Subsidence rings and fracture pattern around dolines in carbonate platforms – Implications for evolution and petrophysical properties of collapse structures

Marine and Petroleum Geology 113 (2020) 104113

Contents lists available at ScienceDirect

Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo

Research paper

Subsidence rings and fracture pattern around dolines in carbonate platforms – Implications for evolution and petrophysical properties of collapse structures

T

Daniel F. Menezesa,b, Francisco H. Bezerrab,∗, Fabrizio Balsamoc, Andrea Arcaric, Rubson P. Maiad, Caroline L. Cazarine a

Petrobras/E&P-EXP, Natal, RN, Brazil Programa de Pós-Graduação em Geodinâmica e Geofísica, Universidade Federal do Rio Grande do Norte, Brazil Dipartimento di Scienze Chimiche, della Vita e della Sostenibilità Ambientale, Università di Parma, Italy d Departamento de Geografia, Universidade Federal do Ceará, Brazil e Petrobras Research Center, Rio de Janeiro, RJ, Brazil b c

ARTICLE INFO

ABSTRACT

Keywords: Dolines Karst Aquifer Oil reservoirs Permeability Collapse risk

This work focuses on the study of collapse dolines, which are the most expressive collapse structures in carbonate rocks, and their relations with preexisting and syn-collapse fractures. The study area has two fracture sets that were formed before folding, early N-S/E-W- and late NE-SW/NW-SE-striking sets, which concentrate most of the dissolution in the region and allow the formation of the dolines. We define subsidence rings as the circular and ellipsoidal concentric zones around collapse structures, which are subjected to subsidence due to major collapses and represent locations where new fractures are formed. In these subsidence rings, the downfaulted topography plunges towards the doline center and reaches more than 10 m in relation to unaffected areas away from dolines. The topographic data indicate that the mean radius of the combined rings is ~twice the radius of the collapse, which corresponds to the closed depression due to failure and downfall of blocks. The subsidence process enlarges, links preexisting fractures, and forms a new set of semicircular concentric opening mode fractures, here named collapse fractures. Increases in the apertures and densities of these fractures occur towards the dolines, which increases fracture porosity around collapse structures. Fractures are reactivated as normal faults close to the main collapse at the doline edge. This increase in fracture intensity could represent an indicator of permoporous quality improvement in these areas. Further, this fracturing increases structural instability, raising the risk of accidents in areas built on soluble carbonate rocks, since the affected area may be much larger than previously predicted. Subsidence rings around collapse dolines could merge with other rings from neighboring collapse structures and potentially increase porosity and permeability, as well as linking areas in carbonate reservoirs.

1. Introduction Dissolution promotes karstification, especially in extremely soluble rocks such as evaporites and carbonates. Among the diverse karst-related structures generated in this type of environment, dolines stand out as one of the most important (Gutiérrez et al., 2008). The genesis and occurrence of dolines are diverse, and their diameters and depths range from tens to hundreds of meters. Therefore, they can be identified from outcrop to seismic scales (Augarde et al., 2003; Palmer and Palmer, 2006; Phung et al., 2017). One of the most common types of doline is generated from subsurface mass removal and collapse of a cave ceiling

∗

(Gabrovšek and Stepišnik, 2011). Dolines are defined as natural depressions that may have been excavated or collapsed in karst terrain (Waltham et al., 2005). Collapse dolines are closed semi-circular depressions due to failure and downfall of blocks, which occurs above an unstable underground cavity (Ford and Williams, 2007). Geometrically, they show subcircular shapes and steep slopes (Williams, 2004; Kranjc, 2013) and are generally filled with debris from surrounding areas, such as the blocks of the collapsed layers, developing different types of breccias (Kranjc, 2013; Silva et al., 2017). Fractures behave as one of the most influential elements in the generation of dolines. They act as conduits for fluids that promote

Corresponding author. E-mail address: [email protected] (F.H. Bezerra).

https://doi.org/10.1016/j.marpetgeo.2019.104113 Received 8 September 2019; Received in revised form 26 October 2019; Accepted 28 October 2019 Available online 01 November 2019 0264-8172/ © 2019 Elsevier Ltd. All rights reserved.

Marine and Petroleum Geology 113 (2020) 104113

D.F. Menezes, et al.

Fig. 1. Location map of studied dolines in the Potiguar Basin, Brazil.

karstified carbonate rocks have great reservoir potential since these processes may induce increases in reservoir permeability (Jeanne et al., 2012; Ali et al., 2016). However, the formation of collapse structures can also cause problems during production when creating paths for aquifer-reservoir communication (Qi et al., 2014). Unfortunately, there is little information available on the generation of new fractures related to collapse dolines, which may include new variables in geological and numerical models. In addition, unpredicted fractures can change the subsurface fluid flow in karstified carbonates and increase reservoir quality (Ali et al., 2016). The investigation of dolines in risk management studies is also important. Catastrophic events involving urban areas built on karst terrains have long been recognized (Galve et al., 2011). Impacts range from immediate effects such as loss of life or damage to properties at the time of collapse to losses arising from subsequent processes (Intrieri et al., 2015). Attempts to predict the occurrence of dolines in areas at risk have been made using numerous techniques, for example, monitoring photographs taken over time to seek changes in topography or identifying possible areas more susceptible to dissolution through field analyses (Gutiérrez et al., 2008; Siska et al., 2016; Panno and Luman, 2018). Moreover, several authors have taken advantage of the improvements in remote sensing to map karst structures across large areas. The use of tools such as a large geographic information systems (GIS) database, LiDAR, thermal and other sensors associated with advanced image processing software allows the characterization of dolines with high accuracy in a short time (Florea et al., 2012; Lee et al., 2016; Silva et al., 2017; Zumpano et al., 2019). Regional tectonic discontinuities seem to control the arrangement of dolines in some areas, as the elongation of the structures appears to indicate a major fracture direction (Florea, 2005; Öztürk et al., 2018; Lipar et al., 2019).

Fig. 2. Conceptual sketch describing the karst features and the vein or stylolite attributes measured with the linear scanlines. V: vein, Va: vein aperture, VL: vein length, Vs: vein spacing, PKV: partially karstified vein, FR: open fracture, ST: stylolite, SL: stylolite length, STK: karstified stylolite, SS: stylolite spacing, Dw: dissolution width, Dh: dissolution height and DL: dissolution length. Spacing between karstified veins or stylolites is not measured from the borders of the karstified area but from the center because the dissolution is assumed to spread from the center of veins or stylolites. Linear scanlines were draw always orthogonal to the main fracture systems.

dissolution and subsequent collapse of the layers above (Florea, 2005; Kaufmann and Romanov, 2016; Xu et al., 2016, Cahalan and Milewski, 2018). The important relationship between dolines and fractures affects oil reservoirs in different ways and has been discussed in several studies (Fournillon et al., 2017; Phung et al., 2017). First, fractured and

2

Marine and Petroleum Geology 113 (2020) 104113

D.F. Menezes, et al.

