Unraveling Petroleum Degradation, Maturity, and Mixing and Addressing Impact on Petroleum Prospectivity: Insights from Frontier Exploration Regions in New Zealand

Determining oil quality is essential to identifying valuable resource accumulations. However, in new areas of exploration, little information is available on the processes affecting resource quality. Geochemical analyses of oil seeps from frontier regions of New Zealand's East Coast illustrate an application of underutilized resource quality assessment techniques. Distributions of n-alkanes and isoprenoids reveal biodegradation, and thus potentially lower oil quality in the southern versus the northern oil seeps. However, sterane and terpane compounds are unaltered, indicating overall biodegradation of these oils is low to moderate. Additionally, lack of 25-norhopane indicates degradation of southern oils may be solely aerobic. Therefore, any subsurface accumulations are potentially unaffected. Investigation of sterane and hopane isomerization ratios and additional sterane and terpane maturity parameters is paired with diamondoid analyses of oil-to-gas conversion and petroleum mixing. Three distinct petroleum mixtures are identified among the sampled seeps: (I) a seep composed of an early/peak oil window component and an intensely cracked condensate/wet gas component, (2) seeps solely containing a peak/late oil window component, and (3) seeps composed of a peak/late oil window component and an intensely cracked condensate/wet gas component. Identified components indicate at least three distinct charges or, stages of petroleum generation. Black oil components might indicate actively producing source rock in all regions represented by the seeps. Intensely cracked components indicate petroleum mixing via thermogenic gas infiltration and suggest an effect on oil quality. Important questions concerning migration pathways and timing, ties to New Zealand's offshore basins, and potential for reservoir entrapment of these petroleum components remain.