Distinguish TSR by FT-ICR MS Combined With Carbon/Sulfur Isotopic Analysis for Condensate Oils in the Tarim Basin, China

Abstract

The maximum H2S content in the gases associated with the deep oil in the Tarim basin is up 40%. Identification and evaluation TSR would be crucial for both deep petroleum exploration and exploitation. Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS) combined with carbon/sulfur isotopic analysis in individual compounds was utilized to investigate TSR in the oils. 1. Significant variations of the composition and relative distribution of the sulfur compounds in the oils were observed based on FT-ICR MS. Most of the oils have abundant S1 class species (one sulfur atom) dominated by the compounds with double-bond equivalent (DBE) value to be 9 (primarily dibenzothiophenes, DBTs). Abnormal sulfur compounds were detected from the condensate oils from wells of M401 and YN2, dominated by S1 class with DBE<4. The predominance of the S1 class with relatively low thermal stability in the high maturity oils, suggests thermochemical sulfate reduction (TSR). Abnormality was also observed for the oil from Well ZS1C, characterized by unusually high concentrations of DBTs and a maximum level of d34S (about ±37‰) for individual sulfur compounds. The completely different kind of sulfur compounds in the oils might be produced by TSR during two different stages and/or have different formation mechanisms. Descending carbon isotopic curves for n-alkanes in the ZS1C and YN2 oils were observed, which might result from carbon isotopic fractionation during TSR. The Cambrian oils from Well ZS1 and ZS5, were suggested to be less altered by TSR and/or mixing with TSR-oil/gas. It seems the TSR-oil/gas could be found in deep horizons and the shallow reservoir mixing with deep hydrocarbons, and the Cambrian genetic oils i.e., M401, YN2 and ZS1C were severely TSR altered. 2. The oils in the Halahatang Oilfield bearing relatively high H2S content (~4%) in the associated gases was suggested to be altered by bacterial sulfate reduction. No abnormal sulfur compounds were detected by FT-ICR MS and so on for the sulfur isotopic analysis. 3. We suggest that FT-ICR MS could be utilized to detect TSR occurrence and/or the TSR-sulfur compounds with lower thermal stability not yet evolved into the compounds with high thermal stability. Sulfur isotopic analysis could be used to identify the occurrence and extent of TSR, especially at earlier stage. These two approaches could be integrated together in order to distinguish TSR more accurately.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015