Fig. 3. Subsidence rings around dolines: (A), (C), (E) and (G) (S1, S3, S5, S8, on Fig. 1, respectively) are UAV images of collapse dolines exposing subcircular geometry. (B), (D), (F) and (H) represent their respective digital elevation models. Each color represents a different topographic elevation range of 30 cm to 10 m. Arrows point to some of the subsidence rings. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

However, several studies have not considered the role of background fracture systems in the dissolution processes (Gabrovšek and Stepišnik, 2011, Kaufmann and Romanov, 2016, Lipar et al., 2019). In addition, a few studies have used the technological advances in remote sensing to acquire high-resolution imagery, seeking to detect features not previously seen (Lee et al., 2016; Silva et al., 2017; Zumpano et al., 2019).

Despite these efforts, authors have not observed how the presence of dolines, preexisting and new fractures structurally disturbs their surroundings. The main aims of the present study are (1) to identify structurally unstable areas surrounding collapse dolines, named here as subsidence rings; (2) to assess the role of preexisting fracture systems in the 3

Marine and Petroleum Geology 113 (2020) 104113

D.F. Menezes, et al.

Table 1 Average radius of subsidence rings by the doline radius. Key: Drd (Doline average radius); SRrd (Subsidence ring average radius). Locations in Fig. 1. Doline

Drd (m)

SRrd (m)

Ratio

S1 S2 S3 S4 S5 S6.1 S6.2 S7.1 S7.2 S7.3 S8 S9

15 6 12 6 17 15 20 2.5 3 2 5 1.5

40 10 35 10 20 15 15 6 7 4 14 4 Mean values

2.67 1.67 2.92 1.67 1.18 1.00 0.75 2.40 2.33 2.00 2.80 2.67 2.00

Table 2 Gradient of topography variation inside subsidence zones. Key: Δtop (Variation of topography); iSZL (Inner subsidence zone average length); oSZL (Outer subsidence zone average length). Locations in Fig. 1.

generation of collapse dolines; and (3) to identify and assess the generation of new gravity-induced fractures/faults, referred to here as collapse fractures, around collapse dolines. The results obtained in this study are very important for karst water and oil reservoirs, as the presence of new fractures can represent increases in the porosity and permeability surrounding dolines. These results could also have implications for risk management in areas built over karst terrains; past studies have ignored the existence of subsidence rings, which represent areas of structural instability much larger than previously predicted.

Doline

Δtop(m)

iSZL(m)

Gradient (%)

S1 S2 S3 S4 S5 S6 S8

5 1 3 6 5 3 3

10 5 10 7 6 10 15 Mean values

50 20 30 85 83 30 20 45

Doline

Δtop(m)

oSZl (m)

Gradient (%)

S1 S2 S3 S4 S5 S6 S8

5 1 5 2 2 3.5 3

20 10 25 6 12 20 10 Mean values

25 10 20 33 16 17 30 21

The sedimentary record is composed of two supersequences: rift and postrift. This study focuses on the Eoturonian postrift Jandaíra Formation, which is mainly composed of carbonate rocks deposited above the fluvial siliciclastic Açu Formation. The Jandaíra Formation represents an event of maximum marine transgression on a large platform/tide-dominated ramp. The sedimentary package of this unit dips smoothly NNE, reaching approximately 600 m in thickness. At the end of the Cretaceous and in the early Paleogene, a strike-slip stress field composed of regional, subhorizontal N-S-trending compression and subhorizontal, E-W-trending extension affected the Jandaíra Formation. The new stress field induced the development of NS-striking fractures that convey fluids (Bertotti et al., 2017; Bezerra

2. Geomorphological and geological settings The study area is in the southwestern region of the Potiguar Basin, where the fill is 5000 m thick in its onshore portion (Fig. 1). This basin originated during the breakup of Pangea in the Early Cretaceous, when the African and South American plates separated (Matos, 1992, 2000). The major faults of the basin are NE-SW-striking normal faults and NWSE-striking strike-slip faults, which were reactivated in the Cenozoic (Bezerra and Vita-Finzi, 2000; Reis et al., 2013; Bezerra et al., 2019).

Fig. 4. Example of how subsidence and fracture rings around dolines are measured. (A) DEM of a doline where the colors represent elevation variations (B). Limits of collapse and subsidence zones. (C) NE-SW topographic profile of a doline. (D) Collapse and subsidence zones represented in the topographic profile. The location of the sinkhole corresponds to S3 in Fig. 1. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

4

Marine and Petroleum Geology 113 (2020) 104113

D.F. Menezes, et al.

Fig. 5. Coalescence of subsidence rings: (A), (B) and (C) (S3, S6, S5, on Fig. 1, respectively) are digital elevation models of some dolines. Color variations represent elevations ranging from 30 cm to 10 m. Examples of dolines and rings are oriented along the NE-SW-striking fracture set. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 6. Structures formed before the doline formation at the outcrop scale. (A) Partially to totally dissolved fractures. (B) E-W-striking stylolite with a low degree of karstification. (C) Calcite-filled veins. In (B) and (C), the vein or stylolite length coincides with the dissolution length. Outcrop located at S2 shown in Fig. 1.

5

Marine and Petroleum Geology 113 (2020) 104113

D.F. Menezes, et al.

Fig. 7. Preexisting fracture attributes at S1, S2 and S3. (A), (E) and (I) Mean bedding planes. (B), (F) and (J) Stereonets of the vein strikes. (C), (G) and (K) Stylolite attitude stereonets. (D), (H) and (L) Attitudes of NE-SW- and NW-SE-striking fractures. Key: n, number of measurements; Mean plane (dip direction, plunge).

et al., 2019). Stable isotope studies indicate that these fluids have a meteoric origin, coming from recharge of the existing aquifer in the sandstones of the Açu Formation and migrating upwards to the Jandaíra Formation (de Graaf et al., 2017). The increase in subsidence allowed the precipitation of vertical N-S- to NNE-SSW-striking calcite veins and generated vertical E-W- and WNW-ESE-striking stylolites, which were formed at depths between ca. 500 and 800 m (Bertotti et al., 2017). Intense karstification events have been identified in outcrops of the Jandaíra Formation in the emergent portion of the basin (Silva et al., 2017). The current semiarid climate in northeastern Brazil started in the late Quaternary. This karstification process has been more effective during rainy periods, when dissolution intensifies along fractures and bedding surfaces, and weathered material is removed from caves. These events cause enlargement of the fracture system, culminating in the various karst structures that can be observed in the Jandaíra Formation (Silva et al., 2017). The intense dissolution of these fracture systems also led to an increase in the permeability of this slightly deformed Jandaíra unit (Bertotti et al., 2017; Bisdom et al., 2017; de Graaf et al., 2017; Silva et al., 2017).

