ALBERT

Description: Large rock inclusions are embedded in many salt bodies and these respond to the movements of the salt in a variety of ways including displacement, folding and fracturing. One mode of salt tectonics is downbuilding, whereby the top of a developing diapir remains in the same vertical position while the surrounding overburden sediments subside. We investigate how the differential displacement of the top salt surface caused by downbuilding induces ductile salt flow and the associated deformation of brittle stringers by an iterative procedure to detect and simulate conditions for the onset of localization of deformation in a finite element model, in combination with adaptive remeshing. The model set-up is constrained by observations from the South Oman Salt Basin, where large carbonate bodies encased in salt form substantial hydrocarbon plays. The model shows that, depending on the displacement of the top salt, the stringers can break very soon after the onset of salt tectonics and can deform in different ways. If extension along the inclusion dominates, stringers are broken by tensile fractures and boudinage at relatively shallow depth. Spacing of the boudin–bounding faults can be as close as 3–4 times the thickness of the stringer. In contrast, salt shortening along the inclusion may lead to folding or thrusting of stringers.

Description: Despite a long history of investigation, several critical issues regarding the glacial history of NW Europe, particularly in currently marine areas, remain unsolved. In this study, we present a comprehensive three-dimensional (3D) seismic interpretation of an area measuring 2000 km2 in the western part of the Danish North Sea that exhibits several buried Quaternary landforms. Well data are used to assign minimum and maximum ages for the studied sedimentary succession. The most prominent buried landforms are three large-scale tunnel valleys of probable Saalian age that extend over more than 20 km across the western and southern part of the study area. These valleys most probably formed through subglacial meltwater erosion close to the termination of a former ice sheet. In the southern part of the study area, an extensive network of small-scale, dendritic seismic lineations interpreted as a palaeo-drainage system characterizes the landward termination of one major tunnel valley. This drainage system was active either contemporaneously or shortly after the development of the tunnel valley. Interpretation of this system as contemporaneous to tunnel-valley formation suggests that steady-state subglacial meltwater discharge was funnelled through a drainage system into the main tunnel valley. In contrast, interpretation of the drainage network as post-incisional points to the development of a post-glacial river system re-using the pre-existing tunnel valley as a downstream fluvial pathway. This uncertainty in the interpretation has important consequences for prediction of the rock content and reservoir characteristics of the tunnel-valley infill, in that either meltwater deposits or fluvial sediments form a considerable part of the tunnel-valley infill.

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: 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.