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

Abstract

Rock-Eval HI values for coals vary with rank and do not give a direct measurement of oil potential. However, oils from coals are characteristically paraffinic and can be considered to derive from a polymethylene (PM) component, so the PM content should provide an estimate of the paraffinic oil potential. A trend apparently representing lignin evolution has been identified on the Van Krevelen diagram which permits the relative proportions of carbon 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 Band. 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 Rank(S) 12 ( R o ca. 0.7%), from which it is suggested that the PM contribution can be obtained using the formula HI PM = 1.15HI max−172 for the suite of NZ coals examined. The onset of oil expulsion can be identified from a variety 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 exceeding ca. 40 mg HC/g TOC. Data from Taranaki Basin coals correlate well with the theoretical relationship between BI/HI PM and HI PM, 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 expulsion, suggesting the HI PM model is reasonably accurate for members of the NZ Coal Band. Kinetic modelling of paraffinic oil generation from vitrinite-rich coals may be best approximated by consideration of PM degradation alone.