Fraturas e halos de subsidência em torno de dolinas, Semiárido do Brasil

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DISSERTAÇÃO DE MESTRADO

FRATURAS E HALOS DE SUBSIDÊNCIA EM

TORNO DE DOLINAS, SEMI-ÁRIDO DO BRASIL

Autor:

Daniel Fernandes De Menezes

Orientador:

Dr. Francisco Hilário Rego Bezerra

Co-orientador:

Dr. Fabrizio Balsamo

Dissertação nr. 235/PPGG

Natal/RN Setembro de 2019

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UNIVERSIDADE FEDERAL DO RIO GRANDE DO NORTE CENTRO DE CIÊNCIAS EXATAS E DA TERRA

PROGRAMA DE PÓS-GRADUAÇÃO EM GEODINÂMICA E GEOFÍSICA

DISSERTAÇÃO DE MESTRADO

FRATURAS E HALOS DE SUBSIDÊNCIA EM

TORNO DE DOLINAS, SEMI-ÁRIDO DO BRASIL

Autor:

Daniel Fernandes de Menezes

Dissertação apresentada em 30 de setembro de 2019, ao Programa de Pós-Graduação em Geodinâmica e Geofísica – PPGG, da Universidade Federal do Rio Grande do Norte - UFRN como requisito à obtenção do Título de Mestre em Geodinâmica e Geofísica.

Comissão Examinadora

Dr. Francisco Hilário Rego Bezerra (orientador) Dr. Vicenzo La Bruna (membro interno)

Dr. Francisco Cezar Costa Nogueira (membro externo)

Natal/RN Setembro de 2019

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Menezes, Daniel Fernandes de.

Fraturas e halos de subsidência em torno de dolinas, Semi-árido do Brasil / Daniel Fernandes de Menezes. - 2019.

68f.: il.

Dissertação (mestrado) - Universidade Federal do Rio Grande do Norte, Centro de Ciências Exatas e da Terra, Programa de Pós-Graduação em Geodinâmica e Geofísica. Natal, 2019.

Orientador: Francisco Hilário Rego Bezerra. Coorientador: Fabrizio Balsamo.

1. Dolina - Dissertação. 2. Colapso - Dissertação. 3. Carbonato - Dissertação. 4. Fraturas - Dissertação. 5.

Reservatório - Dissertação. I. Bezerra, Francisco Hilário Rego. II. Balsamo, Fabrizio. III. Título.

RN/UF/CCET CDU 551.435.83

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UFRN/CCET/PPGG – Dissertação de Mestrado Resumo

Resumo

Rochas carbonáticas são reconhecidas por sua grande heterogeneidade e pela presença de estruturas associadas à dissolução. Isso é muito importante em regiões fraturadas, já que as fraturas podem induzir os processos de dissolução. Entre as consequências geradas se destacam a porosidade secundária e aumento da permeabilidade, que é essencial em regiões com reservatórios de petróleo. Além disso, o aumento na dissolução é responsável pela formação de estruturas de colapso, que podem ocorrer tanto na superfície como na subsuperfície. Os colapsos são responsáveis por diversos problemas em áreas urbanas construídas neste tipo de terreno, como o desmoronamento de edifícios e estradas, o que pode causar sérios problemas sociais e ambientais. O trabalho concentrou-se no estudo de dolinas, que são as estruturas de colapso mais expressivas em rochas carbonáticas, e sua relação com fraturas preexistentes. A ocorrência de dolinas em afloramentos pode ajudar a responder questões intrínsecas aos problemas citados, como se há alguma interferência nas propriedades estruturais e petrofísicas das rochas afetadas, ou mesmo para melhorar a previsão sobre os efeitos que os colapsos geram na topografia. A área estudada possui quatro conjuntos de fraturas preexistentes, um N-S / E-W e outro NE-SW / NW-SE, que concentram a principal dissolução na região. A presença dessas fraturas permitiu a formação das dolinas de colapso. Os dados mostraram que nas áreas onde ocorrem colapsos, há a formação do que está sendo chamado de halos de subsidência. Esta zona está sofrendo subsidência devido ao colapso principal, e o relevo original é afetado, mergulhando em direção à dolina. Foram medidas variações no relevo topográfico maiores de 10 metros em relação às áreas não afetadas. Também foi observada que a área afetada no entorno das dolinas tem em média o dobro do raio das mesmas. Esse processo gera uma mudança no padrão de fraturas da região, com a formação de um novo conjunto, chamado de fratura por colapso. Essas fraturas têm formato circular e ocorrem ao redor das dolinas. Um aumento na abertura e densidade dessas fraturas ao se aproximar das dolinas também foi observado através de scanlines. Isso representa um indicador de melhoria da qualidade permo-porosa nessas áreas. Além disso, mostra que há um aumento na instabilidade estrutural, aumentando o risco de acidentes em áreas construídas em rochas solúveis, uma vez que a área afetada pode ser muito maior do que o previsto anteriormente.

