Insights into Gas Geochemistry of Large Tight Gas Sandstone Reservoirs from Fluid Inclusions

The composition of gas produced from large tight gas sandstone reservoirs in the Rocky Mountains varies areally and stratigraphically within fields and between fields. At Jonah Field (Green River Basin), for example, gas wetness (fraction C2+) varies from 0.00 to 0.38, and the carbon isotopic composition of methane ranges from -27.7 to -44.0 per mil. These differences can be attributed to varying contributions from different sources, gas generation at varying thermal maturities, migration effects and biogenic alteration of gases.

We have investigated the gas composition of fluid inclusions in cuttings samples from two major tight gas fields in the Rocky Mountains: Jonah Field; and the Mamm Creek-Rulison-Parachute-Grand Valley complex in the Piceance Basin, Colorado. This is one component of a major project applying gas geochemistry to understand reservoir filling processes and compartmentalization in large tight gas sand fields. Fluid inclusions contain samples of gases that migrated through the reservoir in the past; by comparing their composition to produced gases, we gain insight into the effects of changing thermal maturity.

At Jonah Field, fluid inclusion gas compositions within the Lance reservoir are distinctly wetter in proximity to the southern fault bounding the structure, a pattern not reflected in present-day produced gases. This fault bounds the Jonah pressure compartment; we suggest this fault in the past had served as a migration pathway for relatively deep Type II source rocks that contributed wet gas and light oil. In the Piceance Basin fields, fluid inclusions record much wetter gases in the eastern field (Mamm Creek), similar to production gas compositions. The Piceance data also record a paleo-oil-water contact in the area of wet gas production, suggesting a complex history of filling, leaking and refilling from increasingly mature source rocks.

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