Rock-Eval pyrolysis is the most widely used method for screening the petroleum generation potential and thermal maturity of organic rich rocks. In using Rock-Eval pyrolysis to evaluate the properties of organic material within a sedimentary unit, a graph Of S2 (hydrocarbons generated by the pyrolysis, measured in milligrams of hydrocarbon per gram of rock) vs. TOC (total organic carbon content of the rock by percent weight) is an effective tool. It provides the correction for HI (miligrams of hydrocarbon per gram of total organic carbon) and indicates the type of kerogen present, and also measures the adsorption of hydrocarbon by the rock matrix. With the S-2 vs. TOC diagram the organic component of different suites of samples may be compared and their petroleum generation potentials established. As an example, the diagrams are used to evaluate the sedimentary environments and petroleum potential of the Middle Triassic-Lower Jurassic sediments (Bak6k, Girmeli, ( amurlu, Telhasan, Dinqer, KozIuca and Yolaqan Formations) of the eastern part of SE Turkey. The organic material contains about 24 (type III), 27 (type II/III), 33 (type II), 36 (type II), 37 (type II), 23 (type III) and 17 (type III) pyrolyzable hydrocarbons respectively. The positive x-intercept (0.014388) has been calculated only in Yolaqan Formation samples according to S-2 vs. TOC diagrams. This value shows a low rock matrix effect. Clay is the main agent of adsorbtion. (c) 2006 Elsevier B.V All rights reserved.