3. Methods and materials 3.1. Remote sensing As the work aimed to study the occurrence of collapsed zones around dolines and their relation to fractures, it was necessary to acquire high-resolution images, allowing the visualization of these structures at larger scales than conventionally applied, such as the previous use of radar and satellite images. Structural data were also acquired in the field to constrain the background fracture pattern and to understand the influence of preexisting fracture system on the dolines. In total, thirteen dolines in different stages of evolution were analyzed, with four main sites containing most structural data (Fig. 1). We performed a photogrammetric survey with unmanned aerial vehicle (UAV) on automatic flights at altitudes between 20 and 100 m above the mean ground surface, with 90% overlap and with a camera positioned 90° from the terrain. Key features of the equipment included a three-axis stabilizer camera and a Complementary Metal-Oxide Semiconductor (CMOS) sensor. Processing the acquired images was possible with the Agisoft Photoscan Professional, Global Mapper 19 and ArcMap 10 software packages. Aerial photographs had a resolution of

6

Marine and Petroleum Geology 113 (2020) 104113

D.F. Menezes, et al.

Fig. 8. Background fracture (veins and stylolites) attributes at S1, S2 and S3. (A), (B) and (C) are vein attributes in S1. (D) and (E) are stylolites measured in S1. (F), (G) and (H) represent veins in S2. (I), (J) and (K) are attributes of veins in S3. (L) and (M) contain stylolite attributes in S3. Locations of sites shown in Fig. 1.

less than 1 cm/pixel. This feature allowed us to use photogrammetry and observe centimetric variations in the mean elevation around the dolines, which was fundamental for the identification of subtle relief variations. Further, the high-resolution drone images were useful to map in detail the fracture pattern in selected field sites, both near and far from the doline.

crosscut the vein and stylolite sets. Therefore, we are calling N-Sstriking fractures as veins, E-W-striking fractures as stylolites, and NESW/NW-SE-striking fractures as joints. For each fracture set, the strike/ dip, length, aperture and spacing between adjacent elements were measured. Where possible, at least 30 data points were collected for reliable statistical analysis. The scanlines were made both in the N-S direction (to intercept the E-W stylolites) and in the E-W direction (to intercept the N-S veins). In places, veins and stylolites were partially or totally karstified, allowing the measurement of karst attributes such as length, height and width of dissolution along these fractures. For example, vein aperture is related to non-karstified fractures filled mainly with calcite, while dissolution width corresponds to the space created after dissolution. All measured fracture and karst attributes were statistically analyzed in order to constrain the amount of deformation and dissolution in each sector and at each site. Quantification of deformation intensity was accomplished using the free-download software FracPaQ (Healy et al., 2017) directly on fieldmapped drone images, i.e., using the whole fracture pattern traced manually in the field. FracPaQ allowed us to use the circular scan area method (Sanderson and Nixon, 2015) to quantify the P20 and P21 parameters as follows:

3.2. Fracture mapping and structural analyses The occurrence of a preexisting orthogonal fracture system in the region allowed us to investigate the relationships between the formation of dolines, their respective subsidence rings, and the collapse fractures. Structural data (fracture types and attitudes) were collected in the field at four main sites characterized by pavement exposures (sites S1, S2, S3 and S4 in Fig. 1). A total of 887 strike/dip fracture data were collected. At site S2, we quantified the background deformation pattern (preexisting fracture systems), whereas in sites S1, S3 and S4, we quantified both the background deformation pattern and the fracture pattern related to the development of doline. Fracture attributes were also recorded in the field along linear scanlines (e.g., Watkins et al., 2015) according to the conceptual scheme in Fig. 2. In this case, N-S-striking fractures represents veins, partially karstified veins and open fractures. The E-W-striking features are stylolites and karstified stylolites. Joints are commonly NE-SW- and NW-SE-striking and

P20 = NL/A[L 2]

7

Marine and Petroleum Geology 113 (2020) 104113

D.F. Menezes, et al.

occurrence of topographic relief below the background topography around dolines, hereafter referred to as subsidence rings (Fig. 3). These rings expand from the edges of the dolines to the surrounding country rocks. Within the rings, the regional topography plunges towards the dolines (Fig. 3B, D, F, G). In these images, each color represents a topographic variation from 30 cm to 10 m (see the color scale of the image to check the precise height). The average radius of the rings decreases towards the doline, which represents an increase in the topographic slope. These radiuses were measured from the edge of the collapses to the undisturbed areas (not including the already collapsed areas). The topographic relief differences between the flat area around the doline and the main collapse (the collapse of the void below ground) can reach more than 10 m. Subsidence rings terminate where the topography reaches the regional relief (red to violet colors), indicating the limits of the area influenced by the formation of the collapse doline. We measured the size of the affected area in relation to the radius of each doline (Table 1). The average radius of the total subsidence rings is generally twice as large as the average radius of the doline. Based on the analysis of topographic relief in the rings, two regions around dolines can be defined: outer and inner subsidence zones (Fig. 4B, D). The inner subsidence zone corresponds to the steep topography located close to the doline (Fig. 4D). This zone concentrates the greatest number of rings, representing larger differences in topography at shorter distances. The topographic gradient varies from 45% in the inner subsidence zone to 21% in the outer subsidence zone (Table 2). The inner subsidence zone is also the area most likely to collapse if the dissolution processes continue and the doline is enlarged. The subsidence outer zone corresponds to the area where the topography is still influenced by the main collapse, i.e., the collapse of a void below ground, but dips smoothly and is hard to identify without the aid of high-resolution images. Subsidence rings are formed by the inner and outer subsidence zones (Fig. 4D). Most dolines occur as isolated structures, representing a sector where dissolution is concentrated. In some cases, dissolution occurs along fractures, where two or more dolines tend to develop. We observe that when the subsidence rings of different dolines coalesce, their rings merge, creating a larger area of subsidence (Fig. 5). The evolution of this process can culminate with the formation of the karst landform known as “uvala” (Kranjc, 2013), representing the coalescence of two or more dolines.

Fig. 9. Example of collapse fractures developed near the dolines. Note the spacing between the fractured blocks (A and B). The collapse increases the apertures of the fractures, causing the rotation and collapse of blocks. Photographs from S1 with location shown in Fig. 1.

