ORGANIC GEOCHEMISTRY AND PETROGRAPHY OF ORGANIC-RICH SEDIMENTS OF SHEMSHAK FORMATION IN EASTERN ALBORZ, NORTH IRAN

Organic geochemistry and petrography investigations for determination of organic matter quality, type and thermal maturity were carried out on shaly sediments of Shemshak Formation (Upper Triassic to Lower Jurassic) from twelve sites in eastern Alborz basin.

The total organic carbon) TOC( content of samples is between 0.2 and 6.7 wt %, indicating a poor to good organic content. So far 165 surface samples have been studied. Organic geochemical analyses (Rock-Eval Pyrolysis) performed on thirteen kerogen concentrates reveal the dominance of organic matter with low hydrogen content with terrestrial or degraded marine origin. The location of studied samples on the modified Van-Krevelen diagram (Tmax-HI) shows the presence of mixture of kerogens type IV and type III. Tmax values obtained from Rock-Eval Pyrolysis ranges from 390 ºC to 587 ºC, indicating immature, mature and over-mature organic matter.

Organic Petrography using transmitted light microscopy suggests organic matter of the studied samples includes continental as well as marine fraction. The continental fraction contains opaque phytoclasts, semi-opaque phytoclasts, translucent phytoclasts and terrestrial sporomorphs. The marine fraction is mostly composed of dinoflagellate cysts. Amorphous organic matter dominate in all samples and it could be derived from the degradation of both terrestrial and marine constituents. Organic Petrography in the studied samples indicates also the presence kerogen type II and one sample type I. The sedimentary organic matter of the Shemshak Formation, in the studied samples corresponds to inertinite, vitrinite, cutinite, spornite and exinite macerals .