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Deep Lacustrine, Sand-Rich, Turbidite Fans: Elevated Heterogeneity and Evidence for Partial Confinement in Examples From the North Falkland Basin, Falkland Islands


As a whole, ancient lacustrine turbidites are rarely documented and only ever briefly described in the literature. In recent years, these sedimentary deposits have been proven to form extensive, good-quality hydrocarbon reservoirs. The Sea Lion Fan Complex, a series of early Cretaceous, deep-lacustrine turbidites, in the North Falkland Basin provides an excellent example of such a system. They form compensationally-stacked, sheet-like, tabular expressions in the seismic data. When RMS amplitudes, derived from 3D seismic data, are examined in plan-view across each individual tabular body, intricate channel-like architectures, fanning outwards geometries and abrupt changes in flow direction are observed. These seismically-based architectures are interpreted to represent geologically-associated heterogeneities, principally controlled by the distribution of sandstone and sand-prone facies. 455 m of conventional core data has been used to test and confirm the seismic-based interpretation of these depositional architectures. Here, we suggest that the combination of intricate channel-like features, fanning outwards geometries and abrupt changes in flow direction represents evidence for partial-confinement. The term partial confinement should be applied when examining a fan whose broad depositional boundaries are not confined by pre-existing palaeo-bathymetry. However, at the intra-fan scale, evidence of flow baffling and sinuosity provides an indication that at least some confinement of internal flow geometries has occurred. The major implication of identifying partial confinement within a lacustrine fan is the likelihood of elevated internal heterogeneities and complexities. Using these observations and evidence for elevated lateral facies variability, we present a model for reservoir sandstone distribution that suggests a previously under-appreciated degree of heterogeneity within deep lacustrine, slope apron, turbidite fans. This is in contrast with conventional marine turbidite systems, where internal facies variability might be typically less-pronounced. Modelling these heterogeneities, particularly within a 3D environment and at a range of scales, is crucial to our understanding of these systems. Furthermore, appreciating their highly heterogeneous nature is vital, considering the recent rise of deep lacustrine turbidites as hydrocarbon reservoir lithologies both in the North Falkland Basin and globally.