P21 = NL Lc/A[L 1]

4.2. Structural data and fracture attributes

where P20 represents the areal frequency (fracture density), P21 is the areal intensity, NL is the number of fractures in each circle of area A and LC is the mean fracture length in each circle. A circle radius of 2.25 m was selected because it contained 30 fracture tips (in the less densely fractured areas) inside the circle perimeter (Rohrbaugh et al., 2002). This scan area method was applied to UAV images in the sectors where the fracture pattern was traced and ground-truthed directly in the field to obtain more reliable estimates of fracture abundance.

4.2.1. Background fractures The Jandaíra carbonates in the study area are characterized by a dense network of fractures that encompasses veins, stylolites, and joints (Fig. 6). In horizontal pavements, all these structures show intense karstification that ranges from mm-wide selective dissolution localized into the veins and stylolites (Fig. 6B and C) to tens of centimeters-wide dissolution, which partially or totally affects the fractures (Fig. 6A). Veins and stylolites are commonly strata-bound, i.e., confined to beds, although some exceptions are observed. In the study area, sedimentary bedding is subhorizontal (Fig. 7A, E, I), dipping ~2° SE at site S1 and NW at sites S2 and S3. Veins, stylolites and joints share the same strikes/dips in these sites (Fig. 7). Veins are subvertical and oriented N-S (Fig. 7B, F, and J), whereas tectonic stylolites are orthogonal to bedding and veins and oriented E-W (Fig. 7C, G, and K). Finally, the study sites are locally characterized by the presence of NE-trending joints and NW-trending cross-joints (Fig. 7D, H,

4. Results 4.1. The use of topographic data to identify subsidence rings 4.1.1. Doline geometry and subsidence rings In the study area, the majority of the dolines exhibit elliptical shapes with NE-SW-trending major axes (Fig. 3A–D). The digital elevation model (DEM) from UAV aerial imagery systematically shows the

8

Marine and Petroleum Geology 113 (2020) 104113

D.F. Menezes, et al.

Fig. 10. Relationship among doline, preexisting fractures and collapse fractures. (A) High-resolution UAV photo of doline at site S1 with characteristics of a collapse resulting from dissolution in the subsurface; the doline exhibits an ellipsoidal shape along the preexisting NE-SW-striking set. Rose diagrams of fractures: blue, stylolites; red, veins; black, collapse fractures. (B) Preexisting and collapse fractures surrounding the doline. Site S1 location shown in Fig. 1. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

and L). Joints are less abundant than veins and stylolites and generally crosscut them. We define the structural and sedimentary attributes at several sites. Sedimentary bed thickness ranges between 30 and 70 cm. Background fracture attributes (vein spacing, length, and aperture and stylolite spacing and length) are summarized in Fig. 8A-M. Vein spacing generally ranges between a few cm and 1 m, with mean values of 25.1 ± 20.7 cm, 39.4 ± 22.2 cm and 31 ± 29.9 cm at sites S1, S2 and S3, respectively (Fig. 8A, F, I). Vein length broadly ranges from tens

of centimeters to 1.5 m, with mean values of 112.6 ± 92.2 cm, 295.9 ± 272.7 cm and 99.9 ± 94 cm at sites S1, S2 and S3, respectively (Fig. 8B, G, J). Vein aperture is generally constant with values between 0.05 and 0.5 cm (Fig. 8C, H, K). Stylolites at sites S1 and S3 have spacing and length values that are comparable to those of the veins; their mean spacings are 59.8 ± 38.9 cm and 36.3 ± 27.9 cm at sites S1 and S3, respectively, whereas their mean lengths are 136 ± 113.6 cm and 58.5 ± 51.3 cm (Fig. 8D, E, L and M).

9

Marine and Petroleum Geology 113 (2020) 104113

D.F. Menezes, et al.

Fig. 11. Collapse fracture attributes. (A) Site S1 collapse fracture aperture. (B) Site S1 collapse fracture height. (C) Site S1 collapse fracture length. (D) Site S3 collapse fracture aperture. (E) Site S3 collapse fracture height. (F) Site S3 collapse fracture length. Locations of sites shown in Fig. 1.

4.2.2. Collapse fractures Within the subsidence rings recognized through topographic elevation analysis (Figs. 3 and 4), a set of opening-mode fractures (i.e., joints) is systematically identified (Fig. 9A and B). These structures, referred to here as “collapse fractures”, differ from the previous vein and stylolite systems as (1) they show curved shapes (Fig. 9B), (2) they systematically cross-cut the veins and stylolites, and (3) their occurrence is limited to the area surrounding the dolines (Fig. 10B). Collapse fractures are developed to 15–20 m from the doline border at sites S1 and S3. This distance is 10–15 m at site S4. These new opening mode fractures are reactivated as normal faults close to the doline borders. The fracture aperture, height, and length were measured along linear scanlines at sites S1 and S3, as reported in Fig. 11. In general, collapse fractures show greater apertures than the strata-bound veins and stylolites because the selective dissolution of collapse fractures is more developed (i.e., height is often greater than the bed thickness) and longer (Fig. 11). The mean fracture aperture (i.e., dissolution) is ~9–10 cm (Fig. 11A, D), the mean height is ~40–45 cm (Fig. 11B, E), and the mean length is ~10 m (Fig. 11 C, F). Collapse fractures often reopen the preexisting background fracture system.

4.3. Fracture density and intensity around dolines Linear and areal scanlines were measured to evaluate the fracture density (number of fractures/meter and P20) and intensity (P21) in the background domains and in the doline subsidence rings. The linear scanlines were acquired at sites S1, S3, and S4, which have the best structural exposures around collapse dolines in the study area (Fig. 12). In all twelve scanlines, we identified an increase in fracture density towards the dolines (Fig. 13), i.e., an area characterized by the presence of background and collapse fractures. Fracture density and intensity in scan areas (P20 and P21 calculated in circular scan areas with a 2.5 m radius) were also calculated at site 1 in four different sectors (Fig. 14). These sectors were selected due to their clear pavement exposures in areas outside and inside the subsidence rings. We field-traced fractures in site S1, where four sectors are present (Fig. 14 A). Sector 1.1 is located ~40 m from the doline border and consists of three scan areas (C1.1, C1.2, and C1.3); sector 1.2 is located ~30 m from the border and consists of three scan areas (C2.1, C2.2, and C2.3); sector 1.3 is located in the outer part of subsidence rings and consists of three scan areas (C3.1, C3.2, and C3.3); and sector

10

Marine and Petroleum Geology 113 (2020) 104113

D.F. Menezes, et al.

Fig. 12. Locations of linear scanlines around three dolines with visible fracture sets at the surface. (A) Six scanlines around doline at S1. (B) Four scanlines around doline at S2. (C) Two scanlines around doline at S4. Locations of sites shown in Fig. 1.

