Abstract The infill history of a salt withdrawal minibasin in the contractional domain of the gravity-driven salt system on the Angolan passive margin has been reconstructed using a high resolution three-dimensional seismic data-set. Well-constrained biostratigraphy has allowed calculation of the growth rate of the basin-bounding structures. Within the interval of stratigraphy investigated, the depositional style of sediments preserved in the basin has changed in response to changes in the rate of growth of the coeval, adjacent salt structures. During the early part of the basin history, sedimentation in the slope system was dominated by a series of erosional channel complex systems, which are 1-3 km wide and contain a preserved infill 100-200 m thick. The creation of sea floor topography by contemporaneous salt movement and the development of salt-cored anticlines caused the channels to be deflected, diverted, off-set, or deeply incised as they interacted with the developing slope topography. Subsequent salt movement, and a concomitant increase in the growth rate of the basin-bounding anticlines, led to more elevated topography and the development of extensive slumping of sediment into the basin centers, forming large mass transport deposits. As salt movement continued, the basin became largely enclosed; bound by a well-developed salt wall to the west and a series of complex salt structures to the east, where salt-cored anticlines developed laterally into salt diapirs. These complex structures controlled the flow pathways of sediment both into and out of the basin. The growth rate of the structures constraining the western margin of the basin slowed at this time, and as a result, sediment transported from the east by feeder channels formed ponded fan systems comprised of sheets of sand each formed by multiple small channels. Understanding the impact that active growth structures can have on sediment distribution and facies development is invaluable in the exploration and production of oil and gas. In this area, the association of the channels with the structure has formed reservoir-trap combinations for four major oil fields, Plutao, Saturno, Marte and Venus, being combined together as the PSVM development. In each field, the specific interaction of the channels with the growing topography has controlled the channel architecture and facies development; this has meant that different development plans, risks and uncertainties are required for each field.

Abstract The Colombian Andes are characterized by a dominant NE structural trend, which is offset by ENE-trending right-lateral and NW-trending left-lateral structures. NE-trending faults are either dip-slip or oblique thrusts, generated as a result of a trans-pressive regime active since at least Palaeogene times. NW-trending faults are considered to be reactivated pre-Cretaceous extensional structures. Right-lateral shear on ENE-trending faults has resulted from oblique convergence between the Nazca Plate and the Northern Andes. Major changes in the geometry of the oblique-plate convergence between the Nazca and South American plates have generated the northward ‘escape’ of the Northern Andes and stress-strain partitioning within the mountain belt. These strike-slip structures have exerted important controls on sedimentation, source-rock distribution, fluid flow and ore mineralization during Cenozoic times. The interpretation of the Northern Andes as a mountain belt affected by strike-slip deformation provides a structural context in which to reassess the exploration plays.

Abstract The numerous sandstone injections found associated with the Gryphon field in the United Kingdom North Sea are mostly small-scale intrusions less than 30 cm (12 in.) thick. The largest intrusions identified in core and wire-line-log data from the Gryphon field are approximately 8 m (26 ft) thick, but these large features are uncommon. The intrusions form two main populations of interconnected steeply dipping dikes (≥60°) and sills (≤20°), with a lesser number of intrusions with moderate dips. Although small, centimeter-scale injections dominate the intrusion population, these small intrusions cluster around thicker dikes and sills (>20-30 cm [>8–11.8 in.] thick) that are localized at the margins and above the field. Sandstone injections are found as much as 170 m (557 ft) vertically above the main Gryphon reservoir sandstone and several hundred meters laterally from the parent sandstone body. Greater numbers of dikes exist in the first approximately 80 m (262 ft) above a top reservoir datum, and at higher levels, sills are more numerous. A well-by-well analysis of the intrusion distributions shows that they cluster at different heights above the top reservoir; injections are not equally spaced. Examination of the total cumulative thickness of intrusions measured in the recovered core and intrusion thickness interpreted from wire-line logs beyond the extent of the core suggests that there may be twice the volume of injected sand on the field flanks, margins, and off-field positions than over the center of the main reservoir sandstone. Integration of the observations from core and wire-line logs allows a new model for the sandstone injection complexes on Gryphon to be developed. This model suggests that a complex hierarchy of intrusion scale exists, with thin intrusions branching off the main intrusive network. The dip distribution of the injection population is influenced by the depth relative to the main reservoir sandstone, and the spatial distribution of the intrusion shows that this network is best developed around the margins of the field. Correlation of core and wire-line-log interpretations with seismic data indicates that a seismically identifiable discordant facies is most likely composed of localized interconnected networks of sandstone dikes and sills (an injection complex) in connection with the main reservoir and is not necessarily a single, simple intrusion made up of 100% intruded sand.

Abstract Several productive Paleogene deepwater sandstone reservoirs in the North Sea show evidence of having undergone post-depositional remobilization and clastic injection, which can result in major disruption of the primary reservoir distribution ( e.g ., Alba, Forth/Harding, Balder, and Gryphon fields). Case studies of deepwater sandstones from UK Quadrants 9, 15, 16 and 21 are presented to illustrate the wide spectrum of remobilization features, which range from centimeters ( e.g ., core-scale) to hundreds of meters ( e.g ., seismic-scale). Most common are clastic injection structures such as dikes and sills. Sills of massive sand, over 20 m thick, have been identified. Intrusions associated with the propagation of syn- to early post-depositional, dewatering-related polygonal fault systems in adjacent deepwater mudrocks are also common. The scale of the clastic intrusion and remobilization has significant impact on reservoir architecture and production performance, including changes in (a) original depositional geometries; (b) reservoir properties; (c) connectivity, (d) top reservoir surface structure, (e) reservoir volumetrics, and (f) recovery/performance predictions. There are several prerequisites for sandstone intrusions to form: the source sediment must be uncemented, and the ‘parent’ sand body must be sealed such that an overpressure with a steep hydraulic gradient can be generated. The seal on the overpressured sand body must then be breached for the sand to fluidize and inject. The stress state within the basin, burial depth, fluid pressure and the nature of the sedimentary host rock all contribute to the final style, geometry and scale of intrusion. At shallow depths, within a few meters of the surface, small irregular intrusions are generated, more commonly forming sills, whereas at greater depth larger and more continuous dikes and sills form clastic intrusion networks. Field examples from the Ordovician in Ireland, and Panoche Hills in California are used to illustrate the control of burial depth/stress on intrusion scale. Earthquake induced liquefaction, tectonics stresses and build-up of excess in-situ pore pressure are the most commonly cited explanations for the occurrence of clastic intrusions. However, our work suggests that the large-scale, ‘catastrophic’ sandstone intrusions within the North Sea Paleogene, which remobilized hundreds of cubic meters of sediment, probably require the presence of fluids migrating from deeper within the basin ( e.g ., gas charge) to drive the injection. Deepwater sand bodies within the North Sea that appear most susceptible to remobilization occur in mud-dominated successions and include (1) narrow, elongate channel or gully-filled sands ( i.e ., non-leveed channel systems), and (2) isolated sand-rich mounds ( e.g ., ‘ponded’ sand bodies and terminal fan lobes). Sand bodies located above rift-related basin-forming faults, which periodically appear to have acted as vertical fluid escape pathways, were especially susceptible to remobilization. Sand remobilization may influence reservoir distribution in other mud-dominated, deepwater depositional systems.