Liassic organic-rich shales in the peri-Tethys area. Valid targets for unconventional oil & gas exploration

Abstract

The extent and geochemical quality of Liassic organic-rich shales at the Tethys and Atlantic boundaries of Europe was studied in various locations. Samples from southern England, western England, southern Germany and central France were collected and analysed for their geochemical characteristics. These results are compared with published geochemical measurements of similar-age rocks. The Liassic organic-rich shales extend across central and southern Europe, under different names: Blue Lias (UK), Schistes carton (France), Posidonienschiefer (Germany, Austria, Switzerland), Posidonia Shales (Italy, Balkans). These rocks were deposited during the Early Jurassic, especially from the Sinemurian to Toarcian. The Blue Lias organic-rich shales charge the largest onshore oilfield in Europe, the Witch Farm field in southern UK. The Liassic organic-rich shales are found along an onshore fairway extending from the Lusitania Basin of Portugal to the Ionian Basin of NW Greece, via northern Spain, southern and central Britain, central and eastern France, Germany, Austria and Italy. Their geochemistry indicates deposition within a strongly anoxic marine environment. An important observation is that the general geochemical and sedimentary characteristics of the Liassic shales remain constant throughout their depositional fairway, which indicates an affinity in their depositional environment. The current model of deposition, involving deposition in semiisolated small basins at the north-western edge of the Lower Jurassic Tethys Ocean is, therefore, put to question. The Liassic shales are often interbedded with limestones, an indication of varying seabed topography and oxygenation. The lack of oxygen in the deeper marine water layers may be explained by local formation of wind-induced oxygen-minimum layers, or by anoxic conditions created by deep water isolation. An additional reason for the Liassic anoxic sea beds in the Tethys Ocean may be the large distance from oxygenreplenishment areas (i.e. the poles). The quality of the Liassic shales makes them an excellent candidate for shale gas & shale oil exploration. At present, Liassic shales are being mined in the Baden-Würrtemberg state of southern Germany and used as fuel instead of coal for electricity production in a cement plant. In the early 1980's, the French government carried out a pilot project of in-situ “fireflooding” Liassic black shales in the Nancy area of NE France, to produce oil by subsurface pyrolysis.

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