STABLE CARBON ISOTOPIC VARIATION OF FLUID INCLUSION GASES FOR CHARGE HISTORY ANALYSIS

Petroleum Inclusion Cluster Counts (PICC), performed on petrographic thin sections from clastic reservoir cores, show that fluid inclusions in a given sample can be highly variable in abundance at a centimeter scale. To explore how petrographic variability of the sample is reflected in stable carbon isotopic composition (δ¹³C), sub-samples with high- and low-abundance of fluid inclusions were physically separated and subsequently analyzed.

Contents of the fluid inclusions were released within an air-tight, off-line crushing vessel. Fluid inclusion gases were then drawn with a syringe and introduced to a GC-FID coupled with an IRMS to measure δ¹³C values for samples of different fluid inclusion abundances. Results from fluid inclusion gases were then compared to those from a production gas sample taken from the same well and reservoir unit.

Both volume percentages and δ¹³C values of hydrocarbon (C1-C5) compounds of fluid inclusion gases are similar among sub-samples, which are also similar to those of the production natural gas sample. This indicates that hydrocarbon gases in both fluid inclusions and production gas possibly have a similar origin. Yet, the discernible carbon isotopic variation (up to 2‰) may reflect maturity progression of different charge episodes, with the current production gas being the latest and most mature charge.

All fluid inclusion gases analyzed contained up to 80% CO₂, relative to only 3% of CO₂ in the production gas sample. High CO₂ in fluid inclusion gases could be inherited from primary quartz-forming processes. The δ¹³C values of fluid inclusion CO₂ are about 5‰ less negative than those of the production gas, apparently due to mixed organic and inorganic CO₂ origins in the inclusions.