pointsets or polylines in order to characterise geological features using 3D CAD-like modules. Other recent and less generic approaches have focused on automated extraction of geological features by using segmentation methods, mostly based on geometric parameters derived from the point cloud, but also aided by attributes captured from the outcrop (intensity, RGB). This paper presents a workflow for the supervised and automated identification and reconstruction of near-planar geological surfaces that have a three-dimensional exposure in the outcrop (typically bedding, fractures, or faults enhanced by differential erosion). The original point cloud is used without modifications, and thus no decimation, smoothing, intermediate triangulation, or gridding are required. The workflow is based on planar regressions carried out for each point in the point cloud, enabling subsequent filtering and classification to be based on orientation, quality of fit, and relative locations of points. A coarse grid preprocessing strategy is implemented to speed up the search for neighboring points, permitting analysis of multimillion point clouds. The surfaces identified are organized into classes according to their orientations and regression quality parameters. These can then be used as seeds for building outcrop reconstructions or further analyzed to investigate their characteristics (geometry, morphology, spacing, dimensions, intersections, etc.). The workflow is illustrated here using a synthetic example and a natural example from a limestone outcrop, in which surfaces corresponding to bedding and three fault orientations were reconstructed.

Synthetic seismic models from outcrop-derived reservoir-scale three-dimensional facies models: The Eocene Ainsa turbidite system (southern Pyrenees)

This study uses one-dimensional convolution seismic models to better understand which features of slope turbidite systems can (or cannot) be observed on real seismic data, aiming to improve subsurface seismic interpretation. Synthetic seismic sections and cubes were built from reservoir-scale three-dimensional facies models of the outcropping Ainsa turbidite system. This turbidite system developed in a foredeep and wedge-top depositional setting within a slope system. The turbidite system consists of laterally and vertically stacked sandstone- and debrite-dominated channel fills, grading into heterolithic and mudstone units, with intercalations of slump-deformed mudstone-rich units. Typical petrophysical values for subsurface Cenozoic sediments were assumed for the seismic models, which are presented at 25-, 50-, and 75-Hz resolution. Seismic models enabled the comparison between architectural and facies distributions observed in the outcrops and the geological models to their possible seismic expression in the subsurface. Comparisons show how seismic expression degrades when seismic resolution decreases. By using models at different geological scales, the effects of each heterogeneity scale are identified. Precise delineation of the internal architecture and facies distribution within channel complexes is beyond the reach of all seismic frequencies. The position of channel complex tops and margins is uncertain because of their gradual character. Differentiating between sandstone- and debrite-filled channels is not straightforward, and bed-scale heterogeneities within the sandstone-dominated channels are barely distinguishable in the seismic data. The net-to-gross predictive capability of root-mean-square amplitude extractions varies depending on the seismic frequency and unit thickness.

3D structure and evolution of an oblique system of relaying folds: the Ainsa basin (Spanish Pyrenees)

The structure of the Eocene Ainsa basin is dominated by kilometre-scale thrusts and folds that have trends at c. 70° to that of the main Pyrenean structures. Late uplift and erosion provide excellent exposure of this fold-and-thrust system and its growth strata. Field observations integrated with 2D seismic and well data support the 3D reconstructions presented in this paper. Structural reconstructions of pre- and syn-growth geometries contribute substantial improvements to the understanding of the structure of the area and provide a unique insight into the timing of Pyrenean structures. This study demonstrates that oblique features in the Ainsa basin can be grouped into two main systems: the La Fueba thrust system, an oblique-lateral ramp to one of the main Pyrenean thrusts, and the kilometre-scale folds of the Sobrabe system, a set of anticlines that grew as part of a relaying system of folds. In this study we propose a chronology of deformation based on the geometry of the basin fill, which has been used to frame the general tectonosedimentary evolution of the Ainsa basin. The relative timing of structures indicates that the La Fueba structures predate the Sobrarbe structures, which originated as orogen-parallel structures, and obliquity developed during the growth of these structures through clockwise vertical-axis rotation.

A geostatistical algorithm to reproduce lateral gradual facies transitions: Description and implementation

Valid representations of geological heterogeneity are fundamental inputs for quantitative models used in managing subsurface activities. Consequently, the simulation of realistic facies distributions is a significant aim. Realistic facies distributions are typically obtained by pixel-based, object-based or process-based methods. This work presents a pixel-based geostatistical algorithm suitable for reproducing lateral gradual facies transitions (LGFT) between two adjacent sedimentary bodies. Lateral contact (i.e. interfingering) between distinct depositional facies is a widespread geometric relationship that occurs at different scales in any depositional system. The algorithm is based on the truncation of the sum of a linear expectation trend and a random Gaussian field, and can be conditioned to well data. The implementation introduced herein also includes subroutines to clean and geometrically characterize the obtained LGFT. The cleaned sedimentary body transition provides a more appropriate and realistic facies distribution for some depositional settings. The geometric measures of the LGFT yield an intuitive measure of the morphology of the sedimentary body boundary, which can be compared to analogue data. An example of a LGFT obtained by the algorithm presented herein is also flow simulated, quantitatively demonstrating the importance of realistically reproducing them in subsurface models, if further flow-related accurate predictions are to be made.

