Study of the Role of the Mineral Matrix in Sedimentary Organic Matter Preservation (Cenomanian Black Shale, Central Italy)

Abstract

Understanding the mechanism(s) which led to sedimentary organic matter (OM) preservation is essential for oil exploration and for the reconstruction of palaeoenvironments and palaeoclimates, as well. In addition to the first model of OM preservation, the degradation-recondensation pathway as defined by Tissot and Welte (1984), two mechanisms have been extensively studied in the last few years and are now well understood, namely the selective preservation and the natural sulphuration pathways. Recently, a new mechanism has been proposed, involving a protective role by minerals: the so-called sorptive protection pathway. Mayer (1994) showed that in sediments from coastal shelves (where most of the OM is preserved in marine environment), outside from zones receiving a large amount of OM, a correlation can be derived between the organic content and the mineral surface area, corresponding to a monomolecular covering of the mineral surface, which was termed monolayer equivalent, or ME. Considering that most of the surface area is composed of pores less than 8 nm width, and that more than 90% of the OM cannot be physically separated from the mineral matrix, this author proposed that the OM is concentrated in these pores, the size of which excludes the hydrolytic enzymes and allows condensation reactions to take place although the latter reactions are characterized by low rates. Up to now, studies on this fourth OM preservation pathway have only been carried out on Recent sediments (see review in Hedges and Keil, 1995). However, a recent study on a Cenomanian black shale (90 Ma) suggested an important role for the mineral phase in OM preservation. Indeed, this study, mainly based on pyrolysis, indicated that the analysed kerogen was formed by condensation of lipidic moieties showing a very low degree of degradation, and that no substantial diagenetic alteration took place in the resulting kerogen, associated with its mineral matrix, till the present time. The aims of the present study are (i) to confirm the hypothesis of a protective role for the mineral phase by morphological observations and (ii) to test whether this phase still plays a protective role for the kerogen, once formed, by comparing pyrolysates of the freshly isolated kerogen and of the same isolated kerogen after one and two years of storage. The pyrolysate of the kerogen isolated from the ground rock and stored for two years was also analysed.