Refference : Geosciences Scientific Quarterly Journal,No.:1,P.:30

Abstract :
Organic-rich shales of early Toarcian age arc widely distributed in the world Their occurrence is well documented in Northern Europe where they are good source rocks. In the present study. selected sections and boreholes f coin Tethyan (= Alpine Mediterranean) margins, Middle East and Northern Europe have been investigated in terms of organic content and depositional conditions. All samples were collected from outcrop as deep as possible within the exposed portion or from boreholes.
The samples were analysed using the Rock-Eval apparatus. This technique gave a quick quantitative evaluation of the total organic carboii (bc) and the petroleum potential of the rock. The main techniques used to charactcri7c the nature of organic matter (OM) were: (I) Rock-Eval pyrolysis which provides the hydrogen and oxygen indices, directly related to the 0/C and H/C ratios, obtained iron (2) the elemental analysis of kerogen: (5) gas chromatography of saturated hydrocarbons extracted by chloroform, (4) optical studies of selected samples in transmitted light. The thermal evolution of OM was estimated from the Rock-Eval Tmax parameter.
From the resultS, a mapping of the OM hasbeen compiled!For the Toarcian stage. During this Lime the Mediterranean. A1 & area had a contrasted paleogeography in relation to the ocan opening aّd...the break-up of the Pangea landinasses. In their present stage, the map show some heterogeneity in the distribution and type of OM which
(i) Marine OM (type II). occurs in different environments of deposition.
(a) High organic contents ! 1'0C! corresponding to thick deposits, occur in the Lower Tharcian within the west-European realm (Jet-Rock in Great Britain, Poseidon Shales in North Sea, Sctiistes cartons in the Paris Basin and Posidonienschicfcr it Germany or Switzerland. These formations are potentially or effective good source rocks and correspond to a shallow water argillaceous deposits.
(b) The OM is less abundant in the Mediterranean area where lower concentrations
(1 to 2% TOC), preserved in Lower Toarcian thin levels, are limited only to the bottom of the basins in extension (Northern Italy, Greece).
(2) The predominarce of continental OM (type III) along the Northern margin of the Tethys (Turkey and Iran) corresponds to a deltaic environment.
(3) The mixture between marine and terrestrial OM within the Norwegian offshore, show the existence of continental organic carbon in put in the high palcolatitudes.
(4) There is a predominance of altered OM ("type IV") within the carbonated platforms around the Mediterranean Tethys (Portugal, Spain, Morocco and Saudi Arabia) and in the oxygenated pelagic basins from Greece (Pindus zone) and southern Italy (Lago Negro).
Sedimentological, geochemical and palynological studies of the Toarcian allow to confirm and complete the palcogeography of the Tehyan domain around Mediterranean Sea and Middle East. The Lower Toarcian Anoxic Event is recorded locally in the Tethys area. All the Toarcian phenomena arc explain by the close connection between several facts during this time: (1) in increase of organic productivity in relation with high sea level. (2) modifications within oceanic circulations. (3) an intense distention promoting the water stagnation in the deeper part of tilted blocks (OM traps): (4) and a generalized argillaceous sedimentation. The end of the Toarcian organic-rich deposits is correlated with the opening of the Atlantic Ocean which have necessary changed the oceanic circulations.