Hypogenic Karst Dissolution in Carbonate Rocks has Implications for Karstified Carbonate Reservoirs (original) (raw)
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Porosity and permeability along fractured zones in carbonates could be significantly enhanced by ascending fluid flow, resulting in hypogene karst development. This work presents a detailed structural analysis of the longest cave system in South America to investigate the relationship between patterns of karst conduits and regional deformation. Our study area encompasses the Toca da Boa Vista (TBV) and Toca da Barriguda (TBR) caves, which are ca. 107 km and 34 km long, respectively. This cave system occurs in Neoproterozoic carbonates of the Salitre Formation in the northern part of the São Francisco Craton, Brazil. The fold belts that are around and at the craton edges were deformed in a compressive setting during the Brasiliano orogeny between 750 and 540 Ma. Based on the integrated analysis of the folds and brittle deformation in the caves and in outcrops of the surrounding region, we show the following: (1) The caves occur in a tectonic transpressive corridor along a regional thrust belt; (2) major cave passages, at the middle storey of the system, considering both length and frequency, developed laterally along mainly (a) NE–SW to E–W and (b) N to S oriented anticline hinges; (3) conduits were formed by dissolutional enlargement of subvertical joints, which present a high concentration along anticline hinges due to folding of competent grainstone layers; (4) the first folding event F1 was previously documented in the region and corresponds with NW–SE-to N–S-trending compression, whereas the second event F2, documented for the first time in the present study, is related to E–W compression; and (5) both folding events occurred during the Brasiliano orogeny. We conclude that fluid flow and related dissolution pathways have a close relationship with regional deformation events, thus enhancing our ability to predict karst patterns in layered carbonates.
The salitre cave karst in the quartzite rocks of diamantina, Minas Gerais, Brazil
This work presents the main morphological characteristics of the Salitre Cave, located in the municipal district of Diamantina -MG and formed in quartzite rocks of the Espinhaço Supergroup, Sopa-Brumadinho Formation as an example of a well-developed karst system. This system now supported by two to three independent small seasonal rivers and displays well-developed dissolution and breakdown structures, as a result of intense intemperance activity. This karst formed, probably before the beginning of Quaternary taking advantage of a system of fractures, normal and thrust faults caused by Espinhaço and Brasiliano events in the Middle to Late-Cambrian. This work attempts at presenting an integrated view of the investigated area and at emphasizing its importance for the understanding of the processes, which occurred in the in carbonatic, quartzitic and silico-carbonatic rocks of Rio San Francisco and Rio Jequitinhonha Basins among the chain of the Espinhaço Range on east and the river valley of San Francisco on the west.
Characterizing and Modelling Fractures and Karst in Carbonate Units - The Porocarste Project
First EAGE/SBGf workshop, 2013
We investigated the Quaternary epigenic karst system in the Jandaíra Formation, a Turonian-Campanian carbonate platform in the Potiguar basin, northeastern Brazil. We concentrated our investigation in the vadose zone of the present-day karst, but also used borehole data. The leaching zones are preferentially concentrated along pre-solution openings composed of faults, joints, and bedding planes. These dissolved and enlarged faults and beddings form a system of caves and sinkholes, which must be included in the architecture of karst systems. This controlled-karst architecture presents a predictable geometry. This process occurs when the carbonate platform is exposed for a long period and can be thus affected by surface processes such as dissolution of erosion. The Quaternary epigenic karst features we describe may correlate with paleokarst systems in other carbonate platforms.
This study is focused on speleogenesis of the Toca da Boa Vista (TBV) and Toca da Barriguda (TBR), the longest caves in South America occurring in the Neoproterozoic Salitre Formation in the São Francisco Craton, NE Brazil. We employ a multidisciplinary approach integrating detailed speleomorphogenetic, lithostratigraphic and geological structure studies in order to reveal the origin of the caves, their functional organization and geologic controls on their development. The caves developed in deep-seated confined conditions by rising flow. The overall fields of passages of TBV and TBR caves represent a speleogenetically exploited large NE-SW-trending fracture corridor associated with a major thrust. This corridor vertically extends across the Salitre Formation allowing the rise of deep fluids. In the overall ascending flow system, the formation of the cave pattern was controlled by a system of sub-parallel anticlines and troughs with NNE-SSW dominant orientation, and by vertical and lateral heterogeneities in fracture distribution. Three cave-stratigraphic storeys reflect the actual hydrostratigraphy during the main phase of speleogenesis. Cavities at different storeys are distinct in morphology and functioning. The gross tree-dimensional pattern of the system is effectively organized to conduct rising flow in deep-seated confined conditions. Cavities in the lower storey developed as recharge components to the system. A laterally extensive conduit network in the middle storey formed because the vertical flow from numerous recharge points has been redirected laterally along the highly conductive unit, occurring below the major seal – a scarcely fractured unit. Rift-like and shaft-like conduits in the upper storey developed along fracture-controlled outflow paths, breaching the integrity of the major seal, and served as outlets for the cave system. The cave system represents a series of vertically organized, functionally largely independent clusters of cavities developed within individual ascending flow cells. Lateral integration of clusters occurred due to hydrodynamic interaction between the flow cells in course of speleogenetic evolution and change of boundary conditions. The main speleogenetic phase, during which the gross cave pattern has been established and the caves acquired most of their volume, was likely related to rise of deep fluids at about 520 Ma or associated with rifting and the Pangea break-up in Triassic – Cretaceous. This study highlights the importance of speleogenetic studies for interpreting porosity and permeability features in carbonate reservoirs.