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UFRN/CCET/PPGG – Dissertação de Mestrado Abstract

Carbonate rocks are recognized for their great heterogeneity and for the presence of structures associated with dissolution. This is very important in fractured regions, since fractures can induce dissolution processes. Among the consequences generated are secondary porosity and increased permeability, which is essential in regions with oil reservoirs. In addition, the increase in dissolution is responsible for the formation of collapse structures, which can occur both at the surface and in the subsurface. The collapses are responsible for several problems in urban areas built on this type of terrain, such as the collapse of buildings and roads, which can cause serious social and environmental problems. The work focused on the study of dolines, which are the most expressive collapse structures in carbonate rocks, and their relation with preexisting fractures. The occurrence of dolines in outcrops can help to answer questions intrinsic to the problems mentioned, such as if there is any interference in the structural and petrophysical properties of the affected rocks, or even to improve the prediction about the effects that the collapses generate in the topography. The studied area has four sets of preexisting fractures, one N-S / E-W and another NE-SE-W / NE-W-SE, which concentrate the main dissolution in the region. The presence of these fractures allowed the formation of the collapse doline. The data showed that in areas where collapses occur, there is the formation of what are being called subsidence halos. This zone is suffering subsidence due to the main collapse, and the original relief is affected, plunging towards the dolines. Topographic relief variations greater than 10 meters in relation to the non-affected areas were measured. It was also observed that the affected area around the dolines has on average twice the radius of the dolines. This process generates a change in the pattern of fractures of the region, with the formation of a new set, called collapse fracture. These fractures are circular in shape and occur around the dolines. An increase in the aperture and density of these fractures when approaching the dolines was also observed through scanlines. This represents an indicator of permo-porous quality improvement in these areas. In addition, it shows that there is an increase in structural instability, rising the risk of accidents in areas built on soluble rocks, since the affected area may be much larger than previously predicted.

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UFRN/CCET/PPGG – Dissertação de Mestrado Agradecimentos

Agradecimentos

Agradeço de forma sincera a todos que contribuíram de forma direta ou indireta com a realização e conclusão deste trabalho, durante as suas diversas etapas.

A meus pais por todo o esforço realizado para garantir uma educação de qualidade a mim e a meus irmãos.

A minha querida e amada esposa Flora por ter estado ao meu lado durante todo o tempo necessário para a conclusão deste projeto, incluindo uma gravidez no meio do processo, e com a benção do nascimento do nosso querido Benjamin, que chegou para transformar nossas vidas e ajudar a enfrentar os desafios com maiores responsabilidades.

A Universidade Federal do Rio Grande do Norte e ao Programa de Pós-Graduação em Geodinâmica e Geofísica pela oportunidade de ingressar no mestrado e utilizar toda a reconhecida excelência da instituição durante o projeto.

A Petrobras, pela liberação parcial das atividades profissionais, permitindo a concentração necessária para a conclusão deste trabalho. Além disso, pelo financiamento do projeto PROCARSTE que incentivou o estudo de rochas carbonáticas em ambientes cársticos no Rio Grande do Norte.

Ao digníssimo professor Dr. Francisco Hilário Bezerra, a quem devo muito do conhecimento que tenho em geologia, por me orientar e guiar mais uma vez em um projeto de crescimento pessoal e profissional, com seus ensinamentos e discussões que são sempre fundamentais.

Ao professor Dr. Fabrizio Balsamo, pela importante contribuição nas discussões técnicas que enriqueceram bastante o trabalho.

A todos os colegas pesquisadores, alunos e funcionários que em algum momento participaram e contribuíram bastante nesta jornada, bem como pela excelente convivência durante as etapas de campo, entre eles Fábio, Rubson, Tomaz, Andrea, Joanderson, Fernando, Pedro, Antônio e Uilson.

