Org. Geochem. Vol. 12, No. 2, pp. 123-135, 1988 0146-6380/88 $3.00+0.00 Printed in Great Britain. All rights reserved Copyright © 1988Pergamon Press pie Geochemical application of sterane and triterpane biomarkers to a description of oils from the Taranaki Basin in New Zealand Z. CZOCHANSKA 1, T. D. GILBERT2, R. P. PHILP2., C. M. SHEPPARD I, R. J. WESTON I, T. A. WOOD l and A. D. WOOLHOUSEIt IChemistry Division,DSIR, PrivateBag, Petone,New Zealand 2CSIRO Divisionof FossilFuels,North Rydc, NSW 2113, Australia (Received 22 July 1987; accepted 27 October 1987) Abstract--The distributions of sterane and terpane biomarkers in a suite of 25 oils from onshore and offshore regions of the Taranaki Basin of New Zealand have been determined by gc-ms. Features of these distributions include the presence of relatively high concentrations of a number of Cu tetracyclic terpanes, the virtual absence of tricyclic terpanes, a predominance of C29 normal-, iso- and dia- steranes and the presence of a number of non-hopanoid triterpanes. In one member of the suite (Urenui) 18,y(H)-oleanane is the most abundant component (m/z 191 ion chromatogram) of the terpane class of biomarkers. Based upon the distributions of 18r,(H)oleanane, the McKee and Moturoa families of oils can be delineated. These terpane distributions are qualitatively similar to those encountered in crude oils from a number of Far-eastern Basins and from the Handil field of the Mahakam delta (Indonesia). The Taranaki oils are of approximately the same thermal maturity. Maturity parameters calculated from the distributions of C29 steranes and of 17~(H),21• (H)- and 17~(H),21/~(H)- hopanes confirm recent findings of incomplete nuclear isomerisation in oils derived from Tertiary sediments. An attempt has been made to correlate the McKee-1 and Maui-4 crude oils with potential source rocks from depths of 3708 m and 3825 m respectively. The rocks studied have attained insufficient maturity to have been the sources of these oils. Key words: biological markers, oils, terpanes, steranes, New Zealand, GC-MS, 18~(H)-oleanane INTRODUCTION Since the discovery of crude oil in shallow wells drilled in the Moturoa field near New Plymouth there has been considerable on- and off-shore exploration activity in the Taranaki region of New Zealand (Fig. 1). Recent activity has located the Kapuni field (1959), the large Maui field (1969), and the McKee field (1980), all of which produce gas in commercial quantities, condensate and oil. In addition, several accumulations of petroleum have been discovered at Urenui, Mangahewa, Stratford, Moki, Kaimiro, Wa- ihapa and Tariki (Fig. 1). Compositional properties, such as the high n-alkane and low sulphur contents, of oils from this region of New Zealand are, in the majority of cases, a reflection of the high input of terrestrial organic matter into their source rocks. In fact, paralic and estuarine coal measure sequences of Late Cretaceous (Pakawau) through Eocene (Kapuni) are generally recognised as being potential source rocks for these oils, which are encountered in Paleocene-Eocene sandstone reservoirs associated with the younger Kapuni group coal measures. In spite of the fact that there is presently much specu- *Present Address: School of Geology and Geophysics, University of Oklahoma, Norman, OK 73019, U.S.A. "['Author to whom correspondence should be addressed. lation regarding the true petroleum potential of this region, little effort has been made to geochemically characterise all of these petroleum occurrences. To date there have been no published reports of system- atic attempts to assess, using data acquired by mod- em organic geochemical techniques, the relationships between the crude petroleum sources from within each of the subbasins of the larger Taranaki graben. This study therefore describes the application of the geochemical biological marker approach to the elucidation of the respective histories of the crude oils from the Taranaki basin of New Zealand. Specific reference has been made to the tetracyclic and pen- tacyclic families of biomarker hydrocarbons since it is now well established that these particular molecu- lar classes provide significant information pertaining to the nature of sedimentary organic matter and likely depositional environment (Huang and Mei- nschein, 1979; Didyk et al., 1978), to the effects of subsequent maturation (Seifert and Moldowan, 1978, 1979; Mackenzie et al., 1981, Mackenzie, 1984; Philp, 1985a, b), migration (Seifert and Moldowan, 1981) and biodegradation processes (Volkman et al., 1983). Regional geology The Taranaki Basin (Fig. 1) is located to the west of the North Island of New Zealand and is the largest of a series of Cretaceous-Cenozoic sedimentary 123 OG. 12/2~C