GEOLOGY AND PETROLEUM PROSPECTS OF RUATORIA AREA, EAST COAST, NORTH ISLAND, NEW ZEALAND.

Abstract

As measured in outcrop sections, more than 30,000 ft. (9,000 m.) of Upper Cretaceous and Tertiary marine sediments overlie basement of Lower Cretaceous age in the area. Mudstones, siltstones, and turbidites make up the majority of this sedimentary section but there are also 3,000 ft. (900 m.) of interbedded greywacke sandstones and siltstones of Maestrichtian age, and 400 ft. (120 m.) of interbedded limestone and greensands of Oligocene age. Both of these latter units are potential reservoir beds.

The majority of the exploration work has been by surface geological mapping. A number of time-rock units have been used which were initially distinguished by fossils, but which could later be distinguished by lithological differences.

Four regional unconformities, base of Upper Cretaceous, near top of Upper Cretaceous, Upper Oligocene, and Upper Miocene have been recognised in the stratigraphic section.

A slump breccia of Upper Eocene age has also been mapped extending over an area 25 mi. (40Km) by 5 mi. (8Km) with a maximum thickness of 7500 ft. (2.25Km.) and contains large lumps of older rocks. The sediments of the Ruatoria area are folded into broad synclines and tight anticlines with average dips ranging from 50 degrees in the Upper Cretaceous to 15 degrees in the Upper Miocene. The trend of the folds is northeast in the southern part of the area and northwest in the northern part, the dividing line being the major northwest trending Hikurangi Fault.

The area abounds in gas seepages some of which have been capped and exploited. There are records of oil seepages also. Within the 453 sq.mi. (1,178 sq. Km) only nine shallow holes have been drilled all of which recorded some oil or gas shows.

Alliance Petroleum has located two wells on the flanks of closed structures outlined by surface geology and shallow structure drilling.