Phase separation and secondary migration of methane-rich gas accompanying uplift of an unconventional tight-hydrocarbon system, Montney Formation, western Canada

Abstract

Tight-hydrocarbon systems can undergo significant depressurization through leakage of fluids, primarily gas, in response to uplift. Leakage at the up-dip margins of these systems may be highly variable if internal gas transmission by secondary migration is focused along fairways with favourable stratigraphic and structural characteristics. In this study of the Montney Formation, regional mapping of the methane content of produced gas reveals two orders of trends. The first-order regional trend is increasing methane content with depth and thermal maturity. Second-order trends are orthogonal or oblique to the first-order trend, and defined by curvilinear fairways with methane contents higher than expected from the first-order trend. They can be mapped for 10s km with some extending up-dip from the dry gas window into the oil window. Methane enrichment along second-order trends reflects fractionation of hydrocarbon fluids by a variety of processes including phase separation, secondary migration, fluid mixing and evaporative fractionation. Gas composition, recombination PVT and carbon isotope ratio data from produced fluids are used in three local case studies to show that methane-rich gas was driven up-dip by buoyancy forces and regional pressure gradient with migration focused along suitably oriented fairways of enhanced permeability and positive local structure. Secondary migration of methane-rich gas resulted in modified hydrocarbon compositions with leaner liquid contents and higher gas-oil ratios than expected from thermal maturity indicators. Recognition of secondary methane migration fairways is thus essential for hydrocarbon evaluation and exploitation in the Montney Formation, and conceivably in other analogous tight-hydrocarbon systems that have undergone significant uplift.