The Upper Devonian Dunkirk Shale – A Late Hydrocarbon Generator
Blood, David R.1 and Lash, Gary G.2
1Dept. of Geosciences, Dept. of Geology, 876 Natural Science Complex, University at Buffalo, Buffalo, NY, 14260
2SUNY-Fredonia, Fredonia, NY 14063
Hydrous pyrolysis experiments were carried out on the basal Upper Devonian Dunkirk black shale of western New York State to assess its hydrocarbon generative potential. The Dunkirk shale at the sample locality is immature to early mature (Ro=0.53%) and composed predominantly of Type II and lesser Type III organic kerogen. Black shale samples were run for 72 hrs at temperatures ranging from 305o C to 357o C; each sample was analyzed for TOC and Rock-Eval parameters. Samples pyrolyzed from 305oC to 344oC showed a modest increase in production index (PI) from 0.11 to 0.22 and, perhaps more significantly, only a minimal increase in Rock-Eval Tmax from 443oC to 446oC. However, the increase in temperature from 344oC to 357oC was accompanied by increases in PI to 0.61 and Tmax to 582oC. Similarly, hydrogen index (HI) over the 305-344oC range diminishes from 356 to 216; however, from 344o to 357o C, HI drops to 11. It appears, then, that organic matter of the Dunkirk shale resists conversion until a threshold level of thermal maturity is reached when the kerogen rapidly decomposes to liquid hydrocarbons. The refractory nature of Dunkirk shale kerogen is confirmed by bulk kinetics analyses of three Dunkirk shale samples of variable Ro (0.53-0.73%) that reveal that the bulk (>80%) of the conversion reactions require activation energies in the range of 5,400-5,500 cal/mole, values that are typical of documented late-oil generators, including the Woodford shale. Further, 10% and peak hydrocarbon generation of the Dunkirk shale would occur at Ro=0.88-0.92% and 1.08-1.14%, respectively.