Southern Tunisia is known to be less deformed and simpler than its neighboring Atlassic domain to the north. This area is complex and basin evolution in the Southern Chotts-Jeffara (SCJ) basin is debated. In this paper we combined surface and subsurface data with low temperature thermochronology (LTT) to reinvestigate the tectono-sedimentary evolution of the SCJ basin from Permian to Jurassic. We reconstruct the present-day architecture of the SCJ basin along two regional sections. In these sections, we focused mainly on regional thickness variations and on internal reflections interpreted from seismic data. We observe three structural elements: (a) A Paleozoic culmination, oriented E-W, capped by Mid-Upper Triassic deposits; (b) the Tebaga of Medenine (ToM), a culmination also oriented E-W but located ∼50 km north of the Paleozoic culmination; and (c) A Triassic culmination in the eastern part of the area, oriented NW-SE. We note the absence of major normal faults along the sections. The LTT data we present are the first published in this area and allow to reconstruct the timing and magnitude of vertical movements. These data prove: (a) exhumation at ∼230 Ma of the Permian and Lower Triassic units associated with the onset of the ToM removing locally about 900 m of pre-Cretaceous sediments; and (b) the development of the Triassic culmination ∼180 Ma removing 2000 m of pre-Cretaceous sediments in the Jebel Rehach. This study demonstrates that vertical movements in the SCJ basin are controlled by long-wavelength processes developed essentially in shortening regimes.

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The heterogeneity of the Upper Jurassic carbonate reservoir (Malm reservoir) beneath the North Alpine Foreland Basin has a significant influence on the mass and heat flow processes during geothermal exploitation. Geophysical borehole data revealed that sub-seismic scale fractures and karstified fractures occur at the inflow zones of deep geothermal wells. However, pressure transient analysis (PTA) in some previous studies concluded that it is difficult to detect the influence of sub-seismic scale features, suggesting that radial flow regime is dominant. Accordingly, a regional thermal-hydraulic model adopted the equivalent porous medium (EPM) approach, homogenizing the sub-seismic scale reservoir heterogeneities; however, unable to detect an early thermal breakthrough (ETB) in a geothermal doublet located SE of Munich. We apply PTA on three buildup tests belonging to that doublet following a deterministic approach to constrain the reservoir type by interpreting the pressure derivative (PD) plots constrained by geophysical and geological data. We derive the magnitudes of the reservoir hydraulic parameters by matching the PD plots with the selected interpretation models. We find that clustered fractures have a significant influence on the reservoir hydraulics, evidenced by trough-shaped curves in the PD plots. Linear flow regime interpreted from the interference test between the two wells indicates permeability anisotropy, which may have caused the ETB. Geophysical data interpretations indicate that these fractures correspond to a coupled fault damage zone and a fracture corridor. Finally, we present a fit-for-purpose 2D discrete fracture network model utilizing the PTA results to match our analytically calibrated model. Our study offers a potential hydraulic explanation to the cause of the ETB highlighting the importance of integrating multi-scale/disciplinary data sets to improve the reliability of dynamic reservoir models, based on which, economic-related decisions are made.

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In this study, a re-evaluation is performed of the well data of the NLW-GT-01 and VAL-01 wells in the Lower Triassic sandstones in the West Netherlands Basin. Core, geophysical and image logs, are compared to document the characteristics of natural fractures distribution, and investigate the geological parameters influencing their development. The main control on the distribution is identified. Natural fractures are concentrated in heterolithic lithological intervals of the VAL-01 and NLW-GT-01 wells. The identification of more fractures in VAL-01 compared to NLW-GT-01 can be explained by the difference in basin location. VAL-01 was located in the centre of the basin where distal playa environments produced fine-grained material alternating with coarser sands. The more proximal NLW-GT-01 was dominated by fluvial sands. The lithological variability produces Young’s modulus variability that seems to be driving increased fracture density rather than that the absolute value of the Young’s modulus. These fluctuations could be the result of concentrated compressional stress, which is supported by the existence of stylolites, indicative of high compressional stresses in the same intervals. The identification of controls on fracture distribution in the targeted stratigraphic interval can be used to optimize the planning of future geothermal doublets and to de-risk upcoming operations.@en

The southern Chotts basin (SCB), Central Tunisia, has shown hydrocarbon potential since the end of the 1980s. This basin records a complex structural history which appears decoupled at the Hercynian or Variscan unconformity. The Paleozoic series is deformed by short to medium wavelength folds (kilometres-multi kilometres scale) and by steep normal faults. The Mesozoic series is largely less deformed. The evolution of the basin through time is still a matter of debate as the preserved Paleozoic series is fragmented (e.g. affected by erosions). In this paper, we proposed a reconstruction of the vertical movements affecting the basin and an evaluation of their magnitude. Using basin modelling techniques, we provided new insights on the possible thermal evolution of the basin that might be used in the future exploration phases. This study was completed by structural restorations allowing the reconstruction of the paleogeography of the basin at the time of deposition of principal reservoir formations.

