Outcrop Analogue Studies Research Articles

Critical evaluation of an Upper Carboniferous tight gas sandstone reservoir analog: Diagenesis and petrophysical aspects

Upper Carboniferous sandstones are one of the most important tight gas reservoirs in Central Europe. We present data from an outcrop reservoir analog (Piesberg quarry) in the Lower Saxony Basin of Northern Germany. This field-based study focuses on the diagenetic control on spatial reservoir quality distribution.The investigated outcrop consists of fluvial 4th-order cycles, which originate from a braided river dominated depositional environment. Westphalian C/D stratigraphy, sedimentary thicknesses and exposed fault orientations (NNW-SSE and W-E) reflect tight gas reservoir properties in the region further north. Diagenetic investigations revealed an early loss of primary porosity by pseudomatrix formation. Present day porosity (7% on average) and matrix permeability (0.0003 mD on average) reflect a high-temperature overprint during burial. The entire remaining pore space is occluded with authigenic minerals, predominantly quartz and illite. This reduces reservoir quality and excludes exposed rocks as tight gas targets. The correlation of petrographic and petrophysical data show that expected facies-related reservoir quality trends were overprinted by high-temperature diagenesis. The present day secondary matrix porosity reflects the telogenetic dissolution of mesogenetic ankerite cements and unstable alumosilicates.Faults are associated with both sealed and partially sealed veins near the faults, indicating localized mass transport. Around W-E striking faults, dissolution is higher in leached sandstones with matrix porosities of up to 26.3% and matrix permeabilities of up to 105 mD. The dissolution of ankerite and lithic fragments around the faults indicates focused fluid flow. However, a telogenetic origin cannot be ruled out.The results of this work demonstrate the limits of outcrop analog studies with respect to actual subsurface reservoirs of the greater area. Whereas the investigated outcrop forms a suitable analog with respect to sedimentological, stratigraphic and structural inventory, actual reservoirs at depth generally lack telogenetic influences. These alter absolute reservoir quality values at the surface. However, the temperature overprint and associated diagenetic modification, which caused the unusually low permeability in the studied outcrop, may pose a reservoir risk for tight gas exploration as a consequence of locally higher overburden or similar structural positions.

Chemostratigraphic and sedimentologic evolution of Wajid Group (Wajid Sandstone): An outcrop analog study from the Cambrian to Permian, SW Saudi Arabia

The Paleozoic age succession in Saudi Arabia represents one of the most prolific petroleum producing systems in the Arabian Peninsula. This succession is also considered important for unconventional tight gas and shale gas reservoirs. The Wajid Group (Wajid Sandstone) in SW Saudi Arabia consists of four formations, namely, Dibsiyah (Lower and Upper), Sanamah, Khusayyayn and Juwayl from bottom to top. This study investigates the major oxides, trace and rare earth elements for the Wajid Group formations in southwestern Saudi Arabia. We characterize and compare the sandstone types, provenance, tectonic setting, and climate. Moreover, we applied the chemostratigraphic technique for stratigraphic differentiation. Concentrations of certain elements indicate that Wajid Group was deposited in a passive continental margin. The geochemical analysis reveals that Wajid Group sediments were likely derived from the upper and bulk continental crust and mafic igneous provenance. The elemental geochemical data has been applied in this study to improve the stratigraphic subdivision and correlation. Using selected elements, geochemical vertical profiles, binary, and ternary diagrams allow clearly distinguishing between Wajid Group formations. Thus supports the established formation boundaries that constructed using lithostratigraphy and sedimentology. The geochemical elements variation between formations can be related to differences in rock-forming minerals, facies change, climate, and provenance. The results of this study may help in constraining and correlating complex facies strata and can be used as a guide for stratigraphic correlations in the subsurface within the Wajid basin and other equivalent stratigraphic successions within Saudi Arabia.

Geothermal Energy from the Main Karoo Basin (South Africa): An Outcrop Analogue Study of Permian Sandstone Reservoir Formations

The geothermal potential of the Main Karoo Basin has not been addressed in the past. A first assessment of Permian sandstone formations in the Eastern Cape Province, including down-hole temperature data from deep boreholes, and evaluation of groundwater temperature and heat flow values from literature leads to 3130 TWh (11.3 EJ) of power generation potential within the central and southern parts of the basin. The low permeability lithotypes may be operated as enhanced geothermal systems (EGS), depending on the fracture porosity in the deeper subsurface. In some areas auto-convective thermal water circulation might be expected and direct heat use becomes reasonable.

