Interprets the covariance of organic carbon and sulfide sulfur in core samples. This covariance results from the catabolism of organic carbon and concomitant reduction of sulfate by sulfate reducing bacteria to form aqueous sulfide which reacts with iron. Defines a central basin area that was the most anoxic-sulfidic (euxinic). This part of the basin is similar to the area of thickest, most organic carbon-rich sediments and has the greatest source-rock potential for petroleum. -from Author