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The positive impact that natural fractures can have on geothermal heat production from low-permeability reservoirs has become increasingly recognised and proven by subsurface case studies. In this study, we assess the potential impact of natural fractures on heat extraction from the tight Lower Buntsandstein Subgroup targeted by the recently drilled NLW-GT-01 well (West Netherlands Basin (WNB)). We integrate: (1) reservoir property characterisation using petrophysical analysis and geostatistical inversion, (2) image-log and core interpretation, (3) large-scale seismic fault extraction and characterisation, (4) Discrete Fracture Network (DFN) modelling and permeability upscaling, and (5) fluid-flow and temperature modelling. First, the results of the petrophysical analysis and geostatistical inversion indicate that the Volpriehausen has almost no intrinsic porosity or permeability in the rock volume surrounding the NLW-GT-01 well. The Detfurth and Hardegsen sandstones show better reservoir properties. Second, the image-log interpretation shows predominately NW-SE-orientated fractures, which are hydraulically conductive and show log-normal and negative-power-law behaviour for their length and aperture, respectively. Third, the faults extracted from the seismic data have four different orientations: NW-SE, N-S, NE-SW and E-W, with faults in proximity to the NLW-GT-01 having a similar strike to the observed fractures. Fourth, inspection of the reservoir-scale 2D DFNs, upscaled permeability models and fluid-flow/temperature simulations indicates that these potentially open natural fractures significantly enhance the effective permeability and heat production of the normally tight reservoir volume. However, our modelling results also show that when the natural fractures are closed, production values are negligible. Furthermore, because active well tests were not performed prior to the abandonment of the Triassic formations targeted by the NLW-GT-01, no conclusive data exist on whether the observed natural fractures are connected and hydraulically conductive under subsurface conditions. Therefore, based on the presented findings and remaining uncertainties, we propose that measures which can test the potential of fracture-enhanced permeability under subsurface conditions should become standard procedure in projects targeting deep and potentially fractured geothermal reservoirs.

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Understanding fractures and fracture networks is essential for the investigation and use of subsurface reservoirs. The aim is to predict the fractures and the fracture network when there is no direct access to subsurface images available. This article presents a universal workflow to numerically compute a discrete fracture network by combining the 1D scanline survey method, processed with the newly written SkaPy script, together with the multiple point statistic method (MPS). This workflow is applied to a potential geothermal site in Mexico called Acoculco. We use Las Minas outcrops and quarries as surface analogues for the Acoculco reservoir, as Las Minas and Acoculco are both formed by the influence of a plutonic intrusion into the Jurassic-Cretaceous carbonate sequence of the Sierra Madre Oriental in the Trans-Mexican volcanic belt (TMVB). The intrusion is associated with contact metamorphism and metasomatic phenomena, providing the basis for the mining activities at Las Minas. The results obtained using this workflow demonstrate the feasibility of the approach, which presents a solution combining the efficiency of data processing and an interpretation-driven approach to build realistic discrete fracture networks. This workflow can be used in the process of estimating the permeability of a fracture controlled reservoir, with using only scanline surveys data as input. This is essential in the process of evaluating the feasibility to develop an enhanced geothermal system.@en

The use of the subsurface and the exploitation of subsurface resources require prior knowledge of fluid flow through fracture networks. For nuclear waste disposal, for the enhancement of hydrocarbon recovery from a field, or the development of an enhanced geothermal system (EGS), it is fundamental to constrain the fractures and the fracture network. This study is part of the GEMex project, an international collaboration of two consortia, one from Europe and one from Mexico. The research is based on exploration, characterization and assessment of two geothermal systems located in the Trans-Mexican volcanic belt, Los Humeros and Acoculco. In Acoculco, two wells reached very high temperatures, but did not find any fluids. For that reason, the Acoculco Caldera is foreseen as an EGS development site, hoping to connect existing wells to a productive zone. This implies that the fluid flow through the geothermal reservoir would be mainly fracture dominated. This study investigates the dependency of fracture permeability, constrained by fracture lengths and apertures, with stress field conditions. Simulations are computed in 2D, using COMSOL Multiphysics^® Finite Elements Method Software, populated with mechanical data obtained in the rock physics laboratory and with dense discrete fracture networks generated from 1D scanline surveys measured in Las Minas analogue outcrops for Acoculco reservoir. The method offers a prediction for multiple scenarios of the reservoir flow characteristics which could be a major improvement in the development of the EGS technology.

