Lithofacies, Depositional Setting, and Reservoir Characteristics of the Farewell Formation, Kupe South Field, Taranaki Basin, New Zealand

摘要

The Kupe Field reservoir interval is the Paleocene Farewell Formation which is up to 1200 m thick. In this study, we re-evaluate the sedimentology of these rocks based on core description, to further refine the interpretation of their depositional setting and assess the reservoir characteristics of these rocks. Over 285 m of core were examined in four of the six exploration wells in the field ― Kupe South-1, -2, -4 and -5. We recognise three relatively coarse-grained lithofacies ― pebbly conglomerate, pebbly sandstone, and very fine to very coarse sandstone, and four fine-grained lithofacies ― mudstone, mudstone with silty-sandy lenses, siltstone-sandstone, and organic-rich lithofacies. These lithofacies occur in fining-upward cycles in each of the cores. We interpret the coarser-grained lithofacies as mixed sandy and gravel bed, braided stream deposits. Parts of Kupe South-5 are interpreted as deposits of meandering streams. Fine-grained lithofacies mostly represent a variety of terrestrial overbank floodplain, paleosol and lacustrine depositional settings, but some containing dinoflagellates, represent marginal marine environments. Our study indicates several encouraging reservoir attributes in the Farewell Formation. The sandstones are moderately to well sorted, with little interstitial clay or silt, and have good porosity and permeability. Permeability is typically in the range 1-1000 mD. Maximum permeability occurs in the very fine-very coarse sandstone lithofacies (maximum 1800 mD, average 800 mD), with greatest variability in the pebbly conglomerate lithofacies (maximum 330 mD, average 26 mD). Although bases of depositional cycles may be erosional and marked by pebbles, they tend not to be mud-lined, and therefore should not significantly impair communication and connectivity within the reservoir. The stacked nature of sand and conglomerate lithofacies within the reservoir favours a high net:gross ratio. Fine-grained lithofacies developed at the tops of depositional cycles are likely to relate to abandonment surfaces in channels or on floodplains, and individual units are probably of restricted lateral extent.