Application of vitrinite-inertinite reflectance and fluorescence (VIRF) to maturity assessment in the East Coast and Canterbury Basins of New Zealand

摘要

Vitrinite reflectance (VR) is the classic maturation indicator, originally favoured because it was considered to have a straightforward relationship with burial temperature. However, VR has a number of problems, and although it is almost always undertaken many basin modellers do not trust the results. Quantitative vitrinite and inertinite reflectance and fluorescence (VIRF) results in a bivariate chart which readily differentiates vitrinite of normal chemistry ― which provides meaningful reflectance data ― from suppressed vitrinite, inertinite, reworked vitrinite, and cavings. VIRF charts are calibrated against data from reference coals, which provide the basis for objective identification of vitrinite in dispersed organic matter. VIRF provides new insights into basin history where sedimentary sequences are dominated by marine strata, which present particular challenges to conventional VR. Whangai Formation source rocks are reliably demonstrated to have achieved maturity in a number of onshore locations in the southern East Coast Basin. Complex variations in maturity within and between some key sections indicate the potential value of a more detailed investigation of maturity trends in this area. Clipper Formation source rocks in Clipper-1 are inferred to have reached post-maturity for oil in the early Paleocene, as a result of thermal intrusion at depth. A dramatic increase in reflectance at the top of Clipper Formation, and evidence of contact metamorphism and hydrothermal activity near target depth, define a zone of convective heating and hydrocarbon distillation which resulted in pervasive bitumen impregnation as temperatures cooled. Clipper Formation source rocks are therefore unlikely to have been responsive to subsequent Tertiary burial. Rocks previously identified as schist at target depth may be contact metamorphosed Clipper Formation. VIRF defines a previously unrecognised 0.15% offset in reflectance at the junction between low and high reflectance gradients in Clipper-1. Further work is needed to assess the significance of this feature.