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ERT AND GPR SURVEY OF COLLAPSED PALEOCAVE SYSTEMS IN THE WESTERN BORDER OF THE POTIGUAR BASIN,

NORTHEAST BRAZIL

Journal: Near Surface Geophysics Manuscript ID: Draft

Manuscript Type: Original Article Date Submitted by the Author: n/a

Complete List of Authors: Reis Júnior, João; Universidade Federal do Rio Grande do Norte, Geologia de Castro, David; Universidade Federal do Rio Grande do Norte, Geologia Himi, Mahjoub; barcelona unversity, GPPG

Casas, Albert; University of Barcelona, Geochemistry, Petrology and Geological Prospecting

Lima-Filho, Francisco; Universidade Federal do Rio Grande do Norte, Geologia

Keywords: GPR imaging, ERT, Carbonate

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ERT AND GPR SURVEY OF COLLAPSED PALEOCAVE SYSTEMS IN THE WESTERN BORDER OF THE POTIGUAR BASIN, NORTHEAST BRAZIL

Authors:

João Andrade dos Reis Júnior1,

*

David Lopes de Castro1

Alberto Casas2 Mahjoub Himi2

Francisco Pinheiro Lima-Filho1

¹Departamento de Geologia, Programa de Pós-Graduação em Geodinâmica e Geofísica – Universidade Federal do Rio Grande do Norte, Campus Universitário S/N, 59078-970, Natal, Brazil

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Department of Geochemistry, Petrology and Geological Prospecting. Faculty of Geology. University of Barcelona. Barcelona, Spain.

*Corresponding author. Tel.: +55318432153808.

E-mail addresses: [email protected] (J.A. Reis Jr), [email protected] (D.L. de Castro), [email protected], (A. Casas), [email protected] (F.P. Lima Filho)

University of Barcelona. Barcelona, Spain.

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ERT AND GPR SURVEY OF COLLAPSED PALEOCAVE SYSTEMS IN THE WESTERN BORDER OF THE POTIGUAR BASIN, NORTHEAST BRAZIL

Abstract

The 3D architecture of a collapsed paleocave system located on the western border of the Potiguar Basin, northeast Brazil, has been mapped accurately. The collapsed paleocaves outcrop in an escarpment that delimits the carbonate platform from the transgressive phase of the basin. Electrical resistivity tomography (ERT) and ground-probing radar (GPR) sections were acquired parallel and perpendicular to a road cut, which served to parameterize the geophysical signatures of the collapsed paleocaves and host rocks. The petrophysical (electrical and dielectrical) properties of the geological units identified in the study area were determined based on the geophysical data and on porosity and permeability measurements obtained in the laboratory. The collapsed paleocaves were mapped based on the identification of high resistivity zones and high-amplitude GPR reflectors. These karst features developed under the influence of the vadose zone and were filled by tufa, breccia and speleothems after the collapse. The mean porosity of this carbonate material is approximately 35%, indicating an incipient burial and poorly developed cementation. The lateral contacts with carbonate soil and host rocks are marked by layers with low to intermediate resistivity (3-1,250 Ω·m) and with a pattern of GPR reflections ranging from low amplitude to transparent. The paleocaves developed in both modern tufa deposits and Cretaceous carbonate rocks, which have now become weathered, forming an unconsolidated white carbonate soil. This carbonate unit lies in conformity on fractured calciferous sandstones deposited in a transitional environment. The karstification processes propagate in depth until they reach the calciferous sandstone layer, whose top represents the lower limit of these karst features. The high resolution of the GPR data allowed the identification of different radar facies within the paleocaves and the host rocks. The resistivity data and the GPR attribute of root square (RS) energy enabled the visualization of the complex 3D distribution of the collapsed paleocaves system. At depths of 10 m, the paleocaves are more spread out and eventually isolated, sometimes vertically connected through shafts. However, in the shallower portions, the paleocaves are interconnected by ducts or coalesced, forming a system of paleocaves hundreds of meters long in an area of 12,000 m2. The results show the detailed 3D geometry of this paleocave system, at a subseismic scale, allowing the identification of the connectivity pattern among these karst features (pipes), the porosity and the total volume of the reservoir. They may function as an outcrop analog for other collapsed paleocave carbonate reservoirs.

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Keywords: Collapsed paleocaves; Ground probing radar; Electrical resistivity tomography; Carbonate reservoir; Potiguar Basin

1. Introduction

Carbonate rocks are significant reservoirs of groundwater and hydrocarbons. Karst aquifers represent extensive groundwater resources around the world, supplying approximately 25% of the world population with water (Ford and Williams 2007). In the petroleum industry, more than a third of the world’s hydrocarbon reserves are located in carbonate reservoirs (Bagdan and Pemberton 2004). In the last few years, this type of reservoir has become even more important due to recent discoveries of large hydrocarbon fields in carbonate rocks, especially in the pre-salt layer found on the continental margins of South America and Africa (Durham 2009; Steven et al. 2010).

