COAL RANK AND TYPE VARIATION IN ROCK‐EVAL ASSESSMENT OF NEW ZEALAND COALS

Rock‐Eval and coal analytical data are compared for New Zealand coal samples ranging from peat to medium‐volatile bituminous coal. Most are humic coals within the New Zealand coal band, which is representative of the Type III maturation series defined for kerogens. However, four were chosen as particularly high‐hydrogen coal types. Relations between individual parameters — principally H/C, O/C, Tmax, HI and OI — are generally similar to those of Espitalié et al. (1985, 1986), but important differences include a broader range of HI values in the peats and lignites, and a differentiation between maturation and coal‐type effects in the relationship of HI to H/C. Correlations of the effects of maturation on HI, OI, Tmax and vitrinite reflectance are made by the use of the independent Rank(S) scale of Suggate (1959). Rank(S)/reflectance relations vary between well sequences. The Rank(S) scale is used to integrate biomarker data on hydrocarbon generation and expulsion. The oil zone (“oil window”) corresponds to the range of bituminous coals, beginning at Rank(S) 10 (Tmax425°C) and ending at Rank(S) 17 (Tmax estimated at 550°C). Release of oil begins at or soon after Rank(S) 12.5 (Tmax 435°C), and culminates in the range Rank(S) 14.5‐ 15.5 (Tmax 460–480°C). For the Taranaki Basin, New Zealand, the results imply that a substantially greater volume of coal measures has been subject to oil generation and migration processes than previously thought.