Outcrop Analogue Research Articles

Analysis of facies proportions as a tool to quantify reservoir heterogeneity

The geometry, orientation and stacking patterns of sedimentary geobodies play a key control on fluid migration within subsurface reservoirs, aquifers, and repositories. Outcrops and more recently virtual outcrops are commonly used as analogues to characterise geobody architecture and stacking. Traditionally, both reservoirs and their analogues are described with respect to the bulk proportion of different facies and architectural elements within the system. These are combined with geometric measurements of the different geobodies to describe heterogeneity.This study presents a new methodology for quantifying the range of facies proportions within architectural element panels taken from outcrop analogues. This method measures the facies proportions within a series of vertical profiles and considers the statistical distribution, rather than the simple mean across the entire panel. The basic premise being that within a layer cake system, the spread in facies proportions will be very narrow whereas in a more “jigsaw” type system the spread will be much wider.To test this hypothesis, a series of nine virtual outcrops from a range of clastic depositional systems including fluvial, shallow marine and deep marine settings, have been analysed. Depositional facies (architectural elements) were interpreted in LIME and a series of depositional strike-orientated orthorectified panels were extracted. The panels were then analysed, and the extracted facies proportions were translated to “net sand” distribution. Results were then compared with conventional analysis of reservoir heterogeneity.Different depositional systems show distinct patterns. When arranged in depositional dip order, there is a systematic decrease in sand distribution spread down dip with significant resets at the boundaries between the major gross depositional environments (continental, shallow marine/shelf, deep marine).

High-resolution analysis of 3D fracture networks from Digital Outcrop Models, correlation to plate-tectonic events and calibration of subsurface models (Jurassic, Arabian Plate)

The performance of different reservoirs, both hydrocarbon and gas storage, is significantly influenced by the presence of natural fracture networks mainly originated in response to tectonic deformation. However, adequate characterization of fracture networks in the subsurface is often difficult since most of the used techniques are either spatially sparse (well and log data) or have limited resolution (seismic and well tests). In this study, we use UAV-based digital photogrammetry to reconstruct a 3D Digital Outcrop Model and characterize natural fracture sets of the late Jurassic Hanifa Formation (Tuwaiq Escarpment, Saudi Arabia). We analyzed the exposed fracture patterns in terms of fracture distribution, intensity, and lateral variability. The fracture dataset was collected using manual sampling approaches since the automatic sampling techniques are highly influenced by outcrop morphology thus not yet reliable to quantify and interpret fracture networks along reservoir outcrop analogues. The well-exposed late Jurassic outcrops in Wadi Birk, southern central Saudi Arabia, are considered analogous in depositional facies to subsurface reservoirs in the region. The observed fracture patterns are congruent with patterns reported from same-age formations in other parts of the Arabian Plate and hence can be related to plate-wide tectonic events that impacted Arabia during the Late Cretaceous (Alpine I) and the late Tertiary Period (Alpine II). This association provides support for using outcrop-based fracture studies as analogue scenarios for the orientation, distribution and intensity of fractures in subsurface reservoirs. The applied methodology for fracture network characterization have potential implications for hydrocarbon development, gas storage (H2) and Carbon Capture Utilization and Storage (CCUS).

Impact of stress regime change on the permeability of a naturally fractured carbonate buildup (Latemar, the Dolomites, northern Italy)