Thermo-physical rock properties of greywacke basement rock and intrusive lavas from the Taupo Volcanic Zone, New Zealand

Greywacke of the Waipapa and Torlesse (Composite) Terrane form the basement of the Taupo Volcanic Zone (TVZ), New Zealand. Together with inferred buried lavas, domes and igneous complexes they are likely to be the dominant rock type prevailing at depths >4km beneath the TVZ. A fundamental understanding of the rock properties of the deep formations is of utmost importance for the exploration of deep unconventional geothermal resources. An outcrop analogue study was conducted to improve the understanding of the thermo-physical rock properties of likely deep buried rock formations beneath the TVZ. A total of 145 core samples were taken at 10 locations inside and outside the TVZ and their grain and bulk density, porosity, matrix permeability, bulk thermal conductivity and specific heat capacity, and the compressional and shear wave velocities measured on oven-dry samples. Additional tests of the unconfined compressive strength were conducted for selected greywacke samples to quantify their mechanical rock strength.The obtained data indicates that the thermo-physical rock properties are mainly controlled by porosity, and minor by mineralogy, texture and grain size. Samples from Waipapa-type and Torlesse-type greywacke exhibit minor rheological differences, with Waipapa-type greywacke having lowest porosity (about 1% vs. 3%) and highest bulk thermal conductivity (2.5Wm−1K−1 vs. 1.7Wm−1K−1) and specific heat capacity (0.8kJkg−1K−1 vs. 0.7kJkg−1K−1). Matrix permeability is <1E-16m2 for all greywacke samples. Tested lavas exhibit heterogeneous rock properties due to their wide range of porosity (<1% up to 32%).The thermo-physical rock properties were tested at laboratory conditions (ambient temperature and pressure), which do not reflect the in situ conditions at greater depth. With depth, thermal conductivity and acoustic wave velocity are likely to decrease caused by micro fractures resulting from thermal cracking of the rock, while specific heat capacity increases.The data presented in this paper are expected to improve the statistical confidence on input data to geophysical and thermo-hydro-mechanical numeric models, and the ability to forecast rock properties at greater depths of the TVZ.

Outcrop analogue study of Permocarboniferous geothermal sandstone reservoir formations (northern Upper Rhine Graben, Germany): impact of mineral content, depositional environment and diagenesis on petrophysical properties

The Permocarboniferous siliciclastic formations represent the largest hydrothermal reservoir in the northern Upper Rhine Graben in SW Germany and have so far been investigated in large-scale studies only. The Cenozoic Upper Rhine Graben crosses the Permocarboniferous Saar–Nahe Basin, a Variscan intramontane molasse basin. Due to the subsidence in this graben structure, the top of the up to 2-km-thick Permocarboniferous is located at a depth of 600–2900 m and is overlain by Tertiary and Quaternary sediments. At this depth, the reservoir temperatures exceed 150 °C, which are sufficient for geothermal electricity generation with binary power plants. To further assess the potential of this geothermal reservoir, detailed information on thermophysical and hydraulic properties of the different lithostratigraphical units and their depositional environment is essential. Here, we present an integrated study of outcrop analogues and drill core material. In total, 850 outcrop samples were analyzed, measuring porosity, permeability, thermal conductivity and thermal diffusivity. Furthermore, 62 plugs were taken from drillings that encountered or intersected the Permocarboniferous at depths between 1800 and 2900 m. Petrographic analysis of 155 thin sections of outcrop samples and samples taken from reservoir depth was conducted to quantify the mineral composition, sorting and rounding of grains and the kind of cementation. Its influence on porosity, permeability, the degree of compaction and illitization was quantified. Three parameters influencing the reservoir properties of the Permocarboniferous were detected. The strongest and most destructive influence on reservoir quality is related to late diagenetic processes. An illitic and kaolinitic cementation and impregnation of bitumina document CO2- and CH4-rich acidic pore water conditions, which are interpreted as fluids that migrated along a hydraulic contact from an underlying Carboniferous hydrocarbon source rock. Migrating oil and acidic waters led to the dissolution of haematite cements in the lower Permocarboniferous formations. During the Eocene, subsidence of the Upper Rhine Graben porosities and permeabilities of the sandstones of these formations were strongly reduced to 2.5 % and 3.2 × 10−18 m2. The second important influence on reservoir quality is the distinct depositional environment and its influence on early diagenetic processes. In early stage diagenesis, the best influence on reservoir properties exhibits a haematite cementation. It typically occurs in eolian sandstones of the Kreuznach Formation (Upper Permocarboniferous) and is characterized by grain covering haematite coatings, which are interpreted to inhibit cementation, compaction and illitization of pore space during burial. Eolian sandstones taken from outcrops and reservoir depths exhibit the highest porosities (16.4; 12.3 %) and permeabilities (2.0 × 10−15; 8.4 × 10−16 m2). A third important influence on reservoir quality is the general mineral composition and the quartz content which is the highest in the Kreuznach Formation with 73.8 %. Based on the integrated study of depositional environments and diagenetic processes, reservoir properties of the different Permocarboniferous formations within the northern Upper Rhine Graben and their changes with burial depth can be predicted with satisfactory accuracy. This leads to a better understanding of the reservoir quality and enables an appropriate well design for exploration and exploitation of these geothermal resources.

