The Geology and Geochemistry of the Crude Oils and Source Rocks of Western New Zealand

Abstract

Many thousands of geochemical analyses have been made during the course of oil exploration in New Zealand. These, together with others made especially for this project by divisions of DSIR have been used to study the problems of oil generation in the Taranaki and West Coast regions of western New Zealand. Source rock analyses have been statistically evaluated for both internal and interlaboratory consistency. For marine rocks of uniform lithology internal consistencies are good as is the comparability between laboratories. However in variable nonmarine sequences interlaboratory analyses correlate poorly with each other due to the deficiencies in the methods of collecting wellsite cuttings samples and to the nonrepresentative nature of the small volume of samples analysed. The large number of analyses available, however, allows useful conclusions to be made for these sequences. Consistent differences in vitrinite reflectance values between laboratories for nonmarine rocks are probably due to measurements of perhydrous coal submacerals being included. Techniques used to study oil generation include Total Organic Carbon, pyrolysis, vitrinite reflectance of source rocks and oil analyses such as quantitative separation, and gas chromatography-mass spectroscopy. Source rock data indicate the marine beds and nonmarine shales have little generative potential and are generally gas prone. The oil sources are indicated as perhydrous coals deposited in a low bacterial anoxic swamp environment. This is borne out by biomarkers within the oils. It is likely that the perhydrous coal submacerals which contribute to oil generation are primarily desmocollinite and vitrodenrite. This is in direct contrast to Australia where oil generation from coals is attributed to their exinite content. Limited biomarker studies on coal extracts have failed to link a specific source rock with the oils even through general biomarker source characteristics are similar for oil and extract. A second conclusion, based on pyrolysis data is that oil generation occurs at maturation levels equivalent to vitrinite reflectance (Ro) of 0.65% but coal studies suggest that migration out of the coal does not begin until Ro = 0.9%. This compares to the values generally accepted by the exploration industry that generation begins at Ro = 0.6% for Type III kerogen with the implication that migration occurs shortly thereafter. Biomarker maturation studies of the oils of the region show they have all reached a similar maximum maturation level of Ro = 0.95 to 1.0%. A map of Taranaki Basin indicates the area where basal sediments have reached mature level (Ro = 0.9% or approximately 5.5-6 km deep) is limited to the central and northern graben. However generation of maps showing the distribution of perhydrous coals at these mature levels is not possible with the present knowledge although shallower perhydrous coals do have a similar distribution to the known oil accumulations.