Abstract. Changing stress regimes control fracture network geometry and influence porosity and permeability in carbonate reservoirs. Using outcrop data analysis and a displacement-based linear elastic finite-element method, we investigate the impact of stress regime change on fracture network permeability. The model is based on fracture networks, specifically fracture substructures. The Latemar, predominantly affected by subsidence deformation and Alpine compression, is taken as an outcrop analogue for an isolated (Mesozoic) carbonate buildup with fracture-dominated permeability. We apply a novel strategy involving two compressive boundary loading conditions constrained by the study area's NW–SE and N–S stress directions. Stress-dependent heterogeneous apertures and effective permeability were computed in the 2D domain by (i) using the local stress state within the fracture substructure and (ii) running a single-phase flow analysis considering the fracture apertures in each fracture substructure. Our results show that the impact of the modelled far-field stresses at (i) subsidence deformation from the NW–SE and (ii) Alpine deformation from N–S increased the overall fracture aperture and permeability. In each case, increasing permeability is associated with open fractures parallel to the orientation of the loading stages and with fracture densities. The anisotropy of permeability is increased by the density and connectedness of the fracture network and affected by shear dilation. The two far-field stresses simultaneously acting within the selected fracture substructure at a different magnitude and orientation do not necessarily cancel each other out in the mechanical deformation modelling. These stresses affect the overall aperture and permeability distributions and the flow patterns. These effects – potentially ignored in simpler stress-dependent permeability – can result in significant inaccuracies in permeability estimation.

Hydrogeological parameterisation of the Daruvar thermal aquifer: integration of fracture network analysis and well testing

Highly fractured Mesozoic carbonate rocks are the main reservoir of many geothermal resources in northern Croatia, being of environmental, cultural, and economic value for the local and regional communities. The Daruvar thermal springs (temperatures < 50°C) represent the outflow area of an intermediate scale, tectonically controlled, hydrothermal system hosted in Triassic carbonate rocks. Several investigations have been conducted in the Daruvar area detailing the architecture of regional and local fracture networks and quantifying the hydrogeological parameters of the thermal aquifer. In this work, an integrated approach based on structural and hydrogeological investigations was employed to model the network of fractures in the reservoir and quantify its impact on the hydraulic properties. Structural investigations were conducted in the Batinjska Rijeka quarry, considered as an outcrop analogue of the thermal aquifer, employing both a classical field approach and the virtual quantitative analysis of a 3D digital outcrop model. Structural analysis of the digital outcrop model allowed identification of two sub-vertical systems of discontinuities, dipping to the NW and the WSW respectively, in accordance with the data collected through direct field measurements. The main geometric features of the discontinuity network and their statistical distributions were employed to construct discrete fracture network models at both the outcrop scale (approximately 100 m) and the aquifer scale in Daruvar (approximately 700 m). Calibration of the input parameters allowed modelling of porosity and permeability values that reproduce the field values assessed through pumping tests, well tests, and well logging. This work highlights the importance of integrating geological and hydrogeological investigations to obtain a more reliable reconstruction and quantification of the processes driving the fluid flow in fractured aquifers and affecting the spatial distribution of their hydraulic properties.

Fault system dynamics and their impact on ordovician carbonate karst reservoirs: Outcrop analogs and 3D seismic analysis in the Tabei region, Tarim Basin, NW China

The Tabei Uplift in the Tarim Basin of NW China represents an area with significant hydrocarbon exploration potential. This paper describes the effects of multi-phase faults on structural and hydrocarbon systems in the Tabei Uplift utilizing high-quality 3-D seismic, geological outcrops and drilling and production data. The interpretation and analysis of 10 fault systems from the Cambrian through the Cretaceous, shows that the deep, middle and shallow faults developed in four stages: the Early Caledonian, Mid-Late Caledonian, Hercynian and Himalayan. Specifically, the Early Caledonian fault system is favorable for the hydrocarbon expulsion of Cambrian source rocks, while Mid-Late Caledonian strike-slip faults facilitate hydrocarbon migration and accumulation. Hercynian and Himalayan strike-slip faults were formed as a result of the reactivation of the Mid-Late Caledonian strike-slip faults and have a destructive effect on the older oil reservoirs. Similarly, Permian volcanic activity also caused varying degrees of damage to Ordovician reservoirs. Oil production along fault zones more than 100 km show that the complex fracture-vuggy (FV) structure is more likely to trap oil and gas when it is damaged than the simple FV structure. Fault-controlled, fracture-vuggy carbonate reservoirs (FVCRs) were classified into six grades, which correlate to the productivity of oil wells in the region. Using this classification system, several drilling locations were selected and resulted in 80% success rate. This classification provides a novel approach for assessing the interplay between fault activity and reservoir formation, enhancing the prediction of hydrocarbon potential in varying strike-slip fault zones. This analysis not only refines our understanding of the region's geodynamic history but also aids in optimizing exploration and development strategies.

