Print this pageLean Source Rock Expulsion Efficiency and Products: Constraints from the Mowry Shale, Northern Powder River Basin
CORRIGAN, JEFF D., and ALTON A. BROWN
The Late Albian-Early Cenomanian? Mowry Shale (initial TOC = approx. 1-3 wt.%, initial HI = approx. 100-400 mg HC/g TOC) is used to constrain lean source rock expulsion efficiency and GOR yield of expelled products as a function of maturity. TOC and Rock-Eval analyses were collected on cuttings intervals through the 200-300 feet thick Mowry interval for 19 wells along a dip transect, and on Mowry core from three well locations expected to span the maturity range. Transformation ratio, derived directly from the slope of highly correlated TOC versus S2 plots for each sample suite, increases approximately linearly from 0.0 at approx. 6000 feet to 0.9 at approx. 13,000 feet. S1 and TOC for individual core sample suites are also highly correlated with near zero intercept indicating that kerogen sorption accounts for nearly all free, volatizable hydrocarbons (S1). Sorption coefficient lower bounds determined from core sample suites at transformation ratios of 0.0, approx. 0.5, and approx. 0.9 are 16, 90, and 55 mg HC/g TOC, respectively. Retention of hydrocarbon within the inorganic pore space is negligible. Expulsion efficiencies at transformation ratios of approx. 0.5 and approx. 0.9 range from 0.0-0.3 and approx. 0.6-0.8, respectively, and are dependent on initial HI. These data indicate that expulsion is very efficient once the kerogen sorption threshold is overcome. The initial GOR of producing Muddy fields with undersaturated oils is generally less than 1000 SCF/STB where the Mowry is at a transformation ratio of 0.5 or less, and increases approximately exponentially to greater than 5000 SCF/STB at a Mowry transformation ratio of greater than 0.9.