1.4 is located near the perimeter of the doline and consists of three scan areas (C4.1, C4.2 and C4.3). The fractures analyzed within these scan areas are represented in Fig. 14B, C, D, and E. In these circles, background fractures are in black, whereas collapse fractures are highlighted in red. Table 3 is related to Fig. 14 and shows the number of fractures, their lengths, the scan area, P20 and P21 for each scan area. Approaching the doline, the mean P20 value increases from 4.08 (sector 1.1) to 7.06 (sector 1.4); similarly, the P21 value increases from 3.88 (sector 1.1) to 5.42 (sector 1.4). We also estimate, for sectors 1.3 and 1.4, increases of 7–8% and 7–13% in P20 and P21, respectively, with respect to the values obtained by removing the collapse fractures, i.e., considering only background fractures. Therefore, the presence of collapse fractures in sectors 1.3 and 1.4 induces an increase in the areal fracture frequency (P20) and the fracture intensity (P21).

influence of these structures. As with every karst system, collapse dolines start with dissolution. Fractures act as drainage nets for fluids to percolate and dissolve rocks in karst terrains such as the carbonate units in the study area. The increase in dissolution in subsurface layers generates structural instability of a cave ceiling, triggering its collapse and consequent formation of a doline (Waltham et al., 2005, Gabrovšek and Stepišnik, 2011, Kaufmann and Romanov, 2016, Cahalan and Milewski, 2018). In the study area, dissolution has been triggered by fluids of epigenic origin, taking advantage of the preexisting fracture systems (Bertotti et al., 2017; de Graaf et al., 2017; Silva et al., 2017). Dissolution causes the widening of fracture apertures and the collapse of small blocks embedded between them, which represents an initial stage in the formation of dolines (Fig. 15A and B). As the fractures are also distributed vertically, the fluids penetrate and dissolve layers in the subsurface, generating small chambers and caves. The mass removal caused by dissolution leads to structural instability in the surface layers, culminating in collapses and consequent formation of large dolines (Fig. 4). These processes affect areas larger than the main collapse, as indicated by the presence of subsidence rings. In the study area, the collapsed material fills the dolines, generating poorly sorted collapse breccias with clasts varying from pebble to boulders. Elsewhere, this

5. Discussion 5.1. Evolutionary model of subsidence rings around collapse dolines The present work develops a conceptual evolutionary model for collapse dolines in a karst carbonate environment, combining different techniques and results that could help predict the occurrence and

11

Marine and Petroleum Geology 113 (2020) 104113

D.F. Menezes, et al.

Fig. 13. Histograms of linear scanlines measured around the dolines. The Y axis represents the number of all types of fractures, whereas the X axis indicates the distance to the doline in meters. SC 1–6 were measured at sites S1, 7–10 at S3 and 11–12 at S4, as shown in Fig. 12.

kind of material represents the main infilling of dolines (Loucks, 1999; Silva et al., 2017). The fracture system is not only responsible for the development of dolines but also affected by them. In areas close to the main collapses, new fractures are created, reactivating preexisting fractures. These new structures are termed collapse fractures and have curvilinear shapes around dolines (Figs. 9 and 10). The surroundings of dolines show increases in fracture density and aperture (Figs. 11 and 13) and are characterized by the presence of collapsed and rotated blocks, which form an area of high structural instability. With the aid of high-resolution digital elevation models, we identified several subsurface dissolution corridors, where many dolines have been generated. The evolution of these processes will probably generate uvalas, causing the coalescence of two or more dolines (Kranjc, 2013). The formation of collapse dolines and the associated structures in an epigenetic karst system can be described by the following stages. (1) Background fractures at the surface dissolve; they are enlarged and

allow fluids to penetrate to subsurface layers (Fig. 15A–D) (Lipar et al., 2019). (2) Fluids dissolve soluble layers and fracture walls in the subsurface, creating dissolution corridors oriented orthogonal to extension. (3) The concentration of dissolution in certain sectors provides continuous mass removal, creating underground voids (Augarde et al., 2003), causing instability of the overlaying layers and generating subsidence reflecting into slight changes in the topography. This stage represents the initial formation of subsidence rings. In this stage, highresolution images may help to identify these areas (Fig. 5A and B). (4) If the dissolution processes continue, structurally unstable layers collapse, and semicircular dolines are generated (Figs. 3, 10 and 16A). Their elongation follows the main fracture system that concentrates dissolution. Subsidence rings are recognized around the dolines at this stage. Collapse joints are also created in this stage because they are induced by the instability generated by the collapse. (5) Incremental dissolution may cause the coalescence of two or more dolines along dissolution corridors (Fig. 16B) controlled by regional fracture systems. These

12

Marine and Petroleum Geology 113 (2020) 104113

D.F. Menezes, et al.

Fig. 14. Measurements of preexisting and collapse fractures. (A) Location of sectors where the circular scanlines were acquired. (B) and (C) Areal frequency (P20) and fracture intensity (P21) from sectors S1.1 and S1.2, away from the doline. (D) and (E) Sectors 1.3 and 1.4, with P20 and P21 calculated closer to the doline, also with the doline collapse-related deformation (collapse fractures). The scanline diameter is 4.5 m. In circles, background fractures are in black, whereas circular fractures are highlighted in red. Doline S1 location shown in Fig. 1. Blank areas without fractures corresponds to areas with soil cover. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

corridors can be predicted with digital elevation models of high-resolution images. The area between dolines in this stage represents a much larger region affected by dissolution and collapses. Subsidence rings and collapse fractures discussed here could evolve to sag geometry and cylindrical faults, respectively, in a paleokarst system, as described by Loucks (1999). The propagation of the collapse fractures and the 3D array of subsidence rings at subsurface levels, however, is a point for further investigation.

5.2. Implications of subsidence rings for oil reservoirs, aquifers and collapse risk The important relationship between fractures and dolines can be observed in several contexts, such as the oil industry. More than 50% of all oil and gas production in the world comes from carbonate rocks (Mazzullo, 2004). In general, these rocks have the natural characteristic of losing primary porosity during cementation and compaction (Halley

13

Marine and Petroleum Geology 113 (2020) 104113

D.F. Menezes, et al.

