CORRELATION OF DIFFERENT MATURITY PARAMETERS IN THE AHMEDABAD-MEHSANABLOCK OF THE CAMBAY BASIN

The potential use of organic matter maturation parameters, in assessin g the diagenetic-catagenetic transition zone, with the help of various known molecular parameters, has been investigated in the Ahmedabad-Me hsana block of the Cambay Basin. A set of 24 conventional cores from t he Eocene shales, covering a depth regime of 922-2500 m, has been stud ied. The oil-generation threshold, i.e. the boundary between the diage netic and catagenetic maturation regimes, is assessed to be at 1800 m based on a combination of parameters including percent hydrocarbons in EOM, saturate/aromatic hydrocarbon ratio in EOM and the Rock-Eval pro duction index. The Pr/n-C-17 ratio. Rock-Eval HI and OI were not suita ble for reliable maturation profiling, nor for independently indexing the stages of maturation. The extent of the catagenetic regime is dist inguished by Ph/n-C18 ratios <0.48 and CPI values <1.28. Maturation pa rameters based on hopane isomerization (22S/22S + 22R of C32 and alpha beta/alphabeta + betaalpha of C30) have shown a distinct advantage in the diagenetic stage over other parameters such as 20S/20S + 20R in C2 9 steranes, infra-red spectral E1630/E1710 ratios for asphaltenes, and MPI-1, because of a sharp inflexion in the depth profiles shown by th e former set of parameters. The C32 hopane ratio (22S/22S + 22R) shows a most distinct inflexion associated with a rapid increase up to the oil generation threshold, followed by very little change after the thr eshold. The next best supplemental parameter is Ts/Tm of C27 hopanes. The 20S/20S + 20R ratio of the C29 steranes, and E1630/E1710 show pote ntial to be used as continuous maturation profiling parameters. The me thylphenanthrene index (MPI-1) shows large scatter with depth in the s tudied maturation regime (VR(o) 0.33-0.63). The regression equation of MPI-1 vs VR(o) is found to be different from those reported by Boreha m et al. and Radke et al. Correlation coefficients between C29 sterane and C32 hopane isomerization, MPI-1 and C29 isomerization, C29 steran e isomerization and Ts/Tm, MPI-1 and Ts/Tm, MPI-1 and C32 hopane and M PI-1 and C30 hopane are 0.85, 087, 0.95, 0.78, 0.89, and 0.94 respecti vely.