Organic geochemistry of the Kapuni Group coal measures using capillary gas chromatographic techniques, Taranaki Basin, New Zealand

Abstract

All known commercial hydrocarbon reserves in New Zealand are contained in the Eocene to Lower Oligocene coal measures of the Kapuni Group within the Taranaki Basin. These coal measures provide the source, reservoir and cap rocks necessary for hydrocarbon accumulation. Possible source rocks are confined to the coals and the intervening carbonaceous shales, which were deposited in the southern and eastern parts of the basin during an Eocene regressive phase. Interbedded quartzose sandstones form the reservoirs. Despite the economic importance of hydrocarbon source rocks in Taranaki, few geochemical data are available concerning the organic matter within the Kapuni Group. Data from the C15+ hydrocarbon extracts of the coal and carbonaceous shale from six onshore wells and one offshore well analysed during the present study are presented here. Analyses were carried out using a Capillary Gas Chromatograph in order to obtain information on the provenance and the thermal maturity of the organic matter present. Pristane to phytane values for the onshore wells Urenui-1, Mangahewa-1, Inglewood-1, Kapuni-1 and McKee-1 range from 3.3 to 15.1, indicating the organic matter within the Kapuni Group to be predominantly terrestrial in origin. Pristane to n-C17 values range from 0.8 to 13.3 indicating peat swamp conditions of deposition for the organic matter. In two wells, however (New Plymouth-2 and Witiora-1), there is evidence suggesting a possible marine influence within the organic matter, although this is not conclusive. CPI values range from 1.7 to 1.0 and generally show an increase with depth, indicating an increase in maturity with depth. Urenui-1, Mangahewa-1 and Inglewood-1 have CPI values within the oil window. Kapuni-1, below 12000 ft, has values which indicate the organic matter to be mature. The values for McKee-1 vary with depth. For the overthrust region CPI values decrease with depth indicating an increase in maturity. However, these values (1.7 to 1.5) would place the organic matter into the immature zone for oil generation. The in situ organic matter CPI values also decrease with depth (1.5 to 1.3), placing the organic matter between 3400 - 3500 metres in the immature zone of oil generation and between 3570 - 3620 metres within the upper zone of oil generation. Hydrocarbons were found in the well and data in this report agrees with data from Robertson Research that the organic matter within Kapuni Group sediments is sufficiently thermally mature to have generated hydrocarbons. The single CPI value obtained from New Plymouth-2 indicates the organic matter in this region is thermally mature. The results from this study agree with those of Analabs with respect to Witiora-1 and suggest that the organic matter below 3900 metres is mature, while that above is immature. The presence of both oil and gas in commercial quantities within the Kapuni Group sediments itself supports the suggestion that these have been generated from associated terrestrial organic matter.