Oil-Stained Sandstones of the Upper Jurassic Naknek Formation and Upper Cretaceous Kaguyak Formation, Kamishak Bay Area, Lower Cook Inlet, Alaska

Herriott, Trystan M.; Wartes, Marwan A.; Stanley, Richard G.; Lillis, Paul G.; Helmold, Kenneth P.; Decker, Paul L.; and Gillis, Robert J.
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The Cook Inlet basin of south-central Alaska hosts a prolific petroleum system that has been producing oil and gas for more than 55 years (e.g., Magoon, 1994). The basin’s producing reservoirs are nonmarine sandstone and conglomerate of Tertiary age (e.g., LePain and others, in press), and oil is principally sourced from organic-rich shales in the Middle Jurassic Tuxedni Group (Magoon and Anders, 1992; Lillis and Stanley, 2011). A persistent and as of yet unanswered question is whether the Upper Jurassic through Cretaceous stratigraphy of Cook Inlet hosts conventional oil reservoirs. Limited direct observations have been made in this regard and include fault- and fracture-controlled oil seeps in Jurassic rocks on the Iniskin Peninsula (Detterman and Hartsock, 1966), oil shows in Upper Cretaceous strata of the Raven No. 1 (see LePain and others, 2012) and Anchor Point No. 1 (unpublished industry data) wells, and oil stain in the Maastrichtian Saddle Mountain section (LePain and others, 2012).

During July–August 2012 fieldwork in the Kamishak Bay area of lower Cook Inlet, we studied an oil-stained outcrop in the Upper Jurassic Naknek Formation and discovered, in modern alluvium, oil-stained cobbles and boulders likely derived from nearby outcrops of the Upper Cretaceous Kaguyak Formation. We report our field observations and initial interpretations herein.

The hydrocarbon-bearing outcrop of Naknek Formation lies immediately north of the south shore of Kamishak Bay on a small, unnamed island near the mouth of Douglas River. The oil seep has previously been noted only briefly in published accounts (Magoon and others, 1975 (p. 19); Lyle and Morehouse, 1977 (p. E-1)). In this area, the Naknek Formation chiefly comprises cross-stratified and bioturbated sandstone, with locally abundant molluscan shells and plant debris. Sedimentary facies and an accompanying trace fossil assemblage suggest that these strata were deposited in a moderate to high-energy marine shoreface setting. Freshly-broken surfaces of the hydrocarbon-bearing sandstone have a strong—yet fleeting—kerosene-like odor. Hand-lens observations suggest that the oil-stained sandstone largely consists of quartz and potassium feldspar, with subordinate plagioclase and heavy minerals. Our observation of quartz-rich Naknek strata—and the likely relative abundance of potassium feldspar versus plagioclase—in the Douglas River area starkly contrasts with the quartz-poor and plagioclase-rich Naknek in the Iniskin–Tuxedni region reported by Helmold and others (2011). The occurrence of quartz-rich and plagioclase-poor Naknek sandstone is significant, because a more mature mineralogy is less susceptible to zeolite cementation and thus more likely to retain primary porosity upon moderate to deep burial (cf. Helmold and others, 2011). Hydrocarbon-stained sandstone at the Douglas River locality indicates that the Naknek Formation, at least locally, may serve as a conventional reservoir of petroleum. Numerous sandstone samples from the Naknek Formation in the Kamishak Bay area are currently being analyzed for reservoir quality, petrology, and organic geochemistry.

During a reconnaissance traverse along an unnamed, north flowing tributary of the Douglas River, we discovered abundant cobbles and small boulders of oil-stained sandstone in the modern stream gravel that were strongly petroliferous, particularly on freshly broken surfaces. The oil-stained rocks largely comprise very fine- to fine-grained sandstone and weather light gray to tan and light greenish-gray. Oil stain occurs as both matrix and fracture fill, with the latter commonly healed. One small boulder of porphyritic andesite was also strongly oil stained along a fracture plane and included visible hydrocarbons within a small vug. The hydrocarbon-bearing clasts in float are dissimilar to the nearby outcrops of Lower Cretaceous Herendeen Formation, and we infer that the clasts were sourced from outcrops of the overlying Kaguyak Formation that are exposed in small catchments immediately upstream to the east and south. We constrained the likely source of the oil-stained sandstone to a zone within the lower Kaguyak Formation that exhibits a similar weathering color and character, although we were unable to directly access the candidate outcrops due to extremely steep and inaccessible terrain. We collected several oil-stained samples from the alluvial cobbles and boulders to be analyzed for reservoir quality, petrology, and organic geochemistry; analytical results are pending.

The observations reported herein are consistent with the preliminary hypothesis that an active petroleum system is present in the southern Kamishak Bay area, and that this petroleum system may include oil-generating source rocks in the Middle Jurassic Tuxedni Group as well as potential oil-bearing reservoir rocks in the Upper Jurassic Naknek Formation and Upper Cretaceous Kaguyak Formation. Additionally, the occurrence of oil-stained and apparently more compositionally mature Naknek Formation sandstone in the Douglas River area of Kamishak Bay suggests that sufficient compositional variability exists within the Naknek for it to serve, at least locally, as a conventional reservoir in the Cook Inlet basin. Furthermore, hydrocarbon-bearing Upper Cretaceous sandstones have now been documented in both wells and outcrop in the lower Cook Inlet basin. Therefore, sandstones in the Upper Jurassic through Upper Cretaceous section of Cook Inlet may yet prove to be viable conventional oil exploration targets.

AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013