Forensic Isotope Methodology for Discriminating among Hydrocarbon and Brine Sources in a Producing Oil and Gas Field

Forensic methods were successful in discriminating among sources of fugitive hydrocarbons in groundwater by using stable isotopic measurements of δ¹¹B, δ³⁴S, δ¹³C, δD in co-migrating inorganic dissolved constituents, as well as the direct determination of δ¹³C, δD, δ¹⁸O, in HC gases, and HC components dissolved in groundwater. Groundwater from a deep water supply well adjacent to a producing gas well pad containing dissolved methane, low levels of BTEX, and elevated salinity in the vicinity of producing gas wells had as many as seven different hypotheses for source contribution, including: natural gas, produced water and condensate, leaching of near-surface organic-rich oil shale, natural brines, fluids from major faults connecting with productive HC reservoirs, reserve pits, or residual components in water well casing such as pipe sealers, and oil residue. The isotopic labels from all potential sources were used along with conventional measurements of concentration to systematically define the sources of each constituent. The dissolved gas was not thermogenic, eliminating the producing wells as a source, and was clearly biogenically derived from the shallow oil shale facies intersecting the well screens of the water well. In contrast the key BTEX components were isotopically different from the oil shale but clearly related to a thermogenic source. The elevated salinity was not derived from any know natural brine in the immediate vicinity nor did it correlate with produced water. The source of all dissolved constituents except for dissolved biogenic methane correlated with residual liquid from a closed reserve pit. The components in the pit were a mixture of produced water, natural brines and completion fluids with a combination of components providing a unique combination of isotopic labels. Following two limited pumping events in the affected groundwater well and continue monitoring, hydrocarbon concentrations have decreased dramatically, eliminating the need for extensive characterization and potential remediation. Additionally, the presence of biogenic methane derived from the shallow oil shale was found to be widely distributed in the groundwater within the producing gas field and further explained the anecdotal occurrences of gas in water wells and gas pockets encountered in drilling locally.

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