--> --> An Under-Filled Rift Basin: Source, Reservoir, and Seal Distribution within the Canterbury Basin, New Zealand

2018 AAPG International Conference and Exhibition

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An Under-Filled Rift Basin: Source, Reservoir, and Seal Distribution within the Canterbury Basin, New Zealand

Abstract

Rift basins are among the most productive petroleum provinces in the world and display a wide range of petroleum system diversity. Under-filled rift basins occur where accommodation space exceeds sediment supply; in these situations syn- and post-rift sedimentation is strongly impacted by rift architecture for protracted lengths of time (~60 M.y). In this study we examine the Canterbury Basin, an under-filled rift basin east of New Zealand ‘s South Island, to quantify its sedimentary fill geometries and processes. The Canterbury Basin formed due to widespread extension along the eastern margin of Gondwana during the Late Cretaceous (~105-83Ma). At the onset of Gondwana break-up at ~83 Ma, the main phase of extension ceased. Topography was primarily controlled by rift faulting, with the rates of displacement on the largest faults exceeding the sedimentation rate. These structural and sedimentological relationships produced under-filled basins separated by footwall highs. We quantify the under-filling of rift basins using an index defined by the ratio of syn-rift thickness and syn-rift fault vertical displacement. Under-filled rift basins have a ratio < 1, a ratio = 1 indicates balanced-filled, and a ratio > 1 suggests an over-filled rift basin. Analysis of 239 fault-controlled basins along seismic reflection lines indicates that the ratio is size dependant with larger faults (e.g. > 1km throw) associated with under-filled basins. Onshore outcrops and core from the Clipper-1 well indicates that initial sedimentary infill was dominated by localised coarse-grained clastics deposited in alluvial fan to fluvial settings potentially flowing into lakes in more distal settings. Continental sediments were followed by more regionally extensive transgressive marine sediments towards the end of rifting in the Late Cretaceous (~83 Ma) until the Paleocene (~34 Ma). The transgressive sediments are varied and include shoreface, deltaic and shelf sandstones, marine mudstones, with less abundant limestone and deep-water clastic fans. Continuity of the regional transgressive seal was likely impacted by paleo-highs that remained emergent until the Paleocene. The rift-drift sedimentary succession offers considerable potential for source, reservoir and seal rocks in sub-basins, however, due to the under-filling of the basin these sedimentary units are likely to be discontinuous and their distribution controlled by rift structures, which may locally impact basin prospectivity.