Veerle Vandeginste

Linking process, dimension, texture, and geochemistry in dolomite geobodies: A case study from Wadi Mistal (northern Oman)

Understanding the distribution and geometry of reservoir geobodies is crucial for net-to-gross estimates and to model subsurface flow. This article focuses on the process of dolomitization and resulting geometry of diagenetic geobodies in an outcrop of Jurassic host rocks from northern Oman. Field and petrographic data show that a first phase of stratabound dolomite is crosscut by a second phase of fault-related dolomite. The stratabound dolomite geobodies are laterally continuous for at least several hundreds of meters (1000 ft) and probably regionally and are one-half meter (1.6 ft) thick. Based on petrography and geochemistry, a process of seepage reflux of mesosaline or hypersaline fluids during the early stages of burial diagenesis is proposed for the formation of the stratabound dolomite. In contrast, the fault-related dolomite geobodies are trending along a fault that can be followed for at least 100 m (328 ft) and vary in width from a few tens of centimeters to as much as 10 m (1–33 ft). Petrography, geochemistry, and high homogenization temperature of fluid inclusions all point to the formation of the dolomite along a normal fault under deep burial conditions during the Middle to Late Cretaceous. The high 87Sr/86Sr ratio in the dolomite and the high salinity measured in fluid inclusions indicate that the dolomitizing fluids are deep basinal brines that interacted with crystalline basement. The dolomitization styles have an impact on the dimension, texture, and geochemistry of the different dolomite geobodies, and a modified classification scheme (compared to the one from Jung and Aigner, 2012) is proposed to incorporate diagenetic geobodies in future reservoir modeling.

Genesis of zebra dolomites (Cathedral Formation: CanadianCordillera Fold and Thrust Belt, British Columbia)

Abstract The late diagenetic zebra dolomites (Middle Cambrian Cathedral Formation: British Columbia, Canada) formed from focalised fluid flow of hot saline fluids (Th maximum = 180–200 °C, Tm,= −18 to −20 °C). δ18O values vary around − 18.0 ℵ (VPDB) and δ13C around − 1.35 ℵ. 87Sr/86Sr values for the grey (a) replacive dolomite laminae (0.70958 to 0.70970) and for the white, partially replacive laminae and dolomite cement (0.71187 to 0.71228), support involvement of radiogenic fluids which interacted with siliciclastics. Zebra dolomitisation post dates cleavage and is controlled by normal faults. Syn-fracturing dolomitisation in an overpressured regime is invoked for the zebra dolomites formation while the associated coarse crystalline beige dolomites reflect the more passive influx of the dolomitising fluids replacing the host rocks. Overpressuring relates to the fluids that became expulsed during Fold and Thrust Belt development along the paleo-structural Kicking Horse Rim, which defines an area where major paleogeographical changes occurred in the Middle Cambrian. However, the exact age of the dolomites is still uncertain. The fact that MVT mineralisations (Kicking Horse and Monarch deposits), which post-dates zebra formation, occur within a similar structural setting adjacent to the zebra dolomites, suggests that similar fluid flow pathways where re-used several times.

Diagenetic Geobodies: Fracture-Controlled Burial Dolomite in Outcrops From Northern Oman

Diagenetic heterogeneities are difficult to predict in the subsurface. Nevertheless, such heterogeneities can be crucial in hydrocarbon exploration. Diagenetic processes can significantly alter petrophysical properties of reservoir rocks, especially in carbonate rocks because of the reactive nature of the carbonate minerals. Dolomitization [i.e., the transformation of calcite (limestone: Lmst) into dolomite] is a common diagenetic process in carbonate rocks. Resulting dolomite bodies have a different pore network than the original Lmst and respond also differently to tectonic stress causing different fracture networks than in the original Lmst. The paper presents an overview of the learning outcomes gained by studying fracture-related dolomite in outcrops of Oman and subsequent laboratory analysis conducted over the last 4 years. A combined structural, petrographic, and geochemical approach was taken to study three dolomite systems occurring in different stratigraphic host-rock (HR) intervals. Structurally controlled dolomitization (i.e., dolomitization along faults and fractures) typically occurs in burial conditions, and the resulting strong permeability anisotropies caused by the dolomite textures can cause major challenges for hydrocarbon exploration. Dolomite bodies in the Precambrian Khufai formation are related to N/S to NNE/SSW fractures, whereas dolomite bodies that mainly occur in the Jurassic HRs occur along reactivated WNW/ESE normal faults. These fracture-related dolomite bodies are generally less than 15 m wide, but can be up to a few hundred meters long. Late-diagenetic dolomite bodies were also recognized in Permian HRs, where they occur at or close to the contact between Permian Lmst and early-diagenetic dolomite. This latediagenetic dolomite system can be traced laterally for at least hundreds of meters and occurs in wadis that are approximately 40 km apart. Our data indicate that there were several dolomitization events in the geological history of the succession, generating dolomite bodies with different characteristics. This paper highlights the need to understand timing and structural setting of dolomite bodies in the subsurface to improve reservoir management.