Multi-disciplinary studies involving lithostratigraphy, structural geology and morphogenetic analysis, were performed in Toca da Boa Vista and Toca da Barriguda, the largest caves in South America, located in the São Francisco Craton, Brazil. The studies were aimed to reveal the origin and the functional organization of the caves and major controls over its architecture. The system, hosted by the Neoproterozoic Salitre Formation, is an analogue of deep-seated carbonate reservoirs dominated by fracture and hypogene karstic porosity and permeability. The rock sequence is gently folded, with a system of high frequency, low vertical magnitude (ca. 5-20 m) elongated anticlines and synclines, superimposed on larger dome-and-basin structures of ca. 30-40 m in magnitude. Five lithostratigraphic units within the part of the formation exposed in the cave system are, from bottom to top: (1) grainstone with cross-stratification, (2) fine grainstone with chert nodules, (3) microbial carbonate, (...
Polyphased karst systems in sandstones and quartzites of Minas Gerais, Brazil
2005
The state of Minas Gerais (Brazil) exhibits several major karst areas located in sandstone and quartzite terrains, that display a complex suite of underground and surface karstic forms. In the Espinhaço Ridge, central Minas Gerais, several caves, up to a few hundred metres long, occur in the surroundings of the town of Diamantina. Some of these caves, such as Salitre, represent swallow-holes and show dome pits. Other horizontal caves are characterized by corrosion forms generated into the phreatic zone. In some places, such as in the Rio Preto area, these phreatic forms have been overprinted by ceiling tubes, suggesting a polyphase karst evolution, prior to the draining of the cave. Relicts of passages, with circular cross section up to a metre in diametre, can be found amidst the residual tower-like surface landforms, which constitute a typical scenery in the landscape. Their dissection is due to a generalised karstification in the area, resulting in closed canyons, megakarrens and kamenitzas. In southern Minas Gerais, close to the Mantiqueira Ridge, the caves of the state park of Ibitipoca can extent 2 km in length. These caves are associated with a very large hanging geological syncline. Several of these caves contain active streams, that flow for hundreds of metres before disappearing in sand-choked passages. Keyhole cross sections characterize steeply descending passages in these caves, indicating a change from slow phreatic flow towards a faster vadose flow responsible for the vertical incision of the passage. Such change is probably related to base level lowering and/or to turn in the direction of the water flow. Several generations of wall-pockets, from a few centimetres to over a metre long, occur into the caves. These features are good indicators of the initial phase of speleogenesis, generating the initial conduits by their coalescence. This mechanism is also responsible for cut-off meanders. The main river in the area, which flows along the syncline axis, cuts through a rock barrier, generating a tunnel-like passage. This cave drains, through resurgences in its walls, part of the water that flows in other caves located in the flank of the syncline. The non-carbonate karst features observed in the state of Minas Gerais demonstrate the complex organisation of polyphase karst systems due to the linkage of underground and surface forms not previously connected. As in carbonate areas, these systems may play an important hydrological role. Fig. 1: Localisation of the studied areas
Hundreds of karstic features (n = 266) and gullies (n = 80) are indentified in an area (56.3 km 2 ) of non-carbonate rocks which include pure quartzite (Q), impure quartzite (Qi) and gneiss (G) within the Santana Basin (286 km 2 ), a part of Middle Paraiba do Sul River Valley, southeastern Brazil. Karstic features include caves (n = 32), collapsed dolines (n = 23), solution dolines (n = 209), Sinkholes (n = 02) and resurgences (n = 02). Gullies include active gullies (n = 46) and stabilized gullies (n = 34). Other general features as knickpoints (n = 06) and captured valleys (n = 03) were also indentified. We used GIS to study correlations between the presence of features, precise lithology and relief to indentify possible controls. The results suggest a lithological control on the karstic features with higher frequency of caves, collapsed dolines, sinkholes and resurgences in pure quartzite (Q) and in areas with high relief, but there is no lithological control on solution dolines. Gullies, knickpoints and captured valleys are also not controlled by lithology or relief. The results suggest a fluvial control of gullies and depositional features in valley bottoms caused by headward erosion, after underground captures or knickpoint disruption.
