ALBERT

Description: Analysis of three-dimensional seismic data from the lower Congo Basin, offshore Angola, reveals numerous fluid-flow features in the Miocene to Holocene succession and the potential for large, shielded traps underneath basinward overhanging salt structures. The fluid-flow evidence includes present-day sea floor pockmarks clustered above salt structures, Pliocene–Pleistocene stacked paleopockmarks and Miocene pockmark fields. Other fluid-flow features include high-amplitude cylindrical pipe structures 60 to 300 m (197–984 ft) wide and 25 to 300 m (82–984 ft) high within lower and middle Miocene strata, thick (〈150 m [492 ft]) high-reflectivity zones within the Pliocene succession associated with bottom-simulating reflections, and subvertical low-amplitude chimneys originating from the deeper section (〉1 km [0.6 mi] beneath the sea floor). The Miocene pockmark fields occur at a specific horizon, suggesting a regional fluid expulsion event at ca. 12 Ma, and the Miocene fluid-flow regime is interpreted to be dominated by thermogenic fluids supplied via carrier beds and leaking vertically above structural highs. The Pliocene–Pleistocene fluid-flow regime was dominated by short-distance vertical fluid migration and expulsion related to early stage diagenetic processes involving biogenic methane and pore water. The present-day fluid-flow regime is inferred to be dominated by thermogenic fluids primarily controlled by kilometer-scale salt-flank-controlled migration. The study emphasizes the use of seismically imaged fluid-flow features in hydrocarbon systems analysis by documenting the evolution of an overburden plumbing system through time, involving several fluid types and flow regimes, depending on the spatiotemporal availability of thermogenic and diagenetic fluids and the tectonostratigraphic occurrence of aquifers, traps, and seals. Katrine Juul Andresen has B.Sc. (2004) and M.Sc. (2007) degrees in geology (Aarhus University) and is currently finishing her Ph.D. studies at Aarhus University, dealing with the 3-D seismic expression of fluid-flow features that originated in hydrocarbon plumbing systems in the North Sea and the Angola Basin, including a description of elongated and stacked paleopockmarks and sand and chalk remobilization. Mads Huuse is a reader in geophysics at the University of Manchester. After a Ph.D. from Aarhus University (1999), he undertook postdoctoral research at Aarhus, Aberdeen, and Cardiff, focusing on basin analysis, sediment remobilization, and fluid flow, before taking up a lectureship at the University of Aberdeen. His interests include the seismic interpretation of basins, their evolution, structures, depositional elements, reservoirs, seals, and fluids. Niels Schødt is a team lead for Angola Regional Studies and New Ventures at Maersk Oil in Copenhagen. He has an M.Sc. degree in geology (Aarhus University) and has worked in the oil industry since 1989. He started at Schlumberger as a wireline engineer and since 1996 has worked in exploration projects in Maersk Oil, primarily focusing on seismic interpretation. Lene F. Clausen is head of the New Ventures Study Teams in Exploration, Maersk Oil, Copenhagen. She has worked with exploration since 1998 mainly in the North and South Atlantic. She received a graduate diploma in geology in 1991 (Australian National University, Canberra), M.Sc. degree in 1993, and a Ph.D. in 1997 (University of Copenhagen). Lars Seidler is a project manager for the Angolan Chissonga Development at Maersk Oil Houston. Seidler has worked in upstream research, development, and production, starting with ExxonMobil in 2000. Since 2006, he has worked in exploration and development with Maersk Oil. He received a Ph.D. in geology in 1999 (University of Copenhagen).