Application of handheld energy-dispersive X-ray fluorescence spectrometry to carbonate studies: opportunities and challenges

Technology development over the last few years has led to significant improvements in the quality and flexibility of portable instruments. Notably, handheld energy-dispersive X-ray fluorescence (ED-XRF) spectrometry has seen a bloom both in terms of technical development and applications, ranging from the field of mineral exploration to archaeology, environmental science, paleoclimatology and forensic science. However, the field of carbonate geoscience has not yet taken the capability and flexibility of this tool to its advantage. This study developed a methodology for the application of handheld XRF to carbonate studies. An assessment was made in terms of measurement time, sample preparation and weathering of outcrops. Correction equations are presented for elemental concentrations of Ca, Ti, Fe, Mn, Zn, Al, K, Mg, Ba, Sr, Rb and Si that were derived from calibration based on a series of carbonate lab standards. Weathering can pose a significant challenge for in situ measurements of carbonate outcrops, since weathering impacts on the concentrations of Ca, Mg, Ti and Al in carbonate rocks. Therefore, we advise that XRF be used on fresh rock chips that are hammered from carbonate outcrops to take advantage of making measurements in situ and at the same time ensuring reliable quantitative results. This method allows rapid and inexpensive geochemical characterization of carbonate, which opens opportunities for stratigraphic, sedimentological, paleoenvironmental and diagenetic studies in extensive study areas.

Dimensions, texture-distribution, and geochemical heterogeneities of fracture–related dolomite geobodies hosted in Ediacaran limestones, northern Oman

Predicting spatial distribution, dimension, and geometry of diagenetic geobodies, as well as heterogeneities within these bodies, is challenging in subsurface applications, and can impact the results of reservoir modeling. In this outcrop–based study, we generated a data set of the dimensions of fracture–related dolomite geobodies hosted in Ediacaran (Khufai Formation) limestones of the Oman Mountains that are up to several hundreds of meters long and a few tens of meters wide. The dolomite formed under burial conditions by fluids that interacted with siliciclastic layers, as demonstrated by the enriched Fe (up to 4.4%) and Mn (up to 0.8%) contents and () signatures. Dolomitization probably occurred during the Hercynian Orogeny (or pre-Permian) because dolomitization predates some folding and pre-Permian rocks have seen intense deformation related to the Carboniferous Hercynian Orogeny. Moreover, dolomitization occurred between the onset and termination of bedding-parallel stylolitization and thus most likely before deep burial related to the Alpine Orogeny. Hence, dolomitization most likely occurred before deep burial related to the Alpine Orogeny and during or following the intense deformation related to the Carboniferous Hercynian Orogeny had affected pre–Permian rocks. The clumped–isotope signature yields a temperature of approximately 260°C (500°F), interpreted as the apparent equilibrium temperature obtained during uplift after deepest burial during the Late Cretaceous. Lateral transects across the dolomite bodies show that zebra dolomite textures are common throughout the body and that vugs are more common at the rim than the center of the bodies. Moreover, a weak geochemical trend exists with more depleted , Fe, and Mn concentrations in the core than at the rim of the dolomite bodies. These results show that minor heterogeneities exist within the dolomite bodies investigated. These data contrast with previous studies, in which more significant variation is reported in width of the dolomitization halo and texture for larger dolomite bodies that formed in host rocks more permeable than the examples from the Oman Mountains.

Ground-based hyperspectral imaging as a tool to identify different carbonate phases in natural cliffs

