The distribution of porphyrin in oils from Jianghan and Biyang Basins

China

was studied. The high contents of C

32

and C

33

DPEP porphyrin isomers with a additional methyl group in five-membered isocyclic ring is the indicator of an unusual type of porphyrin precusor or input occurred in sedimentary environment with very high salinity and low maturity of oils. The major changes of porphyrins under maturity are the gradual decreases of DPEP/(DPEP+ETIO)% and dealkylation/alkylation within ETIO porphyrins. The ratios of DPEP/ETIO or DPEP/(DPEP+ETIO)%

C

31

ETIO-A/C

28

ETIO-B and ETIO-A C

31

/C

30

can be used as thermal maturity parameters.