RELATIONSHIPS BETWEEN HYDROCARBON GENERATION, COAL TYPE AND RANK FOR MIDDLE EOCENE COALS, BULLER COALFIELD, NEW-ZEALAND

The Buller Coalfield in the northern portion of the Paparoa Trough (NW South Island, New Zealand) contains a middle Eocene bituminous coal-b earing succession that exhibits marked variations in both coal rank an d type. The across-basin rank changes result from differential late Pa laeogene subsidence of the Paparoa Trough and subsequent inversion. Su perimposed upon coalification trends are down-seam variations in coal type, evidenced by isorank variation in conventional chemical paramete rs. These type variations are riot a consequence of changes in maceral group proportions, which are dominated by vitrinite. Forty-eight coal samples from fifteen drillholes through the Brunner Coal Measures hav e been examined for vitrinite reflectance, proximate analysis, specifi c energy, and sulphur content. Total Organic Carbon (TOC), Rock-Eval p roperties and the bulk composition of the bitumen were also determined . All the coals analysed lie within the high volatile bituminous B to medium volatile bituminous coalification stages. Variation in analytic al properties within this suite, and more specifically within serial s amples, provides insights into the nature of the organic matter compri sing the coals. While TOC varies systematically, with ash content, ref lecting proximity of the depositional mires to fluvial systems, values increase with rank (68-86% ash free). The Hydrogen Index (HI) decreas es from 334 to 190 mg hydrocarbons/g TOC over a range of 429 to 470 de grees C Tmax, analogous to the volatile matter content and vitrinite r eflectance, respectively. The more perhydrous coal samples at a given rank are characterised by depressed Tmax/vitrinite reflectance and ele vated HI/volatile matter contents. Bitumen and sulphur contents are no t the primary control on perhydrous characteristics. The bitumen conte nt crudely corresponds to the S1 peak plus the initial portion of the S2 peak liberated under low temperatures during Rock-Eval pyrolysis; h owever, the bulk of the generative potential is associated with the re sidual kerogen fraction. Suggate (S) Rank and maximum palaeotemperatur e appear to be the best indicators of coal rank. A period of significa nt hydrocarbon generation and release begins at Tmax similar to 440 de grees C for the Buller coals. The main oil ''window'' is defined by th e interval 440-455 degrees C Tmax or 0.8-1.1% vitrinite reflectance (R o(max)), and by correlation to maximum palaeo-temperatures, 125-155 de grees C. This is further substantiated by mass balance considerations, which indicate that hydrocarbon generation (Petroleum Generation Inde x (PGI) > 0.1) was occurring in the coals at a maturity level of 0.7-0 .9% Ro(max) (Tmax 440-444 degrees C) with an increase in PGI between s imilar to 0.9 and 1.1% Ro(max) (similar to Tmax 445-455 degrees C). Be tween PGI 0.1 and 0.4, the expulsion efficiency rapidly increased, pre sumably due to generation and expulsion of the bulk of the oil.