Coalesced systems of collapsed paleocaves form an important class of carbonate reservoirs arising from shallow karst processes, with subsequent collapse, burial and diagenesis (Loucks and Anderson 1985; McMechan et al. 1998). These systems can extend for hundreds of kilometers and have thicknesses of hundreds of meters (Loucks 1999). Typically, collapsed paleocave reservoirs involve multiple karstification and burial phases, causing pronounced lateral and vertical heterogeneity and complex generation processes (McMechan et al. 1998; Loucks 1999). The carbonate rocks of the Ellenberger Group (Lower Ordovician), in the south central region of the United States, comprise a system of collapsed paleocaves that has been intensively studied since the 1980s and represents the largest oil reservoir of west Texas (Kerans 1988; Loucks 1999; McMechan et al. 2002).

The origin of the epigenic limestone caves is associated with shallow karst processes, in which the dissolution of carbonate rock occurs from the surface, generated by the gravitational movement of acid-rich water both in the vadose zone and below it (Klimchouk 2007). These processes may progress to the formation of extensive cave systems (Ford and Williams 2007; White 2007). The interior of these caves can be filled with material resulting from the precipitation of dissolved calcium carbonate derived from the surrounding limestone rocks, forming sub-horizontal strata (tufa); they can also be filled with speleothems. The term tufa is used here according to Capezzuoli et al. (2014) for continental carbonates composed mainly of calcite in a typical karst area. In addition to tufas and speleothems, sediments and organic debris transported from the surface through ducts may also be deposited inside the caves. As the evolutionary process of the cave continues (sometimes controlled by faults and joints) the caves collapse, thus generating highly fractured carbonate reservoirs. The cave infill may have high porosity and permeability with spatially complex internal architecture (McMechan et al. 1998; Loucks 1999).

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The significant spatial heterogeneity of the collapsed paleocave reservoirs makes their characterization in seismic sections a difficult task, mainly because the ducts and heterogeneities are often on a subseismic scale. Even the study of reservoir analogs, where paleocaves systems outcrop, is largely limited by the partial exposure of such three-dimensional (3D) structures. In recent years, electrical and electromagnetic methods have been applied to obtain high-resolution images of shallow rocky substrates. The 3D geometric data of partially outcropping analogs, acquired using ground penetrating radar (GPR) and electrical resistivity methods, can be linked to data on petrophysical properties obtained from outcropping rocks or boreholes to provide essential parameters for characterization of deep reservoirs. The GPR method has been successfully applied to the mapping and characterization of fractures and faults in carbonate rocks (Pipan et al. 2003; Grasmueck et al. 2005; McClymont et al. 2008), the detection of caves and karst features (Al-fares et al. 2002; Kruse et al. 2006), stratigraphic imaging (Grasmueck and Weger 2002), and studies of limestone outcrops analogous to hydrocarbon reservoirs (McMechan et al. 2002; Takayama et al. 2008; Jesus et al. 2012; Forte et al. 2012). The contrast between the electrical properties of the collapsed paleocaves and of the host rock is crucial for their identification in GPR data (Reis Jr et al. 2014). According to Pipan et al. (2003), the presence of empty spaces in collapsed paleocaves and the filling of these spaces with air (vadose zone) or water (phreatic zone) is responsible for the variations in the electrical properties of the collapsed karst features.

The geoelectrical method has been shown to be one of the most promising geophysical techniques for mapping karst features (Roth and Nyquist 2003; Cardarelli et al 2006) because of the strong resistivity contrast between these features and the carbonate host rock. Examples of the application of this method in limestone rocks can be found in the studies by Schoor (2002), Roth et al. (2002) and Zhou et al. (1999, 2002). In contrast, the use of electrical resistivity combined with GPR surveys makes it possible to correlate the variations in resistivity with the zones in the GPR sections of high and low amplitude of the electromagnetic (EM) signal, as well as to correlate areas where greater attenuation of the EM wave corresponds to areas of lower resistivity.

The use of GPR in combination with electrical resistivity methods has been used in various types of geological environments, including crystalline basement rocks (Beauvais et al. 2004), quaternary glacial sediments (Burke et al. 2012) and the localization and characterization of the geometry of structures typical of volcanic rocks (Gómez-Ortiz et al. 2007). The combined use of these two techniques is also effective in the characterization of shallow aquifers (Doetsch et al. 2011; Turesson 2006). An example of the combined application of these two methods in carbonate rocks can be found in the study by Gómez-Ortiz and Martín-Crespo (2012). However, the combined application of these two methods for characterizing the EM and electrical responses of collapsed paleocave systems is not readily found in the literature.

