Gas Mixing Model in Complex Unconventional Tight Gas Sandstone Formations: Assessment of Long-Distance and Downward Migration

Accurate characterization of the fluid properties is considered one of the most important requirements in determining the potential of unconventional gas reservoirs, reducing exploration risks, predicting migration pathways, and optimizing production plans. Gas generation and migration in unconventional reservoirs are highly complex compared to conventional reservoirs due to their very low porosity and permeability. The objective of this study is to investigate the lateral and vertical fluid variation within the complex tight sandstone formations. To investigate the compositional variations of gases in unconventional reservoirs, carbon isotopic and chemical compositional analysis were carried out on gases from complex tight sandstone formations. Gas chemical composition analysis were conducted using gas-chromatography. Gases were analyzed using a gas chromatograph equipped with stable isotope ratio mass spectrometer. Carbon isotope ratios were measured for individual hydrocarbon compounds from methane to pentane, and for CO2. The results show that the studied gases were sourced from marine source rock and are characterized by a wide range of thermal maturities. Some of the gases were sourced from a non-associated dry gas and others were sourced from oil-associated gas. The carbon isotopic compositions of carbon dioxide and the nitrogen concentration of the two endmembers are distinguishable. An isotopic fractionation model was applied to assess the possibility of mixing between the two endmembers. Data presented in this study suggest that the gases in the tight gas sandstone and shale formations represent two different source rock kitchens with different thermal maturities. Areas with distinguished carbon isotopic compositions of carbon dioxide also indicate differences within the source rock facies (i.e., the presence of calcite cement). Regions with elevated nitrogen concentrations indicate mainly a thermal decomposition of nitrogen-rich organic matter. The results also indicate a downward migration of gases from shallower source into a deeper clastic formation. The results of this study can be further integrated within the reservoir pore network and fractures/faults model to identify migration pathways and predict sweet spot location prior to drilling. These findings could contribute to reduce exploration risks and improve production plans.