Maturity Determinations in the Vermillion Basin-Baxter Shale: Fluid Maturity Parameters Help Interpret Charge and Migration

Michael P. Dolan¹, David G. Majewski², Kevin Ferworn³, and Stephen Brown^3
1Dolan Integration Group, LLC, Louisville, CO
²Kodiak Oil & Gas Corp., Denver, CO
³GeoMark Research, Ltd., Houston, TX

Mud gas isotope analysis (MGIA) has been conducted on three wells in Sweetwater County, Wyoming. The geologic province is known locally as the Vermillion Basin and is part of the greater Sand Wash/Washakie Basins. The Baxter Shale has been a target for some operators as an overpressured, unconventional, fractured shale play. Other deep targets in this area include the Frontier, Nugget and Dakota sandstones.

Organic thermal maturity indicators are used to determine the maximum level of maturity for a given rock unit. Thermal indicators such as measured vitrinite reflectance, thermal alteration index (TAI), and the Rock-Eval parameter Tmax measure the rock unit directly and indirectly and require core or cuttings for analysis. Stable carbon isotopic analyses of mud gas, production gas and headspace gases provide the opportunity to measure the maturity of reservoir fluids. Measuring the maturity of reservoir fluids in conventional and unconventional plays allows for a more complete interpretation of the petroleum system elements such as source maturity, migration of hydrocarbons, and charge history and timing. Results from MGIA have delineated interesting distinctions among petroleum systems. In addition to discrete signatures among charged reservoirs, a clear demarcation of isotopic signature is seen at the top of overpressure. MGIA is a technique that allows sampling of the mud stream gases while drilling to measure δ¹³C isotopic concentrations of C1-5 components. δ¹³C ethane and δ¹³C propane are good thermal maturity indicators and can be derived from MGIA.

Applying a calibrated maturity scale to the fluids analyzed in the Vermillion Basin wells allows for a robust interpretation of charge history. The complex structural history of this area requires efficient and reliable methods for measuring maturity of the fluids in the reservoir. Integrating rock and fluid maturity and determining similarity or differences between rocks and fluids allows for meaningful interpretation of charge and migration.

AAPG Search and Discovery Article #90092©2009 AAPG Rocky Mountain Section, July 9-11, 2008, Denver, Colorado