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In this study, we report the 3D characterization of the internal geometry of a collapsed paleocave system located on the western edge of a Lower Cretaceous carbonate platform in the easternmost portion of northeast Brazil (Fig. 1). High-resolution GPR and geoelectric data were acquired, processed and inverted to obtain the 3D distribution of this paleocave system and of the carbonate host rocks. The GPR and geoelectric signatures of the karst structures are discussed in light of the changes in the physical and petrophysical properties of the sedimentary layers during the karstification processes. Finally, an evolutionary model of this collapsed paleocave reservoir analog is proposed.

2. Geological Aspects

The collapsed paleocave system is located on the western border of the Potiguar Basin, in the easternmost portion of northeastern Brazil (Fig. 1). This basin of Neocomian age extends over an area of approximately 48,000 km2 (considering both its emerged and submerged portions) and is surrounded to the south and west by crystalline basement rocks (Matos 1992; Araripe and Feijó 1994). The sedimentary fill of the Potiguar Basin is associated with the different phases of its tectonic evolution that ended with the opening of the Equatorial Atlantic (de Castro et al. 1998). The rift phase is responsible for a major graben system 200 km in length with depths of up to 7 km. With the transfer of the extensional efforts to the north during the Barremian, the Potiguar rift was aborted, and continental break-up was established in the current shoreline. In the drift phase, subsidence occurred throughout the basin, followed by widespread marine transgression. In the period between the Turonian and Meso-Campanian, an extensive shallow carbonate platform was established, covering the basin with a package of up to 600 m in thickness (Pessoa Neto et al. 2007). The top of this carbonate sequence (Jandaíra Formation) is shaped by an important Neocampanian divergence, which marks the end of the main transgressive sequence. Currently, tertiary and quaternary continental siliciclastic rocks partially cover the carbonate sequence, primarily in the coastal regions.

Currently, the carbonate platform has a geographical boundary in the west marked by an escarpment with a maximum height of several dozen meters. Above and below the escarpment, there is a narrow strip of tufa, variable in width (285-780 m) and 17 km long, deposited in non- thermal waters, where the complex collapsed paleocave system under study was established (Reyes Perez et al. 2003). The studied outcrop is partially exposed on the escarpment of the carbonate platform of the Potiguar Basin.

In the studied area, Jesus et al. (2010) identified three distinct lithofacies (Fig. 2). The lower part of the outcrop exhibits a layer of fractured calciferous sandstone, which is the transition zone

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between the lower siliciclastic (Açu Formation) and the overlying carbonate (Jandaíra Formation) sequences. This sedimentary unit is composed of medium-size sandstone layers, locally brecciated, cemented by calcium carbonate. These layers show low-angle stratification and cross-stratification sets (cross-bedding and tabular cross-bedding), tangential at the base and truncated at the top. Above this layer is a sequence of fine limestone, which underwent an intense pedogenic process, forming a friable soil (2 in Fig. 2). Prior to pedogenesis, these rocks underwent an intensive karstification process, most likely concomitant with tufa deposition, in the upper part of the sedimentary package (3 in Fig. 2).

The collapsed paleocaves were filled mainly by tufa, ceiling fragments (stalactite), stalagmites and the walls of the cave. Tufas were deposited according to tabular strata with slightly plane-parallel stratification, marked by macrophyte molds. Speleothems and rock fragments from the cave ceiling and walls are found inside the paleocave, arranged in a chaotic pattern, with layers of irregular geometry composed of stalagmite and stalactite fragments (Jesus et al. 2012). Measurements of the petrophysical properties obtained in rock samples from the paleocaves indicate porosity intervals of 30-40% and permeability between 130-136 mD (Table 1). In calciferous sandstone, these values do not exceed 8% porosity, with almost zero permeability.

3. Geophysical Data

The geoelectric and GPR data were collected along six profiles, distributed throughout the studied outcrop (Fig. 3). The geophysical line parallel to the road cut allowed a direct correlation with the karst features and the outcropping sedimentary facies. Two common midpoint (CMP) surveys were performed to analyze the propagation velocity of the EM waves. Additionally, a differential global positioning system (GPS) was used to determine the precise geographic position of the geophysical data and the altimetric variations along the survey lines.

3.1. Electrical resistivity tomography

The geoelectric profiles were carried out using a multi-electrode resistivity-meter SYSCAL Pro Switch (IRIS Instruments). Altogether, 571 m of geoelectric profiles were surveyed using 48 electrodes spaced every 2 m to acquire the northeast-southwest lines and every 1 m for the northwest-southeast lines. The Wenner-Schlumberger array enabled eight levels of investigation, generating apparent geoelectric pseudo-sections with depths of approximately 18 and 9 m, respectively.

