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2019
Title Author Source Short Description

Becker I,

Müller B,

Koehrer B,

Jelinek W,

Hilgers C

Marine and Petroleum Geology, 103:320-330, doi

Understanding the orientation and connectivity of fracture systems in tight reservoirs is essential to reduce uncertainties in reservoir development and production. However, the actual flow potential of the fractures can be controlled by the present-day stress field. Dilation- and slip tendency analyses can improve fracture permeability evaluations and thus, aid in estimating the hydrocarbon recovery of a field.

This study focuses on a naturally-fractured, relatively tight (matrix permeabilities of 0.01–1 mD) and gas-producing Late Permian (Zechstein) reservoir in the Southern Permian Basin, northern Germany. Fracture data are obtained from resistivity image log data of a 775 m long horizontal well, and show principal NE – SW orientation with main dip angles steeper than 70°. To quantify the potential of those fractures that contribute to the flow within the reservoir, dilation (Td)- and slip (Ts) tendencies are calculated. Two different stress scenarios are defined based on known variations in the orientation of the horizontal stresses between formations below and above the Zechstein salt in an offset field approximately 5 km away.

Results of Td and Ts strongly depend on the strike and dip angle of the fractures, with sub-vertical fractures showing highest potential to dilate while fractures with dip angles of 60° are favorably oriented for slip. Fractures with orientations parallel to the principal horizontal stress have highest slip- and dilation tendencies. Adding this information to the calculation of fracture permeability results in a significant reduction of the calculated flow potential due to the influence of the stress field. Thus, incorporating stress field data helps reducing risks in field development planning.

Merz L,

Almqvist BSG,

Grimmer JC,

Kontny A

Tectonophysics, 751:212-228, doi

The COSC-1 project drilled the several hundred meters thick basal shear zone of the Lower Seve nappe with mylonites in mica schists, amphibole schists and gneisses. In zones of high magnetic susceptibility from 1910 to 2450 m we studied magnetic and petrographic fabrics, and magnetic mineralogy. Borehole imaging allowed for geographic reorientation of the samples and offered the opportunity to study anisotropy of magnetic susceptibility (AMS) in relation to tectonic evolution of the Seve thrust. We measured AMS at room temperature, added low-temperature and field-dependent AMS for a subset of samples, and compared magnetic with petrographic fabrics. Triaxial and prolate magnetic fabrics with degree of anisotropy (P′) up to 3.2 together with abundant S-C fabrics and strain partitioning around porphyroclasts indicate dominant simple shear until 2300 m. Magnetite and ilmenohematite mimic the rock fabric due to fabric parallel alignment and/or magnetic interaction and either contribute to increase or decrease of P′, depending on the dominating rock fabric elements. Field-dependency of pyrrhotite and magnetite in kmax-direction further increases P′. Homogeneous and oblate petrographic and magnetic fabrics in the greenschist-grade overprinted rocks below 2300 m with subhorizontal kmax-kint-girdle distributions indicate dominant flattening. AMS depicts shear fabrics including magnetite and ilmenohematite, and is additionally increased by retrograde magnetite-rutile intergrowth in ilmenohematites. We interpret that shape and degree of AMS are controlled by (a) tectonic deformation and strain, (b) alteration and magnetic grain interaction, and (c) field-dependency of deformed pyrrhotite and/or magnetite. We observed that all petrographic and magnetic subfabrics are coaxial, and lineations are mainly E-W to SE-NW directed confirming the transport direction of the Caledonian allochthonous. From our microstructural and AMS results we suggest that thrusting of the Lower Seve unit commenced under simple shear conditions at higher metamorphic grades and subsequently switched to more pure shear under greenschist-grade conditions.