Microfacies-Based Depositional Model for the Maastrichtian Upper Member of Thaqab Formation in Northern Oman: A Potential Outcrop Analog for Deep-Marine Slope Succession

Microfacies-Based Depositional Model for the Maastrichtian Upper Member of Thaqab Formation in Northern Oman: A Potential Outcrop Analog for Deep-Marine Slope Succession

Lateral heterogeneity of basin‐plain turbidites of the Cloridorme Formation, Quebec, Canada: Implications for horizontal well prediction

AbstractFacies models for basin‐plain turbidite systems often depict very simplistic event‐bed geometries that are tabular at the kilometre scale. However, recent studies have demonstrated more complex facies architectures, including rapid changes in event‐bed thickness and facies composition. This lateral event‐bed heterogeneity can have a significant impact on reservoir heterogeneity prediction in basin‐plain turbidite systems developed for hydrocarbon production, carbon sequestration or geothermal energy. Coastal outcrops on the Gaspé Peninsula in Quebec expose the Middle Ordovician Cloridorme Formation, a synorogenic ‘flysch’ turbidite system developed in the Taconic foreland basin. The formation is interpreted to occupy a basin‐floor position due to long‐distance (tens of kilometres) correlations of bedsets in the direction of palaeocurrent. This outcrop‐based study of the Cloridorme Formation utilises drone photogrammetry, centimetre‐scale graphic logs and handheld gamma ray scintillometry data to better understand the detailed turbidite and hybrid event‐bed architecture in a basin‐plain setting. While most beds in this outcrop study can be traced for 500 m or more in a downcurrent direction, these results indicate significant intra‐bed and inter‐bed lateral complexity, including changes in bed thickness, grain‐size distribution and mud content. The quantification of these lateral changes and comparison with other well‐constrained outcrop analogues refines the environment of the Cloridorme Formation and aids in the prediction of subsurface heterogeneity in conventional and unconventional hydrocarbon reservoir systems through reservoir model parameterisation, as well as the characterisation of lateral heterogeneity important for horizontal‐well geosteering and completion strategies.

The role of salt basins in the race to net zero: a focus on Australian basins and key research topics

Globally, many salt basins host highly productive fossil fuel resources and provide excellent opportunities for developing economically viable clean energy systems such as (1) energy storage in salt caverns, including hydrogen, helium, natural gas, and other economic gases; (2) permanent sequestration of carbon dioxide; (3) development of geothermal energy; (4) critical mineral exploration and extraction, and (5) natural hydrogen production. Despite the high potential to deploy financially viable clean energy solutions related to the formation and evolution of salt basins, our current knowledge regarding critical aspects of salt basin characterisation in Australia is limited. New research is necessary to develop these sustainable energy systems and achieve net zero emissions; therefore, it is critical to re-evaluate the geology of salt basins. Key research areas to enable these opportunities relate to the precipitation, deposition, and deformation of salt basins. This paper reviews the potential for a range of energy systems within salt basins, outlines emerging research topics, and demonstrates the value of Australian salt basin outcrop analogues for improved subsurface interpretation globally.

Tidal versus fluvial point bars: Key features from the integration of outcrop, core and wireline log information of Triassic examples

