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New insights into the Lower Cook Inlet DGGS-led team reports on results of 2015 field investigations of the underexplored Mesozoic rocks of the oil and gas bearing basin ALAN BAILEY Petroleum News
Alaska’s Division of Geological and Geophysical Surveys has published a report detailing the findings from field work conducted during the summer of 2015 on the western coast of the lower Cook Inlet. A DGGS-led team is conducting a multiyear investigation into the petroleum geology of the Cook Inlet basin - the idea is to assemble publicly available data that can aid in the search for new hydrocarbon resources.
Widely exposed rocks Field results described in the new report focus on strata of Mesozoic age that are widely exposed along the coast on the west side of the inlet in the region of the Iniskin Peninsula. Although the producing oil and gas fields of the Cook Inlet basin have reservoirs in rocks of Tertiary age, the oil appears to have originated from older and deeper Mesozoic rocks that extend under the Tertiary strata. Over the years there has been much speculation about the potential for finding new oil and gas resources in the deep Mesozoic section of the upper Cook Inlet, or in the shallower Mesozoic of the lower Cook Inlet to the south. The Mesozoic rock exposures on the west side of the inlet provide opportunities to evaluate the petroleum potential of the Mesozoic sequence.
Unlike the Tertiary strata, which were laid down from rivers and lakes in a terrestrial environment, much of the Mesozoic sequence appears to have formed in a marine situation. However, the DGGS team found during their 2015 fieldwork that a part of the Talkeetna formation, the Horn Mountain tuff of lower Jurassic age, showed clear evidence of a terrestrial environment. In particular, conglomerates within this rock unit contain large fossilized tree logs, while sandstones contain fossil leaves. The nature of the sediments indicates that they were formed from volcanic ash, with rivers forming small channels through the vegetated ash deposits, while periodic floods shifted material across the terrain.
Oil source? The team has also been investigating surface exposures of the Red Glacier formation, a rock unit towards the bottom of the middle Jurassic Tuxedni group and laid down in a marine setting. Much of the Cook Inlet oil is thought to have originated in the Tuxedni. And, of six rock formations within the Tuxedni, the Red Glacier formation is the only unit to have, in outcrop, fine-grained rocks that appear to be rich in organic material, the DGGS report says. Outcrops of the rock are seen between Tuxedni Bay and the Lateral and Red glaciers.
The team has found significant variations in the nature of the rocks within this formation, depending, it appears, on whether the sediments that formed the rocks were deposited near an ancient river delta or farther offshore, in deeper water. These variations have implications for the distribution of oil-prone source rocks in the subsurface, the DGGS report says.
Sandstones in the Red Glacier formation consist of material eroded from an ancient volcanic arc. Given the chemical and mechanical instability of the volcanic material in these rocks, the rock formation is unlikely to operate as a conventional oil or gas reservoir, the DGGS report says. But, with some of the material having decomposed into clays, the formation could hold tight gas. There is also the possibility that the formation could host continuous oil accumulations, analogous to those in the Bakken formation of North Dakota - a similar possibility has been suggested for the overlying Gaikema sandstone, the report says.
Oil staining The DGGS team also investigated the middle Jurassic Chinitna formation, above the Tuxedni group, in the Tuxedni Bay area. The team noted oil staining in this formation at one location in Chinitna Bay, demonstrating at least local migration of oil though the rock, although any role that the formation may have in Cook Inlet petroleum systems remains unclear. Sediments in this formation have formed sandstones and conglomerates suggestive of a high-energy deltaic environment.
It appears likely that when these sediments were laid down significant volumes of coarse detritus spread into areas beyond the current rock outcrop belt, leading to the possibility of prospective oil and gas reservoir rocks, deposited in a marine environment and now lying in the subsurface of the Cook Inlet basin, the DGGS report says.
The upper Jurassic Naknek formation, above the Chinitna formation, contains fine-grained sands and silts and is interpreted as having been predominantly laid down on the slope and floor of a marine basin. Ancient erosion channels cut through the sediments. And geologists have found evidence of the existence of deep-water canyons that existed at the time that the sediments were deposited. The study conducted by the DGGS team yields insights into the distribution of coarse-grained strata that might host oil and gas, the DGGS report says.
The DGGS team has also studied rock fracturing in the Chinitna and Naknek formations, finding that the fractures tend to be predominantly oriented northwest-southeast. With the modern-day principal stresses in the rocks being oriented in roughly the same direction, these fractures would seem likely to have acted as natural conduits for subsurface fluid migration, the DGGS report says.
Cretaceous reservoir potential Moving higher in the geologic succession, rocks of Cretaceous age in the lower Cook Inlet region tend to contain more quartz grains, rather than volcanic detritus, and hence appear more promising as conventional oil and gas reservoirs than do the older and deeper rocks in the Mesozoic succession. The DGGS team has observed a wedge of conglomerate and sandstone with residual oil staining and silty coal in Cretaceous rocks at Shelter Creek, to the northeast of the Iniskin Peninsula. And there are long-known oil-bearing Cretaceous sandstones near Saddle Mountain, a few miles further northeast.
Upper Cretaceous strata are widespread in the subsurface of the Cook Inlet. Although relatively few wells have penetrated these rocks, three of the wells showed non-commercial quantities of oil - the broad distribution of these rocks suggests that the rocks may form conventional oil and gas reservoirs in the underexplored Mesozoic stratigraphy, the DGGS report says.
In 2015 the DGGS team also investigated the geometry and history of folding and faulting of the rocks along the Bruin Bay fault, the major geologic fault that marks the northwestern side of the Cook Inlet basin.
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