Busch B,

Becker I,

Koehrer B,

Adelmann D,

Hilgers H

Marine and Petroleum Geology, 100:376-390, doi

Cyclic fluvial, coal-bearing sandstones from the Upper Carboniferous are important tight-gas reservoirs in the Lower Saxony Basin, NW Germany. Two wells from two fields located 10 km apart, both comprising of Westphalian C/D fluvial sandstones were studied. Well A was affected by approximately 1.2 km and well B of approximately 2.8 km of basin inversion after maximum burial depths larger than 5 km. Both fields display significant differences in recovered resources. An integrated core study combining sedimentological, petrographic and petrophysical data illustrates the impact of different diagenetic pathways. While permeability values appear in the same range of 0.01–1 mD for both fields, lithologies from well A of field A have average matrix porosities of 6% in a generally unfractured reservoir, whereas lithologies from well B of field B have matrix porosities of 10% in an overall fractured reservoir. Intense siderite/ankerite cementation in matrix and fractures is present in sandstones from both fields and can be related to fluid migration during basin inversion. In contrast, the strong alpine tectonic inversion affecting field B results in a lower thermal exposure since the late Cretaceous, resulting in smaller quartz cement volumes than in field A and larger matrix porosities in field B. Thus, matrix porosity is overall better due to smaller total cement volumes in field B. The study indicates a strong impact of fractures on tight sandstone reservoir quality and quartz cementation on hydrocarbon production performance from the sandstone matrix. Observations may be applicable to similar, strongly inverted tight gas sandstone fields and may directly affect exploration strategies.



2018
Title Author Source Short Description

Becker I,

Koehrer B,

Hilgers C

Zeitschrift der Deutschen Gesellschaft für Geowissenschaften, 169:517-531, doi

Understanding the lateral distribution of carbonate facies is essential to understand reservoir continuity, geometry and quality, and, as a consequence, can help improving development strategies as reservoir characteristics vary between dif- ferent types of depositional environments. This study focuses on three outcrops of Zechstein-2-Carbonates (second cycle, Stassfurt, Ca2) along the SW margin of the Harz Mountains. The carbonates were deposited on the Eichsfeld-Altmark Swell, a NNE–SSW striking palaeohigh at the southern margin of the Southern Permian Basin. For each outcrop, different gross depositional environments (GDE) from slope to platform margin to inner platform deposition are interpreted and correlated over a lateral distance of 25 km. Reservoir characteristics are analysed with regard to the interpreted type of depositional environment and exemplarily compared to subsurface data of a gas-producing Ca2 field, representing one interpreted GDE type, at a distance of approximately 130 km to the NW in the Lower Saxony Basin. Porosity values of the slope carbonates in outcrop are in good accordance with the provided subsurface slope data. Both show a similar range of values and average porosities of approximately 6 % (outcrop) and 4 % (reservoir). In contrast, per- meability values of the subsurface samples are increased compared to outcrop values which may be due to micro-fracturing. Platform-related grainstone facies show best matrix porosities and permeabilities, and highest proportions of those outcrop carbonates are located in the centre of the studied Eichsfeld-Altmark Swell. Porosity-permeability relationships are derived for the interpreted gross depositional environments. In combination with the presented lateral GDE correlation lengths of 25 km, they can be incorporated into models of Ca2 reservoirs in NW Germany. Subsurface information about the depositional environment distribution potentially derived from core descriptions are the basis to choose the correct outcrop analogue for future studies of the structural inventory and lateral reservoir quality variations.

Prajapati N,

Selzer M,

Nestler B,

Busch B,

Hilgers C

Journal of Geophysical Research: Solid Earth, 123:6378-6396, doi

The present work investigates the dynamics of quartz precipitation from supersaturated formation fluids in sandstone using a multiphase-field model. First, we simulate the unitaxial growth of quartz in geological fractures in two dimensions and examine the role of misorientation and crystal c- to a-axis ratio (c/a) in the formation of quartz bridge structures that are extensively observed in nature. Based on this sensitivity analysis, we choose a realistic value of c/a to computationally mimic the three-dimensional (3D) anisotropic sealing of pore space in sandstone. The simulated microstructures exhibit similarities related to crystal morphologies and remaining pore space with those observed in natural samples. Further, the phase-field simulations successfully capture the effect of grain size on (I) development of euhedral form and (II) sealing kinetics of cementation, consistent with experiments. Moreover, the initially imposed normal distribution of pore sizes evolves eventually to a lognormal pattern exhibiting a bimodal behavior in the intermediate stages. Furthermore, computational fluid dynamics analysis is performed in order to derive the temporal evolution of permeability in numerically cemented microstructures. The obtained permeability-porosity relationships are coherent with previous findings. Finally, we highlight the capabilities of the present modeling approach in simulating 3D reactive flow during progressive sealing in porous rocks based on innovative post-processing analyses and advanced visualization techniques.

