We have interpreted twelve depositional sequences (3rd and 4th order) within the Middle to Upper Eocene strata. The base of our study interval marks a relative transgression within the Middle Eocene, and is overlain by composite sequences and parasequences, represented by cyclically stacked packages. Overall regression occurs up to the top of the Middle Eocene, which is overprinted by intermittent, shorter-term transgressions. The overlying Upper Eocene strata are characterised by overall shoreline retreat with final transgression over the entire Taranaki Basin in the latest Eocene/Early Oligocene. Stratigraphic cyclicity within the terrestrial to shallow marine facies is noted from paleoenvironmental interpretations determined from well log, core, biostratigraphic and coal sulphur data. Whilst eustasy may have played a role in development of the Taranaki stratigraphy, tectonically-induced subsidence was probably the over-riding factor, especially in the Upper Eocene.
Petroleum-producing reservoirs occur at several distinct horizons within the Middle to Upper Eocene succession, interpreted here to represent significant events during sequence development. Most reservoir sandstones were deposited as channelised facies following 3rd order lowstands. Sequence boundaries are interpreted at the base of many of these sandstone intervals, some of which may represent Type 1 boundaries developed as incised-valley systems.
This regional study has used an integrated approach to show how sequence stratigraphy can be applied to a non-marine to shallow marine succession to assist with exploration and reservoir development. Through our recent and multi-disciplinary work, we have been able to discriminate subtle base level fluctuations and greater marine control of depositional architecture than has previously been recognised, with implications for source rock quality, fairway exploration, reservoir charge and production effectiveness.