Description: Because of their potentially large volumes and excellent reservoir properties, winglike clastic intrusion complexes may represent stand-alone exploration targets. However, determining the three-dimensional (3-D) geometry of such complexes is problematic because of limited exposure in the field and insufficient seismic resolution and well coverage in the subsurface. In this study, high-quality 3-D seismic reflection data from offshore Norway are used to determine the 3-D geometry of winglike intrusion complexes adjacent to a deep-water slope-channel complex. Intrusions form sheets or “wings” that are developed almost continuously along both margins of the 15-km (9-mi)-long channel complex. Intrusions dip up to 20° (more commonly 〈10°), crosscut up to 90 m (30 ft) (undecompacted) of the overlying stratigraphy and extend up to 1095 m (3593 ft) away from the channel complex. Three styles of intrusion are observed: type 1: dikes lacking sills at their upper tips; type 2: dikes that pass upward into sills at their upper tips; and type 3: “stepped sills” that consist of linked sill and dike segments. These different styles of intrusions pass laterally into one another, leading to extreme complexity both between and along-strike individual intrusion complexes. Although the mechanisms driving initial overpressure development and injection itself are unclear, this study indicates that 3-D seismic data are a powerful tool in understanding the 3-D geometry of winglike clastic injection complexes and suggests that the true geometry of these features is more complex than previously documented. This study also has implications for potential reservoir geometries and hydrocarbon exploitation of winglike clastic intrusion complexes. second revised manuscript received June 14, 2010 Christopher Jackson is currently the Statoil senior lecturer in basin analysis at Imperial College. He obtained a B.Sc. degree and Ph.D. from the University of Manchester. During 2002–2004, he held a position as a geologist in the research center at Statoil. His current research interests are in the tectonostratigraphic evolution of rifts and the application of 3-D seismic reflection data to a range of structural and stratigraphic issues. Mads Huuse received his Ph.D. from the University of Aarhus (1999). He was then a researcher at the Universities of Aarhus, Cardiff, and Aberdeen (1999–2005), where he also held a lectureship (2005–2009). He is currently a reader at the University of Manchester. His research interests are in the seismic interpretation of fluid-flow phenomena, sediment remobilization, glaciogenic deposits, and cool-water carbonates. Gillian Barber is currently an exploration geologist at BP. In 2007, she received an M.Sci. degree in earth sciences from Imperial College. Her research interests are in the seismic interpretation of deep-water depositional systems and large-scale clastic intrusions.

Description: We present an interpretational framework for the analysis of a diverse set of geological structures that breach sealing sequences and allow fluids to flow vertically or subvertically across the seal. In so doing, they act as seal bypass systems (SBS). We define SBS as seismically resolvable geological features embedded within sealing sequences that promote cross-stratal fluid migration and allow fluids to bypass the pore network. If such bypass systems exist within a given seal sequence, then predictions of sealing capacity based exclusively on the flow properties (capillary entry pressure and hydraulic conductivity) of the bulk rock can potentially be negated by the capacity of the bypass system to breach the grain and pore network. We present a range of examples of SBS affecting contrasting types of sealing sequences using three-dimensional (3-D) seismic data. These examples show direct evidence of highly focused vertical or subvertical fluid flow from subsurface reservoirs up through the seal sequence, with leakage internally at higher levels or to the surface as seeps. We classify SBS into three main groups based on seismic interpretational criteria: (1) fault related, (2) intrusion related, and (3) pipe related. We show how each group exhibits different modes of behavior with different scaling relationships between flux and dimensions and different short- and long-term impacts on seal behavior. Joe Cartwright received his B.A. degree and his D.Phil. in geology from Oxford University. He is a research professor of geophysics and director of the 3DLab at Cardiff University. His research interests focus on three-dimensional seismic interpretation in basin analysis, with special emphasis on seal integrity analysis, the genesis of polygonal faults, the emplacement of sandstone and igneous intrusions, and silica diagenesis. Mads Huuse received his Ph.D. from the University of Aarhus in 1999. In 1999–2005, he was a postdoctoral researcher, first at the University of Aarhus, then at the University of Aberdeen, working on injected sands, and finally, at Cardiff University, working on cap rocks. Currently, he is lecturer of geophysics at the University of Aberdeen. His special interest is in the seismic imaging of fluid-flow features, glacial deposits, and cool-water carbonates. Andrew Aplin received his Ph.D. from Imperial College in 1983. He was a Royal Society European Fellow at Centre de Recherches Pétrographiques et Géochimiques Nancy from 1983 to 1984 and worked at BP as a research geochemist from 1984 to 1990. Since 1990, he has worked at the University of Newcastle, where he is currently professor of petroleum geoscience. His main research interests are in the physical and fluid-flow properties of fine-grained sediments.