AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Carbon and Hydrogen Isotope Systematics in Gases from Horizontal Bakken Shale Wells
(1) GasConsult International. Inc., Berkeley, CA.
(2) North Dakota Geological Survey, Bismarck, ND.
We report here composition and isotope data from horizontal Bakken shale wells in North Dakota that penetrated different sections of different maturities and found that ethane carbon isotopes in oil-dissolved gases are excellent predictors of maturity of the organic matter of the source rock kerogens, thus opening opportunities for assessing migration of oils and gases.
The 8 wells of this study are in two maturity domains based on Bakken Shale Rock-Eval hydrogen indices. The less mature domain around Squaw Gap and Bicentennial (“SGB) has high hydrogen indices (HI~450 to 530mg/g), whereas the more mature domain around Pierre Creek, Buckhorn and Elkhorn Ranch (“PCBE) has lower hydrogen indices (HI≈170 to 270mg/g). Gases from the less mature SGB wells(GOR~350 to 390) and high %C2+ concentrations (27 to 48%) whereas gases in the more mature PCBE area have higher GOR's (~560 to 720) and lower %C2+ (~30%). In the less mature wells at Squaw Gap (HI~535), the produced gas has ethane carbon values del13C2 of -38 ‰, in contrast to del13C2 of -34.2‰ at the more mature Elkhorn Ranch well 44-25H . Similarly, other isotopic properties such as the differences of del13C2-3 and del13CiC4-nC4 as well as isotopic differences between gas components and oil tend to decrease with maturity. Ethane del13C values are an excellent proxy for all maturity-related properties of gases and associated oils as well as rock properties such as HI. In particular, hydrogen isotope values in ethane and propane are excellent predictors of the onset of kerogen conversion. Therefore, our gas isotope-HI relationship can be useful in various ways in exploration and production of Bakken Shale plays: The comparison of HI in Lower and Upper Bakken Shale along the trace of a horizontal well with those calculated from the gas isotopes in the oils will allow: a determination of the indigenous nature of the oils (no significant migration) or, if a discrepancy is found, an estimate of the migration distance using existing HI distribution maps published by the North Dakota Geological Survey. A lower maturity determined from gas isotopes compared to oil biomarker maturity could indicate fracture zones from which gases from the locally less mature Bakken Shale bleed into the oil that is derived from more mature Bakken source rocks.