ALBERT

Description: This Article reports results from a field experiment investigating the influence of volcanic tephra coverage on glacier ablation. These influences are known to be significantly different from those of moraine debris on glaciers due to the contrasting grain size distribution and thermal conductivity. Influences of tephra deposits on glacier ablation have hardly been studied so far. For the experiment, artificial plots of two different tephra types from Eyjafjallajökull and Grimsvötn volcanoes were installed on a snow-covered glacier surface of Vatnajökull ice cap, Iceland.Ablation was automatically monitored along with atmospheric variables and ablation on a non-tephra covered reference site over the summer season 2015. For each of the two volcanic tephra types, three plots (~ 1.5 mm, ~ 8.5 mm and ~ 80 mm) were monitored. After limiting the records to a period of reliable measurements, a 50-days dataset of hourly records was obtained, which can be downloaded from the Pangaea data repository (https://www.pangaea.de; https://doi.org/10.1594/PANGAEA.876656). The experiment shows a substantial increase of ablation under the ~ 1.5 mm and ~ 8.5 mm tephra plots when compared to uncovered conditions. Only under the thick tephra cover some insulating effects could be observed. This result is in contrast to other studies which depicted insulating effects for much thinner tephra coverson bare-ice glacier surfaces. Differences between the influences of the two different petrological types of tephra exist but are small.

Description: In addition to seismically mapped fault structures, a large number of faults below the limit of seismic resolution contribute to subsurface deformation. However, a correlation between large- and small-scale faults is difficult because of their strong variation in orientation. A workflow to analyze deformation over different scales is described here. Based on the combination of seismic interpretation, coherency analysis, geostatistical analysis, kinematic modeling, and well data analysis, we constrained the density and orientation of subseismic faults and made predictions about reactivation and opening of fractures. We interpreted faults in seismic and coherency volumes at scales between several kilometers and a few tens of meters. Three-dimensional (3-D) retrodeformation was performed on a detailed interpreted 3-D structural model to simulate strain in the hanging wall at the time of faulting, at a scale below seismic resolution. The modeling results show that (1) considerable strain is observed more than 1 km (0.62 mi) away from the fault trace and (2) deformation around the fault causes strain variations, depending on the fault morphology. This strain variation is responsible for the heterogeneous subseismic fracture distribution observed in wells. We linked the fracture density from the well data with the modeled strain magnitude and used the strain magnitude as a proxy for fracture density. With this method, we can predict the relative density of small-scale fractures in areas without well data. Furthermore, knowing the orientation of the local strain axis, we predict a fault strike and opening or reactivation of fractures during a particular deformation event. Tina Lohr graduated in geology at Freiberg University, Germany. She is currently completing her Ph.D. at the GeoForschungsZentrum (GFZ) Potsdam. As a Marie-Curie fellow, she joined the Fault Dynamics Research Group at the Royal Holloway University of London for 5 months. Her research is focused on seismic interpretation, fault analysis, and structural restoration and modeling. Charlotte Krawczyk is now at the Leibniz Institute for Applied Geosciences and is a professor for geophysics, with focus on seismics at Technical University Berlin. From 1995 to 2007, she was a senior scientist at GFZ Potsdam. She did her Ph.D. at GEOMAR, Research Center for Marine Geosciences, Kiel, and received her diploma in geophysics from Kiel University. David Tanner earned his B.Sc. degree at Liverpool University (1988), his M.Sc. degree at Imperial College, London (1989), and his Ph.D. at Giessen University, Germany (1995). His main research interest is three-dimensional structural and geometrical modeling of seismic and outcrop data at all scales. Ramin Samiee received his M.S. degree in geology at Heidelberg University and his Ph.D. at Erlangen University (1998) in Germany. His interests are facies and diagenesis of carbonates and siliciclastics, log analysis, and seismic interpretation. He worked as a consultant for Shell, PanTerra, BEB, and Trappe Erdoel Erdgas Consultant (TEEC) and is now at RWE Dea AG. Heike Endres received her diploma in geophysics in 1995 from Muenster University, Germany. She worked as a geophysicist for Western-Geco and TEEC. For this project, she was part of the working group of RWTH Aachen University. Peter Thierer received his diploma in geology from Kiel University, Germany, in 2001. He worked as a research associate at GEOMAR Research Center for Marine Geosciences, and, since 2006, for TEEC. Onno Oncken received his diploma and Ph.D. at Cologne University, followed by postdoctoral research at Muenster and Frankfurt Universities, and a professorship for structural geology at Wuerzburg University. In 1992, he joined the GFZ in Potsdam. He is the director of the Geodynamics Department and holds a faculty position at Free University Berlin. Henning Trappe received his Ph.D. from Kiel University, Germany, in 1986. He worked at BEB from 1986 to 1992 as a geophysicist. Since 1992, he is the head of the self-founded TEEC and TEECware. Raik Bachmann received his diploma in geology from Freiberg University, Germany. Presently, he is finishing his Ph.D. at GFZ Potsdam and Free University Berlin. His work focuses on exhumed convergent plate boundaries and fossil seismicity. Peter Kukla graduated in geology from Wuerzburg University, Germany, and received his Ph.D. from Witwatersrand University, South Africa. His professional career included positions at Witwatersrand University (1986–1990), Shell International Exploration and Production (1991–2000), and RWTH Aachen University (since 2000) as a full professor of geology, head of the department, and director of the Geological Institute, with research focus on petroleum reservoir geology.