Facies, sequence stratigraphy, reservoir and seal potential of the Mafraq Formation, Sultanate of Oman: An integrated outcrop analogue study

ABSTRACT The mixed carbonate-siliciclastic Lower to Middle Jurassic Mafraq Formation unconformably overlies the Triassic Mahil Formation in outcrops of the Oman Mountains (pre-Mafraq Unconformity, known as pre-Marrat unconformity in other regions of Arabia). Together with the overlying Dhruma Formation, it is part of the Sahtan Group. This study provides: (1) a detailed facies analysis based on sedimentological logging of 12 outcrops. Twenty-four facies types were established and grouped into five facies associations, which can also be recognized in subsurface core intervals; (2) a detailed sequence-stratigraphic framework of the Mafraq Formation. Facies stacking and log patterns reveal cycle hierarchies on four scales from m-scale cycles, to several m-thick cycle sets, to tens of m-thick, high-frequency sequences, to 100 m-thick composite sequences; and (3) a documentation of potential reservoir and seal units. The study follows an approach from 1-D (outcrop sections) to 2-D (correlations and potential reservoir dimensions). The Mafraq outcrop type section, located in Wadi Sahtan is documented in an integrated way (facies, litho-, bio-, chemo- and sequence stratigraphy), together with additional outcrops of the Mafraq Formation throughout North Oman. 2-D correlation of the Mafraq Formation throughout North Oman is essentially based on cycle sets and provides key information about the lateral paleogeographic development of the formation. A general proximal-distal trend, from south to north, has been proposed by Ziegler (2001); outcrop data from the Oman Mountains confirms this trend and adds an EW-deepening component. The mixed carbonate-clastic Lower Mafraq Member (Sequence) with a coastal/estuarine to shallow-marine environment forms onlaps onto the pre-Mafraq Unconformity below, and thins out completely after some 10s of kilometers towards the southeast. The Upper Mafraq Member (Sequence) seems to be continuous over 10s of kilometers with less thickness decrease. Instead, a transition from a more distal carbonate shoal - backshoal environment in the northwest to a proximal clastic coastal/estuarine/terrestrial environment in the southeast can be observed. On a 100s km-scale significant thinning and a change towards terrestrial clastic facies can be observed southeast of the Oman Mountains area. Combined results from lateral/vertical logging, paleoenvironmental interpretations and correlation provided 3-D information about the dimensions of potential reservoir and seal units. Several potential reservoir/seal intervals and their dimensions in dip direction could be identified: (1) Lower Mafraq Sequence: various types of sandbodies, most of them with a lateral extent ca. 5 km, sealed by shales. (2) Upper Mafraq Sequence, northwestern part: oolitic grainstones, laterally correlative over 10–20 km, sealed by shales. (3) Upper Mafraq Sequence, southeastern part: channelized sandstones units, lateral extent up to km, sealed by shales.

Reservoir characterization of the Upper Jurassic geothermal target formations (Molasse Basin, Germany): role of thermofacies as exploration tool