Wuestefeld P,

de Medeiros M,

Koehrer B,

Sibbing D,

Kobbelt L,

Hilgers C

AAPG Bulletin, 102:2355-2387, doi

Understanding natural fracture networks in the subsurface is highly challenging, as direct one-dimensional borehole data are unable to reflect their spatial complexity, and three-dimensional seismic data are limited in spatial resolution to resolve individual meter-scale fractures.

Here, we present a prototype workflow for automated fracture detection along horizontal scan lines using terrestrial light detection and ranging (t-LIDAR). Data are derived from a kilometer-scale Pennsylvanian (locally upper Carboniferous) reservoir outcrop analog in the Lower Saxony Basin, northwestern Germany. The workflow allows the t-LIDAR data to be integrated into conventional reservoir-modeling software for characterizing natural fracture networks with regard to orientation and spatial distribution. The analysis outlines the lateral reorientation of fractures from a west–southwest/east–northeast strike, near a normal fault with approximately 600 m (∼1970 ft) displacement, toward an east–west strike away from the fault. Fracture corridors, 10–20 m (33–66 ft) wide, are present in unfaulted rocks with an average fracture density of 3.4–3.9 m−1 (11.2–12.8 ft−1). A reservoir-scale digital outcrop model was constructed as a basis for data integration. The fracture detection and analysis serve as input for a stochastically modeled discrete fracture network, demonstrating the transferability of the derived data into standard hydrocarbon exploration-and-production-industry approaches.

The presented t-LIDAR workflow provides a powerful tool for quantitative spatial analysis of outcrop analogs, in terms of natural fracture network characterization, and enriches classical outcrop investigation techniques. This study may contribute to a better application of outcrop analog data to naturally fractured reservoirs in the subsurface, reducing uncertainties in the characterization of this reservoir type at depth.

Prajapati N,

Selzer M,

Nestler B,

Busch B,

Hilgers C

Geothermal Energy, 6:7, doi

The present work investigates the influence of crack opening rates on the development of four important calcite vein morphologies, namely fibrous, elongate-blocky, partially open, and euhedral, as a result of bitaxial growth in syntaxial veins using a multiphase-field model. The continued fracturing that occurs during synkinematic cementation in these veins is simulated using the geometric shift algorithm. The stark resemblance of the numerically sealed vein microstructures with the natural samples in terms of structural characteristics as well as remaining pore space signifies a dominant role of crack opening rates in the resulting morphological patterns. Further, simulation results of slow crack opening rates reveal that non-uniform fibers of variable lengths are formed when initial crack aperture is small, due to suppression of growth competition and vice versa.

Kontny A,

Reznik B,

Boubnov A,

Göttlicher J,

Steininger R

Geochemistry, Geophysics, Geosystems, 19:921-931, doi

We studied the effect of 973 K heating in argon atmosphere on the magnetic and structural properties of a magnetite‐bearing ore, which was previously exposed to laboratory shock waves between 5 and 30 GPa. For this purpose magnetic properties were studied using temperature‐dependent magnetic susceptibility, magnetic hysteresis and low‐temperature saturation isothermal remanent magnetization. Structural properties of magnetite were analyzed using X‐ray diffraction, high‐resolution scanning electron microscopy and synchrotron‐assisted X‐ray absorption spectroscopy. The shock‐induced changes include magnetic domain size reduction due to brittle and ductile deformation features and an increase in Verwey transition temperature due to lattice distortion. After heating, the crystal lattice is relaxed and apparent crystallite size is increased suggesting a recovery of lattice defects documented by a mosaic recrystallization texture. The structural changes correlate with modifications in magnetic domain state recorded by temperature‐dependent magnetic susceptibility, hysteresis properties and low‐temperature saturation isothermal remanent magnetization. These alterations in both, magnetic and structural properties of magnetite can be used to assess impact‐related magnetic anomalies in impact structures with a high temperature overprint.

Becker I,

Koehrer B,

Waldvogel M,

Jelinek W,

Hilgers C

Marine and Petroleum Geology, 95:228-245, doi

Natural fracture networks strongly control hydrocarbon flow paths in tight carbonate reservoirs. An improved understanding of their geometries regarding orientations and distribution may result in reduced uncertainties in reservoir modeling and well planning. Outcrop analogs provide supplementary information about fracture networks below seismic resolution. We introduce a suitable analog for a gas-producing Zechstein reservoir in the Stassfurt carbonates (Ca2) in the Southern Permian Basin, northern Germany. Dolomite represents the main lithology in outcrop and reservoir rocks, which were deposited on a similar carbonate platform slope environment, and both locations were subject to the same events of diagenetic and tectonic overprint.