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UFRN/CCET/PPGG – Dissertação de Mestrado Índice

Resumo ... i

Abstract ... ii

Agradecimentos ... iii

Índice ... iv

Lista de Figuras ... v

Capítulo 1 - Introdução ... 1

Capítulo 2 - Geologia Regional ... 5

Capítulo 3 - Metodologia ... 8

3.1 - Sensoriamento Remoto ... 8

3.2 - Análise Estrutural ... 9

Capítulo 4 - Artigo Científico ... 11

1. Introduction ... 14

2. Geomorphological and geological settings ... 16

3. Methods and Materials ... 18

3.1 Remote Sensing ... 18

3.2 Fracture mapping and structural analyses... 19

4. Results ... 21

4.1 The use of topographic data to identify subsidence rings... 21

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UFRN/CCET/PPGG – Dissertação de Mestrado Índice

4.2 Structural data and fracture attributes... 26

4.2.1 Background fractures …………... 26

4.2.2 Collapse fractures ... 29

4.3 Fracture density and intensity around dolines... 33

5. Discussions………... 37

5.1 Evolutionary model of subsidence rings around collapse

dolines... 37

5.2 Implications of subsidence rings for oil reservoirs, aquifers and

risk of collapse……….………... 40

6. Conclusions …………...……….……….………... 44

References ... 46

Capítulo 5 - Conclusões ... 53

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UFRN/CCET/PPGG – Dissertação de Mestrado Lista de Figuras Figura 1 - Dolina formada por colapso do teto de cavernas devido à processos

de remoção de material em subsuperfície (modificado de Waltham, 2005) ... 2

Figura 2 - Mapa de localização da área de trabalho. A imagem apresentada é

um SRTM mostrando as principais feições de drenagem regionais, com direção principal NE-SW. Em branco estão representados os afloramentos onde foram encontradas as treze dolinas estudadas, notar também seu alinhamento NE-SW, coincidindo com a principal direção de dissolução na região ... 4

Figura 3 - Mapa da Bacia Potiguar com as formações aflorantes na porção

terrestre. Notar em azul a ocorrência da Formação Jandaíra, carbonatos alvos do trabalho ... 5

Figura 4 - Modelo deposicional esquemático da Formação Jandaíra, em

ambiente de plataforma / rampa carbonática representando o ápice da transgressão marinha durante o eoturoniano na Bacia Potiguar. (Modificado de Monteiro & Faria, 1988) ... 6

Figura 5 - Evolução dos processos de carstificação através do alargamento dos

principais sistemas de fraturas na região. (Maia et al., 2012) ... 8

Figura 6 - Principais ferramentas de aquisição de dados no trabalho. (A)

Equipamento de aquisição de imagens aéreas de alta resolução, drone DJI Phantom IV. (B) Dados estruturais adquiridos com a utilização de bússola Brunton e scanlines ... 10

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UFRN/CCET/PPGG – Dissertação de Mestrado Introdução

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1 – Introdução

Ambientes cársticos representam áreas com intensa dissolução, especialmente em rochas extremamente solúveis, como evaporitos e carbonatos. Dentre as diversas estruturas geradas nesse tipo de ambiente, as dolinas se destacam como um dos mais importantes (Gutierrez et al., 2008). Sua formação tem sido associada a processos de colapso do teto de cavernas e remoção de massa em subsuperfície (Gabrovšek e Stepišnik, 2011) (Fig. 1). Elas são definidas como depressões naturais que podem ter sido escavadas ou colapsadas em terrenos cársticos (Waltham et al, 2005). Sua gênese e ocorrência são bastante diversas e seus diâmetros e profundidades variam de dezenas a centenas de metros, portanto podem ser identificados desde afloramentos até imagens sísmicas (Augarde et al, 2003; Palmer e Palmer, 2006; Phung et al., 2017).

As fraturas se comportam como um dos elementos mais significativos na geração de dolinas. Elas atuam como condutos de fluidos que irão gerar dissolução e subsequente colapso das camadas acima. (Florea, 2005; Kaufman e Romanov, 2016; Xu et al., 2016, Cahalan e Milewsky, 2018).

A importante relação entre dolinas e fraturas é perceptível no estudo de reservatórios, uma vez que essas estruturas não apenas se encontram espalhadas pelo mundo (Fournillon et al., 2017, Phung et al., 2017), mas também se correlacionam de forma controversa. Primeiramente, rochas carbonáticas fraturadas e carstificadas possuem grande potencial de reservatório, uma vez que podem induzir um aumento na permeabilidade do mesmo (Jeanne et al., 2012, Ali et al., 2016). No entanto, a formação de estruturas de colapso nessas regiões também pode prejudicar a produção quando estas criam caminhos para a comunicação entre o aqúifero e o reservatório (Qi et al., 2014). Infelizmente, há pouca informação disponível sobre a geração de novas fraturas devido à presença de dolinas de colapso, o que pode incluir novas variáveis nos modelos existentes. Fraturas não previstas podem alterar o fluxo de fluidos nessas áreas, assim como aumentar a qualidade do reservatório.