Description: In the South Oman salt basin (SOSB), diapirs of infra-Cambrian Ara Salt enclose isolated, commonly overpressured carbonate reservoirs. Hydrocarbon-impregnated black rock salt shows that it has repeatedly lost and then regained its sealing capacity. The black staining is caused by intragranular microcracks and grain boundaries filled with solid bitumen formed by the alteration of oil. The same samples show evidence for crystal plastic deformation and dynamic recrystallization. Subgrain-size piezometry indicates a maximum differential paleostress of less than 2 MPa. Under such low shear stress, laboratory-calibrated dilatancy criteria indicate that oil can only enter the rock salt at near-zero effective stresses, where fluid pressures are very close to lithostatic. In our model, the oil pressure in the carbonate reservoirs increases until it is equal to the fluid pressure in the low but interconnected porosity of the Ara Salt plus the capillary entry pressure. When this condition is met, oil is expelled into the rock salt, which dilates and increases its permeability by many orders of magnitude. Sealing capacity is lost, and fluid flow will continue until the fluid pressure drops below the minimal principal stress, at which point rock salt will reseal to maintain the fluid pressure at lithostatic values. Johannes Schoenherr received his diploma from the Technical University of Darmstadt, Germany, with main emphasis in structural geology. Johannes is currently a Ph.D. student at Rheinisch-Westfälische Technische Hochschule (RWTH) Aachen University, Germany. His research is focused on the diagenesis and organic geochemistry of intrasalt carbonates and evaporites from the South Oman salt basin. His further interests are in microtectonics involving the geomechanics of rock salt. Janos L. Urai is currently a professor of structural geology, tectonics, and geomechanics at RWTH Aachen University and program director of the Department of Applied Geoscience, Oman-German University of Technology in Muscat, Oman. He is interested in basic and applied aspects of rock deformation in the presence of fluids at a wide range of scales in hydrocarbon reservoirs. Peter A. Kukla graduated in geology from Wuerzburg University, Germany, and Witwatersrand University, South Africa (Ph.D.). His professional career included positions at Witwatersrand University (1986–1990), Shell International E&P (1991–2000), and at RWTH Aachen University (since 2000) as full professor of geology and head of the department and director of the Geological Institute, with research focus on applied sedimentology, reservoir geology, and quantitative geodynamics. Ralf Littke is a professor of geology and geochemistry of petroleum and coal at RWTH Aachen University, Germany. Ralf's current research topics include dynamics of sedimentary basins, with special emphasis on temperature and pressure history; generation of hydrocarbon gases and nonhydrocarbon gases as well as petroleum; transport and accumulation of methane and carbon dioxide; and development of new tools in petroleum system modeling. Zsolt Schléder received his M.Sc. degree from the Eötvös University, Budapest, Hungary, in 2001 and his Ph.D. from the RWTH Aachen University, Germany, in 2006. He is currently working at Midland Valley Exploration, Ltd., as a structural geologist. His research efforts are focused on deformation and recrystallization mechanisms in rock salt. His current interest is in two- and three-dimensional structural restoration technology. Jean-Michel Larroque has a Ph.D. in structural geology from Montpellier University (France) and joined the Shell structural geology team in 1988. He had assignments in the United Kingdom, Germany, and Oman as South Oman Exploration team leader. Previously, he was Shell Exploration chief geoscientist for the Middle East and the Caspian. He is now exploration manager for Shell Syria. Mark J. Newall is a senior exploration geologist in Frontier Exploration in Shell, Egypt. He has a Ph.D. from Liverpool University (1990) and, since joining Shell, has worked as an explorationist in Holland, Malaysia, and Oman, before moving to Cairo in 2005. He is currently exploring for gas in the Nile delta. Nadia Al-Abry holds a Ph.D. (2002) from the University of Edinburgh. Nadia joined Petroleum Development Oman in October 2002 and since then has been working on the Precambrian intrasalt Ara carbonate stringers first as an exploration team geologist and seismic interpreter and then as a production geologist. Her research interests are in the tectonic evolution of basins and its influence on sedimentation and reservoir architecture. Hisham A. Al-Siyabi holds an M.S. degree (1994) and a Ph.D. (1998) from the Colorado School of Mines. Hisham joined Petroleum Development Oman in 1999 and, since 2001, has worked as a geologist and seismic interpreter exclusively on the terminal Proterozoic intrasalt Ara stringers. In 2005, Hisham joined Shell Exploration and Production Company in the United States as an exploration geologist. Zuwena Rawahi is a senior carbonate geologist in Petroleum Development Oman and has been working on the Precambrian stringer play on the South Oman exploration team for the last 3 years. Prior to that, she worked for 7 years on the Shuaiba Formation. Her main interest is related to carbonate sedimentology and diagenesis.

