Thermal Alteration of Crude Oils in the Central Tarim Basin, Northwest China, as Revealed by Molecular Isotopic Compositions of Gasoline Range Hydrocarbons

Abstract

The gasoline hydrocarbons are the important intermediate products during the thermal degradation of sedimentary organic matter, and ultimately to methane and graphite with increasing temperature. Theoretically, gasoline hydrocarbons from kerogen are directly degraded from macromolecular network and will migrate out of the source rock via generation and expulsion once hydrocarbon concentration reaches up to the saturated threshold of rock, while gasoline hydrocarbons from oil-cracking are formed via C-C bond cleavage of high molecular weight compounds, cyclization and aromatization, and involved an isotopic kinetic fractionation, although they will be destroyed under very high temperature. Therefore, gasoline hydrocarbons in condensates from oil-cracking should be characterized not only by highly relative concentrations, but also by the 13C enrichments of cycloalkanes and monoaromatics, suggesting that molecular distributions and stable carbon isotope compositions of gasoline hydrocarbons can be a measure to characterize genetic types of condensates. In last two decades, large reserves of natural gas and condensates have been discovered from the Central Tarim Basin, NW China. Although exploration efforts have greatly enhanced our understanding of source, maturation and secondary alteration of crude oils in this region, genetic types of gas and condensates discovered to date is still ambiguous and vigorously debated. Most published work concluded that gas and condensates were derived from thermal degradation of highly mature kerogens and/or evaporation fractionation due to gas intrusion, while others argued that it was from deep buried in-reservoir oil cracking. Here, molecular carbon isotopic compositions of gasoline hydrocarbons are measured and compared with that from typical kerogen-sourced oils in the Palaeozoic petroleum systems. The results suggest that there is a significant contribution from secondary thermal alteration in unpenetrated, deep-buried oil reservoirs to account for the enrichment of condensates. Integrated with geological data, the distribution and magnitude of thermal alteration in the Central Tarim Basin are discussed.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015