&lt;p&gt;&lt;strong&gt;Abstract.&lt;/strong&gt; The Upper Jurassic carbonates of the southern German Molasse Basin are the target of numerous geothermal combined heat and power production projects since the year 2000. A production-orientated reservoir characterization is therefore of high economic interest. Outcrop analogue studies enable reservoir property prediction by determination and correlation of lithofacies-related thermo- and petrophysical parameters. A thermofacies classification of the carbonate formations serves to identify heterogeneities and production zones. The hydraulic conductivity is mainly controlled by tectonic structures and karstification, whilst the type and grade of karstification is facies related. The rock permeability has only a minor effect on the reservoir's sustainability. Physical parameters determined on oven-dried samples have to be corrected, applying reservoir transfer models to water-saturated reservoir conditions. To validate these calculated parameters, a Thermo-Triaxial-Cell simulating the temperature and pressure conditions of the reservoir is used and calorimetric and thermal conductivity measurements under elevated temperature conditions are performed. Additionally, core and cutting material from a 1600 m deep research drilling and a 4850 m (total vertical depth, measured depth: 6020 m) deep well is used to validate the reservoir property predictions. Under reservoir conditions a decrease in permeability of 2–3 magnitudes is observed due to the thermal expansion of the rock matrix. For tight carbonates the matrix permeability is temperature-controlled; the thermophysical matrix parameters are density-controlled. Density increases typically with depth and especially with higher dolomite content. Therefore, thermal conductivity increases; however the dominant factor temperature also decreases the thermal conductivity. Specific heat capacity typically increases with increasing depth and temperature. The lithofacies-related characterization and prediction of reservoir properties based on outcrop and drilling data demonstrates that this approach is a powerful tool for exploration and operation of geothermal reservoirs.&lt;/p&gt;

Characterization and quantification of aquifer heterogeneity using outcrop analogs at the Canadian Forces Base Borden, Ontario, Canada

Accurate characterization of internal structures and geometries of aquifers is critical for evaluation of plume migration and dispersion of contaminants. For this reason, high-resolution transport study field sites, such as the Waterloo Groundwater Research Site at the Canadian Forces Base Borden, Ontario, Canada, have been established. However, geological characterization at this site is based primarily on cores and shallow geophysical data. Outcrop analog studies offer detailed horizontal data that can help to build quantified models of aquifer heterogeneity. We used a combination of ground-based light detection and ranging (LiDAR) and high-resolution photographs at a sand quarry cut into Borden aquifer sediment to evaluate and quantify the distribution of lithofacies and hydrofacies that control flow properties in the Borden aquifer. We exposed sixteen ∼20 m × 1.5 m outcrops, LiDAR surveyed, photographed, and field mapped lithofacies at each exposure, and segmented the lithofacies from photographs to produce maps of facies distributions. We grouped different lithofacies into hydrofacies based on overall facies geometry and estimated hydraulic properties. We used LiDAR scans and the segmented facies maps to produce a digital outcrop model (DOM) of the site. The resultant DOM combines lithofacies and hydrofacies derived from field observations with high-resolution (5 mm) LiDAR data to reconstruct hydrofacies distributions in a three-dimensional (3-D) geomodel. The DOM offers a relatively complete horizontal correlation structure that was used in transition probability geostatistical modeling to create realizations of hydrofacies distributions in the aquifer at the study site.

Shale gas formation research: from well logs to outcrop – and back again

Mart Zijp, Johan ten Veen, Roel Verreussel, Jan ter Heege, Dario Ventra and John Martin show the added value of outcrop analogue studies to understand and characterize the heterogeneity of gas shale formations. Intense exploration activities over the past 75 years have resulted in an impressive amount of publically available subsurface data for the Netherlands. About 100 exploration wells penetrate the Early Jurassic Posidonia Shale Formation (PSF) in the Netherlands, which is considered the most important oil source rock in the Netherlands. More recently, this approximately 30m-thick black shale has attracted attention as a possible target for shale gas. Since outcrops and tangible subsurface data are virtually absent, Dutch geoscientists face the reality that shale-gas research typically takes place on a work station. For instance, petrophysical analysis shows that the PSF stands out from the surrounding shales by conspicuous patterns on wire-line logs: high gamma ray, high sonic, low density and high resistivity. Higher order trends in the well logs can be traced across large distances and are thought to represent a remarkable variation in lithology that is important for the characterization of potential shale gas targets. Ideally this reservoir characterization should be carried out at different scales: from an analysis of fault networks and responses to local stress regimes at reservoir scale, to analyses of hydro-mechanical properties and fracture networks at bed scale, finishing at a characterization of compositional and sedimentological heterogeneity at the scale of laminae. Thus, the common practice of lithology deduction via petrophysical methods is incomplete and requires ground truthing, which can be best provided by outcrop studies.

