Nicola Marsh, StatoilHydro (Norway)
Jonathan Imber, University of Durham (United Kingdom)
Robert Holdsworth, University of Durham (United Kingdom)
Paul Brockbank, StatoilHydro (United Kingdom)
The structural evolution of the Halten Terrace has implications for advancing production and exploration in this economically important hydrocarbon province. Disagreement exists concerning the spatial and temporal evolution of the Halten Terrace, with different authors emphasising the role of two end-member processes: normal faulting with basement fault reactivation; and salt tectonics and fault-propagation folding. We present a new structural interpretation of the Halten Terrace based on interpretations of high quality 3D seismic data, regional 2D lines and well data. We use these data to establish a new, regionally consistent structural framework from the Permo-Triassic to the Early Cretaceous, and propose a new model for the structural evolution of the area.
Permo-Triassic basement rocks on the Halten Terrace are overlain by a thick sequence of Triassic evaporites, separating basement and cover stratigraphy. The presence of evaporites strongly influences the structural evolution, resulting in varying degrees of linkage between basement and cover deformation. We demonstrate that basement faults were not active during the interval immediately prior to evaporite deposition, as previously proposed in the literature, but instead Permo-Triassic basement faults were initiated during the Earliest Jurassic as recorded by the formation of broad monoclinal flexures in the cover. Normal fault activity in the cover also initiated during the Early Jurassic (Hettangian) interval, which is earlier than previously recognised, and continued into the Early Cretaceous when new faults formed oblique to existing structures.
Despite the synchronous onset of extension in basement and cover stratigraphy, we have mapped distinctly different fault patterns above and below the evaporite interval, highlighting the role of evaporites in decoupling strain in brittle-ductile systems during extension. We demonstrate that two mechanisms - crustal extension and gravity-sliding - control the evolution of post-evaporite cover faults on the Halten Terrace resulting in the development of both thick-skinned, basement-rooted faults and thin-skinned faults that detach on mechanically weak evaporites. We describe the distribution and geometry of both structural styles and suggest that three factors defining each structural style, (i) the degree of coupling between basement and cover faults, (ii) the dominant structural orientation, (iii) and the duration of extension, can be used to distinguish faults that form due to crustal extension from those that are gravity-driven.
Our results have implications for constraining the distribution of cover faults that offset important reservoir intervals. We can use the spatial and temporal evolution of faults and folds to predict the magnitude and distribution of structures in prospective hydrocarbon provinces, which can additionally be used to assess the degree of reservoir compartmentalisation in new and existing prospects on the Halten Terrace.