Molecular composition and isotope mapping of natural gas in the British Columbia Natural Gas Atlas

Abstract

This thesis provides a geochemical interpretation of natural gas resources in north eastern British Columbia (NEBC), Canada. The work is part of the three-year project, British Columbia Natural Gas Atlas (BC-NGA) to collect samples and compile data on molecular (C1 to C5) and stable isotope ratio (δ13C and δ2H) compositions of natural gases in NEBC. The primary objective of the BC-NGA project is to produce a comprehensive, public, web database with maps of the gas geochemical data from a variety of gas tests including mudgas collected during drilling, downhole flow tests, production gas, and gas collected from surface emissions. The area of study in NEBC is a large portion of the Western Canadian Sedimentary Basin (WCSB) with Paleozoic, Mesozoic, and Cenozoic strata of thousands of meters thickness. Within this stratigraphic package there are numerous depositional hiatus and regional aquitards complicating the generation of regional maps and profiles. This M.Sc. thesis utilizes the geochemical gas parameters to characterize the range of gases in the BC-NGA database. The thesis found that the petroleum sources and active generation processes are not uniform in the NEBC. In some cases, the original gas signatures have been overprinted by localized processes in specific strata. The results of this new data plus compilation of existing data in the BC-NGA dataset indicate that many classical interpretive diagrams, e.g., Bernard Diagram (C1/[C2+C3] vs. δ13C1) and CD Diagram (δ13C1 vs. δ2H-C1), confirm the microbial/ thermogenic nature of the gases, but lack the resolution for detailed stratigraphic interpretation of gas sources and migration. A particularly interesting finding is that δ13Ckerogen (-33 ‰) estimated from δ13C1 observed for most strata in NEBC is 13C depleted compared with conventional kerogens and the data supports new calibration of the methane isotopes. This δ13Ckerogen value is an unlikely value and therefore the offset observed compared with conventional natural gases requires a different explanation, including commingling of 13C depleted methane from microbial sources. Enhanced characterization is obtained by combinations of the gas parameter ratios: δ13C1, δ13C2, δ13C3, C2/C3, C2/iC4, (e.g., ‘Berner-Faber Diagram’, ‘Prinzhofer Diagram’, ‘Lorant Diagram’). In addition, a new plot of δ13C2-δ13C1 versus iC4/nC4 ratio was developed in this thesis.