Characteristics of Organic Matter Distribution and Hydrocarbon Expulsion in Carbonate Source Rocks

Key words: 
• carbonate rock /
• source rock /
• expulsion /
• primary migration /
• stylolite seam /
• pressure solution

Abstract: Core analysis of Ordovician carbonate rocks from a well in Tarim basin in Northwest China showed that the carbonate source rock consisted of three main components,namely pure carbonate matrix,argillaceous carbonate band and stylolite seam.From organic carbon analysis,it can be seen that the organic carbon contents in argillaceous carbonate band and stylolite seam were very high,with the average of 1.14% and 1.92% respectively and that in pure carbonate matrix was very low,with the mean value of 0.098%.Whereas,the distribution of S1／TOC value from Rock Eval analysis was completely different,with highest value in pure carbonate samples and much lower values in argillaceous carbonate band and stylolite seam samples.But the three kinds of samples from the same depth in the well did not show much differences in S2／TOC and Tmax values,indicating that the kerogen contained in these three kind samples from the same depth had the same thermal maturity and the same hydrocarbon potential.The significant differences in S1／TOC values among the three kinds samples were resulted from different efficiency of hydrocarbon expulsion in different kinds of carbonate components. The lack of mechanical compaction and the low hydrocarbon generation amount due to the low organic carbon content resulted in the lack of expulsion force in pure carbonate source rock,so the expulsion efficiency was very low and most generated hydrocarbons still remained in the source rock,which accounted for the high S1/TOC value in pure carbonate samples.Whereas,the powerful driving force for hydrocarbon expulsion in argillaceous carbonate band and stylolite seam,resulted from the intensive mechanical compaction owing to their high mud content and the high hydrocarbon generation amount due to the high organic carbon content,made the most generated hydrocarbons migrate out of argillaceous band and stylolite seam and caused the low values of S1/TOC. Stylolite seam and argillaceous band were the components of high organic carbon content in carbonate source rocks.The high organic carbon content in stylolite seam was the result of the accumulation of organic matter in the seam by pressure solution,and the high organic carbon content in argillaceous band was mostly from original sedimentation.In the thickness of 650m Ordovician carbonate rocks in the above well in Tarim basin,the total thickness of stylolite was 20m and that of argillaceous carbonate band was 130m. They were the most important contributors to the Ordovician carbonate source rock in Tarim basin.