The scope of this study is the evaluation of fracture characteristics by a direct comparison of three similar datasets of manual outcrop and digital outcrop data with borehole resistivity image log data from a horizontal gas development well. Manually measured fracture data of the exact same digital scanlines in the outcrop are used to successfully validate terrestrial laser scanning (t-LiDAR). T-LiDAR data is used to generate a high-resolution digital outcrop model, and we introduce a novel workflow, firstly applied to carbonate rocks, to detect fractures from that data set along artificial horizontal wells following E&P industry best practice.

Results of both outcrop and subsurface data suggest W – E orientations for well path planning due to dominant northerly striking open tectonic fracture directions. Higher fracture intensities (P10 values describing the amount of fractures intersecting the scanline) of conventional scanline measurements of 4.3 m−1 indicate a bias in the t-LiDAR (P10: 2.6 m−1) dataset inversely related to limitations in fracture detection abilities along the horizontal well. As for the horizontal well (P10: 2.7 m−1), fractures paralleling the borehole appear systematically underrepresented. Fractures with almost perpendicular orientation to the outcrop wall can only be occasionally detected in the t-LiDAR data set due to their limited exposed surface. However, outcrop fracture characteristics are in good correlation with subsurface results and thus, can help reducing uncertainties in reservoir characterization during field development and well planning.

Busch B,

Hilgers C, 

Landers RH,

Bonell LM,

Adelmann D

AAPG Bulletin, 102:293-307, doi

Silicate reaction kinetics provide a complementary means to other established paleothermal indicators such as organic maturation for evaluating thermal reconstructions. In this study we combine the use of an organic maturation model with kinetic models for quartz and illite cementation to evaluate burial history scenarios for five subsalt wells in lithologically and structurally complex Rotliegendes reservoirs. Models for organic maturation are most sensitive to maximum temperature and provide no direct evidence for the time of peak temperature or the overall duration of high temperatures. By contrast, the kinetics of quartz cementation are much more strongly influenced by the duration of exposure to high temperatures compared with organic indicators. Kinetic models for fibrous illite formation similarly are sensitive to time and temperature and provide predictions for the time of illite formation that can be compared with radiometric dates. Used collectively, these organic and inorganic paleothermal indicators provide improved constraints on thermal evolution compared with conventional approaches. In this study we use these indicators to evaluate two alternative burial history scenarios. Scenario 1 incorporates a hypothesized Jurassic heat flow peak together with significant Late Jurassic deposition and subsequent erosion. Scenario 2 omits the Jurassic heat flow peak and omits the deposition and erosion of the Upper Jurassic. Although both of these scenarios are consistent with organic maturation data, scenario 2 leads to a far better match with quartz cement volumes and fibrous illite K-Ar dates.



2017
Title Author Source Abstract

Manss Y,

Hilgers C,

Buddenbaum H,

Stanjek H

Zeitschrift der Deutschen Gesellschaft für Geowissenschaften, 168:403-412, doi

It is often difficult to identify and quantify mineralogical heterogeneities on the centimetre to metre scale by optical means, when these heterogeneities have apparently similar colours. Therefore, a general image processing workflow combined with mineralogical analytical methods is presented by using contact-free hyperspectral imaging techniques on a mudstone concretion. The application of spectral algorithms reveals a distinct texture in a visually homogeneous black rock. The pattern is confirmed by optical and electron microscopy as well as X-ray diffraction, showing a layered texture of varying amounts of siderite and clay minerals. The rapid and contact-free visualisation of rock anisotropies such as mineralogical heterogeneities in overall homogeneous rocks may thus enable a better prediction of mechanical and petrophysical heterogeneities on mm- to 100 m scale.

