Abstract: Detrital Chert in the Ivishak Sandstone, Prudhoe Bay, Alaska: Provenance and Origin of Microporosity

N.T. Harun, Earle F. McBride

The Ivishak Sandstone contains a large percentage of microporous chert clasts that can contribute significantly to total porosity. Over 80 distinct chert categories have been identified in the Ivishak sandstone and conglomerate in five wells, M-07, Wetw, 17-1, D-06, X-04. Major types include spiculitic chert, radiolarian chert, brecciated chert, and carbonate-replacement chert. Carbonate-replacement chert, spiculitic chert and brecciated chert are probably derived from the underlying carbonate units of the Lisburne Group and the Katakturuk Dolomite. However, the source of radiolarian chert in northern Alaska has yet to be identified.

Chert type is an important control on the predisposition of clasts to become microporous. Within a single sample, some distinct chert types contain no porosity, whereas other types contain over 40% microporosity. Radiolarian chert rarely develops microporosity, whereas brecciated chert is commonly microporous.

Incomplete crystallization of microquartz or dissolution of microquartz during weathering in chert source rocks can not be discounted as a cause of microporosity. However, the lack of broken and partially microporous rinds suggests that microporosity developed in situ. High microporosities and local total dissolution of chert clasts indicates that a large amount of dissolution occurred after deposition. Siderite cement surrounds partially dissolved chert grains, but does not fill open pore space within the grains, suggesting that chert dissolution at burial depths followed siderite cementation.

The percentage of microporous chert clasts and their degree of microporosity does not increase toward the Lower Cretaceous unconformity. Production zone 2 contains at least 40% microporous chert, whereas the overlying production zone 3 contains less than 10% microporous chert.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994