The results obtained in the present study have significant implications. The generation of collapse fractures around dolines (Figs. 9 and 10), as well as the increases in fracture density in these zones, caused by the sum of preexisting background fractures and the collapse fractures in the subsidence rings (Fig. 13), are indicators of high porosity and permeability zones (subsidence rings). As these structures can be mapped on seismic scales (Phung et al., 2017), wells could be targeted to increase production using these premises. Finally, collapse fractures differ geometrically from the preexisting structural pattern, which could affect fluid flow in reservoirs and aquifers in ways previously not predicted by existing models (Bauer and Tóth, 2017; Fournillon et al., 2017). Petrophysical analysis of the rocks affected by collapse fractures could also help to quantify the impact of these structures in reservoirs. Karst terrains are further recognized for their relation to natural disasters, especially sudden collapses in urban areas (Galve et al., 2011). The occurrence of dolines in these areas generates a variety of risks, such as losses of life, destruction of properties, and contamination of aquifers (Intrieri et al., 2015). To mitigate these risks, a number of studies have sought to map and catalog dolines and their evolution over the years, as well as to attempt to predict their occurrence (Gutiérrez et al., 2008; Siska et al., 2016; Panno and Luman, 2018). We identified large areas around the dolines that are being affected by the main collapse, here named subsidence rings (Fig. 3). High-resolution images and their respective digital elevation models revealed that the rings cover on average areas twice as large as the doline radius (Table 1). Approaching the main collapse, the topographic slope and the density of fractures increase (Figs. 4 and 13), denoting areas of high structural instability. The presence of subsidence rings suggests hazards not previously accounted for in risk management studies in urban areas built on karst terrains. The risks for locations in doline areas may be underestimated. For further studies, it would be interesting to analyze the presence of subsidence rings on other types of dolines and the changes after burial and compaction. The occurrence of dolines can be linked with regional tectonic discontinuities, which may concentrate dissolution in some areas (Florea, 2005). These structures show semicircular geometry, with the major axis oriented parallel to the main fracture direction (Öztürk et al., 2018). Despite progress on doline evolution, several of these studies, however, lack information on the role of background fracture patterns in the formation of dolines. The results in the present study indicate that a NE-SW local trend is responsible for the main dissolution in the area. This fracture set leads to the creation of dissolution corridors close to the surface, which are identified in the digital elevation models as places where several dolines have been generated (Fig. 5). The identification of these background fracture patterns and collapse trends is fundamental to predicting the occurrence of additional dolines below seismic resolution. To improve these results, the relationships between rock petrophysical properties and the propagation of fractures in specific areas are important parameters to be analyzed. This relationship could reveal information about why some areas are more susceptible to collapses than others. However, this topic is beyond the goals of the present study and should be investigated in future studies. Other collapse structures, such as breccia pipes, have similar development processes. Their formation can be associated to dissolution trends, gravity-induced collapses, and formation of new fault planes (Broughton, 2017). This may indicate the occurrence of subsidence rings in its surroundings, what could help determine the size of the damage zone around breccia pipes and its impact on fluid flow. This opens new possibilities to other similar structures to be investigated, and to correlate the results presented here to other environments and settings.

Table 3 Circular scanlines results. Key: NL (Number of fractures in circles); Lc (Length of fractures in circles); A (Area). Data acquired in S1. Sector 1.1 Scan

Nl

LC(m)

A (m2)

P20(L−2)

P21(L−1)

C- 1.1 C- 1.2 C- 1.3

59 77 59 Mean values

1.07 0.82 1

15.9 15.9 15.9

3.711 4.843 3.711 4.088

3.970 3.971 3.711 3.884

Scan

NL

LC(m)

A (m2)

P20M

P21M

C - 2.1 C - 2.2 C - 2.3

73 67 89 Mean values

0.88 0.96 0.8

15.9 15.9 15.9

4.591 4.214 5.597 4.801

4.040 4.045 4.478 4.188

Sector 1.2

Sector 1.3 (including collapse fractures) Scan

NL

LC(m)

A (m2)

P20(L−2)

P21(L−1)

C - 3.1 C - 3.2 C - 3.3

79 85 103 Mean values

0,93 0,9 0,85

15,9 15,9 15,9

4969 5346 6478 5597 +8.09%

4621 4811 5506 4979 +7.55%

Sector 1.4 (including collapse fractures) Scan

NL

LC(m)

A (m2)

P20(L−2)

P21(L−1)

C −4.1 C −4.2 C −4.3

105 135 97 Mean values

0,78 0,75 0,78

15,9 15,9 15,9

6604 8491 6101 7065 +7.32%

5151 6368 4758 5426 +13.22%

and Schmoker, 1983), reinforcing the importance of fractures, which increase the reservoir quality potential (Ali et al., 2016). Nevertheless, collapse structures such as dolines can cause production problems by creating communication zones between nearby aquifers and the main reservoir (Qi et al., 2014). However, as previously mentioned, there is a lack of information regarding how new fractures are generated around dolines and how they affect the existing reservoir models. Paleokarst reservoirs are formed in environments, where emergent portions of carbonate platforms are subaerially exposed to hot and humid conditions, generating interconnected cave systems (Dou et al., 2011), which may collapse and form a coalesced system (Loucks, 1999) with structures such as those described in this paper. Vertical successions in paleokarst reservoirs show unconformities, various sets of fractures, and several generation of breccias (Esteban and Wilson, 1993), which adds other complexities to the karst system. The occurrence of fractures in tight reservoirs such carbonates is an important factor because fractures can help increase the permeability and porosity of rocks and facilitate the migration of oil and gas, acting as conduits to fluid flow (Jeanne et al., 2012; Qi et al., 2014; Xu et al., 2016). The coalescence of subsidence rings provides a large area of communication between collapses, with consequent concentrations of fractures in these areas (Fig. 16B). The presence of such structures in a reservoir could represent a large pathway for fluid flow, helping to increase the production of an oil/water field.

14

Marine and Petroleum Geology 113 (2020) 104113

D.F. Menezes, et al.

Fig. 15. Evolution of dissolution from fractures to doline: (A), (C) and (E) (sites S7 for A and C, S9 for E, locations shown in Fig. 1) are UAV images of dolines at an initial stage of evolution, with dissolution of fractures exposed at the surface. (B), (D) and (F) represent their respective digital elevation models. Note that the color variation represents elevation changes from 30 cm to 4 m. Figure order emphasizes the evolution of dissolution, from the collapse of fractures around fractures (A–B) to the formation of an ellipsoidal doline (E–F). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

6. Conclusions

subsidence zones. The inner subsidence zone concentrates the greatest number of rings, which represents higher topographic differences. In the studied dolines, the inner subsidence zones have twice as many rings as the outer subsidence zone. On average, topographic gradient varies from 45% in the inner subsidence zone to 21% in the outer subsidence zone. (c) Preexisting background fractures are reopened in the subsidence rings. These fractures are enlarged and linked to one another. In addition, new opening mode fractures are formed and their intensity increases towards the border of the doline, where they are reactivated as normal faults. (d) The new opening mode fractures are named collapse fractures here. The density of fractures around dolines increases compared to nonaffected areas, where only background fractures exist. With circular

The present study identifies subsidence rings, which increase the size of the area influenced by collapse dolines and consequently the associated risks. These rings can be identified with the help of highresolution images and field surveys. The present study indicates the following conclusions: (a) Subsidence rings are shown in digital elevation models, where each color represents a topographic elevation interval. Each ring ranges from 30 cm to 1 m in the study area. Topographic relief differences between the flat area around the doline and the main collapse can reach 10 m or more. (b) Subsidence rings are divided into two zones: inner and outer 15

Marine and Petroleum Geology 113 (2020) 104113

D.F. Menezes, et al.

