An outcrop‐based study of the economically significant Late Cretaceous Rakopi Formation, northwest Nelson, Taranaki Basin, New Zealand

Abstract

The Late Cretaceous Rakopi Formation (Pakawau Group) represents one of the most important petroleum source rock units and a potential reservoir unit in the highly prospective Taranaki Basin. This paper presents a predominantly outcrop‐based study of the sedimentology, petrography, stratigraphy, and depositional environment of the Rakopi Formation in the Paturau River and Pakawau areas of northwest Nelson, southern Taranaki Basin, together with some preliminary insights into the stratigraphie architecture of the Pakawau Group on a more basin‐wide scale. The Rakopi Formation is interpreted here as a terrestrial deposit, representing sedimentation in fluvial channels and their associated overbank and levee environments. However, the presence of dinoflagellates, glauconite, and elevated coal seam sulfur contents is evidence for periodic marine influence during deposition. This could be explained by a low‐gradient coastal plain paleogeography, crossed by a series of rivers and their associated floodplain deposits, episodically inundated by marine incursions during successive transgressions. A modern analogue setting from the present‐day Hauraki Graben, North Island, New Zealand, indicates that marine influence within coastal plain systems can extend several tens of kilometres inland. Given such a physiography, relatively small increases in relative sea level could potentially move the shoreline several kilometres or tens of kilometres farther inland, sufficient to introduce the type of marine influence on sedimentation that we suggest for the Rakopi Formation. The results from this study suggest a greater marine influence within the Rakopi Formation, northward into the greater Taranaki Basin, than has previously been recognised. This raises the possibility of both different reservoir facies as well as potentially a greater proportion of marine mudstones, which would have implications for both reservoir and trapping of hydrocarbons. In addition, marine‐influenced coaly rocks within the Rakopi Formation are expected to have greater petroleum generative potentials and to be more oil‐prone than their fully non‐marine counterparts.