Predicting generation and expulsion of paraffinic oil from vitrinite-rich coals

Rock-Eval HI values for coals vary With rank and do not give a direct measu rement of oil potential. However, oils from coals are characteristically pa raffinic and can be considered-to derive from a polymethylene (PM) componen t, so the PM content should provide an estimate of the paraffinic oil poten tial. A trend apparently representing lignin evolution has been identified on the Van Krevelen diagram which permits the relative proportions of carbo n in lignin and PM to be determined for coals that approximate a mixture of these two components, such as the members of the New Zealand (NZ) Coal Ban d. On the basis of this compositional model, HI values can be calibrated to provide an alternative estimate of the paraffinic oil potential. A maximum in HI is generally reached in coals near the onset of oil generation, at R ank(S) 12 (R-o ca. 0.7%), from which it is suggested that the PM contributi on can be obtained using the formula HIPM = 1.15HI(max)-172 for the suite o f NZ coals examined. The onset of oil expulsion can be identified from a va riety of geochemical measurements, and occurs in the Rank(S) range ca. 12.0 - 14.5 (R-o ca. 0.7-1.1%) for coals with paraffinic oil potentials exceedi ng ca. 40 mg HC/g TOC. Data from Taranaki Basin coals correlate well with t he theoretical relationship between BI/HIPM and HIPM, using bitumen index ( BI = S1/TOC) values of 10 mg HC/g TOC at the start of oil generation (i.e. bitumen inherited from diagenesis) and 40 mg HC/g TOC at the onset of oil e xpulsion, suggesting the HIPM model is reasonably accurate for members of t he NZ Coal Band. Kinetic modelling of paraffinic oil generation from vitrin ite-rich coals maybe best approximated by consideration of PM degradation a lone. (C) 1998 Elsevier Science Ltd. All rights reserved.