Fig. 16. Conceptual model of a collapse doline developed in a carbonate karst system. (A) Preexisting and collapse karstified fractures. Preexisting karstified fractures (pkf) formed before the doline collapse. Next, the structural instability generated when the doline collapsed was the main factor for the formation of curvilinear-shaped collapse fractures (cf). Around the doline, there is a larger area of subsidence generated by the collapse, named the subsidence ring (sr), here shown with red dots. The dissolution also occurs along more soluble layers (kl). Inside the doline, the infilling material, generally collapsed blocks (cb) mixed with siliciclastic materials, generates different types of breccias (br). (B) Collapse rings coalesce and form highly fractured zones parallel to the major fracture sets. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

and modeling of karstified reservoirs), coordinated by FHB (UFRN) and CLC (Petrobras).

scanlines, we estimate increases of 7–8% and 7–13% in P20 and P21, respectively, with respect to the values obtained by removing the circular fractures (i.e., considering only background fractures). These fractures differ from the background fracture systems in that (1) they show curved shapes, (2) they systematically cross-cut veins and stylolites, and (3) their occurrence is limited to the areas surrounding the dolines, up to an average distance of 10–20 m from the border. These new opening mode fractures are reactivated as normal faults close to the doline edges. (e) Subsidence rings can coalesce and increase the fractured area around collapse structures. This fact may have implications for aquifer and oil reservoir porosity and permeability. The risk of collapse in these areas increases.

Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.marpetgeo.2019.104113. References Ali, H.H., Belghithi, H., Ouali, J.A., Touir, J., 2016. Relationship between fracturing and porogenesis in a carbonate reservoir: example from the Middle Turonian Bireno Member in Jebel M'rhila, Central Tunisia. J. Afr. Earth Sci. 121, 342–357. Augarde, C.E., Lyamin, A.V., Sloan, S.W., 2003. Prediction of undrained doline collapse. J. Geotech. Geoenviron. Eng. 129, 197–205. Bauer, M., Tóth, T.M., 2017. Characterization and dfn modelling of the fracture network in a Mesozoic karst reservoir: Gomba oilfield, Paleogene basin, central Hungary. J. Pet. Geol. 40, 319–334. Bertotti, G., de Graaf, S., Bisdom, K., Oskam, B., Vonhof, H.B., Bezerra, F.H.R., Cazarin, C., 2017. Fracturing and fluid-flow during post-rift subsidence in carbonates of the Jandaíra Formation, Potiguar Basin, NE Brazil. Basin Res. 29, 836–853. Bezerra, F.H.R., Vita-Finzi, C., 2000. How active is a passive margin? Paleoseismicity in northeastern Brazil. Geology 28, 591–594. Bezerra, F.H.R., Castro, D.L., Maia, R.P., Sousa, M.O.L., Moura-Lima, E.N., Rosseti, D.F., Bertotti, G., Souza, Z.S., Nogueira, F.C.C., 2019. Postrift stress field inversion in the Potiguar Basin, Brazil – implications for the petroleum systems and evolution of the equatorial margin of South America. Mar. Pet. Geol. 111, 88–104. Bisdom, K., Bertotti, G., Bezerra, F.H., 2017. Inter-well scale natural fracture geometry and permeability in low-deformation carbonate rocks. J. Struct. Geol. 97, 23–36. Broughton, P.L., 2017. Breccia pipe and sinkhole linked fluidized beds and debris flows in the Athabasca Oil Sands: dynamics of evaporite karst collapse-induced fault block collisions. Bull. Can. Petrol. Geol. 65, 200–234. Cahalan, M.D., Milewski, A.M., 2018. Doline formation mechanisms and geostatisticalbased prediction analysis in a mantled karst terrain. Catena 165, 333–344. de Graaf, S., Reijmer, J.J.G., Bertotti, G.V., Bezerra, F.H.R., Cazarin, C.L., Bisdom, K.,

Acknowledgments We thank two anonymous reviewers and Marine and Petroleum Geology editor Enrique Gomez-Rivas for comments and suggestions, which substantially improved the manuscript. AA thanks the Overworld Program of the University of Parma (Dipartimento di Scienze Chimiche, della Vita e della Sostenibilità Ambientale, Università di Parma), which allowed him to performed part of his MSc in Brazil. DFM thanks the Graduate Program on Geodynamics and Geophysics (Programa de PósGraduação em Geodinâmica e Geofísica, Universidade Federal do Rio Grande do Norte – UFRN), where he developed his MSc thesis and Petrobras Brasil for allowing him to develop this study. We thank the Brazilian Agency of Oil, Gas and Biofuels (Agência Brasileira de Petróleo, Gás e Biocombustíveis, ANP). This work was sponsored by Petrobras as part of the Procarste Project (Advanced Project of Acquisition and Interpretation of parameters to the characterization 16

