Isotopic fingerprints of shallow gases in the Western Canadian sedimentary basin: tools for remediation of leaking heavy oil wells

Abstract

Thousands of wells have been drilled in northeastern Alberta and adjacent Saskatchewan in order to develop the heavy oil reservoirs of the Lower Cretaceous Mannville Group. In a large number of these wells, vertical migration of gas from unknown sources to the surface via well casings and surrounding soils represents a serious environmental problem. Mud samples from new wells drilled in the area were collected at approximately 50 m intervals from the surface to the completion depth, and the gases contained within the muds (‘mud-gases’) were analyzed for 13C/ 12C composition. Gases from the various Mannville Group sands do not show unique isotopic signatures; they comprise bacterial methane ( δ 13C C1=−70‰ to −60‰ (PDB)), with trace C 2+ components that are residues from biodegradation processes. Gases from each of the overlying Upper Cretaceous Colorado Group shale units are, however, isotopically distinct. The δ 13C of methanes in the shales have values equivalent to those within the Mannville sands, but the δ 13C of the C 2+ components are unambiguously different. C 2+ gases from the shallower Colorado shales are incipient thermal gases and fit the isotopic maturity diagram of James, A.T., 1983. AAPG Bull. 67, 1176–1191, and also of Chung, H.M., Gormly, J.R., Squires, R.M. 1988. Chem Geol. 71, 97–103. We believe these gases may include the most immature suite of thermogenic gases reported ( δ 13C C2=−63‰ to −32‰, δ 13C C3=−42‰ to −22‰, δ 13C nC4=−37‰ to −23‰). Carbon isotope fingerprints from mud-gases are used to identify specific source depths of migrated gases. Comparisons between the isotopic characteristics of the shallow gases and the migrated gases collected near surface infer that most of the migrated gases are actually generated in the Colorado Group shales, and not the Mannville sands in which the wells were completed. The ability to estimate the source depth of the migrated gases greatly aids remediation of ‘leaking’ wells within the Western Canadian Sedimentary Basin (WCSB).