Near Surface Geophysics, 2015
Collapsed paleocave systems are carbonate reservoirs with high internal spatial complexity that are the result of several stages of karst processes. Paleocave-related reservoirs can be spread over large areas with significant thicknesses that are favourable for hydrocarbon exploration. Nevertheless, few studies have provided a detailed understanding of the strong lateral heterogeneity of these reservoirs and their complex karst-controlled development using modern karst terrain analogues. To elucidate this issue, the internal architecture of a collapsed paleocave system has been mapped accurately at the western border of the Potiguar Basin in Northeastern Brazil. The collapsed paleocaves outcrop in an escarpment that delimits the carbonate platform from the transgressive phase of the basin. Electrical resistivity tomography (ERT) and ground-penetrating radar (GPR) sections were acquired parallel and perpendicular to a road cut and served to parameterize the geophysical signatures of the collapsed paleocaves and the host rocks. The collapsed paleocaves were mapped by identifying high-resistivity zones and high-amplitude ground-penetrating radar reflectors. In contrast, the host rocks are marked by low to intermediate resistivity and ground-penetrating radar reflections that range from low amplitude to almost transparent. The resistivity data and the ground-penetrating radar attribute of the rootsquare energy enabled the mapping of the complex spatial distribution of the collapsed paleocaves system. At depths of approximately 20 m, the paleocaves are more spread out and eventually become isolated while sometimes being connected vertically through shafts. However, at shallower levels, the paleocaves are interconnected by ducts or coalesce into a system of paleocaves that are hundreds of metres long over an area of 12000 m 2 . The results of the study show the detailed internal geometry of this paleocave system at a subseismic scale, which enables the identification of the connectivity pattern among these karst features and the porosity and total volume of the reservoir. This system could serve as an outcrop analogue for other collapsed paleocave carbonate reservoirs worldwide.
Karsts in sandstones and quartzites of Minas Gerais, Brazil
The state of Minas Gerais (Brazil) is characterized by significant karst regions, which develop in both sandstone and quartzite terrains and display complex suites of underground and surface forms. In the Espinhaço Ridge, Central Minas Gerais, several caves of up to a few hundred metres long, occur in the surroundings of the town of Diamantina. Some of these caves, such as Salitre actually consist of swallow-holes. Other horizontal caves are characterized by corrosion forms generated in the phreatic zone. In some places, such as in the Rio Preto area, these phreatic forms are overprinted by ceiling tubes, suggesting a polyphase karst evolution, prior
Fractured and karstified carbonates are often major aquifers and hydrocarbon reservoirs. The Toca da Boa Vista (TBV) and Toca da Barriguda (TBR) caves in NE Brazil, which form the longest cave system in South America, developed in a gently folded, layered and fractured Neoproterozoic carbonate sequence made of different lithological units (limestones versus siliciclastic rocks and marls) with variable bed properties (bed thickness, surface roughness of bed-to-bed interlayers) and fracture attributes (spacing, height). The functional organization of the hypogenic cave system has been recently revealed, with subdivision in three speleo-stratigraphic storeys which, from bottom to top, acted as recharge, lateral redistribution and outflow components of the rising fluid flow system, respectively. Structural analysis and new fracture data from TBV and TBR indicate that the stratigraphic setting controlled the fracture pattern development during two tectonic phases, which determined the fluid flow anisotropy during speleogenesis. We found that mechanical and fracture stratigraphy, in conjunction with bed thickness and bed-to-bed interlayer characteristics, controlled the spacing and height of fractures and, as a result, strongly influenced the fluid flow pattern and functional organization of the cave system. In particular, bed-normal rising fluids localized along m-to tens of m-spaced throughgoing fractures developed in the lower and upper storeys, whereas horizontal and bed-parallel fluid flow was focused in a package of thin carbonates, siliciclastics and marls with cm-spaced, strata-bound fractures. This fluid flow anisotropy is attained also due to the presence of rough-walled, bedding-parallel interlayers (i.e., burial stylolites) in the lower massive grainstone which hampered bed-parallel flow, and smooth, planar bedding-parallel interlayers which enhanced lateral fluid movements in the well-layered and fractured middle storey units. Therefore, we conclude that in heterolithic layered carbonate packages, the intricate hypogenic karst pattern is related to heterogeneous fluid flow behaviour associated with the structural complexity imparted by the mechanical versus fracture stratigraphy.