Browsing by Author "Musgrave, J. A."
Now showing 1 - 2 of 2
Results Per Page
Sort Options
- New evidence concerning the thermal history of Devonian and Carboniferous rocks in the South Portuguese ZonePublication . Fernandes, Paulo; Musgrave, J. A.; Clayton, Geoff; Pereira, Zélia; Oliveira, José Tomás; Goodhue, R.; Rodrigues, B.The Late Palaeozoic rocks of southern Portugal have a complex thermal history. Vitrinite reflectance determinations from 90 samples confirm that the organic maturity of Late Palaeozoic rocks in SW Portugal is very high, mainly corresponding to meta-anthracite coal rank. The optic fabric of vitrinite in oriented coal samples from the Brejeira Formation suggests maturation under simple, non-tectonic, burial conditions with peak temperatures being attained prior to Variscan deformation. The lack of any increase in vitrinite reflectance with depth through c. 1 km of section in borehole AC-1 is not consistent with conductive heat transfer and is interpreted as the result of late synorogenic to post-orogenic advective heating. This heating episode generated temperatures sufficiently high to produce levels of maturity corresponding to metaanthracite coal rank through much of the terrane now exposed, but not high enough to result in overprinting of the pre-deformation optic fabric of vitrinite in the thin coals. The slightly lower maturity of older, platformfacies rocks in the Aljezur–Bordeira area may reflect either deposition on a basement high followed by less burial than adjacent parts of the basin, or the area being the site of a sink for descending relatively cool fluids.
- The thermal history and hydrocarbon source rock potential of the mid Carboniferous Quebradas Formation in SW Portugal and its correlatives in western Atlantic offshore basinsPublication . Clayton, Geoff; Fernandes, Paulo; Goodhue, R.; McCormack, Niall; Musgrave, J. A.; O'Donoghue, E. P.The mid Carboniferous Quebradas Formation of the ‘South Portuguese Zone’ (SPZ) comprises 80m of post-mature black mudrocks with a mean TOC of 2.5%. Lithostratigraphic units of similar facies and age such as the Holywell Shale, the Edale Shale and the Bowland Shale are important HC source rocks in the UK, having sourced a considerable proportion of the hydrocarbons in the East Irish Sea, East Midlands and Formby oilfields respectively. The kerogen content of the Quebradas Formation is mixed but slightly more oil-prone in its lower part. At outcrop, it is strongly post-mature with vitrinite reflectance (Rr) ca. 4%. Illite crystallinity results from the Quebradas Formation and associated units suggest lower maturity than vitrinite reflectance. Analysis of the optic fabric of very thin coal lenses within the Brejeira Formation which overlies the Quebradas Formation suggests that peak temperatures were attained before the Variscan (late Carboniferous – early Permian) deformation. Triassic rocks unconformably overlying the Carboniferous sequence are much less mature, with Rr ca. 1.2%. Although the the Quebradas Fm has no HC source potential onshore due to its high maturity, Carboniferous rocks offshore may not have experienced the same extreme thermal history as the SPZ.