Geothermal reservoir potential of volcaniclastic settings: The Valley of Mexico, Central Mexico

The geothermal potential of the Valley of Mexico has not been addressed in the past, although volcaniclastic settings in other parts of the world contain promising target reservoir formations. An outcrop analogue study of the thermophysical rock properties of the Neogene rocks within the Valley of Mexico was conducted to assess the geothermal potential of this area. Permeability and thermal conductivity are key parameters in geothermal reservoir characterization and the values gained from outcrop samples serve as a sufficient database for further assessment. The mainly low permeable lithofacies types may be operated as stimulated systems, depending on the fracture porosity in the deeper subsurface. In some areas also auto-convective thermal water circulation might be expected and direct heat use without artificial stimulation becomes reasonable. Thermophysical properties of tuffs and siliciclastic rocks qualify them as target horizons for future utilization of deep geothermal reservoirs.

A combined study of radar facies, lithofacies and three‐dimensional architecture of an alpine alluvial fan (Illgraben fan, Switzerland)

AbstractAlluvial fans serve as useful archives that record the history of depositional and erosional processes in mountainous regions and thus can reveal the environmental controls that influenced their development. Economically, they play an important role as groundwater reservoirs as well as host rocks for hydrocarbons in deeply buried systems. The interpretation of these archives and the evaluation of their reservoir architecture, however, are problematic because marked heterogeneity in the distribution of sedimentary facies makes correlation difficult. This problem is compounded because the accumulated sedimentary deposits of modern unconsolidated fan systems tend to be poorly exposed and few such systems have been the focus of investigation using high‐resolution subsurface analytical techniques. To overcome this limitation of standard outcrop–analogue studies, a geophysical survey of an alpine alluvial fan was performed using ground‐penetrating radar to devise a scaled three‐dimensional subsurface model. Radar facies were classified and calibrated to lithofacies within a fan system that provided outcropping walls and these were used to derive a three‐dimensional model of the sedimentary architecture and identify evolutionary fan stages. The Illgraben fan in the Swiss Alps was selected as a case study and a network of ca 60 km sections of ground‐penetrating radar was surveyed. Seven radar facies types could be distinguished, which were grouped into debris‐flow deposits and stream‐flow deposits. Assemblages of these radar facies types show three depositional units, which are separated by continuous, fan‐wide reflectors; they were interpreted as palaeo‐surfaces corresponding to episodes of sediment starvation that affected the entire fan. An overall upward decline in the proportion of debris‐flow deposits from ca 50% to 15% and a corresponding increase in stream‐flow deposits were identified. The uppermost depositional unit is bounded at its base by a significant incision surface up to 700 m wide, which was subsequently filled up mostly by stream‐flow deposits. The pronounced palaeo‐surfaces and depositional trends suggest that allocyclic controls governed the evolution of the Illgraben fan, making this fan a valuable archive from which to reconstruct past sediment fluxes and environmental change in the Alps. The results of the integrated outcrop–geophysical approach encourage similar future studies on fans to retrieve their depositional history as well as their potential reservoir properties.

Assessing accuracy of gas-driven permeability measurements: a comparative study of diverse Hassler-cell and probe permeameter devices

Abstract. Permeability is one of the most important petrophysical parameters to describe the reservoir properties of sedimentary rocks, pertaining to problems in hydrology, geothermics, and hydrocarbon reservoir analysis. Outcrop analogue studies, well core measurements, and individual sample analysis take advantage of a variety of commercially available devices for permeability measurements. Very often, permeability data derived from different devices need to be merged within one study (e.g. outcrop minipermeametry and lab-based core plug measurements). To enhance accuracy of different gas-driven permeability measurements, device-specific aberrations need to be taken into account. The application of simple one-to-one correlations may draw the wrong picture of permeability trends. For this purpose, transform equations need to be established. This study presents a detailed comparison of permeability data derived from a selection of commonly used Hassler cells and probe permeameters. As a result of individual cross-plots, typical aberrations and transform equations are elaborated, which enable corrections for the specific permeameters. Permeability measurements of the commercially available ErgoTech gas permeameter and the TinyPerm II probe permeameter are well-comparable over the entire range of permeability, with R2 = 0.955. Aberrations are mostly identified in the permeability range &lt; 10 mD, regarding the TinyPerm II and the minipermeameter/Hassler-cell combination at Darmstadt University, which need to be corrected and standardized. Applying standardizations which consider these aberration intervals strongly improves the comparability of permeability data sets and facilitates the combination of measurement principles. Therefore, the utilization of such correlation tests is highly recommended for all kinds of reservoir studies using integrated permeability databases.