Busch B,

Hilgers C, 

Landers RH,

Gronen L,

Adelmann D

Journal of the Geological Society, 174:855-868, doi

Quartz cementation in sandstones is closely linked to grain coating phases and diagenetic alteration. Grain coatings consisting of illite–smectite stained with iron oxides and hydroxides are able to preserve large amounts of porosity by preventing the formation of syntaxial quartz overgrowth cement. The Penrith Sandstone Formation was chosen as an analogue for Rotliegend reservoirs to test the impact of grain coatings on quartz cementation. This adds to an existing model of cementation. Differences of grain coating coverage can be linked to grain size. Extensive grain coatings are present in finer grained laminae in some samples. Coarser grained laminae contain less extensive grain coatings. The analysis of grain coatings based on standard petrographic analyses is combined with high-resolution QEMSCAN® analyses. Structural features include deformation bands of different ages. Diagenetic alterations around faults, recorded by grain coatings, allow the delineation of relative temporal relations, revealing at least two generations of deformation band formation associated with normal faulting. In the Vale of Eden succession one normal faulting event postdates burial diagenetic quartz cementation as is evident by fault focused fluid flow and associated bleaching of iron and absence of quartz overgrowth.

Becker I,

Wuestfeld P,

Koehrer B,

Felder M,

Hilgers C

Journal of Petroleum Geology, 40:363-389, doi

Understanding the spatial variability of reservoir properties in tight gas sandstones can significantly reduce uncertainties in reservoir characterization. This study focuses on two outcrops of Upper Carboniferous (Westphalian D) fluvial sandstones in the southern part of the Lower Saxony Basin, NW Germany, which are analogues for tight, gas‐producing reservoir rocks at fields in the north of the basin. Large‐scale differences in reservoir quality occur between the two outcrops which are separated by a distance of approximately 15 km and by 600 m of stratigraphy. Smaller‐scale heterogeneities in the form of channel geometries occur within individual fluvial cycles, as evidenced in a 30 m (high) by 150 m (long) porosity‐permeability profile at one of the outcrops studied.

In the Woitzel quarry, lower Westphalian D deposits consist of fining‐upwards cycles of channel and bar sandstones with intercalated floodplain sediments and coal seams. In abandoned quarries at the Hüggel location, upper Westphalian D strata are composed of grey and red coloured sandstones which suggest deposition in a fluvial – alluvial plain environment under increasingly arid conditions.

Reservoir quality is higher in the upper Westphalian D sandstones at Hüggel (permeability: up to 1 mD; mean porosity: 19%) than in the lower Westphalian D sandstones at Woitzel (permeability: <0.1 mD; mean porosity: 15%). Due to the high degree of compaction of the upper and lower Westphalian D deposits (intergranular volumes of 21.9% and 19.4% respectively) and the high proportions of pseudomatrix, porosity in these sandstones is mainly secondary with up to 80% microporosity. Reservoir quality is in general higher in the upper Westphalian D sandstones where kaolinite is present. In contrast, intense illitization has degraded the reservoir quality of the lower Westphalian D sandstones.

A positive correlation between permeability and grain size is exhibited by the kaolinite‐bearing upper Westphalian D sandstones. No such correlation is evident in the illite‐bearing lower Westphalian D sandstones, unless samples with permeabilities of <0.01 mD are excluded from the correlation. However, such a low permeability cut‐off is not related to sorting, grain size or authigenic quartz cementation.

At a small scale, permeability varies laterally by two orders of magnitude from 0.001 mD to 0.1 mD in a single, approximately 3 m thick tight sandstone bed over a distance of 150 m. Repeated, centimetre‐scaled fining‐upwards grading and bed‐internal erosion contribute to lateral variations in reservoir flow characteristics.

Integrated studies of the depositional environment, diagenesis and sedimentary geometries of the deposited units are a key to the understanding of heterogeneities in tight fluvial sandstones and thus to the successful development of Upper Carboniferous reservoirs in the Lower Saxony Basin and elsewhere.

Wüstefeld P,

Hilse U,

Koehrer B,

Adelmann D,

Hilgers C

Marine and Petroleum Geology, 86:689-710, doi

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.

 

Hammann E,

Madlener R,

Hilgers C

Energy Procedia, 105:3798-3805, doi

Since the liberalization of electricity markets power prices are considered as highly volatile. Thus investment in new power plants is exposed to higher risks. Traditional capital budgeting methods lack the integration of flexibility. This can be overcome by applying real options analysis. In the context of an increasing electricity production by highly volatile renewable power plants, new solutions need to be found to ensure security of supply and to accommodate intermittent generation. Energy storage is seen as a possible solution. The Compressed Air Energy Storage (CAES) technology provides many key features that are relevant to deal with arising problems by renewables. Lately an advanced adiabatic version with higher roundtrip efficiencies has been discussed. However, high capital expenditures limit the economically viable implementation. As conventional CAES uses natural gas for production, it is exposed to price fluctuations on two markets. Depending on the configuration of component sizes, CAES storage units can be used for different purposes. A price model is set up to produce, in combination with the application of a Monte Carlo simulation, possible future price paths for power, natural gas and demand rate for minute reserve. Based on these price paths costs and revenues for different CAES applications are calculated. For the economic evaluation three different configurations are considered for both diabatic and adiabatic CAES. Investment in a diabatic CAES used for load-leveling purposes is found to be the most economical option.