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A ocorrência de dolinas em estudos de gerenciamento de risco também possui grande relevância. Eventos catastróficos envolvendo áreas urbanas construídas em terrenos cársticos são reconhecidos há muito tempo (Therolf, 2005, Galve et al., 2011). Os impactos variam desde os efeitos imediatos, como a perda de vidas ou danos às propriedades no momento do colapso, até as perdas decorrentes de processos subseqüentes (Intrieri et al., 2015). Tentativas de prever a ocorrência de dolinas em áreas de risco foram feitas por meio de inúmeras técnicas, como por exemplo, o monitoramento de fotos tiradas ao longo do tempo buscando mudanças na topografia ou identificação de possíveis áreas mais suscetíveis à dissolução em análises de campo, dentre outras (Gutierrez et al. , 2008; Siska et al., 2016; Panno e Luman, 2018). Além disso, vários autores aproveitaram as melhorias no sensoriamento remoto para mapear estruturas

Figura 1 - Dolina formada por colapso do teto de cavernas devido à processos de remoção de material em subsuperfície (modificado de Waltham, 2005).

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cársticas em grandes áreas. O uso de ferramentas como grandes bancos de dados no formato GIS, LiDAR, sensores térmicos e outros associados à softwares avançados de processamento de imagens permitiu caracterizar as dolinas com alta precisão e em menor tempo (Florea et al., 2012; Lee et al., 2016; Silva et al. ., 2017; Zumpano et al., 2019). Apesar de identificarem várias estruturas, elas não perceberam a presença de variações topográficas sutis no entorno das dolinas, o que aumenta o tamanho da área influenciada pelos colapsos e, consequentemente, os riscos associados. Essas variações só podem ser identificadas com a ajuda de imagens de alta resolução.

Estruturas tectônicas regionais parecem controlar o arranjo de dolinas em algumas áreas, já que o alongamento das mesmas parece indicar uma direção tectônica principal (Florea, 2005; Ozturk et al., 2018; Lipar et al., 2019). No entanto, muitos não levam em conta o papel dos sistemas de fraturas que atuam nos processos de dissolução, e suas relações com novas estruturas geradas durante os colapsos. Essas fraturas podem indicar grandes corredores de dissolução em subsuperfície que serão responsáveis por orientar a formação das dolinas, ou mesmo a permitir a coalescência de sistemas maiores, assim como à ocorrência de novos sistemas de fraturas no entorno de dolinas pode alterar propriedades permo-porosas das rochas nessas áreas.

Os principais objetivos do presente estudo são identificar áreas estruturalmente instáveis ao redor de dolinas, bem como entender a influência de dolinas em sistemas de fraturas preexistentes e a geração de novas fraturas ao redor dos colapsos.

Os resultados são muito importantes e aplicáveis em estudos de reservatórios cársticos, pois a presença de novas fraturas pode representar um aumento na porosidade e permeabilidade em áreas com dolinas. Também podem haver implicações para o gerenciamento de riscos em áreas construídas em terrenos cársticos, onde estudos anteriores ignoraram a existência de zonas estruturalmente instáveis ao redor de dolinas, que podem representar uma área de instabilidade estrutural muito maior do que o previsto anteriormente.

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UFRN/CCET/PPGG – Dissertação de Mestrado Geologia Regional

2 – Geologia Regional

A área de estudo está localizada na porção terrestre da Bacia Potiguar, importante província petrolífera da região Nordeste do Brasil, entre os municípios de Felipe Guerra e Gov. Dix-Sept Rosado, no Estado do Rio Grande do Norte (Fig 2).

Figura 2 - Mapa de localização da área de trabalho. A imagem apresentada é um SRTM mostrando as principais feições de drenagem regionais, com direção principal NE-SW. Em branco estão representados os afloramentos onde foram encontradas as treze dolinas estudadas, notar também seu alinhamento NE-SW, coincidindo com a principal direção de dissolução na região.

A Bacia Potiguar está inserida entre os Estados do Rio Grande do Norte e Ceará, limitada a NNW pelo Alto de Fortaleza, e a SSE pelo Alto de Touros. As seções sedimentares foram depositadas sobre um embasamento Pré-Cambriano pertencente à Província Borborema, atingindo espessuras na ordem de 5000m na porção terrestre. Esta bacia originou-se durante o colapso do supercontinente Gondwana, no limite Jurássico-Cretáceo, desconectando os continentes da África e da América do Sul (Matos, 1992; Matos, 2000).