Description: The megacrack pattern of the ephemeral north Panamint dry lake, California, United States, is characterized by variably sized polygons with diameters ranging from hundreds of meters to meters. The evolution and subsurface extent of this polygonal pattern and a probable tectonic link are examined by ground resistivity measurements and surface mapping. Crack development is initiated by the shrinking of clays caused by changes in water content near the surface. For crack evolution, the following processes are proposed: Cavities develop at approximately 1-m (∼3-ft) depth during a subsurface phase, followed by the collapse of the overburden into the existing cavities to form the surface cracks. Cracks are filled by wind-blown sand and dried-out lake sediments from collapsing crack walls. Following burial, differences in competence between crack-fill and surrounding playa-lake sediments provide zones of structural weakness that might channelize stress release and faulting. Ground resistivity measurements confirmed the extent of the cracks to a depth of more than 3 m (〉9 ft). The megacrack pattern is compared to a Rotliegende (Upper Permian) tight gas field, located in the southern Permian Basin of northwestern Germany, situated in a comparable geologic setting. There, a multidirectional polygonal pattern is recorded on horizon slices of three-dimensional seismic data and compares well to our observations from the Panamint Valley. The Rotliegende pattern is associated with low-offset faults, which are proposed to be responsible for subtle reservoir compartmentalization.

Description: Conventional basin and petroleum systems modeling uses the vertical backstripping approach to describe the structural evolution of a basin. In structurally complex regions, this is not sufficient. If lateral rock movement and faulting are inputs, the basin and petroleum systems modeling should be performed using structurally restored models. This requires a specific methodology to simulate rock stress, pore pressure, and compaction, followed by the modeling of the thermal history and the petroleum systems. We demonstrate the strength of this approach in a case study from the Monagas fold and thrust belt (Eastern Venezuela Basin). The different petroleum systems have been evaluated through geologic time within a pressure and temperature framework. Particular emphasis has been given to investigating structural dependencies of the petroleum systems such as the relationship between thrusting and hydrocarbon generation, dynamic structure-related migration pathways, and the general impact of deformation. We also focus on seal integrity through geologic time by using two independent methods: forward rock stress simulation and fault activity analysis. We describe the uncertainty that is introduced by replacing backstripped paleogeometry with structural restoration, and discuss decompaction adequacy. We have built two end-member scenarios using structural restoration, one assuming hydrostatic decompaction, and one neglecting it. We have quantified the impact through geologic time of both scenarios by analyzing important parameters such as rock matrix mass balance, source rock burial depth, temperature, and transformation ratio.

Description: Layered evaporite sequences (LES) comprise interbedded weak layers (halite and, commonly, bittern salts) and strong layers (anhydrite and usually non-evaporite rocks such as carbonates and siliciclastics). This results in a strong rheological stratification, with a range of effective viscosity up to a factor of 105. We focus here on the deformation of competent intrasalt beds in different modes of salt tectonics using a combination of conceptual, numerical and analog models, and seismic data. In bedding-paralell extension, boudinage of the strong layers forms ruptured stringers, within a halite matrix, that become increasingly isolated with increasing strain. In bedding-parallel shortening, competent layers tend to maintain coherency while forming harmonic, disharmonic, and polyharmonic folds, with the rheological stratification leading to buckling and fold growth by bedding-parallel shear. In differential loading, extension and the resultant stringers dominate beneath suprasalt depocenters while folded competent beds characterize salt pillows. Finally, in tall passive diapirs, stringers generated by intrasalt extension are rotated to near vertical in tectonic melanges during upward flow of salt. In all cases, strong layers are progressively removed from areas of salt thinning and increasingly disrupted and folded in areas of salt growth as deformation intensifies. The varying styles of intrasalt deformation impact seismic imaging of LES and associated interpretations. Ruptured stringers are often visible where they have low dips, as in slightly extended salt layers or beneath depocenters, but are usually not imaged in tall passive diapirs due to steep dips. In contrast, areas of slightly to moderately shortened salt typically have well imaged, mostly continuous intrasalt reflectors, although seismic coherency decreases as deformation intensifies. Similarly, wells are most likely to penetrate strong layers in contractional structures and salt pillows, less likely in extended salt because they might drill between stringers, and unlikely in tall passive diapirs because the stringers are near-vertical. Thus, both seismic and well data may be interpreted to suggest that diapirs and other areas of more intense intrasalt deformation are more halite rich than is actually the case.