AbstractThe Triassic red beds of the Tabular Cover of the Iberian Meseta are an excellent reservoir outcrop analogue, a direct consequence of high‐quality exposures, which offer exceptional three‐dimensional outcrops, as well as a wide variability of depositional environments. Fluvial and transitional with tide‐influenced and wave‐influenced settings are recognised. Three point bar geobodies of similar scale, but influenced by different processes, were selected from this succession. Point bar geobody 1 was influenced by purely fluvial processes while geobodies 2 and 3 were tide‐influenced. Both types of geobody were developed as point bar deposits in sinuous channels. A fully integrated study was carried out on these geobodies, utilising both outcrop and subsurface‐based approaches, to characterise the key differences between fluvial and tidal point bars in the sedimentary record. The outcrop‐based component involved traditional field data collection methods alongside digital techniques and data capture, including the use of digital outcrop models. Additionally, subsurface‐based methods were employed, utilising core and wireline logs obtained from wells drilled in close proximity to the outcrop. The integration of these approaches aims to accurately differentiate the depositional settings of the three different geobodies, which while apparently very similar in many key respects also exhibit considerable differences when considered from the perspective of subsurface management of potentially similar geobodies. This study also emphasises the need to clearly distinguish high‐sinuosity deposits based on their depositional sub‐environment in order to properly evaluate their potential for subsurface management. Additionally, it highlights the presence and importance of internal baffles that may well influence fluid migration and indeed even compartmentalise geobodies. Three point bar geobodies of similar scale, but influenced by different processes, have been selected in this succession. A fully integrated study was carried out on these geobodies, utilising both outcrop‐based and subsurface‐based approaches, to characterise the key differences between fluvial and tidal point bars in the sedimentary record.

Granitoids of the western Himalaya and Karakoram as potential geothermal reservoirs – A petrological, geochemical and petrophysical study

Hot springs in various granitoid and gneissic complexes in northern Pakistan indicate elevated geothermal gradients, which warrants their evaluation for geothermal applications. Evaluation of such prospects requires extensive knowledge of rock properties and their interaction with reservoir fluids. Our contribution provides first-order information on petrophysical, geochemical, and petrographic descriptions derived from outcrop analogs. About seventy samples of (mostly) granitoids and gneisses from the Nanga Parbat Massif (NPM), the Kohistan-Ladakh Batholith (KLB), and the Karakoram Batholith (KB) were collected. Biotite, K-feldspar, and plagioclase in the granitoid and gneiss show weak to moderate alteration, which increases in shear zones, suggesting fracture-assisted fluid interaction. Geochemical results indicate that the gneisses and granites of the NPM are mostly peraluminous S-type and show enrichment in most LREEs and depletion in HREEs. In addition, depleted Sr and Ba and enriched Rb and U in granites indicate partial melting and high fractionation. On the contrary, granitoids from the KLB are of calc-alkaline I-type and characterized by the depletion of REEs and the enrichment in Ba and Sr. The granitoids of the KB, due to their different magmatic histories, are more diverse, ranging from older I-type calc-alkaline granodiorite, and younger I-type alkaline syenite and S-type calc-alkaline granite. They show an overall enrichment in HREEs along with Ba, Th, Ta, Sr, and Lu. Allanites from syenite of the KB show significant concentrations of Th and U.Petrophysical measurements reveal low matrix porosities (0.6–3.5 %), with primarily average thermal conductivities (1.48–3.37 W m−1K −1) and thermal diffusivities (0.68–1.95 ∙10–6 m2 s−1), with minor variation in specific heat capacities (744–767 J kg−1 K−1). The NPM displays higher average thermal conductivity, thermal diffusivity, and lower porosity than the KB, while the petrophysical properties of the KLB range between these two domains.The geothermal systems in the area operate due to the thickening of a granitoid-dominant crust, which is enriched with radiogenic elements. The fault zones provide channels for meteoric water to access deeply these hot and thermally conductive rocks in the subsurface. After heat exchange, the water is discharged back to the surface as hot springs. The present data set provides a better understanding of regional geothermal regimes from petrological, geochemical, and petrophysical perspectives and assists in numerical modeling for potential geothermal assessment.

