Pyrolytic assessment of oil generation and expulsion from a suite of vitrinite-rich New Zealand coals

Abstract

Pyrolysis–gas chromatography (PY–GC) of Tara-1 coals (Great South Basin, New Zealand) suggests a mean paraffinic oil potential of ∼85 kg/t C (range ∼70–130 kg/t C), based on n-alkane/alkene generation in the C 6–C 31 range and typical proportions of these compounds in related oils from New Zealand. This maximum in the depth trend of pyrolysate yield is reached at ∼0.75–0.80% R 0, comparable to the previously reported maximum in hydrogen index (HI). Thermal extraction–GC (TE–GC) provided higher yields than Rock-Eval S1pyrolysis, but without the uniform trend exhibited by S1/C below the ∼3.0 km below sea floor (∼0.75–0.80% R 0) onset of expulsion. In addition, the saturates:aromatics ratio in the thermal extract decreased with depth, indicating preferential retention of aromatics, which may affect the expulsion efficiency of paraffins. Thermal extracts suggest that at most ∼15 kg/t C of paraffinic oil is retained within the Tara coals, which is lower than previous estimates of saturation threshold. However, it is likely to be adversely affected (lowered) by loss of the least volatile components during recovery and storage of cuttings and incomplete recovery of high molecular weight components (particularly C 40+ paraffins) using TE–GC. Isopimarane generation appears to be restricted to the depth zone corresponding to the early oil window, although it is unclear whether this is directly related to the maturity range required for such generation or that only the latest Cretaceous coals in Tara-1 exhibit particularly high abundances of such gymnosperm-derived diterpanes.