Wüstefeld P,

Hilse U,

Lüders V,

Wemmer K,

Koehrer B,

Hilgers C

 Marine and Petroleum Geology, 86:288-303, doi

Upper Carboniferous sandstones make one of the most important tight gas reservoirs in Central Europe. This study integrates a variety of geothermometers (chlorite thermometry, fluid inclusion microthermometry and vitrinite reflection measurements) to characterize a thermal anomaly in a reservoir outcrop analog (Piesberg quarry, Lower Saxony Basin), which is assumed responsible for high temperatures of circa 300 °C, deteriorating reservoir quality entirely. The tight gas siliciclastics were overprinted with temperatures approximately 90–120 °C higher compared to outcropping rocks of a similar stratigraphic position some 15 km to the west. The local temperature increase can be explained by circulating hydrothermal fluids along the fault damage zone of a large NNW-SSE striking fault with a displacement of up to 600 m in the east of the quarry, laterally heating up the entire exposed tight gas sandstones. The km-scale lateral extent of this fault-bound thermal anomaly is evidenced by vitrinite reflectance measurements of meta-anthracite coals (VRrot ∼ 4.66) and the temperature-related diagenetic overprint. Data suggest that this thermal event and the associated highest coalification was reached prior to peak subsidence during Late Jurassic rifting (162 Ma) based on K-Ar dating of the <2 μm fraction of the tight gas sandstones. Associated stable isotope data from fluid inclusions, hosted in a first fracture filling quartz generation (T ∼ 250 °C) close to lithostatic fluid pressure (P ∼ 1000 bars), together with authigenic chlorite growth in mineralized extension fractures, demonstrate that coalification was not subject to significant changes during ongoing burial. This is further evidenced by the biaxial reflectance anisotropy of meta-anthracite coals. A second event of quartz vein formation occurred at lower temperatures (T ∼ 180 °C) and lower (hydrostatic) pressure conditions (P ∼ 400 bars) and can be related to basin inversion. This second quartz generation might be associated with a second event of illite growth and K-Ar ages of 96.5–106.7 Ma derived from the <0.2 μm fraction of the tight gas sandstones.

This study demonstrates the exploration risk of fault-bound thermal anomalies by deteriorating entirely the reservoir quality of tight gas sandstones with respect to porosity and permeability due to the cementation with temperature-related authigenic cements. It documents that peak temperatures are not necessarily associated with peak subsidence. Consequently, these phenomena need to be considered in petroleum system models to avoid, for example, overestimates of burial depth and reservoir quality.

 

Reznik B,

Kontny A,

Uehara M,

Gattacceca J,

Solheid P,

Jackson M

Magnetism and Magnetic Materials, 426:594-603, doi

Magnetic properties, texture and microstructure of cohenite grains from Morasko iron meteorite have been investigated using electron backscattered diffraction, Bitter pattern technique, magneto-optical imaging method and magnetic force microscopy. Cohenite shows much stronger magnetic contrast compared to kamacite because it is magnetically harder than the Fe-Ni alloy, and thus causes higher stray fields. A surprising result is the high stability and reversibility of the global stripe-like magnetic domain structure in cohenite when applying high magnetic fields up to 1.5 T, and exposing it to high temperatures above the Curie temperature of about 220 °C. Heating up to 700 °C under atmosphere conditions has shown that cohenite remains stable and that the global magnetic domain structures mainly recover to its preheating state. This observation suggests that magnetic domains are strongly controlled by the crystal anisotropy of cohenite. Branching magnetic domain structures at the grain boundary to kamacite can be annealed, which indicates that they are very sensitive to record deformation. EBSD observations clearly demonstrate that increasing deviation from the easy [010] crystallographic axis and stress localization are the main factors controlling the distortion of Bitter patterns, and suggest a high sensitivity of the cohenite magnetic domain structure to local microstructural heterogeneities. The results of this study substantiate the theory t