Geochemical Biomarker Studies of Oils and Potential Source Rocks of the Taranaki Basin, New Zealand

Abstract

The use of biomarkers for study of oil generation and source rocks has become widespread in recent years. Sterane and triterpane biomarkers are now commonly used for maturity assessment of both oils and source rocks. In addition to maturity assessment biomarkers have also been used to assess the extent of oil-source rock correlation, migration and biodegradation of the oils. This study characterises oils and possible source rocks from the Taranaki Basin, New Zealand, and aims to identify the source rocks of the oils in the southern part of the basin. Collier (1985) and Maidment (1985) carried out preliminary biomarker geochemical studies of oil generation in the southern and onshore northern parts of the Taranaki Basin, respectively. Further, more detailed work was deemed to be necessary and is the basis of this study. Also a study by Czochanska et al (1988) and Weston et al., (1989) showed that the oils of the Taranaki Basin to have similar maturity levels and that there was significant terrestrial input to their source rocks. Triterpane, sterane and diterpane biomarkers are studied in extracts from selected coals and shales from the Maui-1, Maui-4, Kapuni-2, Kapuni-8, Toko-1 and Kapuni Deep-1 wells and the Maui-4 oil, Maui-1 oil, Maui and Kapuni condensates. Little difference was apparent between the source characteristics of most coal and shale pairs. Hopane and sterane biomarkers were used to assess maturity levels, while other triterpanes and diterpanes were used predominantly for source characterisation. In addition to the saturated biomarkers, methylphenanthrenes were also used to assess maturity. However the methylphenanthrene indices (MPI) did not show a definitive maturity trend in many of the samples and thus, overall were not very useful for maturity assessment in sediments from the Taranaki Basin. The results presented in this thesis show that prospective source rocks from the Kapuni Group are generally of terrestrial origin, with a greater marine influence (higher C27 sterane abundances) in the Upper Kapuni Group, consistent with the marine transgression which occurred during the Late Oligocene (King and Robinson, 1988). Most of the oil reservoirs in the Taranaki Basin are found in Upper Kapuni Group sandstones. However as this marine influence is not evident in the oils, it indicates that the source of the oils is not in fact the Upper Kapuni Group sediments. Maturity assessment of sediments from the Maui-4, Toko-1 and Kapuni Deep-1 wells indicates that the source rocks for the oils and condensates are deep Kapuni or Pakawau Group sediments. The maturity levels required for oil generation are reached between 4 and 5.5 km dependant on the age and post-Miocene uplift experienced by the sediments. When the extent of uplift which has occurred in the area studied is taken into account the source rocks must have been buried to depths of 5 to 6 km to reach a maturity consistent with generation of oil in the Taranaki Basin. The extent of oil-source rock correlations for diterpane biomarkers in the Maui-4 well indicates that the Pakawau Group sediments are the likely source rocks for the Maui-4 oil. This suggests that the source rocks are probably located slightly deeper than those encountered in the Maui-4 well but are otherwise very similar in character. The Kapuni Deep-1 sediments of cycle A, Kapuni Group, have similar diterpane distributions to that of the Kapuni condensate. Even though the triterpane distributions do not show the same degree of correlation, the correlation is sufficient to indicate that the source rock of the Kapuni condensate is likely to be an extension of the Kapuni Group (cycle A) sediments, but about 500 m deeper (as indicated by maturity measurements). The conclusion of a deep source for oil in the Taranaki Basin suggests there may be some as yet unrecognised deeper reservoirs which could be exploration targets in the future. The present reservoirs in the Upper Kapuni Group sandstones, and their distance from prospective source rocks indicates that migration of hydrocarbons is important for the accumulation of commercial quantities of hydrocarbons in the Taranaki Basin. Thus the location of possible migration pathways should also be considered when evaluating possible exploration targets.