ALBERT

Description: Structural elements of deformation-band fault zones are implemented as volumetrically expressed building blocks, that is, fault facies, in a series of synthetic reservoir geomodels and simulation models. The models are designed and built to reproduce a predefined range of fault system configuration, sedimentary facies configuration, and fault zone architecture. Using petrophysical properties derived from published field studies, the geomodel realizations are run in a reservoir simulator to monitor reservoir responses to variations in modeling factors. The modeled fault zones act as dual barrier-conduit systems, resulting in simulation models that can capture contrasting waterfront velocities, changes in waterfront geometries, and flow channelizing and bifurcation in the fault envelopes. The simulation models also show the development and sweep efficiency of bypassed oil and poorly swept regions because of the presence of the fault zones. Statistical analysis reveals that the fault facies modeling factors can be ranked according to impact on reservoir responses in the following descending order: fault core thickness, the type of displacement function, sedimentary facies configuration, the fraction of total fault throw accommodated by fault core and damage zones, fault system configuration, and maximum damage zone width. Fault core thickness is the most important factor because it governs the space available for fluid flow in the fault-dip direction. Other modeling factors affect the reservoir responses by controlling the geometry and continuity of fluid flow paths in the modeled fault zones.

Description: Faults play a key role in reservoirs by enhancing or restricting fluid flow. A fault zone can be divided into a fault core that accommodates most of the displacement and a surrounding damage zone. Interpretation of seismic data is a key method for studying subsurface features, but the internal structure and properties of fault zones are often at the limit of seismic resolution. We have investigated the seismic response of a vertical fault zone model in sandstone, populated with fault facies based on deformation band distributions. Deformation bands reduce the porosity of the sandstone, and they condition its elastic properties. We generate synthetic seismic cubes of the fault facies model for several wave frequencies and under realistic conditions of reservoir burial and seismic acquisition. Seismic image quality and fault zone definition are highly dependent on wave frequency. At a low wave frequency (e.g., 10 Hz), the fault zone is broader and no information about its fault facies distribution can be extracted. At higher wave frequencies (e.g., 30 and 60 Hz), seismic attributes, such as tensor and envelope, can be used to characterize the fault volume and its internal structure. Based on these attributes, we can subdivide the fault zone into several seismic facies from the core to the damage zone. Statistical analyses indicate a correlation between the seismic attributes and the fault internal structure, although seismic facies, due to their coarser resolution, cannot be matched to individual fault facies. The seismic facies can be used as input for reservoir models as spatial conditioning parameters for fault facies distributions inside the fault zone. However, relying only on the information provided by seismic analyses might not be enough to create high-resolution fault reservoir models.

Description: The Putumayo foreland basin (PFB) is an underexplored, hydrocarbon-bearing basin located in southernmost Colombia. The PFB forms a 250-km long segment of the 7000-km-long corridor of Late Cretaceous-Cenozoic foreland basins produced by eastward thrusting of the Andean mountain chain over Precambrian rocks of the South American craton. We use ∼4000 km of 2D seismic data tied to 28 exploratory wells to describe the basin-wide structure and stratigraphy of an underexplored hydrocarbon basin. Based on seismic interpretation and comparison with published works from the southward continuation of the PFB into Peru and Ecuador, three main across-strike, structural zones include: 1) the 20-km-wide, Western structural zone closest to the Andean mountain front characterized by inversion of older, Jurassic half-grabens during the late Miocene; 2) the 45-km-wide, Central structural zone characterized by moderately-inverted Jurassic half-grabens; and 3) the 120-km-wide, Eastern structural zone characterized by the 40-km-wide, N-S trending Caquetá arch. The five mainly clastic tectonosequences of the PFB include: 1) Lower Cretaceous pre-foreland basin deposits; 2) Upper Cretaceous-Paleocene foreland basin deposits; 3) Eocene foreland basin deposits related to the early uplift of the Eastern Cordillera; 4) Oligocene-Miocene underfilled, foreland basin deposits; and 5) Plio-Pleistocene overfilled, foreland basin deposits. We used 3D flexural modeling to identify the elastic thickness (Te) of the lithosphere below the PFB, in order to model the location of the sedimentary-related and tectonically-related forebulges of Cretaceous to Oligocene age. Flexural analysis shows two pulses of rapid, foreland-related subsidence first during the Late Cretaceous-early Paleocene and later during the Oligocene-Miocene. Despite the present-day oblique thrusting of the mountain front, flexure of the PFB basement has produced a tectonic forebulge now located in the Eastern structural zone and controls a basement high that forms the eastern, updip limit for most hydrocarbons found in the PFB.

