The Mikkelsen Bay State #1 well has one of the few completely cored Lower Cretaceous successions of the North Slope region between the NPRA and the ANWR. Forty-seven samples were collected over 30 m of section in this well to evaluate the variation in petroleum source potential and lithofacies, and to determine controls on their deposition. The sampled section includes the top Kemik Sandstone, the pebble shale unit, and possibly the basal Hue Shale (known as the gamma-ray zone or GRZ).
To determine lithofacies, unusually thin polished thin sections were prepared and analyzed using combined optical and backscattered electron imagery. The base of the studied succession, the upper part of the Kemik Sandstone, comprises medium to fine-grained sandstone and bioturbated muddy sandstones. These are abruptly overlain by burrow-mottled, thin (2-5 mm), relict-bedded, silt and pyrite-bearing clay and organic-rich (2-6% TOC) mudstones with rare “floating” sand grains (up to 500 microns) of the pebble shale unit and possibly basal Hue Shale. Lapilli tuffs begin to appear in the middle of the pebble shale, becoming more common near the top of the sampled core. Carbonate-dominated and clay-bearing pyrite-rich mudstones appear towards the top of the studied succession.
Forty-five samples were analyzed by Rock-Eval pyrolysis. These data can be divided into 3 units that are consistent with borehole geophysical log signatures and lithofacies. The lower unit, which is the upper part of the Kemik Sandstone, is a non-source interval that contains migrated hydrocarbons. The overlying mudstone succession comprises 2 distinct units which both contain primarily type II, oil-prone kerogens. The lower part of the pebble shale unit is just entering the oil window and hasn’t reached peak hydrocarbon generation, whereas the upper part of the pebble shale unit and possibly the lower part of the Hue Shale have generated hydrocarbons. The original S2, TOC, and HI values for this upper unit were probably closer to or greater than present values for the lower part of the pebble shale unit.
This high resolution study illustrates the dramatic variability in lithofacies and organic facies present over a short interval of the Lower Cretaceous mudstone succession of the North Slope. This variability is attributed to changes in sediment transport mechanisms (fluvial + storms versus ice-rafting); sediment provenance (detrital clay versus volcanic ash, and possibly north versus south source); position along the sediment transport path (silt to clay ratio); bottom water oxygen (episodically oxic versus anoxic); diagenesis; and organic productivity.
