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Macroscopic to Microscopic Criteria for Assessing Spatial Changes in Material and Bounding Surface Character of Mass-Transport Deposits and Implications for Seal Quality

Abstract

Mass-transport deposits (MTDs) play a critical role in petroleum systems, acting as reservoirs, seals, and trap elements. A huge variety of character exists worldwide in MTDs due to the wide number of settings and sediment sources that host MTDs. Previous work has focused on the material nature of MTDs as an indicator of reservoir and/or seal quality, but less emphasis has been placed on deformation of the surrounding material or the nature of surfaces/contacts between the MTD and surrounding material. This research is focused on the nature of these surfaces and the deformation associated with MTD emplacement. To assess this problem, we examined an MTD outcrop of the Miocene Mount Messenger Formation in the Taranaki basin, New Zealand. This MTD overlies submarine fan deposits, and outcrops semi-obliquely to the basin paleoflow direction and is continuously exposed for nearly 4 km along coastal cliffs ~10-15 m height. The MTD is interpreted as a detached-MTD sourced from toe-of-slope deposits. In order to characterize the MTD's spatial facies heterogeneity, we employ outcrop facies description, as well as thin-sections, FE-SEM images, and XRD and spectroscopy from samples collected in the outcrop. Methods developed by Burgess (2016) were employed to quantify the degree of strata order by comparing vertical facies successions with a random model via Monte Carlo. Results show that the nature of the shear basal contact (sbc) of the MTD is heterogeneous and varies spatially from non- to weakly-erosive (~10-20 cm deformation zone around the sbc) near the proximal domain to highly-erosive (~100-200 cm deformation zone around the sbc) in the distal domain. This transient erosive behavior is the result of changes in boundary shear stress during MTD emplacement; likely influenced by confinement along its pathway, entrainment of ambient water, and interaction with differing underlying lithologies. The degree of erosion by the MTD into the underlying deposits is important because as the MTD entrains sand and silt, its seal capacity is compromised. Unlike the basal incision, the degree of disorder within the MTD varies non-systematically —the proximal and distal domains show highly disordered strata, separated by a translational domain showing partly ordered strata. Microscopic techniques are currently being employed on samples from the outcrop to aid in recognition of such spatial changes in character on conventional industry data (e.g., core and well logs).