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The sedimentary succession exposed in the Northern Dalmatia Islands mainly consists of Cretaceous to Neogene shallow water carbonates, folded and imbricated within the External Dinarides thrust belt. During Cretaceous times, carbonate sediments were deposed on a heterogeneous, tectonically-influenced carbonate platform, which was then uplifted and eroded, as evidenced by a regional unconformity embracing the Late Cretaceous and Paleocene. Sedimentation resumed during the Eocene, when the area was part of the foreland basin of the Dinaric belt. With our geological and structural map of the southeastern Pag Island at the 1:25,000 scale, we refined the stratigraphic and structural setting and the tectono- sedimentary evolution of the area.@en

Representing fractures explicitly using a discrete fracture network (DFN) approach is often necessary to model the complex physics that govern thermo-hydro-mechanical-chemical processes (THMC) in porous media. DFNs find applications in modelling geothermal heat recovery, hydrocarbon exploitation, and groundwater flow. It is advantageous to construct DFNs from the photogrammetry of fractured outcrop analogues as the DFNs would capture realistic, fracture network properties. Recent advances in drone photogrammetry have greatly simplified the process of acquiring outcrop images, and there is a remarkable increase in the volume of image data that can be routinely generated. However, manually digitizing fracture traces is time-consuming and inevitably subject to interpreter bias. Additionally, variations in interpretation style can result in different fracture network geometries, which, may then influence modelling results depending on the use case of the fracture study. In this paper, an automated fracture trace detection technique is introduced. The method consists of ridge detection using the complex shearlet transform coupled with post-processing algorithms that threshold, skeletonize, and vectorize fracture traces. The technique is applied to the task of automatic trace extraction at varying scales of rock discontinuities, ranging from 10° to 102m. We present automatic trace extraction results from three different fractured outcrop settings. The results indicate that the automated approach enables the extraction of fracture patterns at a volume beyond what is manually feasible. Comparative analysis of automatically extracted results with manual interpretations demonstrates that the method can eliminate the subjectivity that is typically associated with manual interpretation. The proposed method augments the process of characterizing rock fractures from outcrops.

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Natural fracture network characteristics can be establishes from high-resolution outcrop images acquired from drone and photogrammetry. Such images might also be good analogues of subsurface naturally fractured reservoirs and can be used to make predictions of the fracture geometry and efficiency at depth. However, even when supplementing fractured reservoir models with outcrop data, gaps will remain in the model and fracture network extrapolation methods are required. In this paper we used fracture networks interpreted from two outcrops from the Apodi area, Brazil, to present a revised and innovative method of fracture network geometry prediction using the multiple-point statistics (MPS) method.

The MPS method presented in this article uses a series of small synthetic training images (TIs) representing the geological variability of fracture parameters observed locally in the field. The TIs contain the statistical characteristics of the network (i.e. orientation, spacing, length/height and topology) and allow for the representation of a complex arrangement of fracture networks. These images are flexible, as they can be simply sketched by the user.

We proposed to simultaneously use a set of training images in specific elementary zones of the Apodi outcrops in order to best replicate the non-stationarity of the reference network. A sensitivity analysis was conducted to emphasise the influence of the conditioning data, the simulation parameters and the training images used. Fracture density computations were performed on selected realisations and compared to the reference outcrop fracture interpretation to qualitatively evaluate the accuracy of our simulations. The method proposed here is adaptable in terms of training images and probability maps to ensure that the geological complexity in the simulation process is accounted for. It can be used on any type of rock containing natural fractures in any kind of tectonic context. This workflow can also be applied to the subsurface to predict the fracture arrangement and fluid flow efficiency in water, geothermal or hydrocarbon fractured reservoirs.@en

It sounds counter-intuitive to consider contraction features such as stylolites as potential conduits for flow. However, this idea has grown since 1980, with geoscientists finding many examples principally in carbonate reservoirs where stylolites can be considered as fluid-efficient features. Among others, these features can be reactivated stylolites, can generate positive porosity and permeability anomalies, can drive corrosive fluids or can remain open in an overpressured system. Conversely, stylolites can also be closed forever. These impermeable stylolites can generate permeability anisotropy that may impact fluid movements. Stylolites require particular attention to evaluate whether they act as drains or as barriers to flow (compartmentalisation). We review some of the key studies of the past thirty years with a special attention to the most recent ones. We end-up considering their mechanical origin, their nucleation and growth, their past and present impact on reservoir properties and performances as key factors influencing the flow efficiency differentiation of these features. This short review presents the latest theories and observations about stylolites with respect to the key factors aforementioned. The authors support herein that a distinction should be made between processes occurring in the past and the present-day impact the stylolite had on reservoir properties.@en