High-resolution anatomy of a grainstone package in Khuff Sequence KS4, Oman Mountains, Sultanate of Oman

ABSTRACTThis study is part of a large-scale outcrop analog study on Middle Permian to Lower Triassic Khuff-equivalent strata in the Oman Mountains, Al Jabal al-Akhdar, Sultanate of Oman. The Khuff outcrop equivalent can be divided into six sequences (Khuff sequences KS6 to KS1, from base to top). The main focus of this study is the description of the internal anatomy of the shoal grainstone bodies in the lower part of Sequence KS4 (“lower KS4”). High-resolution sedimentological logging of three outcrop sections in wadis Sahtan, Bani Awf and Mistal yielded eight lithofacies types that were grouped into five facies associations. Lower KS4 strata were mainly deposited within a “shoal complex” of an epeiric carbonate ramp, resulting in a thick pile of up to 70 m of grainstones that, on first sight, appear relatively homogeneous. However, detailed facies and microfacies analysis revealed their heterogeneous architecture on various scales: (1) Minor changes in depositional environments directly affected the type of carbonate grains (ooids versus peloids/cortoids versus bioclasts), leading potentially to highly variable pore systems (moldic versus interparticle versus intraparticle). (2) Vertically, detailed sequence-stratigraphic analysis revealed a higher-order of cyclicity (“mini-cycles”) on a decimeter- to meter-scale. Four mini-cycle types were recognized. (3) Laterally, facies changes, the amalgamation of grainstone beds and mini-cycle pinch-outs were observed in 2-D correlations on a scale of a few kilometers. These different types of heterogeneities may contribute to varying production rates commonly observed in the subsurface KS4 reservoir.

Characterizing heterogeneity in a glaciofluvial deposit using architectural elements, Limehouse, Ontario, Canada

Major hydrocarbon and groundwater reservoirs are commonly hosted within coarse-grained alluvial deposits that contain a high degree of sedimentary heterogeneity. This paper presents a detailed characterization of the sedimentary heterogeneity of fluvial–deltaic deposits using architectural element analysis (AEA). Sedimentological data collected from outcrop faces exposing Late Quaternary glaciofluvial deposits in southern Ontario, Canada, is recorded in 31 sedimentary logs. These logs are used to identify nine different facies types, including gravel facies (Gm, Gp, Gt), sand facies (Sr, Sp, St, Ss), and fine-grained facies (Fl and Fd). Variations in facies associations and geometries are defined by five architectural elements (AEs): sand complex (SC), gravel sheet (GS), fine-grained sheet (FS), gravel foreset body (GFB), and concave fill (CF) elements. The spatial arrangement of bounding surfaces (first- to fifth-order) and AEs allows the classification of six EAs, which, in this study, are defined as the largest-scale architectural subunits that allow for architectural-based mapping over a large area. EAs delineated in this study are sandy braided-river (EA1), delta-front (EA2), gravelly braided-river to delta-top (EA3), delta-front to lacustrine (EA4), braided-river to deltaic (EA5), and sand-dominated fluvial (EA6). AEA is utilized here to capture three levels of heterogeneity, which allow detailed reservoir characterization based on geometric objects and can be readily used for computer-based modelling. Outcrop analogue studies such as this one provide insight to the geometries of more deeply buried coarse-grained deposits that form potential reservoirs and enhance paleoenvironmental reconstruction of subsurface alluvial deposits in Canada and elsewhere.

Reservoir properties and petrophysical modelling of carbonate sand bodies: outcrop analogue study in an epicontinental basin (Triassic, Germany)

Abstract This paper represents the second part of an integrated study that is focussed on the development and distribution of reservoir bodies and properties in epeiric carbonate systems. It is based on outcrop analogue data from Triassic ‘layer-cake’ carbonates in the South German Basin, which were deposited along an epicontinental, very gently inclined carbonate ramp. The reservoir facies consists of skeletal and oolitic carbonate grainstones (Φ max 23%, K max 700 mD), which are organized in a pronounced hierarchy of stratigraphic cycles. Based on outcrops, cores, gamma ray (GR) logs and thin sections, a high-resolution, geocellular 3D facies model was generated, which covers the area of a Middle East giant gas field (25×36 km). The spatial distribution of reservoir properties was systematically investigated on different scales. The lateral distribution of reservoir properties remains in the same order of magnitude for hundreds of metres, within in the same stratigraphic position. However, on a kilometre scale, facies bodies, diagenetic trends and thus reservoir properties show gradual lateral changes. Vertically, in contrast, properties change commonly on a decimetre scale and are largely controlled by stratigraphic cycles. Petrophysical modelling enhanced the understanding of key factors and processes controlling both reservoir quality and quantity.