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Estruturalmente se destacam grandes falhas normais NE-SW e falhas transcorrentes NW-SE, com ambos os sistemas tendo sido reativados durante o Cenozoico (Bezerra & Vita-Finzi, 2000; Reis et al., 2013).

O registro sedimentar é composto por três supersequências: Rifte, pós-rifte e dpós-rifte. Na porção terrestre, a fase pós-rifte é composta pelos sedimentos lacustres da Formação Pendência. Sobre estes foi depositada a Formação Alagamar, num ambiente transicional. A área de estudos abrange os sedimentos Eoturonianos da Formação Jandaíra (Fig. 3), depositados sobre sedimentos fluviais da Formação Açu. Eles representam um evento de máxima transgressão marinha, com a instalação de uma grande plataforma / rampa carbonática dominada por maré (Fig. 4). Este pacote sedimentar mergulha suavemente para NNE, atingindo aproximadamente 600m de espessura em algumas áreas da bacia.

Figura 3 - Mapa da Bacia Potiguar com as formações aflorantes na porção terrestre. Notar em azul a ocorrência da Formação Jandaíra, carbonatos alvos do trabalho.

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UFRN/CCET/PPGG – Dissertação de Mestrado Geologia Regional Figura 4 - Modelo deposicional esquemático da Formação Jandaíra, em ambiente de plataforma / rampa carbonática representando o ápice da transgressão marinha durante o eoturoniano na Bacia Potiguar. (Modificado de Bertani et al., 1990)

Apesar de localizada no Semiárido nordestino, região de raras e esparsas chuvas, existem diversas evidências diretas de forte dissolução sobre essas rochas. Eventos de carstificação intensos foram identificados em afloramentos na porção emersa (Pessoa Neto et al., 2007; Silva et al., 2017), atraindo a atenção de muitos pesquisadores nos últimos anos, uma vez que esta região tem se mostrado um excelente análogo para reservatórios carbonáticos fraturados e dissolvidos.

No final do Cretáceo e início do Terciário, mudanças no regime regional de tensões afetaram estruturalmente as rochas da Formação Jandaíra. O novo regime compressivo permitiu a dissolução de carbonatos, com fluxo de fluido através de fraturas recém-formadas. Os estudos isotópicos permitiram identificar a origem destes fluidos, caracterizando-os como de origem meteórica, provenientes da recarga do aquífero existente nos arenitos da Formação Açu, migrando ascendentemente para os carbonatos da Formação Jandaíra. O aumento da subsidência permitiu a precipitação de veios de calcita N-S e NNE-SSW, bem como estilólitos de direção E-W e WNW-ESE, ambos sub-verticais.

Em termos de processos cársticos, o clima semiárido do final do quaternário até os dias atuais no nordeste do Brasil permitiu um processo de

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preenchimento cíclico / remoção de massa que causou uma ampliação do sistema de fraturas, culminando nas diversas estruturas cársticas que podem ser observadas atualmente (Fig. 5). A intensa dissolução desses sistemas de fraturas também permitiu um aumento na permeabilidade dos carbonatos originalmente fechados da Formação Jandaíra. (Carneiro et al., 2015; Bertotti et al., 2017; Bisdom et al., 2017; de Graaf et al., 2017; Silva et al., 2017).

Figura 5 - Evolução dos processos de carstificação através do alargamento dos principais sistemas de fraturas na região. (Maia et al., 2012)

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UFRN/CCET/PPGG – Dissertação de Mestrado Metodologia

3 – Metodologia

Trabalhos de aquisição de imagens aéreas têm passado por uma expansão nos últimos anos com a popularização dos veículos aéreos não-tripulados, também comumente chamado de drones. Entre as facilidades que essa tecnologia traz se destacam principalmente a rápida aquisição e processamento dos dados, além da altíssima resolução conseguida com o uso de câmeras modernas e voos de baixa altitude. Muito disso se deve ao formato compacto e uso intuitivo dos equipamentos, que podem por exemplo ser controlados através de smartphones comuns.

Esses avanços tem permitido diversos autores explorarem a relação entre fraturas e ambientes cársticos em escalas antes não estudadas. Na área de estudos, (Silva et al., 2017) mapearam e identificaram diversas estruturas cársticas em variadas escalas usando imagens “LiDAR” (Light Detection And Ranging) em auxílio com as obtidas com drones. (Bertotti et al., 2017) utilizaram imagens de alta resolução para caracterizar a evolução das fraturas encontradas na região.

