Ground-Penetrating Radar And Its Potential To Image Underground: Application In Studying Ancient Subsurface Deposits And Implications For Petroleum Reservoir Characterization
Ground-penetrating radar (GPR) is a geophysical tool that allows the visualization of the shallow subsurface (1-70 m deep) in relatively high resolution (from cm-scale to 100s of m scale). The method is based on the injection of an electromagnetic current into the ground, which is reflected by surfaces/features with different electrical properties and received by an antenna back at the surface (Neal et al., 2002). It has been extensively used as a non-invasive and fast method in archaeology, civil construction, and forensic sciences. In stratigraphy and sedimentology, GPR can provide a unique view of the architecture and internal character of ancient depositional and erosive features, especially in sand-rich deposits where resistivities are predominantly high.
In the present research published in Sedimentary Geology, GPR was used as a base to determine the distribution, depositional controls, and evolution of Quaternary coastal systems in central Santos Basin, south-southeast Brazil. Furthermore, the results can be used for analog studies in the petroleum industry, as they provide an intermediate scale of visualization between high-resolution outcrop and low-resolution deep geophysical tools such as seismic.
The database included 108 GPR sections covering an area of approximately 310 km², acquired with a 200 MHz antenna from Geophysical Survey Systems Inc. (GSSI SIR-3000) using the Common Offset Method. The interpretation of radargrams is based on the determination of radarfacies, i.e. patterns of reflections, and reflection terminations. While radarfacies lead to the geological interpretation of depositional and/or erosive elements, reflection termination patterns allow for the interpretation of the evolution of depositional systems.
The Santos Basin is in the southeastern Brazilian continental margin, with evolution associated with the breakup of Gondwana in the early Cretaceous and the development of a passive margin from the Albian to present. The Quaternary record of the basin is exposed in the coastal province of Paraná, with a coastal plain divided by the Pleistocene and Holocene regressive barriers, and paleolagoonal deposits (Angulo and Lessa, 1997; Souza et al., 2012). The Pleistocene barrier is associated with a marine regression that started 120 ky BP and was interrupted by a phase of sea level rise that reached a maximum of 5 ky BP. Following this transgression, regressive conditions were re-established, and the Holocene barrier was built from 5 ky BP to present (Angulo and Lessa, 1997).
Radarfacies and Associations
13 radarfacies were interpreted from the radargrams, as zones with similar reflector patterns considering geometries, size, continuity, amplitude, frequency, terminations, and bounding surfaces. Their interpretation is based on the correlation of these characteristics with deposits exposed in outcrops along the coast and elsewhere. Additionally, local and regional truncation surfaces were interpreted, as they are related to erosive processes that also affect the dynamic of a depositional system. The groups of radarfacies are divided into two different successions of seaward migration of coastal systems and separated from each other by a regional truncation surface. The upper succession is up to 15 m thick, while the lower succession was not completely imaged.
Radarfacies were grouped into three associations that point to the development of barred coastal systems with deposition and evolution controlled by the action of waves and longshore wave currents. The main radar signatures are related to the progradation of beach deposits, while other radarfacies are related to subaerial and subtidal bedforms. Internal truncations among beach-face deposits represent depositional breaks associated with variable environmental energy, formed during episodes of anomalous energy that resulted in the erosion of the coast. This interpretation is reinforced by the interpretation of storm-related depositional bodies in outcrops (Souza et al., 2012).
The first radarfacies association corresponds to laterally-elongated sand-rich deposits with a prograding pattern that can be correlated to strandplains. The evolution of these barred coasts often result in the building of extensive coastal plains where the migration of backshore (subaerial dunes), foreshore (beach front), and shoreface (shallow marine) deposits form sandy wedges with a surface morphology of ridges and swales. The only expressive concentrations of mud and organic matter occur within the swales, in the form of interstrand marshes and/or confined fluvial channels that run parallel to the coast.
The second radarfacies association is also correspondent to elongated deposits with a progradational pattern, but a migration normal to the shore direction is also registered. These deposits, smaller in area when compared with strandplains, are interpreted as spit-inlet systems formed by the longshore migration of tidal channels and coastal sand bodies. Although waves are the controlling factors for the evolution of such systems, tidal processes also play a major role in deposition within the inlets, resulting in a muddier and more heterogeneous composition. The third association is correspondent to zones with transparent to chaotic configuration, indicating a muddy composition that results in low resistivity. Landward-migrating sandy wedges are associated with these zones. The association is interpreted as backbarrier lagoonal deposits with washover fans formed during high energy episodes such as storms.
Stratigraphy and Implications for Reservoirs
The two successions of radarfacies separated by a regional truncation surface are interpreted as stratigraphic units that record different stages of evolution of the coast. As both units are essentially seaward-migrating, they are the result of a positive sediment budget, controlled in this case by wave processes. The truncation surface indicates a period of partial erosion of the lower unit and displacement of the shoreline landward and is correlated to a wave ravinement surface formed during the Holocene sea level rise, when the coast was flooded (Angulo and Lessa, 1997). Therefore, the lower succession corresponds to Pleistocene regressive coastal deposits, while the upper succession corresponds to the Holocene regression that lasts until today. The two units are coarsening-up successions separated by a transgressive surface, thus correspondent to parasequences in the stratigraphic classification.
Parasequences are commonly regarded as the basic units for deep subsurface studies, as their dimensions are compatible with the resolution of tools used for deep subsurface visualization (e.g., Ainsworth, 2005). The parasequences identified in the research are sand-rich and laterally elongated, which are important characteristics for petroleum reservoirs. However, heterogeneities in the form of muddy sediment would compromise the quality of the reservoir in the subsurface (e.g., Cook et al., 1999).
In the petroleum industry, these types of heterogeneities result in compartmentalization and in baffles or barriers to the oil flow. The parasequences in the study area are often limited by continuous muddy lagoonal deposits that would represent major barriers in a reservoir. Additionally, the inter-digitation of strandplain and spit-inlet systems within the same parasequence implies internal compartmentalization and in different permeability pathways, affecting flow trends.
GPR proved to be an accessible and versatile tool for the imaging of the shallow subsurface, especially in areas with poor exposition and limited subsurface control. The results demonstrate not only its potential to image sediment bodies in the subsurface but also the potential to interpret the origin, composition, depositional context, depositional processes, and evolution of sedimentary systems.
It is thus ideal for studies of stratigraphic evolution of ancient depositional systems, allowing the interpretation of relatively high-frequency depositional trends. The optimal resolution of radar data in sandy deposits make it also a potential method for the detailed investigation of analog reservoirs in petroleum geology, representing an intermediate scale of visualization between outcrops and deep geophysical tools, and adding another level of heterogeneities for the evaluation of a petroleum reservoir.
These findings are described in the article entitled Quaternary coastal plains as reservoir analogs: Wave-dominated sand-body heterogeneity from outcrop and ground-penetrating radar, central Santos Basin, southeast Brazil, recently published in the journal Sedimentary Geology.
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