Marine and Petroleum Geology 113 (2020) 104113

D.F. Menezes, et al. Vonhof, H.B., 2017. Fracturing and calcite cementation controlling fluid flow in the shallow-water carbonates of the Jandaíra Formation, Brazil. J. Mar. Pet. Geol. 80 382–39. Dou, Q., Sun, Y., Sullivan, C., Guo, H., 2011. Paleokarst system development in the san Andres formation, Permian basin, revealed by seismic characterization. J. Appl. Geophys. 75, 379–389. Esteban, M., Wilson, J.L., 1993. In: Introduction to Karst Systems and Paleokarst Reservoirs. SEPM Core Workshop, vol. 18 Paleokarst Related Hydrocarbon Reservoirs (chapter 1). Florea, L.J., 2005. Using State-wide GIS data to identify the coincidence between dolines and geologic structure. J. Cave Karst Stud. 67, 120–124. Florea, L.J., Paylor, R.L., Simpson, L., Gulley, J., 2012. Karst Gis advances in Kentucky. J. Cave Karst Stud. 64, 58–62. Ford, D., Williams, P., 2007. Karst Hydrogeology and Geomorphology. John Wiley & Sons Ltd, Chichester. Fournillon, A., Bellentani, G., Moccia, A., Jumaeucourt, C., Terdich, P., Siliprandi, F., Peruzzo, F., 2017. Characterization of a paleokarstic oil field (Rospo Mare, Italy): sedimentologic and diagenetic outcomes, and their integration in reservoir simulation. In: EuroKarst 2016, Neuchatel, no.5. Gabrovšek, F., Stepišnik, U., 2011. On the formation of collapse dolines: a modelling perspective. Geomorphology 134, 23–31. Galve, J.P., Remondo, J., Gutiérrez, F., 2011. Improving doline hazard models incorporating magnitude–frequency relationships and nearest neighbor analysis. Geomorphology 134, 157–170. Gutiérrez, F., Cooper, A.H., Johnson, K.S., 2008. Identification, prediction, and mitigation of doline hazards in evaporite karst areas. Environ. Geol. 53, 1007–1022. Halley, R.B., Schmoker, J.W., 1983. High-porosity Cenozoic carbonate rocks of South Florida: progressive loss of porosity with depth. AAPG (Am. Assoc. Pet. Geol.) Bull. 67, 191–200. Healy, D., Rizzo, R.E., Cornwell, D.G., Farrell, N.J.C., Watkins, H., Timms, N.E., Smith, M., 2017. FracPaQ: a MATLABTM toolbox for the quantification of fracture patterns. J. Struct. Geol. 95, 1–16. Intrieri, E., Gigli, G., Nocentini, M., Lombardi, L., Mugnai, F., Fidolini, F., Casagli, N., 2015. Doline monitoring and early warning: an experimental and successful GBInSAR application. Geomorphology 241, 304–314. Jeanne, P., Guglielmi, Y., Lamarche, J., Cappa, F., Marie, L., 2012. Architectural characteristics and petrophysical properties evolution of a slip fault zone in a fractured porous carbonate reservoir. J. Struct. Geol. 44, 93–109. Kaufmann, G., Romanov, D., 2016. Structure and evolution of collapse dolines: combined interpretation from physico-chemical modelling and geophysical field work. J. Hydrol. 540, 688–698. Kranjc, A., 2013. Classification of closed depressions in carbonate karst. In: In: Schroder, J.F., Frumkin, A. (Eds.), Treatise on Geomorphology, vol. 6. Karst geomorphology. San Diego: Academic, pp. 104–111. Lee, E.J., Shin, S.Y., Ko, B.C., Chang, C., 2016. Early doline detection using a drone-based thermal camera and image processing. Infrared Phys. Technol. 78, 223–232. Lipar, M., Stepišnik, U., Ferk, M., 2019. Multiphase breakdown sequence of collapse doline morphogenesis: an example from Quaternary aeolianites in Western Australia. Geomorphology 327, 572–584.

Loucks, R.G., 1999. Paleocave carbonate reservoirs: origins, burial- depth modifications, spatial complexity, and reservoir implications. AAPG (Am. Assoc. Pet. Geol.) Bull. 83, 1795–1834. Matos, R.M.D., 1992. The Northeast Brazilian rift system. Tectonics 11, 766–791. Matos, R.M.D., 2000. Tectonic evolution of the equatorial South Atlantic. Atlantic rifts and Continental Margins. Geophys. Monogr. 115, 331–354. Mazzullo, S.J., 2004. Overview of porosity evolution in carbonate reservoirs. Kansas Geol. Soc. Bull. 79 (1 and 2). Öztürk, M., Şener, M.F., Şener, M., Şimşek, M., 2018. Structural controls on distribution of dolines on mount Anamas (Taurus Mountains, Turkey). Geomorphology 317, 107–116. Palmer, A.N., Palmer, M.V., 2006. Hydraulic considerations in the development of tiankengs. Speleogenesis Evolut. Karst Aquifers 4, 1–8. Panno, S., Luman, D., 2018. Characterization of cover-collapse doline morphology on a groundwater basin-wide scale using lidar elevation data:a new conceptual model for doline evolution. Geomorphology 318, 1–17. Phung, P. Van, The, A.V., Thanh, T.N., 2017. The development of Palaeokarst systems in the Middle Miocene carbonate reservoir, South Song Hong basin. Int. J. Appl. Eng. Res. 12, 12844–12851. Qi, J., Zhang, B., Zhou, H., Marfurt, K., 2014. Attribute expression of fault-controlled karst — fort Worth Basin, Texas: a tutorial. Interpretation 2 (3), SF91–SF110. Reis, Á.F.C., Bezerra, F.H.R., Ferreira, J.M., Do Nascimento, A.F., Lima, C.C., 2013. Stress magnitude and orientation in the Potiguar Basin, Brazil: implications on faulting style and reactivation. J. Geophys. Res.: Solid Earth 118, 5550–5563. Rohrbaugh, J.B., Dunne, W.M., Mauldon, M., 2002. Estimating fracture trace intensity, density, and mean length using circular scan lines and windows. AAPG (Am. Assoc. Pet. Geol.) Bull. 86, 2089–2104. Sanderson, D.J., Nixon, C.W., 2015. The use of topology in fracture network characterization. J. Struct. Geol. 72, 55–66. Silva, O.L., Bezerra, F.H.R., Maia, R.P., Cazarin, C.L., 2017. Karst landforms revealed at various scales using LiDAR and UAV in semi-arid Brazil: consideration on karstification processes and methodological constraints. Geomorphology 295, 611–630. Siska, P., Goovaerts, P., Hung, I., 2016. Evaluating susceptibility of karst dolines (dolines) for collapse in Sango, Tennessee, USA. Prog. Phys. Geogr. 1–19. Waltham, T., Bell, F.G., Culshaw, M.G., 2005. Dolines and Subsidence: Karst and Cavernous Rocks in Engineering and Construction. Springer Praxis Publishing, Chichester, UK, pp. 379. Watkins, H., Bond, C.E., Healy, D., Butler, R.W.H., 2015. Appraisal of fracture sampling methods and a new workflow to characterise heterogeneous fracture networks at outcrop. J. Struct. Geol. 72, 67–82. Williams, P., 2004. Dolines. In: Gunn, J. (Ed.), Encyclopedia of Caves andKarst Science. Fitzroy Dearborn, New York, NY, and London, pp. 304–310. Xu, W., Bernardes, S., Bacchus, S.T., Madden, M., 2016. Mapped fractures and dolines in the coastal plain of Florida and Georgia to infer environmental impacts from aquifer storage and recovery (ASR) and supply wells in the regional karst Floridan aquifer system. J. Geogr. Geol. 8 (2). https://doi.org/10.5539/jgg.v8n2p76. Zumpano, V., Pisano, L., Parise, M., 2019. An integrated framework to identify and analyze karst dolines. Geomorphology 332, 213–225.

17