Como o trabalho objetivou estudar a ocorrência de zonas de colapso ao redor de dolinas e sua relação com as fraturas, foi necessário adquirir imagens com altíssimas resoluções, permitindo a visualização dessas estruturas em escalas maiores do que as convencionalmente feitas com o uso prévio de imagens de radar e satélite. Dados estruturais também foram adquiridos no campo para obter o padrão de fraturas preexistente e entender a influência das dolinas sobre o mesmo. Para o levantamento da imagens aéreas foi utilizado um veículo aéreo não-tripulado, também chamado de drone, da empresa DJI, enquanto que os dados estruturais foram adquiridos com o uso de uma bússola

Brunton e aplicação da técnica de scanlines (Fig. 6). No total, treze dolinas em

diferentes estágios de evolução foram analisadas. 3.1. Sensoriamento remoto

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O reconhecimento inicial das dolinas foi feito com o auxílio de modelos de elevação digital gerados a partir de imagens “LiDAR”, que é uma tecnologia que se utiliza das propriedades da luz refletida visando medir a distância de um objeto a certa distância. Também foi acessada a base de dados do CECAV (Centro Nacional de Pesquisa e Conservação de Cavernas), buscando-se a identificação de dolinas já mapeadas.

A união dessas duas fontes de informação permitiu a seleção das dolinas a serem investigadas. A partir desses dados, foram feitos trabalhos de levantamento de imagens aéreas dessas estruturas. O equipamento utilizado foi um Phantom 4 Pro®, da empresa DJI. Entre as principais características se destacam uma câmera com estabilizador de três eixos e sensor CMOS (Complementary Metal-Oxide Semiconductor) de 20 megapixels. O drone tem sistema de vôo autônomo com o auxílio de sensores infravermelho para desvio de obstáculos. Para cada dolina houve pequenas variações em parâmetros como altura de vôo e ângulo de disparo, de acordo com as particularidades de cada região. De maneira geral, os vôos variaram em altutides de 20 a 100m, com 90% de sobreposição e câmera entre 80 e 90 graus. Isto permitiu a obtenção de imagens de altíssimas resoluções, alcançando escalas menores que 1cm/pixel, trazendo nível de detalhes muito acima dos obtidos tradicionalmente com imagens de satélite e/ou radar.

O processamento das imagens adquiridas foi possível com a ajuda de softwares como o Agisoft Photoscan Professional®, Global Mapper 19® e ArcMap 10®. Como resultado obtivemos fotografias aéreas e seus respectivos modelos digitais de elevação para cada dolina. Isso nos permitiu observar variações centimétricas da topografia em torno das estruturas. Além disso, as imagens de alta resolução foram utilizadas para mapear em detalhes padrões de fraturas em alguns lugares específicos, tanto longe quanto perto das dolinas. 3.2 Análise Estrutural

Como citado, um dos objetivos do trabalho era buscar entender a relação das dolinas com os padrões de fraturas preexistentes. A partir disso, foram coletados diversos dados estruturais, possibilitando a aquisição de parâmetros

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como direção, mergulho, densidade e abertura das fraturas. Os dados foram coletados principalmente em quatro áreas com melhores exposições de fraturas. No total, mais de 800 dados de mergulho e direção de foram coletados. O principal método de coleta foi a utilização de scanlines lineares, ortogonais aos principais sistemas de fraturas da área, veios N-S e estilólitos E-W. Em alguns lugares, essas fraturas encontram-se dissolvidas pelo sistema de carstificação na região. A fraturas dissolvidas também foram analisadas. Em laboratório também foi realizada a análise de scanlines circulares nas imagens adquiridas.

Os resultados adquiridos foram plotados em planilhas e gráficos, além da geração de estereogramas com o auxílio do software Stereonet.

Figura 6 - Principais ferramentas de aquisição de dados no trabalho. (A) Equipamento de aquisição de imagens aéreas de alta resolução, drone DJI Phantom IV. (B) Dados estruturais adquiridos com a utilização de bússola Brunton e scanlines.