Description: The concept of fault facies is a novel approach to fault description adapted to three-dimensional reservoir modeling purposes. Faults are considered strained volumes of rock, defining a three-dimensional fault envelope in which host-rock structures and petrophysical properties are altered by tectonic deformation. The fault envelope consists of a varying number of discrete fault facies originating from the host rock and organized spatially according to strain distribution and displacement gradients. Fault facies are related to field data on dimensions, geometry, internal structure, petrophysical properties, and spatial distribution of fault elements, facilitating pattern recognition and statistical analysis for generic modeling purposes. Fault facies can be organized hierarchically and scale independent as architectural elements, facies associations, and individual facies. Adding volumetric fault-zone grids populated with fault facies to reservoir models allows realistic fault-zone structures and properties to be included. To show the strength of the fault-facies concept, we present analyses of 26 fault cores in sandstone reservoirs of western Sinai (Egypt). These faults all consist of discrete structures, membranes, and lenses. Measured core widths show a close correlation to fault displacement; however, no link to the distribution of fault facies exists. The fault cores are bound by slip surfaces on the hanging-wall side, in some cases paired with slip surfaces on the footwall side. The slip surfaces tend to be continuous and parallel to the fault core at the scale of the exposure. Membranes are continuous to semicontinuous, long and thin layers of fault rock, such as sand gouge, shale gouge, and breccia, with a length/thickness ratio that exceeds 100:1. Most observed lenses are four sided (Riedel classification of marginal structures) and show open to dense networks of internal structures, many of which have an extensional shear (R) orientation. The average lens long axis/short axis aspect ratio is about 9:1. Alvar Braathen is a professor in structural geology at the University Center in Svalbard and an adjunct professor at the Department of Earth Science, University of Bergen. He received his M.S. degree and his Ph.D. from the University of Tromsø, Norway. His research covers aspects of fold and thrust belts and extensional tectonics, with a current focus on fault description and the importance of faults for fluid flow. After receiving his Doctor of Science title from the University of Bergen, Jan Tveranger worked as a polar Quaternary scientist for several years before being engaged by Saga Petroleum and subsequently Norsk Hydro as a reservoir geologist. He is currently employed as a senior researcher and research coordinator at the Center for Integrated Petroleum Research, University of Bergen, focusing on description and modeling of reservoir properties of faults and paleokarst features. Haakon Fossen received his Ph.D. from the University of Minnesota in 1992. He joined Statoil in 1986 and, since 1996, has been a professor in structural geology at the University of Bergen. His scientific interests cover the evolution and collapse of mountain ranges, the structure of rift basins, and petroleum-related deformation structures at various scales, with current focus on deformation bands and subseismic faults. Tore Skar received his M.S. degree and Ph.D. in geology from universities in Bergen and Amsterdam. After some years as a senior researcher at the University of Bergen, he moved to the senior geologist position in StatoilHydro. His scientific interests cover sedimentology and structural geology. Nestor Cardozo received his B.S. degree in geology from the Universidad Nacional de Colombia in 1994 and his Ph.D. in geology from Cornell University in 2003. He is an associate professor at the University of Stavanger. His scientific interests cover faults, their related deformation, and their implementation in reservoir models. Siv Semshaug works as an exploration geologist in rock source in Bergen. In parallel, she is undertaking her Ph.D. through the Center for Integrated Petroleum Research, University of Bergen. She also received her M.S. degree in structural geology from the same university. Her current research focuses on fault siliciclastic rocks and their importance for reservoir performance. Eivind Bastesen is a Ph.D. student at the Center for Integrated Petroleum Research at the University of Bergen. He also received his M.S. degree in structural geology from the same university. His current research interests are extensional faults in carbonates and siliciclastic rocks, with a focus on field descriptions and quantification of fault zones. Einar Sverdrup is the exploration manager of Dana Petroleum Norway. He received his M.S. degree and Ph.D. degrees from the University of Oslo, Norway. His research topics cover sedimentology and diagenesis, fault properties, and flow characterization of seismic to subseismic reservoir heterogeneities.

Description: Structural elements of deformation-band fault zones are implemented as volumetrically expressed building blocks, that is, fault facies, in a series of synthetic reservoir geomodels and simulation models. The models are designed and built to reproduce a predefined range of fault system configuration, sedimentary facies configuration, and fault zone architecture. Using petrophysical properties derived from published field studies, the geomodel realizations are run in a reservoir simulator to monitor reservoir responses to variations in modeling factors. The modeled fault zones act as dual barrier-conduit systems, resulting in simulation models that can capture contrasting waterfront velocities, changes in waterfront geometries, and flow channelizing and bifurcation in the fault envelopes. The simulation models also show the development and sweep efficiency of bypassed oil and poorly swept regions because of the presence of the fault zones. Statistical analysis reveals that the fault facies modeling factors can be ranked according to impact on reservoir responses in the following descending order: fault core thickness, the type of displacement function, sedimentary facies configuration, the fraction of total fault throw accommodated by fault core and damage zones, fault system configuration, and maximum damage zone width. Fault core thickness is the most important factor because it governs the space available for fluid flow in the fault-dip direction. Other modeling factors affect the reservoir responses by controlling the geometry and continuity of fluid flow paths in the modeled fault zones. 2nd revised manuscript received April 25, 2010 Muhammad Fachri earned a B.Sc. degree in geophysics and an M.Sc. degree in geology from the Institute Technology Bandung (Indonesia). In 2006, he entered the University of Bergen, where he is currently a Ph.D. candidate. His scientific interests include faults and fluid flow, reservoir modeling, upgridding, and upscaling. He is currently employed as staff geologist at Weatherford Petroleum Consultants AS. Jan Tveranger was awarded his D.Sc. degree in geology from the University of Bergen, Norway in 1995. He is currently employed as research manager of geosciences at the Centre for Integrated Petroleum Research (CIPR) in Bergen. His scientific interests include sedimentology, structural geology, and reservoir modeling. Nestor Cardozo received his bachelor's degree in geology from the Universidad Nacional de Colombia and his Ph.D. in geology from Cornell University. He is an associate professor at the University of Stavanger, Department of Petroleum Engineering. His scientific interests cover faults, their related deformations, and their implementation in reservoir models. Øystein Pettersen is a principal researcher and professor in computational mathematics (reservoir engineering and rock mechanics) at the Centre for Integrated Petroleum Research (CIPR) in Bergen. His main research areas are within rock mechanics modeling and simulation, and reservoir simulation. His former career includes 18 yr with Statoil in different positions, such as advising and specialist reservoir engineer, and leader of Statoil simulation network.