The authors present the effects of the total organic carbon (TOC inert ) during pyrolysis and how it overall affects the estimates of the hydrocarbon index (HI) using the cross plot S 2 vs． TOC graph． A total of 21 drillcore samples consisting of the non- carbonaceous mudstones，carbonaceous mudstones，coaly mudstones and coals with S 2 ＞ 0． 35 are selected and used． The average TOC for the sampled population is 10． 36 wt%． TOC inert measured from the S 2 vs． TOC graph is 2． 97 wt% and the average pyrolysable TOC live is 7． 39 wt%． The HI is 102 mg HC/g TOC and the pyrolysable HI is 175 mg HC/g TOC，indicating that the TOC inert or mi- neral matrix adsorbs some of the hydrocarbons liberated during pyrolysis resulting into the low HI values which affects the correct determination of the source rock potential． Other applications of the S 2 vs． TOC cross plot have been used to determine the depositional system tracts． The high stand system tract is also determined through this method．