HYDROUS PYROLYSIS OF A TYPE-III SOURCE - FRACTIONATION EFFECTS DURINGPRIMARY MIGRATION IN NATURAL AND ARTIFICIALLY MATURED SAMPLES

A core from the Cambay Shale Formation of the Cambay Basin, containing immature Type III organic matter, was pyrolysed at 300 degrees C for different durations of time to different maturation levels. Fractionat ion effects were studied employing a three-step extraction technique a fter removal of the expelled pyrolysate. The extractable organic matte r (EOM) obtained on extraction of the whole core is assumed to be that present in open pores, while that obtained on finely crushing the sam ple is assumed to be that present in closed pores. The EOM obtained fr om 1 cm chips is termed EOM from semi-open pores. The gross compositio n of the pyrolysates expelled during pyrolysis is not similar to the o ils reservoired in the area, and there is no significant fractionation observed between expelled pyrolysates and unexpelled EOM. Our study i ndicates movement of fluids between closed, semi-open and open pores. In both systems, there is a higher concentration of EOM in open pores than in semi-open and closed pores, and the fraction of EOM in open po res is much greater in the artificial system than in the natural syste m. Fractionation effects on n-alkane and isoprenoid hydrocarbon-based parameters were also studied. n-Alkenes are present in semi-open and c losed pores of the immature core and in the core after it was pyrolyse d to 300 degrees C for 6 and 48 h, but are absent in the open pores. n -Alkenes are present in closed pores in the naturally matured core. Pr esence of n-alkenes in the pyrolysates expelled during the 6 and 48 h experiments, but their absence in the open pores of the core, indicate s that expulsion also occurs through temporary microfractures during l aboratory pyrolysis, whereas in the natural system expulsion from clos ed pores seems to be only via semi-open and open pores. (C) 1997 Elsev ier Science Ltd.