An outcrop-based comparison of facies modelling strategies in fan-delta reservoir analogues from the Eocene Sant Llorenç del Munt fan-delta (NE Spain)

ABSTRACT Sedimentary facies is a key control on petrophysical properties within subsurface reservoirs and facies modelling is a critical aspect of reservoir modelling. Several modelling methods exist and selecting the best approach for a specific case is challenging and time-consuming. Outcrop analogues provide detailed information on architecture, geometry and facies connectivity that is not directly available in the subsurface. By modelling outcrop data it is possible to test and compare different modelling strategies systematically in a case where the geology is well constrained. The Eocene aged, Sant Llorenç del Munt fan-delta complex (NE Spain) is a well exposed, transgressive–regressive fan-delta. Outcrop data were used to test a variety of modelling strategies in which the density of conditioning wells, stratigraphic subdivision, modelling algorithm and trends were all varied. The results of these modelling exercises were compared against themselves and against a close-to-deterministically built Base Case reconstruction using a series of static measures including the distribution of fan-delta front reservoir facies, directional connectivity and reservoir-to-well connectivity. The results highlight the following: (a) the impact of the conditioning well density on improving the stratigraphic architecture reproduction in the different modelling approaches; (b) that surface-based modelling subdivisions including the maximum flooding surface to separate independent grids for modelling the transgressive and regressive sequence sets can be detrimental when compared to using only the top and base of the composite sequence unless additional constraints are included; (c) that an algorithm combining a linear trend and a Gaussian field is the most suitable algorithm for reproducing this type of architecture, but requires defining a 3D trend; and (d) the need for using trends to reproduce the architecture when well data are sparse. These results provide guidelines for modelling analogue fan-delta reservoirs in the subsurface.

Quantifying and correcting errors derived from apparent dip in the construction of dip-domain cross-sections

This paper deals with the errors introduced in the construction of cross-sections due to the use of apparent dips and thicknesses when projecting data onto the plane of section. These errors are analyzed under the perspective of cross-sections constructed with the dip-domain method. A method to evaluate variations of unit thicknesses due to distortion during projection is presented, and modifications to the dip-domain method are proposed to account for them, including the use of apparent bisectors. Methodology is discussed for inclined section planes, and the equations to calculate apparent thickness on inclined sections are presented. An analytical definition of cylindrical folding is also derived.

A Method for Producing Photorealistic Digital Outcrop Models

Outcrop studies can be substantially improved by use of photorealistic digital outcrop models. A method is presented that has been tested for decametre to hectometre long outcrops. The models have accuracy and resolution around one decimeter and the models are conveniently scaled and oriented, allowing for comparison to geological data that were directly acquired in the field. The gear involved is lightweight and low cost when compared to LIDAR. The geologist can backpack this gear, decide acquisition on the flight, and finish the computer processing back in the base camp or in the office. The experiments have been carried in a series of Spanish outcrops that represent a variety of depositional settings: alluvial fan, fluvial fan, point bar deposits, fan-delta, and deepwater. The output models will be discussed in in terms of their quality by report to LIDAR data.

Orientation domains: A mobile grid clustering algorithm with spherical corrections

An algorithm has been designed and tested which was devised as a tool assisting the analysis of geological structures solely from orientation data. More specifically, the algorithm was intended for the analysis of geological structures that can be approached as planar and piecewise features, like many folded strata. Input orientation data is expressed as pairs of angles (azimuth and dip). The algorithm starts by considering the data in Cartesian coordinates. This is followed by a search for an initial clustering solution, which is achieved by comparing the results output from the systematic shift of a regular rigid grid over the data. This initial solution is optimal (achieves minimum square error) once the grid size and the shift increment are fixed. Finally, the algorithm corrects for the variable spread that is generally expected from the data type using a reshaped non-rigid grid. The algorithm is size-oriented, which implies the application of conditions over cluster size through all the process in contrast to density-oriented algorithms, also widely used when dealing with spatial data. Results are derived in few seconds and, when tested over synthetic examples, they were found to be consistent and reliable. This makes the algorithm a valuable alternative to the time-consuming traditional approaches available to geologists.

ABSTRACT: Controls on the Variability of the Turbidite Systems in the Ainsa Slope Complex (South-Central Pyrenees, Spain)

The Ainsa slope complex developed during the lower and middle Eocene in a foredeep to piggy-back setting in south-central Pyrenees. It is preserved as a 40 Km long, 30 Km wide and 4000 m thick, mudstone-dominated clastic wedge, which includes coarser grained turbidite lithosomes. The latter correspond mostly to channel fills and associated overbank and splay deposits. Our research in the Ainsa slope complex has produced a stratigraphic and structural dataset resolving from the basin fill to the reservoir scale. From the analysis of this dataset we offer a scheme of stratigraphic subdivision which covers the entire range of scales mentioned above plus tabulated parameters and related graphs for each of the elements in the scheme. The basin fill is first divided into four major depositional cycles (2-5 m.y. each) which are bounded by unconformities and occur stepped several kilometers towards the foreland due to the propagation of the thrust front. Within a major depositional cycle, several turbidite systems occur which are bounded below by unconformities and above by a mudstone cap. From system to system, in a younging direction, there is an increase in N/G and reductions on stratigraphic thickness, amount of forelandwards migration of system axis, volume of bypass facies and maximum sediment caliber. These changes can all be explained by an overall decrease in gradient from base to top within a major depositional cycle and fit into a model characterized by progressively reduced rates of forelimb rotation along thrustrelated frontal anticlines.