Organic geochemical results from the shales of the South Portuguese Zone, southern Portugal

Abstract

This work aims to present the laboratorial results from Rock-Eval pyrolysis of Carboniferous shales sampled on the South Portuguese Zone (SPZ).

The SPZ is located in the southernmost sector of the pre-Mesozoic Iberian Variscan Fold Belt, which is composed by highly deformed upper Paleozoic rocks, of low to very low grade metamorphism. However, in some areas, it is possible to observe the same rocks practically undeformed.

The SPZ includes the Flysh Group with three turbiditic units, organized from proximal to distal areas, Mértola, Mira and Brejeira Fm. SPZ includes also the Iberian Pyrite Belt (IPB) comprising late Devonian to middle Carboniferous units. The IPB includes a Volcano-Sedimentary (VS) sequence containing a pelite-organic black shale and fine grained sandstone sequence (known as Paraíso Fm. at Aljustrel mine zone).

A total of 31 samples were collected - 5 in Paraíso Fm., 22 in Brejeira Fm., 3 in Mira Fm. and 1 in Mértola Fm.. The laboratorial analyses were performed by Weatherford Laboratories and Polish Geological Institute and were financed by Repsol E&P, Partex Oil&Gas and Polish Geological Institute.

These recently sampled surface black shales provide a new insight regarding the basic geochemical characteristics of the shales and their petroleum potential. Samples revealed over-mature phase of hydrocarbon generation in range of a gas window zone. The shale samples are dominated by depleted type III kerogen, reflecting they went through the whole oil window and the uppermost part of the gas window zones. Consequently, the samples reflect fully completed hydrocarbon potential as a result of probable hydrocarbon expulsion. The black shales are dominated by low TOC and low S2 values, with the residual organic carbon content reaching up to 98 wt% of the TOC.

To study the potential of these shales, additional geochemical analysis are necessary preferably in the less mature sectors, giving a better insight into the pyrolyzable organic carbon content and petroleum potential. Additional studies, showing values of the key pyrolysis parameters on the earlier phases of hydrocarbon generation or even before generation (late diagenesis), are crucial for determination of the unconventional potential of the shales from the SPZ.

AAPG Datapages/Search and Discovery Article #90226 © 2015 European Regional Conference and Exhibition, Lisbon, Portugal, May 18-19, 2015