Finding clues for lower Cook Inlet oilState survey publishes results of an investigation of a rock sequence with oil-bearing sandstone on the west side of the inlet Alan Bailey Petroleum News
During its summer 2011 Cook Inlet field season a team of geologists headed by Alaska’s Division of Geological and Geophysical Surveys investigated an upper Cretaceous rock sequence that outcrops near Saddle Mountain, on the west side of the lower Cook Inlet. This particular rock sequence, known from fieldwork done by Cook Inlet geologists several decades ago, includes a sand layer that contains residual oil. The rock sequence provides some intriguing clues regarding the petroleum geology of the lower Cook Inlet, a region with a very limited history of oil and gas exploration — all known Cook Inlet oil and gas fields are in the upper Cook Inlet to the north.
The state survey has now published a report on its Saddle Mountain findings.
The exposed upper Cretaceous rock sequence, some 250 feet thick, includes sandstones, conglomerates, siltstones and thin coal seams, all apparently deposited on land rather than under the sea, the report says.
Friable sandstone The oil is found in a friable sandstone layer some seven to eight feet thick. When undisturbed the sandstone emits a faint hydrocarbon odor, but when the rock is freshly excavated the odor, although fleeting, becomes strong to very strong.
A U.S. Geological Survey laboratory extracted and analyzed hydrocarbons from a sandstone sample that the field team collected. The chemical composition of the oil turned out to essentially lie within that of oil known elsewhere in the Cook Inlet basin to have been sourced from the Tuxedni group within the Jurassic rock sequence of the basin. The Tuxedni is considered the prime source of oil in the Cook Inlet oil fields.
The state survey has determined that the friable oil-bearing sandstone has significantly higher porosity and permeability, and hence better potential oil reservoir characteristics, than other sandstones in the Saddle Mountain sequence. Those other sandstones tend to be cemented with calcite and have pores that are clogged with clay material, the state survey report says. Insights into the distribution and timing of the clay and calcite formation would be particularly important in understanding the oil potential of the lower Cook Inlet region, the report says.
Offshore wells Fossil pollen grains in the Saddle Mountain rocks indicate the upper Cretaceous age of the rocks and enable the rock sequence to be correlated with non-marine rocks encountered in the COST No. 1 well, drilled in 1977 in federal waters of the inlet about 31 miles to the south of the Saddle Mountain location.
In addition, the lower Cook Inlet Raven well, drilled in 1980 by ARCO about 25 miles to the south-southeast of Saddle Mountain, penetrated upper Cretaceous strata, including both non-marine rocks similar to those at Saddle Mountain and sedimentary rocks formed in a marine environment. The well encountered oil-stained rocks and “movable hydrocarbons” between depths of 1,760 feet and 3,700 feet, the report says
An analysis done some time ago of oil from the Raven well suggested a mixed source, with some oil coming from the Tuxedni and some from an older Triassic source. However, a more recent analysis indicates just a Tuxedni source, the report says.
The presence of hydrocarbons at widely scattered locations in the lower Cook Inlet area, including the Saddle Mountain location, the Raven well and some known oil seeps indicates the presence of mature oil source rocks at depth in the region, the report says. An understanding of what controls the distribution of oil between different rock units in the region would be significant for oil exploration.
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