Stable Carbon Isotopes of Pyrolysates from Various Marine Hydrocarbon Source Material from Southern China

Hydrous pyrolysis experiments of kerogen, solid bitumen and liquid hydrocarbons in southern China were carried out in order to study the processes of gas generation and derive geochemical indicators of gas genesis in geological pressure and temperature regimes. The results indicate that gas generation productivity of different marine material decreased in the order of crude oil (light oil and condensate), dispersed soluble organic matter (solid bitumen and heavy oil), and kerogen. Under identical temperature-pressue regimes, pyrolysates derived from kerogen and dispersed soluble organic matter display drastically different geochemical characteristics. For example, the δ13CCO₂-δ13C1 values of gaseous products from dispersed soluble organic matter are greater than 20%, while those from kerogen are less than 20%. The δ13C1 values of pyrolysates from different marine hydrocarbon sources generally increase with pyrolysis temperature, but are always lower than that of the source precursors. The δ13C values of ethane and propane in the pyrolysates also increase with increasing pyrolysis temperature, eventually approaching that of their sources, at peak hydrocarbon generation. At high-over mature stages, the δ13C values of ethane and propane are often greater than that of their sources but close to those of coal gases, and thus become ineffective as gas genetic indicators. Prior to peak hydrocarbon generation, the Ln (C2/C3) ranges of gaseous products from crude oil (dispersed soluble organic matter) cracked gas and kerogen degraded gas are relatively small, with a large Ln (C1/C2) range. At peak generation to overmature stage, the Ln (C2/C3) change of gaseous products has a significant difference between manifold kerogen and crude oil (dispersed soluble organic matter). Moreover, with increasing thermal simulation temperature, the Ln (C2/C3) increasing rate of crude oil (dispersed soluble organic matter) cracked gas is obviously bigger than that of kerogen degraded gas. The linear gradient of crude oil cracked gas is bigger than that of kerogen degraded gas. So the Ln (C2/C3) could serve as an effective indicator for distinguishing kerogen degradation gas from crude oil (soluble organic matter) cracked gas, particularly in peak generation to overmature stage. Ln(C1/C2)- (δ13C1-δ13C2)can be an effective indicator for distinguishing gases derived from oil- and dispersed soluble organic matter cracking.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California