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11

UFRN/CCET/PPGG – Dissertação de Mestrado Resultados

4 - Resultados (Artigo Científico)

Subsidence rings and fracture pattern around dolines in carbonate

platforms – implications for evolution of collapse structures and reservoir properties

Submetido à revista Marine and Petroleum Geology

Daniel F. Menezes1,2_{, Francisco H. Bezerra}2*_{, Fabrizio Balsamo}3_{, Andrea}

Arcari3_{, Rubson P. Maia}4_{, Caroline L. Cazarin}5

1 – Petrobras/E&P-EXP, Natal – RN, Brazil

2 – Programa de Pós-Graduação em Geodinâmica e Geofísica, Universidade Federal do Rio Grande do Norte, Brazil

3 – Dipartimento di Scienze Chimiche, della Vita e della Sostenibilità Ambientale, Università di Parma, Italy

4 – Departamento de Geografia, Universidade Federal do Ceará, Brazil 5 – Petrobras Research Center, Rio de Janeiro - RJ, Brazil

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UFRN/CCET/PPGG – Dissertação de Mestrado Resultados Highlights

• Subsidence rings are highly fractured zones around dolines prone to collapse

• Gravity-induced, semicircular open-mode collapse fractures occur in the rings

• Collapse fractures reactivate and reopen preexisting background fracture sets

• The total subsidence ring width is ~ twice the radius of the related doline • Subsidence rings coalesce and form highly fractured areas around collapse structures

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Abstract

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 preexisting fracture sets, 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 form and topography plunges towards the doline. In these subsidence rings, the downfaulted topography 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. The subsidence process enlarges and links preexisting fractures and forms a new set of semicircular concentric structures, here named collapse fractures. Increases in the apertures and densities of these fractures occur towards the dolines, which increases porosity around collapse structures. This increase could represent an indicator of permo-porous 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.

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UFRN/CCET/PPGG – Dissertação de Mestrado Resultados 1. Introduction

Karstification promotes dissolution, 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 (Gutierrez 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). Geometrically, they show subcircular shapes and steep slopes (Williams, 2004; Kranjic, 2013) and are generally filled with debris from surrounding areas, such as the blocks of the collapsed layers, developing different types of breccias (Kranjic, 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 dissolution and subsequent collapse of the layers above (Florea, 2005; Kaufman and Romanov, 2016; Xu et al., 2016, Cahalan and Milewsky, 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 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 hinder production when creating paths for aquifer-reservoir communication (Qi et

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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 (Gutierrez 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; Ozturk et al., 2018; Lipar et al., 2019). However, several studies have not taken into account the role of background fracture systems in the dissolution processes (Gabrovšek and Stepišnik, 2011,

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Kaufman 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 and 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 generation of collapse dolines; and (3) to identify and assess the generation of new gravity-induced fractures, 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.

2. Geomorphological and Geological settings

The study area is located in the southwestern region of the Potiguar Basin, where the fill is 5,000 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 NW-SE-striking strike-slip faults, which were reactivated in the Cenozoic (Bezerra and Vita-Finzi, 2000; Reis et al.,

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2013; Bezerra et al., 2019). The sedimentary record is composed of three 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 N-S-striking fractures that convey fluids (Bertotti et al., 2017; Bezerra et al., 2019). Stable isotope studies indicate that these fluids have a meteoric origin, coming from recharge of the existing aquifer

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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 (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, which started in the late Quaternary, allowed a cyclic filling/mass removal process that caused an 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 collapse 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 the dolines on the preexisting fracture system. In total, thirteen dolines in different stages of evolution were analyzed, with four main sites containing the majority of structural data (Fig. 1).

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We performed a photogrammetric survey with drones on automatic flights at altitudes between 20 and 100 m above the mean ground surface, with 90% overlap and with a camera positioned 90 degrees 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 less than 1 cm/pixel. This feature allowed us to observe centimetric variations in the topography 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.

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 induced by well-developed dolines.

Fracture attributes were also recorded in the field along linear scanlines (e.g., Watkins, 2015) according to the conceptual scheme in Figure 2. For each

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fracture set (veins and stylolites), 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.

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Quantification of deformation intensity was accomplished using the free-download software FracPaQ (Healy and Rizzo, 2017) directly on field-mapped 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:

P20 = NL/A [L-2] P21 = NL Lc/A [L-1]

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, 2002). This scan area method was applied to drone 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. 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 drone aerial imagery systematically shows the 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

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towards the dolines (Fig. 3B, D, F, G). In these images, each color represents a topographic height between 30 and 50 cm (see the color scale of the image to check the precise height). The widths of the rings decrease towards the doline, which represents an increase in the topographic slope. The subsidence rings were measured initially from the edges of the dolines. The topographic relief differences between the flat area around the doline and the main collapse 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 width of the total subsidence rings is generally twice as large as the average radius of the doline.

Table 1 - Subsidence rings versus doline radius data. Key: Rd (Doline average radius); SRL (Subsidence ring average length). Locations in figure 1.

