Diagenesis and Reservoir Characterization
The carboniferous lisburne group in the fourth range, shublik, and sadlerochit mountains of the northeastern Brooks Range, Arctic National Wildlife Refuge, Alaska, U.S.A., consists of carbonate strata of the Alapah and WahooLimestones from the Sadlerochit high to paleotopographically lower areas 65 km to the southwest. The southward paleoslope is documented by onlapping geometry, erosional topography, and lateral facies variation. Twelve surfaces of subaerial exposure were identified in the Alapah Limestone and six in the Wahoo Limestone. Two can be traced over about 100 m of topography; several others disappear southward into subtidal rocks, indicating paleoslope. A major unconformity lies along the top of the Lisburne Group and predates deposition of Permian strata.
Crosscutting relationships show that early cathodoluminescence (CL)-banded calcite cement was precipitated during subaerial exposure events; thus it is meteoric in origin. Low Mg and Sr contents and a depleted mean δ18O of -6%o support the meteoric interpretation. Correlation of CL zones illustrates that meteoric calcite occluded much of the primary porosity. Cements commonly decrease in abundance downward beneath exposure surfaces. Traces of evaporites and the character of exposure surfaces within the Alapah Limestone indicate an arid climate during deposition, whereas dissolution features within the Wahoo Limestone suggest a humid climate. Variations in cement abundance cannot be related to changes in climate. The proportion of porosity reduced by meteoric calcite is higher in upslope areas than in downslope areas, indicating that duration of exposure was an important control.