Imaging along-strike variability in fault structure; insights from seismic modelling of the Maghlaq Fault, Malta

Seismic modelling studies of outcrop analogues have proven a powerful method to provide a link between geology as observed at outcrop, and the interpretation of subsurface geology from seismic data. Seismic imaging of fault zones is inherently challenging, as they may be associated with steep dips, narrow zones of heterogeneously deformed rocks, and complex geometries at the limit of seismic resolution. We here investigate how along-strike variations in fault geometry and structural style are imaged in reflection seismic images, based on seismic modelling of well-exposed outcrops of the Maghlaq Fault hosted in the Miocene-Oligocene carbonate rocks in Malta. Using two-dimensional seismic modelling, selected structural cross-sections have been modelled, focusing on variations in fault- and hangingwall bed geometries, and dominant seismic signal frequency. The seismic models show great variability in resolution and, thus, level of resolved geologic detail. The modelled seismic images show that, for all signal frequencies, the fault itself is not imaged, but is manifested by terminations of footwall reflections as well as fault-related strain in the form of drag folding of hangingwall beds, in addition to fault-proximal seismic imaging artefacts. Rotated and ‘drag-folded’ hangingwall beds along the fault produces a high-amplitude fault parallel reflection bundle in the immediate hangingwall of the fault. The illumination issues that dominate in low dominant frequency seismic models are less prevalent in the higher-resolution, higher dominant frequency models. The results of this outcrop-to-seismic study offer insight to the relationship between faults and seismic images, which may improve our ability to accurately interpret faults in the subsurface form reflection seismic data.

Lithocodium mound identification using logging-while-drilling image log and quantified cutting analysis — Validation with analogues

Lithocodium mounds are early Cretaceous sedimentary structures described in the literature from outcrops, however, never described in the subsurface. The objective of this work is to identify and characterize Lithocodium mounds in the subsurface along a 25,000 ft horizontal well. Drill cuttings sampled at a 100 ft interval are observed in thin sections to define and quantify key sedimentary indicators (bioclasts, facies, and texture). Logging-while-drilling (LWD) gamma ray (GR), density, neutron, and resistivity logs are acquired along with the LWD high-resolution borehole image (BHI) log. Bedding dips from BHI data, interpreted along the horizontal well, enabled the reconstruction of the reservoir paleotopography. In particular, the alternation of dip azimuth combined with the facies interpretation from the thin sections supported the interpretation of eight distinct mound structures. An assessment of their overall geometry confirmed the mound shape to be subcircular, consistent with the subcircular geometries observed in Oman at the outcrop. The inferred dimensions of the mounds are comparable with the Aptian Lithocodium mounds in Oman (30–40 m), and their intermound organization resembles that of the Albian mounds in Texas. This work demonstrates the value of analyzing cuttings to complement image log interpretation and the value of outcrop analogues for interpreting sedimentary structures. For the first time, the subsurface identification and characterization of Lithocodium mounds and intermounds are achieved.

Predicting fluid flow in reservoirs: analysis of fracture clustering in outcrop analogues

Researchers have used lacunarity, a parameter that quantifies scale-dependent clustering in patterns, to differentiate fracture networks that belong to the same fractal system. In a previous study, the authors showed that lacunarity is efficient in representing connectivity and fluid flow in fractal-fracture models with the same fractal dimension. The objective of this research is to investigate if the concepts thus developed are applicable to outcrop analogues that are representative of subsurface fractured reservoirs. A set of nested fracture networks belonging to a single fractal system but mapped at different scales and resolutions is considered in this study. Lacunarity and connectivity values of these maps are evaluated using geospatial data analysis techniques. Fracture continuum (FC) models are built from these fracture maps and a streamline simulator, TRACE3D, is used to flow simulate these maps. Results show that although the fractal dimension of these maps is same, differences exist in the values of lacunarity, percolation connectivity and also fluid recovery values. It is further noted that the clustering, connectivity and fluid recovery values can be pairwise correlated very well for these natural fracture maps. Thus, the overall results indicate that connectivity in fracture maps, and hence their flow properties, are controlled by lacunarity or scale-dependent clustering attributes. Therefore, there could be novel applicability of the lacunarity parameter in calibrating discrete fracture network (DFN) models with respect to connectivity in natural fracture maps and the prediction of flow behaviour in fractured reservoirs. Thematic collection: This article is part of the Digitally enabled geoscience workflows: unlocking the power of our data collection available at: https://www.lyellcollection.org/topic/collections/digitally-enabled-geoscience-workflows

