Metodi di valutazione delle proprietà naftogeniche originarie di una roccia madre.

The knowledge of the original potential is of primary importance in order to obtain a good geochemical model for the petroleum system modeling. For mature source rocks it is possible to measure in laboratory only the residual features: petroleum potential – PPR, total organic carbon content – TOCR, hydrogen index – HIR. In case of extremely mature rocks only the TOC value remains as a proof of an early naphthogenic appeal. Historically, there were at least three different methods to evaluate the original petroleum potential. After a bibliographical review on the subject, the different evaluation hypotheses introduced by Claypool (in Hunt, 1995), Scotti (1998, Agip internal report) and Jarvie (2012) have been verified and compared. The common base is the utilization of the residual features, which can be measured in laboratory, and the analysis of the original quality of the organic matter, or more precisely of the kerogen, its diagenetic product, in terms of one more plausible value of the hydrogen index - HI. Once the reliability of the results in different theoretical conditions has been tested, we have verified the different approaches on real sample of source rocks, considering the samples from the Hils Syncline in the Posidonia Shale (Germany). The intrusion of the laccolitic body in this region allowed different conditions of maturity within the same formation, making possible the measurement of the naphthogenic residual features, through the decrease of the HI in samples that can be considered similar. The more recently proposed method (Jarvie, 2012) is also the most accurate, even if of complex application. Claypool (1995) proposed a formula that is even more complex and of difficult application, since it needs many parameters sometimes indefinable; moreover, in particular situations, it can lead to poor quality results. Scotti's method (1998) offers results that are identical to those obtained with Jarvie's formula (2012) only in case of overmature source rocks; if the maturity of the source rocks is intermediate and the kerogen is of excellent quality, we can observe a gap in comparison to Jarvie's method (2012). Using a simpler version of this formula, the calculation often converge towards the more precise formula proposed by Jarvie (2012). This seems to be of easier use, since it does not consider a really critical parameter that is the quantity of free hydrocarbon present in the sample, which can be sometimes hard to identify due to the presence of migrated hydrocarbons, in addition to indigenous ones. The study allowed to define the limits and the peculiarities of each proposed method and to establish which is the more correct work flow in order to evaluate the original potential of a source rock.