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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. On average, topography varies 0.5 m/m in the inner subsidence zone, while it ranges 0.2 m/m in the outer subsidence zone (Table 2).

Figure 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 doline corresponds to S3 in Fig. 1.

The inner subsidence zone is also the area most likely to collapse if the dissolution processes continue and the doline is enlarged. The subsidence zone corresponds to the area where the topography is still influenced by the main collapse but dips smoothly and is hard to identify without the aid of high-resolution

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images. Subsidence rings are formed by the inner and outer subsidence zones (Fig. 4D).

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

Most dolines occur as isolated structures, representing a sector where dissolution is concentrated. In some cases, dissolution occurs along fracture corridors, where two or more dolines tend to develop. We observe that when the subsidence rings of different dolines coalesce, their rings are superposed, creating a larger area of subsidence (Fig. 5). The evolution of this process can

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culminate with the formation of the karst landform known as an “uvala” (Kranjc, 2013), representing the coalescence of two or more dolines.

Figure 5 - Coalescence of subsidence rings: (A), (B) and (C) (S3, S6, S5, on Fig. 1, respectively) are digital elevation models of some dolines. These rings correspond to elevations ranging from 30 – 90 cm. Examples of dolines and rings are oriented along the NE-SW-striking fracture set.

4.2 Structural data and fracture attributes

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 (fractures) (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, C) to tens of centimeters-wide dissolution, which partially or totally affects the fractures (Fig. 6A). Veins and stylolites are commonly

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strata-UFRN/CCET/PPGG – Dissertação de Mestrado Resultados

27

bound, i.e., confined to beds, although cross-bedding elements are also 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, and L). Joints are less abundant than veins and stylolites and generally cross-cut them.

Figure 6 – Structures formed before the doline formation at the outcrop scale. (A) Partially to totally dissolved fractures, where the measurement of the vein length includes the dissolved portion. (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.

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UFRN/CCET/PPGG – Dissertação de Mestrado Resultados Figure 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).

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 Figure 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

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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).

Figure 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 Figure 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, 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

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(Fig. 10B). Collapse fractures are developed to a distance of 15-20 m from the doline border at sites S1 and S3. This distance is 10-15 m at site S4.

Figure 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 Figure 1.

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Figure 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 Figure 1.

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The fracture aperture, height, and length were measured along linear scanlines at sites S1 and S3, as reported in Figure 11. In general, collapse fractures show greater apertures than the strata-bound veins and stylolites because the selective dissolution of collapse fractures is more persistent (i.e., height is often greater than the beds) 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.

Figure 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 figure 1.

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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.

Figure 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.

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UFRN/CCET/PPGG – Dissertação de Mestrado Resultados Figure 13 - Histograms of linear scanlines measured around the dolines. The Y axis represents the number 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 Figure 12.

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

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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 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 Figure 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 Figure 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).

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UFRN/CCET/PPGG – Dissertação de Mestrado Resultados Figure 14 – Measurements of preexisting and collapse fractures. A) Areal frequency (P20) and fracture intensity (P21) from sectors S1.3 and S1.4 at site S1 and sector S3.3 at site S3. B) Areal frequency and fracture intensity from sectors S1.3, S1.4 and S3.3 calculated 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.

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Table 3 - Circular scanlines results. Key: NL (Number of fractures in circles); Lc (Lenght of fractures in circles); A (Area). Data acquired in S1.

5. Discussions

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 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

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layers generates structural instability of a cave ceiling, triggering its collapse and consequent formation of a doline (Waltham, 2005, Gabrovšek and Stepišnik, 2011, Kaufman and Romanov, 2016, Cahalan and Milewsky, 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; 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, 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.

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, 10). The surroundings of dolines show increases in fracture density and aperture (Figs. 11, 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).

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Figure 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 50 to 90 cm. 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).

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

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layers and fractures in the subsurface, creating dissolution corridors oriented to the main tectonic direction. (3) The concentration of dissolution in certain sectors provides continuous mass removal, creating empty chambers such as caves (Augarde et al., 2003), causing instability of the overlaying layers and generating slight changes in the topography. This stage represents the initial formation of subsidence rings. In this stage, high-resolution images may help to identify these areas (Figs. 5A, 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 fractures 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 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 fields, such as the oil industry. More than 50% of all oil and gas

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production in the world comes from carbonate rocks (Mazzulo, 2004). In general, these rocks have the natural characteristic of losing primary porosity during cementation and compaction (Halley 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 hinder production 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.

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.

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

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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.

Figure 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.

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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 (Gutierrez 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 (Ozturk 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