Abstract: The authors report the organic matter in seep-carbonates from the block GC 185 (Sample GC-F) of the upper slope and the block AC 645 (Sample AC-E) of the lower slope of Gulf of Mexico, where water depth is 540 m and 2200 m, respectively. Their results show that both organic contents and alkane compositions are different between Samples AC-E and GC-F. Sample AC-E has low organic content, with relatively high fraction contents of normal alkanes, steranes and terpanes. It is dominated by low molecular weight alkanes and also with kinds of tricyclic terpanes and pentacyclic triterpanes that are maximized at 17α(H)，21β(H)-hoptane, indicating that the organic matter of the lower slope (Sample AC-E) mainly originated from bacteria and marine algae, with a little hydrocarbon seep from the underlying reservoir and slight microbial degradation. Sample GC-F has higher organic content, but its content of normal alkanes is low with mainly unresolved complex mixture (UCM). It also has kinds of tricyclic terpanes and pentacyclic triterpanes that are maximized at 17α(H)，21β(H)-nor-hoptane, suggesting that the organic matter of the upper slope (Sample GC-F) is microbial-degraded and mainly from seep hydrocarbon of the underlying reservoir. They also investigate the composition and distribution of normal alkanes, steranes and terpanes in seep carbonates, and discuss the relationship between organic matter source and cold seep.

Key words: seep carbonate')">seep carbonate, organic matter, microbial process, Gulf of Mexico