Fracture analysis and low-temperature thermochronology of faulted Jurassic igneous rocks in the Southern Colombian Andes: Reservoir and tectonic implications

Fractured basement hydrocarbon reservoirs are widely distributed worldwide in more than 30 basins, where volcanic and plutonic rocks have produced significant quantities of oil and gas. The Upper Magdalena Valley Basin (UMVB) is a mature and productive hydrocarbon basin located in the Colombian Andes. In this basin, plutonic and volcaniclastic Jurassic rocks are thrusted on top of Cretaceous to Cenozoic sedimentary rocks, which constitute the petroleum system. Multi-scale fracture analysis together with petrography, petrophysics, and low-temperature thermochronology were conducted in and outcrop analogue of this basement to assess the structural evolution and investigate the main factors controlling the development of fracture properties in igneous basements. Thermochronological data indicate the occurrence of three exhumation events between the Early Cretaceous and Miocene, suggesting that most of the fracture networks within the Agrado-Betania hanging-wall were likely formed before hydrocarbon migration. Structural analysis has identified a fault damage zone with a width of approximately 746 m. Volcanic breccias and ash tuffs exhibit slightly higher areal fracture intensities (P21 > 30 m/m2) compared to plutonic and clastic rocks (P21 < 20 m/m2). Furthermore, the fracture networks exhibit good connectivity, with connection per branch (CB) values exceeding the 1.5 percolation threshold. Petrophysical calculations of matrix-fracture properties indicate high permeabilities (ranging from 1000mD to 10000000mD) and low porosities (<10%). The structural position and the diagenetic transformation of the volcanic rocks are the primary factors controlling fracture intensity in the igneous rocks within the Agrado-Betania fault. Results from this outcrop analogue also show that polyphasic structural histories and the burial history positively influence the quality of fractured basement reservoirs.

Fault locking and alternate fault activity revealed by simple tools and approaches — Examples from the Western Canadian Sedimentary Basin with implications for hydrocarbon exploration and production

The geology toolkit that is used to reveal faults and fractures is much wider than before. This is due to 3D and 4D views in exploratory statistics programs and to the availability of user-friendly GIS software. These tools allow us to visualize a multitude of parameters that will be briefly explored here. A review of many geologic and nongeologic parameters led to evidence of fault locking and alternate fault activity. It also resulted in new structural models for the Western Canadian Sedimentary Basin (WCSB). The presented data sets include earthquakes, drilling, production, well data, aeromagnetic data, and more. Various integrated approaches reveal well-defined fault patterns that are typical of a strike-slip regime and the existence of previously unrecognized detachments that are important for hydrocarbon exploration. Some of the new geometries and associated mechanisms are illustrated here with outcrop analogues and present-day cross sections, maps, and 3D views. Only the most recent of the two identified strike-slip regimes is covered in this paper. Some emphasis is given to the recognition of detachments at various scales. Among these is the importance of megadetachments displacing the sedimentary cover by up to 16 km with respect to the aeromag. Hence, there is a need for reconstruction before making conclusions. The WCSB has a lot more to offer to explorers who understand faults, fractures, and migration paths. Integrating many types of information in map or 3D views offers new tools to identify and characterize faults.

Facies prediction in a mature oil field of Cretaceous age in the Calumbi Formation (Sergipe-Alagoas Basin, Brazil) by using an outcrop analogue approach

