Global Characterization of Modern Depositional Environments for Reservoir Analogues
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The advent of significant volumes of freely available, global scale, remote sensing and Geographical Information Systems (GIS) data has revolutionized the study of modern depositional systems, especially with reference to their application as analogues for hydrocarbon reservoirs. While it is possible to browse Google Earth (or similar packages) and identify modern analogues, to date there have been no systematic studies which have mapped and categorized depositional systems on a global scale. Furthermore the application of data from modern systems in reservoir studies has commonly been undertaken in a qualitative way. Where measurements of geobodies are made it is often manually and prone to user error and bias. The work presented in this study details a systematic approach to the challenge of utilizing modern data as reservoir analogues from the global mapping of sedimentary basins, through the global classification of shallow marine shorelines to the automated description of sediment body geometries to their implementation in reservoir models. In addition to improved reservoir analogue studies, many aspects of this work, such as the mapping of basins and the quantification of shoreline types, have implications for broader understanding of modern systems and their preservation in the stratigraphic record.
Given that the rock record is wholly comprised of deposits that were laid down in sedimentary basins, it is appropriate to only select analogous for the rock record from modern systems that sit within basins. To this end it was necessary to map the distribution of modern sedimentary basins. This was achieved using GIS algorithms that combine data on surface gradient, geology, climate and tectonics. The key result is the first known map of the World’s current terrestrial basins. Analysis of the results suggest that only about 16% of the Earth land surface lies within a basin, the rest is in upland areas that are in long tern net erosion. Within that 16%, 60% is within an arid climate which is significantly more than 27% of the total land proportion. Analysis of tectonic setting suggests that intracratonic and foreland basins are the greatest proportion by area.
Paralic systems are major reservoirs. Clastic coastlines are classified on the relative importance of wave, tide and fluvial processes in transporting and depositing sediments. While this classification has existed for over 40 years, there have been no previous attempts to classify the entire coastline of the World. Using a series of GIS datasets describing factors such as mean wave height, tidal range, fluvial discharge and shoreline shape it has been possible to produce a map within which the coastline is subdivided into 927,577 5 km segments, each of which is categorized on the primary and secondary controlling mechanism. This dataset shows that the World’s depositional coastlines are primarily wave dominated (62%) with a significant component being tide dominated (35%) and only 3% being dominated by fluvial deltas. Further analysis suggests that shoreline type is heavily influenced by complex interplay between climate, latitude, shelf width, tectonic setting, ocean basin morphology and plate tectonics. The data have been analyzed by two parameters which are easily definable in the ancient (palaeo-climate and tectonic setting) to produce a probability matrix for a given shoreline type.
The last two articles explore methodologies for the extraction of geometric information from modern depositional environments and outcrop data in a quantitative, objective and automated workflow. This shows a methodology for describing spatial polygons by a centerline and a series of deviations from that centerline. The method described in the third article is applied to two modern systems (the Mitchell Delta in the Gulf of Carpentaria and the Congo River in the Democratic Republic of the Congo) to show significant improvements in comparison to standard GIS routines. A linear regression line comparing a method based on the minimum bounding box (MBB) of a shape with the method developed here shows that MBB width was exaggerated up to 4.5 times and MBB length underestimated by as much as 0.31 times between the two datasets. As width and length of features are important parameters in object-based reservoir modeling, the potential skewed relationship of width to length ratio could reflect in associated reservoir models.
By expanding on the method proposed in Paper III, the fourth paper shows that mapped 2.5D geobodies from virtual outcrops can automatically describe shape, length, width and centerline deviation. Those attributes are used to define complex shapes for object based modelling. The potential of this approach is illustrated on the non-marine portion of the Cretaceous Blackhawk Formation of the Book Cliffs in eastern Utah where data were extracted and a reservoir model was built.
The results of the study are a step towards the more systematic use of modern systems as reservoir analogues and also shed light on the fundamental controls on the stratigraphic record.