Interpretation of source rock depositional environment and age from seep oil, east coast of New Zealand

Abstract

Biomarker fingerprints of crude oil samples from four onshore East Coast Basin oil seeps were analyzed to assess source rock characteristics including type of organic matter input, redox conditions, sedimentary facies, and age. Results show that samples generally form two groups, correlating with geographic location: a northern and a southern group. Source rocks associated with all seep samples are interpreted to be marine. However, results suggest northern samples had more terrigenous organic matter input to their source rock(s), while southern samples had more marine input. Results suggest northern sample source rock(s) had more oxic depositional environments, whereas southern sample source rock had more reducing environments. A shale source rock sedimentary facies was indicated for all samples. These observations suggest that southern samples may be derived from slightly higher quality source rocks (higher HI, deposited in more reducing conditions), although source rocks in both regions are oil prone. Biomarker age parameters suggest that the northern oil samples are from a younger (Cenozoic) source rock, whereas the southern oil samples are from an older (Cretaceous) source rock. Source rock characteristics (depositional environment and age) point to the presence of two different source rocks. We postulate that northern oils samples from younger source rock with more terrigenous organic matter input represent the upper Paleocene Waipawa Formation, whereas southern oil samples from older source rock with more marine organic matter input represent the Upper Cretaceous to Paleocene Whangai Formation.