Natural fractures conduct fluids in subsurface reservoirs. Quick and realistic predictions of the fracture network organization and its fluid flow efficiency from limited amount of data is critical to optimize resources productivity. We recently developed a method based on multiple point statistics (MPS) technique to produce geologically-constrained fracture network simulations. The method allows to account for the intrinsic non-stationarity of these networks by considering a multivariate input data instead of averaged distribution of fracture parameters. In addition, the method considers probability maps reflecting the influence of fracture drivers in the network variability. Consequently, the simulated fracture networks derived from the innovative MPS approach are geologically better constrained than in classical discrete fracture network modelling approaches. This paper proposes to apply this method in subsurface conditions where available data are sparsely distributed. We developed a workflow where data are gathered from wellbore and from additional sources (outcrops). These data are used to extrapolate a network around the borehole as training images and themselves are extrapolated at the reservoir scale following a geological probability map.This work also presents innovations on the way how training images and probability maps that may integrate more geology constrain than relying almost entirely on available data.@en

Abundant literature has documented the ecological impact of very high-energy (hurricane-type) events and high-energy (storm-type) events on modern coral reefs and sediment transport. In previous studies, sedimentary processes are often deduced from geophysical image analysis and scuba diving observations. Few field-based studies have been devoted to the impact of (very) high-energy disturbance on the structure and ecology of Holocene and Pleistocene coral reefs, and very few to the sedimentary processes acting around these reefs. This research examines the structure of the late Pleistocene (end of MIS5e stage) reefs of La Désirade Island, Guadeloupe Archipelago, Lesser Antilles, and focuses on the poorly known reworking processes and related coral-breccia sedimentation surrounding these reefs. Fieldwork was performed to document two coral-fragment-rich breccia interbedded with coral patches. The breccia predominantly show the same petrography but differ in their internal structure, showing a “massive breccia” type and a “stratified breccia” type. This work infers that these breccia are the result of gravity-induced high-density mass flows triggered by high to very-high energy events such as storms and hurricanes. Damage and reworking vary according to the type of (very) high-energy event. Prior to transport, storms are thought to have broken the heads of branching and columnar coral colonies, and broken and overturn isolated lamellar and massive colonies. Hurricanes (particularly their eye walls) may have reworked much more sediment than storms, and may have also destabilised entire several metre-sized columnar colonies from the flank of coral patch reefs. We propose the name “coral debrite” for these coral breccia deposits caused by gravity-induced debris flows. The reworked deposits are interbedded between two coral reef levels, of which the main coral-species vertical succession should record the end of Marine Isotope Stage 5e sea-level rise. Thus, reworking is believed to occur during the transitional stage between a glacial and an interglacial period.@en

Improving the understanding of fluid flow through fracture networks is crucial to the optimum recovery of hydrocarbons in naturally fractured reservoirs. In explicit dual continuum representations of matrix and fractures, it is not just the intensity of fracturing and the topological relationships in the network but also connectivity of hydraulically open fractures that determines effective permeability of the system. Fractured folds often form prolific reservoirs owing to the structural closure they afford and the additional porosity and permeability due to the fold related fracturing. The fracture patterns are of specific interest owing to the complex geometries associated with folding. In this work we utilize a combined outcrop based and numerical approach to characterize fracture patterns, fracture apertures and fluid flow sensitivities using a folded ‘box-type’ anticlinal structure example from the Pag Island, Croatia. The Pag Island is part of the main Adriatic-Dinaridic Carbonate Platform in the External Dinarides region that persisted from the Triassic to the Eocene with Paleocene-Miocene Dinaridic deformation phases. The current day geology consists of up to 1 km thick Cenomanian - Senonian shallow water rudists bearing carbonates and about 650m thick Eocene-Oligocene Nummulitic carbonate successions. We present a 3D structural model of the Pag Island with a Discrete Fracture Network (DFN) digitized representation built from drone photogrammetry. The multiscale fracture patterns in this multilayer folded reservoir analog are classified on the basis of folding stages (pre-, syn-, post folding). FEM geomechanical modeling is used to quantify stress-sensitive heterogeneous fracture apertures with the Barton-Bandis empirical model. Permeability sensitivities of the fracture network are tested using coupled single phase fluid flow and poroelasticity simulations. The results show that hydraulic apertures are extremely sensitive to fracture orientation with respect to the shortening direction and reservoir pressure transients. Our novel approach for coupled flow & geomechanics simulations yields effective permeability tensors that are functions of both matrix and fracture conductivities and also fracture aperture variations in time. We test dynamic closure & dilation of fractures in depletion and injection scenarios and our results stress the importance of dynamic fracture apertures in structurally complex fractured reservoirs.@en