International Geologiical Congress - Oslo 2008

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GEC-01 General contributions to coal geoscience

 

The evolving petroleum potential of humic coals

 

Tiem Vu Thi Anh, GeoForschungsZentrum Potsdam (Germany)
Brian Horsfield, GeoForschungsZentrum Potsdam (Germany)
Richard Sykes, Institute of Geological & Nuclear Sciences (New Zealand)
 

 

The New Zealand Coal Band, covering the complete maturity range from peat to high volatile bituminous rank, has been studied in order to relate its petroleum potential to elemental composition and structural evolution. The evolution of New Zealand coal in the van Krevelen diagram is marked by a significant decrease in O/C ratio (0.34- 0.07), and only a minor change in H/C values (0.90- 0.78) with increasing maturity. A Transformation Ratio of CO2 (TRCO2), calculated by a novel mass balance approach, has been formulated here in order to quantify the loss of CO2 for any given coal type resulting from decarboxylation reactions. The study revealed about 10 to 105 mg CO2, equivalent to 0.23 to 2.4 millimoles CO2, per gram of total organic carbon, are released during maturation from peat to high volatile bituminous rank. The increase in hydrogen index from 120 to 280 (mg/g TOC) prior to oil expulsion has here been shown to be entirely due to the loss of oxygen during diagenesis, as revealed from its empirical relation with atomic H/C ratio. For coals with O/C ranging from 0.07 to 0.34, the evolution of increasing hydrogen index is strongly controlled by O/C atomic ratios. For coals with O/C < 0.1, both H/C and hydrogen index decrease, arising from the generation of hydrocarbons.
The pyrolysate composition yields present the overall increasing petroleum potential of New Zealand coals as function of rank. With increasing rank, the primary gas potential increases from 30 to 50 (mg/g TOC), the liquid hydrocarbon potential (Bulk C6+) increases from 90 to 170 (mg/g TOC) that is consequently equivalent to increasing secondary gas yields, which increase from 60 to 120 (mg/g TOC) with increasing maturity (R0 = 0.2- 0.8%).

 

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