Outcrop gamma-ray logging is crucial for understanding subsurface reservoirs, but it is rarely used on outcrops, especially for correlating gamma-ray data from borehole profiles. This case study focuses on a mature onshore reservoir in the Sergipe-Alagoas Basin, Brazil, which is currently discarded by the oil industry. The study used an ideal outcrop (M-17) to gather stratigraphic, sedimentological, and gamma-ray data to better understand the lateral heterogeneity and distribution of reservoirs thought to have formed in offshore environments in the Sergipe-Alagoas Basin. Seismic and paleontological data were used to reduce uncertainty in outcrop data. The results showed a paleoenvironment compatible with marine sedimentary deposits on the continental shelf, supported by evidence from Thalassinoides and Ophiomorpha ichnogenera. The model suggests sand was continuously deposited following cycles of accommodation generation, resulting in a single succession of rocks organized into three system tracts: lowstand, transgressive, and highstand. The gamma-ray response identified and correlated the Cidade de Aracaju Oilfield using a pseudo-well and a well in the Cidade de Aracaju Oilfield. The methodological approach was seen as useful as a correlation tool, revealing previously unknown exploratory potential in the surrounding areas of the overexploited Cidade de Aracaju Oilfield. This approach is both unprecedented and pivotal, revealing previously unknown exploratory potential in the surrounding areas of the overexploited Cidade de Aracaju Oilfield.

3D geophysical image translated into photorealistic virtual outcrop geology using generative adversarial networks

Outcrop analogues play a pivotal role in resolving meter-scale depositional facies heterogeneity of carbonate strata. Two-dimensional outcrops are insufficient to decipher the 3D heterogeneity of carbonate facies. Near-surface geophysical methods, notably ground-penetrating radar (GPR), can be employed to step into 3D and extend the dimensionality of the outcrops to behind the outcrop. However, interpreting geophysical images requires specific geophysical expertise, often unfamiliar to field geologists who are more familiar with the actual rock than the geophysical data. A novel generative adversarial network (GAN) application is presented that constructs a photorealistic 3D virtual outcrop behind-the-outcrop model. The method combines GPR forward modeling with a conditional generative adversarial network (CGAN) and exploits the apparent similarities between outcrop expressions of lithofacies with their radargram counterparts. We exemplified the methodology and applied it to the open-source GPR data acquired from the Late Oxfordian-Early Kimmeridgian Arabian carbonate outcrop. We interpret a 4 km long outcrop photomosaic from a digital outcrop model (DOM) for its lithofacies, populate the DOM with GPR properties, and forward model the synthetic GPR response of these lithofacies. We pair the synthetic GPR with DOM lithofacies and train them using CGAN. Similarly, we pair the DOM lithofacies with outcrop photos and train them using CGAN. We chain the two trained networks and apply them to construct an approximately 2 km long 2D and an approximately 60 m2 3D volume of photorealistic artificial outcrop model. This model operates in a visual medium familiar to outcrop geologists, providing a complementary instrument to visualize and interpret rock formation instead of geophysical signals. This virtual outcrop replicates the visual character of outcrop-scale lithofacies features, such as the intricate bedding contacts and the outline of reef geobodies.

A data-driven hyperspectral method for sampling of diagenetic carbonate fabrics – A case study using an outcrop analogue of Jurassic Arab-D reservoirs, Saudi Arabia

Diagenetic alteration commonly overprints depositional fabrics in carbonate-dominated sediments and impact reservoir quality. Dolomitization is a prevalent diagenetic process observed in subsurface reservoirs that profoundly alters the depositional precursor's pore network, thereby influencing subsurface storage capacity and fluid flow behavior. Typical workflows to characterize the dolomitized sequences, textures, degree and extent of dolomitization rely on mapping, spatial sampling, and time-consuming geochemical, petrographic and petrophysical analysis. In this study, we propose a hyperspectral data-driven workflow for identifying dolomitized horizons and extracting sample sets optimized to characterize textural and chemical variations. Hyperspectral imaging (HSI) data was acquired with 1.5 mm spatial sampling along a 50 m long core drilled behind an outcrop of the Late Jurassic Jubaila-Arab sequence in Wadi Daqlah, Saudi Arabia. Spectral features in the visible (VNIR), shortwave (SWIR), mid-wave (MWIR), and long-wave (LWIR) infrared regions were then used to classify carbonate mineralogy, allowing for the rapid identification of dolomitized zones, and k-means clustering applied exclusively to the dolomitized areas used to identify intra-dolomite variations and suggest representative sample locations. Petrographic and geochemical analyses were carried out on these samples, revealing that clusters identified with the hyperspectral data represent four distinct diagenetic fabrics. These results demonstrate the value of HSI for objective and data-driven sampling, reducing the number of samples required for petrographic, geochemical and geophysical analysis and hence time and costs required to spatially characterize diagenetic alteration.