The apparent electrical resistivity data recorded in the field were processed with the RES2DINV (Loke, 2002) including topography. Covariance matrix is commonly used to assess the

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accuracy of the inversion for models that consist of a small number of parameters but RES2DINV, like most nonlinear inversion programs, carries out an optimization process that tries to reduce the difference between the calculated and measured apparent resistivity values. The inversion routine used by the program is based on the smoothness-constrained least-squares method (de Groot-Hedlin and Constable 1990). The average root-mean-square (RMS) error was of 5.6% for a maximum of 16 iterations. Figure 4 shows the geoelectric pseudo-sections of the L1 profile, showing an increase in resistivity of approximately 3,500 Ω·m from bottom to top in all profiles. The maximum local resistivity, close to the surface, is coincident with the location of the collapsed paleocaves, showing the geoelectric signature pattern of these karst features, while the carbonate host rocks have lower resistivity (Table 1).

3.2. Ground probing radar (GPR)

The GPR data were acquired using SIR-3000 equipment (GSSI Inc.) with a 200 MHz antenna, using a time window of 220 ns with samplings of 512 samples per trace and 50 traces per meter. In total, 612 m of GPR lines was obtained. Two sections of CMP array were also surveyed (yellow triangles in Fig. 2) to analyze the propagation velocity of the electromagnetic waves and thus estimate the dielectric constant of the collapsed paleocaves. The CMP was acquired using bi- static 80 MHz antennas with a time window of 350 ns and 512 samples per trace. For this acquisition, the antennas were initially spaced at 1.0 m, which was gradually increased by 0.2 m for each new reading until a maximum opening of approximately 35 m was reached.

The purpose of processing the GPR data was to highlight the geophysical responses of the studied karst features, using the profile acquired parallel to the road cut as reference. We adopted the following processing routine: time zero correction, background removal, dewow, removal of gain in acquisition, energy decay gain, bandpass filter and topographic correction. Finally, the time to depth conversion was performed using velocities obtained by applying the semblance method to the CMP data. The result of the GPR data processing is exemplified in the initial part of the L1 profile (Fig. 5). The GPR section was corrected to time zero, and the direct waves and low- and high-frequency noise were removed. The effects related to electromagnetic induction and signal attenuation with depth were corrected, highlighting areas of high amplitude related to the paleocave regions and recovering the signal from deeper layers, at approximately 6 m depth.

To improve the mapping of the collapsed paleocaves, the root square (RS) energy or root mean square (RMS) amplitude attribute was applied to all processed GPR sections. This attribute is based on the calculation of the square root of the energy, which is calculated by dividing the sum of the squares of the amplitudes by the number of samples for a predetermined time interval (dGB

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Earth Sciences 2011). This time window must have a length close to the period of the central frequency of the signal. This attribute is very sensitive to high amplitude values and thus can highlight regions of contrasting EM impedance and help to distinguish lithologies with different amplitude ranges. The application of the RS energy attribute to the GPR section of the L1 profile is shown in Figure 5C. The mean values of the RS energy are shown in Table 1.

In the CMP sections, only dewow filter to eliminate the effect of the electromagnetic induction of the antennas and energy decay gain to suppress the effects of the GPR signal attenuation with depth were applied. Using the Semblance method, velocities between 0.103 and 0.144 cm/ns were obtained (Fig. 6). For the time-to-depth conversion in the GPR sections, a mean velocity of 0.106 m/ns was chosen. This velocity allowed the best correlation between the GPR data and the sedimentary structures observed in the road cut (Fig. 2). Additionally, we estimated a mean dielectric constant of 5.0 for the material that fills the collapsed paleocaves, based on the mean velocity of 0.135 m/ns (Table 1), while the limestone host rock has a higher dielectric constant, approximately 8.5 for a velocity of 0.103 m/ns.

4. Mapping of Karst Features

The interpretation of the GPR and ERT sections was initially based on the direct association between the geophysical signal along the parallel profile and the sedimentary features found in the road cut (Fig. 2). This procedure also served to calibrate the depths of the reflectors and validate the interpretations of the GPR sections. In the L1 profile, the three geological layers can be easily recognized by the resistivity distribution and the patterns of the GPR reflections (Fig. 7). The bottom layer (1) of calciferous sandstone is characterized by low resistivity values, ¿? ranging from 3 to 190 Ω·m, and by parallel and short reflectors with amplitudes ranging from low to medium in the GPR section. In both geophysical sections, this layer becomes shallower toward the northeast, reflecting the observations in the road cut. The intermediate layer of carbonate soil (2) shows