ABSTRACT Recent research has shown hyperspectral imaging to be a powerful tool to distinguish carbonate phases with slight compositional differences on quarry cliff faces. The traditional remote sensing set-up uses an optimal short distance between the hyperspectral camera mounted on a tripod and a quarry wall characterized by a planar, mostly unweathered surface. Here we present results of a modified workflow geared to the application of ground-based hyperspectral imaging of rough and weathered cliff faces in order to map large scale dolomite bodies from a distance of up to several kilometres. The goal of the study was to determine unique spectral properties of fracture-controlled dolomite bodies in order to be able to distinguish them from a dolomitic host rock. In addition, the impact of weathering on carbonate phases and thus, the modification of the spectral signature between altered and unaltered carbonates is assessed. The spectral analysis is complemented by ICP-AES (inductively coupled plasma atomic emission spectroscopy) measurements of the spectrally measured powders. Furthermore, we examined the detection limits and characterisation potential of dolomite bodies from hyperspectral images captured at varying distances from cliff faces in the study area. Hyperspectral images of 10 natural cliffs distributed across the Central Oman Mountains were obtained with a Push broom scanner system. The high resolution of 5.45 nm (288 bands in total) enabled the visualization of small-scale changes in the near infrared continuous spectrum of all present lithofacies types. The determination of dolomite bodies of varying sizes (metre to hundreds of metres) on natural cliffs was achieved with the hyperspectral mapping approach and mapping results have been tested with the position of visually defined dolomite bodies on field panoramas. Spectra of natural cliffs contain a strong absorption peak indicative for iron which is absent in spectra of unweathered sample powders. However, ICP-AES analysis of powders revealed relatively low contents of iron of 12,392 ppm. The strong peaks in field images are interpreted as linked to intensive weathering associated with the precepitation of goethite, hematite, specularite and manganese as well as intensive dedolomitization. Dedolomitization is indicated by calcitic spectra derived from the dolomite bodies. The spectral difference of laboratory and field spectra interferes significantly the application of laboratory spectra of powdered samples for the identification of dolomite bodies in the field. Furthermore, the process of late dedolomitization puts an additional challenge on the determination of dolomite bodies. Due to these strong spectral variations between laboratory and field spectra, we recommend that the mapping approaches should not solely rely on spectral algorithms but also consider normal light field panoramas and representative outcrop analysis. We also note that the quality of resolution is too low for the determination of small-scale variations of diagenetic phases at distances larger than 4 km. However, when the limitations mentioned are taken into account, hyperspectral imaging proves to be a powerful tool that helps in the determination of the distribution of diagenetic phases, even in challenging conditions.

Carbonate Reservoir Analogues and Clumped Isotopes: How Combined Geometries and Geochemistry of Outcrops Help Reservoir Management in the Middle East

Petroleum geologists working in carbonate plays are facing two common and inter-connected challenges linked to optimizing production. First, constraining the geometry, spatial distribution and inter-connectivity of reservoir geobodies is crucial as these properties can control the permeability anisotropy of reservoirs zones. This is difficult to do at the inter-well scale due to the limited resolution of seismic methods (20 meters or higher) compared to the size of typical reservoir geobodies (tens of centimers to meters and higher) and the very heterogeneous nature of carbonate reservoirs. Furthermore, diagenetic transformations are very important in carbonate reservoirs. Being able to fingerprint the process and timing of diagenetic transformation is crucial to a correct assessement of the distribution of cemented zones in the subsurface. The issue of diagenesis is also important for organic matter maturation and the timing of oil migration, and therefore the second challenge faced by reservoir geologists in carbonate plays is one of constraining as well as possible the thermal history of the targeted basin. This paper reports on the results of a major long-term research effort that addresses some aspects of this double challenge in the Middle East, and that focused on novel isotopic methods to constrain the thermal history of carbonate phases in the context of the geometry of geobodies measured at the outcrop. Geological work under the Qatar Carbonates and Carbon Storage Centre (QCCSRC), funded jointly by Qatar Petroleum, Shell and the Qatar Science & Technology Park, has as its long-term research goals to improve characterization of subsurface anisotropies in carbonate reservoirs, notably for CCS operations.

Diagenetic Geobodies: Fracture-Controlled Burial Dolomite Bodies in Outcrops from Northern Oman

Diagenetic heterogeneities are difficult to predict in subsurface. Nevertheless, such heterogeneities can be crucial in hydrocarbon exploration. Diagenetic processes can significantly alter petrophysical properties of reservoir rocks, especially in carbonate rocks because of the reactive nature of the carbonate minerals. Dolomitization, i.e. the transformation of calcite (limestone) into dolomite, is a common diagenetic process in carbonate rocks. Description: an overview of the learning outcomes gained by studying fracture-related dolomite in outcrops of Oman and subsequent laboratory analysis over the last four years. A combined structural, petrographic and geochemical approach was taken to study three dolomite systems occurring in different stratigraphic host rock intervals. Application: Structurally-controlled dolomitization (i.e. dolomitization along faults and fractures) typically occurs in burial conditions, and the resulting strong permeability anisotropies caused by the dolomite textures can cause major challenges for hydrocarbon exploration. Results and Conclusions: Dolomite bodies in the Precambrian Khufai Formation are related to N-S to NNE-SSW fractures, whereas dolomite bodies that mainly occur in the Jurassic host rocks occur along reactivated WNW-ESE normal faults. These fracture-related dolomite bodies are generally less than 15 m wide, but can be up to a few hundred meters long. Late-diagenetic dolomite bodies were also recognized in Permian host rocks, where they occur at or close to the contact between Permian limestone and early-diagenetic dolomite. This late diagenetic dolomite system can be traced laterally for at least hundreds of meters and occurs in wadi’s about 40 km apart. Our data indicate that there were several dolomitization events in the geological history, generating dolomite bodies with different characteristics. Technical Contributions: This highlights the needs to understand the timing and structural setting of dolomite bodies in subsurface to improve reservoir management.