Diagenetic effect on reservoir quality of siliciclastic and volcaniclastic sandstones from a Paleogene volcanic rifted margin, East Greenland

ABSTRACT Siliciclastic and volcaniclastic sediments in a volcanic rifted-margin succession may experience a complex diagenetic history during burial that can have a large impact on sandstone reservoir properties. To understand such changes, variations in initial sediment composition and succeeding diagenetic changes have been studied for a Paleogene outcrop analogue in the Kangerlussuaq area, East Greenland. The nature of the mafic volcanics-bearing succession, which consists of intra-volcanic sandstones, accommodated over quartz-rich pre-volcanic fluvial sandstones, are comparable to the settings of recently discovered hydrocarbon-producing sandstones in the Faroe–Shetland Basin on the conjugate Atlantic margin. Our petrographic and provenance investigations of the pre- and intra-volcanic sandstones are supported by geochemical and X-ray diffraction analyses. The intra-volcanic sandstones were deposited in shallow marine environments with mixed siliciclastic and volcaniclastic input, the latter rich in felsic to mafic volcanic rock fragments and feldspar grains. Similar zircon age distributions of pre- and intra-volcanic sandstones support a continued supply from the same siliciclastic sediment source after the onset of volcanism. Variations in initial detrital grain and pore-fluid (fresh to marine) compositions resulted in different diagenetic changes in the pre- and intra-volcanic sandstones. However, where siliciclastic sandstones were overlain by volcaniclastic rocks rather than massive lava flows, the diagenetic changes resemble those of the intra-volcanic sandstones. The cementing phases are typically quartz, illite (probably illitized kaolinite), and rare anatase in the pre-volcanic sandstones. Chlorite, calcite, zeolite/feldspar, opal/quartz, and titanite are characteristic authigenic phases in the intra-volcanic sandstones. Precipitation of different minerals in the pre- and intra-volcanic sandstones show that the detrital composition (and to a lesser extent depositional environment) played a major role during early and late diagenesis after deep burial (up to 6–8 km). Inter-eruptive siliciclastic units may prove to form highly valuable reservoirs when they are composed of mixed siliciclastic and volcaniclastic material. In the stratigraphically youngest intra-volcanic sandstones and pre-volcanic sandstones overlain by hyaloclastite or tuff, there is a high potential for preservation of interparticle porosity during burial (&amp;lt; 5 km) due to early chlorite rims and the generation of secondary porosity after the dissolution of early zeolite cement.

An integrated 3D digital model of stratigraphy, petrophysics and karstified fracture network for the Cristal Cave, NE-Brazil

Digital Outcrop Models (DOMs) are virtual representations of geological features. Although DOMs are widely used tools in geosciences, their integration with other datasets remains relatively underexplored. We combined a DOM, derived from a photogrammetric survey of a carbonate sequence, with lithostratigraphic, petrophysical (porosity, permeability and uniaxial compressive strength), fracture distribution, and karst dissolution information to compose a single integrated three-dimensional digital model. The study site is one of the entrances of the Cristal Cave (São Francisco Craton, Northeastern Brazil), which has been used as a structural and diagenetic outcrop analog for the Brazilian pre-salt carbonate reservoirs. Data from fracture distributions, measured on the exposed surfaces of the cave, were used to build a Discrete Fracture Network, based on the solution of the stereology inverse problem. Fracture apertures were then modified to generate different scenarios of karstification, thus composing Discrete Fracture and Karst Networks. This integrative approach brought relevant insights into the cave development due to dissolution along fracture clusters. Our methodology offers better geological data handling to build static models to be used in a fluid flow modeling environment, contributing to bridge the gap between geophysics/geology and engineering approaches.