|
The pebble shale oil source potential Team of geologists reports on research centered on surface outcrops of a prominent and widespread North Slope oil source rock unit ALAN BAILEY Petroleum News
The pebble shale, lower Cretaceous in age, is one of an assemblage of rock units, including the Hue shale and the highly radioactive or gamma-ray zone, that forms the youngest of the major oil sources in the prolific petroleum system of Alaska’s North Slope. But although the pebble shale is widespread at depth under the North Slope, surface exposure of the rock is mostly limited to the northeastern Brooks Range, adjacent the Arctic National Wildlife Refuge, according to a report published by Alaska’s Division of Geological and Geophysical Services. The report documents an investigation by a team of geologists from DGGS and the University of Alaska Fairbanks of rocks from surface outcrops of the pebble shale, to better understand the rock’s oil source potential in the area of those surface exposures.
Distinctive rock sequence The report says that the pebble shale occurs directly above the Kemik sandstone, a rock that in northeastern Alaska sits on top of a major rock sequence discontinuity known as the lower Cretaceous unconformity. The highly radioactive or gamma-ray zone, generally abbreviated to HRZ or GRZ, a particularly distinctive horizon above the pebble shale, forms the base of the Hue shale. The complete assemblage of rocks is highly variable in composition but has characteristics indicating deposition on an ancient marine shelf, the DGGS report says.
When encountered in well bores the pebble shale tends to contain rounded and frosted quartz grains, together with chert and quartzite pebbles that float in a fine-grained matrix and that have given the rock its name.
But there has been some past confusion concerning whether well cores identified as coming from the pebble shale and used in oil source evaluations have, in fact, come from the pebble shale or from the Hue shale. However, the complete assemblage of Cretaceous rocks associated with the HRZ is a potential source of 28 percent of the oil in the Prudhoe Bay oil field, according to a research paper published in 2001.
Three locations The team of geologists conducting the study now reported by DGGS investigated the pebble shale at three outcrops, one on the west side of the Canning River; one on an unnamed tributary of the Katakturuk River, inside the Arctic National Wildlife Refuge, 23 miles or so east of the Canning River; and one on Marsh Creek, about 7 miles farther east. The team collected 115 rock samples from the three locations for laboratory analysis of the organic content of the rocks, the nature of the organic material and the rocks’ thermal histories. The team compared the results with a previous analysis of pebble shale samples collected from the Mikkelsen Bay State No. 1 well, to the northwest of the study area.
Unlike in the Mikkelsen well, where the total organic content of the rock scatters around a relatively constant level, the team found that in the rocks exposed at the surface the content consistently increases, upwards from the base of the rock unit. At two of the locations there is a subsequent decrease in the organic content at the top of the pebble shale. The geologists attribute these variations in organic content to factors such as changes in the sea level at the time the sediments were laid down, to changes in the sediment supply and to changes in the sub-sea ecosystem.
Reached high temperatures An analysis of the thermal histories of the surface rock samples confirmed that the rocks had at some time in the past reached temperatures fairly high for oil generation and high enough for the generation of natural gas.
And, while the rocks in the study area contain sufficient total organic content for sourcing hydrocarbons, and while the rocks may at some time in the past have generated oil and gas, it appears that past heating of the rocks has degraded the organic material to the point where the rocks now have poor source rock potential.
It seems that the high temperatures reached in the pebble shale in the area of the surface study can be attributed to exceptionally deep past burial of the rocks, with the rock subsequently having been exhumed at the surface, the DGGS report says.
|