Volcanic settings and their reservoir potential: An outcrop analog study on the Miocene Tepoztlán Formation, Central Mexico

The reservoir potential of volcanic and associated sedimentary rocks is less documented in regard to groundwater resources, and oil and gas storage compared to siliciclastic and carbonate systems. Outcrop analog studies within a volcanic setting enable to identify spatio-temporal architectural elements and geometric features of different rock units and their petrophysical properties such as porosity and permeability, which are important information for reservoir characterization. Despite the wide distribution of volcanic rocks in Mexico, their reservoir potential has been little studied in the past. In the Valley of Mexico, situated 4000 m above the Neogene volcanic rocks, groundwater is a matter of major importance as more than 20 million people and 42% of the industrial capacity of the Mexican nation depend on it for most of their water supply. Here, we present porosity and permeability data of 108 rock samples representing five different lithofacies types of the Miocene Tepoztlán Formation. This 800 m thick formation mainly consists of pyroclastic rocks, mass flow and fluvial deposits and is part of the southern Transmexican Volcanic Belt, cropping out south of the Valley of Mexico and within the two states of Morelos and Mexico State. Porosities range from 1.4% to 56.7%; average porosity is 24.8%. Generally, permeabilities are low to median (0.2–933.3 mD) with an average permeability of 88.5 mD. The lavas are characterized by the highest porosity values followed by tuffs, conglomerates, sandstones and tuffaceous breccias. On the contrary, the highest permeabilities can be found in the conglomerates, followed by tuffs, tuffaceous breccias, sandstones and lavas. The knowledge of these petrophysical rock properties provides important information on the reservoir potential of volcanic settings to be integrated to 3D subsurface models.

Integration of digital outcrop models (DOMs) and high resolution sedimentology – workflow and implications for geological modelling: Oukaimeden Sandstone Formation, High Atlas (Morocco)

ABSTRACT Outcrop analogue studies provide key information for reservoir modelling which is difficult to obtain from traditional subsurface datasets (i.e. seismic data, well data). Terrestrial laser scanners or LiDAR (light detection and ranging), combined with digital photography, provide a new technique to create high resolution 3D digital outcrop models (DOMs). These DOMs generate exhaustive information which is used to build more realistic three-dimensional facies-based geocellular models and populate the different model zones. This paper documents the use of an extensive dataset, which combines high resolution traditional field data and DOMs. We provide an accurate description of the workflow followed in the geocellular modelling of a fluvial-dominated continental formation. Geocellular facies association and connectivity models are well constrained by outcrop observations and a number of different techniques are used in the quality control of the final model. The study also qualitatively discusses the uncertainties identified during the workflow and proposes methods to reduce them. The workflow and results shown in this paper can be applied in similar analogue systems in order to help improve model building for subsurface reservoirs.

Three-dimensional facies modeling of carbonate sand bodies: Outcrop analog study in an epicontinental basin (Triassic, southwest Germany)

This article is the first part of an integrated study to characterize the anatomy and geometries of carbonate shoal reservoir bodies in epicontinental settings. It is based on outcrop analog data from Triassic layer-cake carbonates in the south German Basin, which were deposited along an epicontinental, very gently inclined carbonate ramp. The database of this study consists of 56 measured sections supplemented by six shallow cores and wireline logs, covering an area of 25 by 36 km (15 by 22 mi). The potential reservoir bodies consist generally of midramp shoal and shoal-fringe facies types, which are composed of skeletal and oolitic carbonate packstones and grainstones with significant amounts of porosity. The upper Muschelkalk is composed of a lower transgressive and an upper regressive interval; within these, shoal bodies show not only similarities but also major differences in character, geometry, and distribution. These reflect the conditions of an epeiric system, which reacts strongly to small changes in accommodation. The accommodation seems to be mainly triggered by the interaction of hierarchically organized large-, medium-, and small-scale relative sea level changes and a subtle paleorelief. At the level of medium-scale cycles, distinct two-dimensional cyclicity styles can be differentiated, which record the lateral facies distribution along the depositional gradient. Different styles of medium-scale cycles include different types of shoal bodies: (1) transgressive crinoidal shoal style: shoal bodies (mean width: 21 km [13 mi], mean length: 37 km [23 mi], mean thickness: 4.2 m [13.7 ft]) with low facies differentiation, deposited on top of subtle paleohighs; (2) skeletal sheets style: thin reservoir sheets (mean width: 5.1 km [3.1 mi], mean length: 11.1 km [6.8 mi], mean thickness: 0.3 m [0.9 ft]) of reservoir facies on top of the paleohighs; (3) regressive oolitic shoal style: shoal bodies (mean width: 14.6 km [9.0 mi], mean length: 20.8 km [12.9 mi], mean thickness: 0.69 m [2.2 ft]) with high facies differentiation on the flanks of paleohighs; and (4) low-accommodation style: patchy and mosaic distribution of shoal bodies (mean width: 11.2 km [6.9 mi], mean length: 26 km [16 mi], mean thickness: 0.69 m [2.2 ft]). All data were loaded into three-dimensional (3-D) modeling software to distribute 14 facies types and model the 3-D stratigraphic architecture. The resulting facies distribution implies that volume and dimensions of the shoal bodies are mainly controlled by the combination of stratigraphic cycles and a subtle paleorelief, which is indicated by overall thickness changes of succession. High-energy shoal facies types occur only in and around areas with a reduced overall thickness, whereas areas with a thicker development are dominated by low-energy, muddy facies types. All observations combined point to the presence of a subtle paleorelief, which could be induced by slight differential subsidence of inherited paleotectonic basement blocks. On the kilometer to regional scale, previous studies suggest simple layer-cake stratal patterns. However, in the full 3-D view, the apparent layer-cake stratigraphy turned out to be a pseudolayer cake, with very gentle (0.01–0.001 dip) clinoform geometries. These very subtle offlapping, pinch-out geometries may have been overlooked in epeiric reservoir systems elsewhere.

