Reservoir characterisation of syn-rift and post-rift sandstones in frontier basins: An example from the Cretaceous of Canterbury and Great South basins, New Zealand

Abstract A paucity of well data in frontier basins presents serious challenges for reservoir characterisation studies around the world. In this paper, we provide the first regional study addressing reservoir distribution, composition and quality in the frontier Canterbury Basin (CB) and Great South Basin (GSB) of New Zealand, and show how the results can be used to help reduce uncertainties associated with reservoir presence and effectiveness. Cretaceous strata in Canterbury and Great South basins represent two stages of basin history. The oldest Cenomanian–Coniacian (mid-Cretaceous) strata are syn-rift deposits that developed in response to the breakup of the eastern margin of Gondwana, while overlying Santonian–Maastrichtian (late Cretaceous) strata are post-rift deposits that signify the start of rift–drift transgression. These Cretaceous deposits are characterised by a change in facies from predominantly alluvial–fluvial to marginal–shallow marine that is recognisable in outcrop and wells, and from seismic response. Based on our petrographic examination of 99 Cretaceous samples from well and outcrop locations, we conclude that there are significant differences in sandstone texture and mineralogy that are related to differences in depositional facies, basin stratigraphy, geographic location, and degree of diagenetic alteration. Sandstones from the earliest part of the syn-rift succession largely reflect the composition of the underlying basement geology; typically granitoid or quartzose Western Province rocks in GSB, and lithic-rich Eastern Province metasedimentary rocks in CB. Younger strata, representing part of the post-rift succession, are generally better sorted, with detrital compositions reflecting sediment mixing of several basement source areas. Based on available data, reservoir quality is directly related to sandstone texture and detrital composition (factors of provenance and facies), compaction (a factor of burial depth/temperature), and degree or type of diagenetic alteration. Best quality sandstones are predicted to occur in the late Cretaceous post-rift succession of GSB and locally in CB, either where sediment is derived from local granitoids, or where high-energy depositional processes have resulted in quartz-enriched compositions.