Use of Tmax as a thermal maturity indicator in orogenic successions and comparison with clay mineral evolution.

The relationship between three parameters, the T, given by Rock-Eval pyrolysis, the illite content in illite-smectite mixed layers (I-S) and the Kubler Index (KI) has been investigated in the Cretaceous-Neogene, sedimentary syn-orogenic successions in the Northern Apennines (Italy). A strong relationship was found between maturity stages of kerogen, illite content in I-S and KI. The oil formation zone for continental organic matter (Type III), delimited by T-max between 434 and 465 degrees C, corresponded to rocks with short-range ordering RI, I-S with illite content between 60 and 85% and KI values in the range 0.85-0.65 (degrees Delta 2 theta). Over-mature rocks were characterized by T-max >465 degrees C, a long-range ordered I-S with an illite content >85% and KI in the range 0.65-0.45 (degrees Delta 2 theta). The relationship permits use of both mineralogical parameters and T-max to estimate palaeotemperatures in sedimentary successions and it can be exploited in hydrocarbon research to evaluate the petroleum potential.