Host rock dolomitization and secondary porosity development in the Upper Devonian Cairn Formation of the Fairholme carbonate complex (South‐west Alberta, Canadian Rockies): diagenesis and geochemical modelling

AbstractThe Upper Devonian carbonate reefs in West‐central Alberta are important petroleum reservoirs that are well‐known for their extensive secondary porosity. An outcrop analogue study indicates that an early matrix‐selective dolomitization event occurred which is characterized by a major Late Devonian sea water component with increased salinity because of evaporation. It is interpreted that the matrix (replacive) dolomite formed during the Famennian as the result of a combination of both seepage and latent reflux dolomitization, although an additional type or overprinting of later intermediate burial dolomitization cannot be excluded. Formation of the moulds is attributed mainly to the dissolution of undolomitized fossil cores, most typically stromatoporoids. Geochemical modelling indicates that carboxylic acid fluids have the highest potential for dissolving residual calcite in this case. Geochemical models consistent with this analysis and interpretations can reproduce the secondary porosity and suggest a viable dolomitization process for the localities studied.

Multiple-scale facies and reservoir quality variations within a dolomite body – Outcrop analog study from the Middle Triassic, SW German Basin

This study focuses on a porous dolomite body in the proximal part of an epeiric carbonate ramp (Triassic Muschelkalk, South German Basin), analogous to hydrocarbon-bearing dolomite reservoirs in the Middle East (e.g. Khuff and Arab Formations). The dolomite body is made up of shoal, lagoonal and peritidal facies types and is built by a three-fold hierarchy of cycles (3rd-order to 5th-order). The best reservoir quality occurs in finely crystalline lagoonal dolo-mudstones (mean: K = 37.8 mD; φ = 20.2%) with micro-intercrystalline and vuggy porosity. Coarsely crystalline, shoal-related dolo-grainstones show variable reservoir potential (mean: K = 30.1 mD; φ = 4.4%) due to late diagenetic modification. Reservoir quality significantly decreases in the non-dolomitized foreshoal section (mean: K = 4.5 mD; φ = 3.6%). Stratigraphically, the most porous dolomite occurs in the upper regressive portion of the 3rd-order cycle. Reservoir quality progressively decreases towards the limy bottom of the section. Outcrop mapping showed high lateral continuity for the distribution of facies types and poroperm values on the scale of several 10s of meters. Within correlated 5th-order cycles, considerable facies and poroperm heterogeneity is already observed on the 100s of meters-scale. On a 1000s of meters-scale, the facies and lithology distribution is controlled by the geometry of a gently inclined carbonate ramp. During regression, a progradational pattern is recognized, marked by a basinward migration of backshoal facies and a down-dip decrease of dolomite body thickness. Lithology, facies and poroperm data from outcrops and cores were used for 3D-modeling with the software Petrel. The modeled variations in the poroperm distribution are consistent with the degree of heterogeneity expected for a combination of laterally persistent lithology and highly variable facies. Results of this study may help to better understand the reservoir heterogeneity of inner ramp dolomite bodies and can be applied to improve characterization and modeling of similar dolomite reservoirs.