Geochemical and petrographic analyses of new petroleum source rocks from the onshore Upper Jurassic and Lower Cretaceous of Lebanon

The recent interest in the Lebanese petroleum potential led to new studies mainly concerning marine petroleum source rocks, without taking in consideration the dysodiles and other associated facies from the Upper Jurassic and the Lower Cretaceous. These organic-rich outcrops of metric thickness (max. 3 m) are of limited lateral extension and often found next to volcanic deposits. Sixty-three samples were collected from 25 different localities, and were subdivided into three facies that are presented in this paper: (i) dysodiles formed by the superposition of millimetric layers that are easy to separate; (ii) dysodiles-like with distinct, but more compacted layers; (iii) non-dysodiles where the layering is not visible. The aim of this work is to assess the origin of these organic-rich rocks, and to evaluate their source rock potential. We used herein Rock-Eval pyrolysis, elemental analysis, palynofacies observation, organic petrography and molecular geochemical analyses. Rock-Eval pyrolysis reveals a very good to excellent source-rock potential for all above cited rock types, with average total organic carbon (TOC) of 14 +/- 7% for the dysodiles, 19 +/- 12% for the dysodiles-like and 13 +/- 9% for the nondysodiles. Hydrogen index (HI) and oxygen index (OI) values indicate a type I to II kerogen for the dysodiles (average HI = 649 +/- 162 mg HC/g TOC), type I to II kerogen for the dysodiles-like (average HI = 340 +/- 182 mg HC/g TOC) and type III kerogen for the non-dysodiles (average HI = 145 +/- 42 mg HC/g TOC). The dominance of fluorescent amorphous organic matter (AOM) and alginite macerals confirm an algal/ bacterial source of organic matter for the dysodiles. The presence of opaque AOM, phytoclasts, sporinites and cutinites macerals, suggests a variable terrestrial organic matter input for the dysodiles-like and non-dysodiles. Maturity parameters such as Tmax values (on average 428 +/- 7 degrees C for dysodiles, 429 +/- 6 degrees C for dysodiles-like and 424 +/- 10 degrees C for non-dysodiles), vitrinite reflectance (< 0.5%) and Carbon Preference Index (> 1) indicate that the organic matter is still thermally immature. The present study sheds light on the long forgotten dysodile shales and similar material, and their importance as potential source rocks or oil shales. A comparison with local and regional source rocks is provided in order to emphasize the specific character of the dysodile deposits.