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Home My WebLink AboutGMC Data Report No. 195Evgtluation of basalt samples (17,859' - 17,888') from the Husky Oil NPRA Tunalik No. 1 well, Western Alaska. Received 5 March 1992 Alaska Geologic Materials Center Data Report No. 195 Total of 71 pages in report December 30, 1991 Shell Development Company Hostage. 7x ~ Mr. O. FI. Worrall Chief Geologist Shell Western E&P Inc. P. O. Box 4252 Houston, TX 77001-4252 Oear Mr. Worrall: REFERENCE: SHELL OIL COHPANY/SHELL WESTERN E&P INC. RESEARCH AGREEMENT · ,. . At your request members of our staff examined basalt samples from the Tunalik #1 well, western Alaska, for stratigraphic contrOl. The primary goals were to determine whether the basalts were intrusive or extrusive and if K/Ar age dating was viable. Textural relations indicate that the basalts were quenched and therefore cooled at or near the earth's surface. K/Ar dating will not yi:eld reliable results due to the absence of K'bearing minerals and' the high degree of alteration in the basalts..The age of the basalt is therefore beSt constrained by its stratigraphic position. Thin section photomicrographs along with a more detailed discussion are forwarded to R. R. Smith. Very t.rulY yours, Oean N. Italouta Manager Geology Research TRT:ml · Enclosure cc: Shell Western E&P Incj R. R. Smith (w/o enclosure) GMC Data Report No. 195 Page 1/71 EVALUATION OF BASALT SAMPLES FROH THE HUSKY OIL NPRA TUNALIK NO. 1 tIELL, ~;ESTERIq AlaSKA T. R. Taylor Reservoir Geology Section L. T. Bryndzia Basin Analysis Section L. Garmezy Structure and Tectonics Section Purvose: At the request of S~IEPI Alaska Division Exploration six core samples of basalt fro~ the Husky NPRA Tunalik ~1 weI1 were examined in hand sample and chin section. The samples were taken from depths of 17,859' to 17,888' and are underlain and overlain by sedimentary rocks of Pezmian age. These igneous rocks are of interest for their potential use in regional stratigraphic calibration and prediction. Three questions were addressed in chis study: 1) Are these basalts extrusive, having cooled at or near the .earth's surface or are they intrusive having crys~allized at depth? 2). If the .basalts are extrusive, did they crystallize on the seafloor or on land? 3) Can the basalts be .dated using F/Ar techniques and are previously determined K/Ar data reliable? Sample Description: All the samples examined are relatively fine-grained, amygdaloidal basalt. The original mineralogy consisted of acicular plagioclase, olivine, pyroxene, an opaque phase (magnetite? ilmenlte?), and glass. The samples exhibit varying degrees of alteration. The alteration products are chlorite, calcite and quartz. Pyroxene phenocrysts are commonly altered to chlorite, often from the core outward. Fresh pyroxene is relatively rare. Irregular patches of chlorite and radial fibrous chlorite chat lines amygdules most likely formed by hydration of glass. In rare occurrences, glass remains in a relatively unaltered state. Amygdules are filled with calcite, chlorite or in some cases quartz. Calcite-filled veins are also present. K/A{ Dating: Due to the high degree of alteration and the absence of K-bearin~ minerals such as amphiboles, plagioclase represents the only phase from which a K/Ar date could be derived. Four l~Ar ages were previously determined from plagioclases by the University of Alaska. The ages are Tertiary (50.3 m.y., 50.4 m.y.), Jurassic (186 m.y.), and Devonian (385 n.y.). None of the ages can be considered as a reliable crystallization age for the basalt. The t~o youngest ages, alrJaough overlapping viChin the errors of the technique, are from altered plagioclases rich KzO values of 2.095%, far in .excess of chat in pristine diabasic plagioclase. Typical values are less than 0.3%. Therefore, most of the argon chaC is neasu~ed is froa poCassiun added co the syscea du~tng alteration. The un~eli, abiliCy of the data is also reirlecCed in the radiogenic yields of the samples. Cood K/A~ data has radiogenic yields o£ ,over 90%. The lovez values reported in these analyses. (51 - 671) suggest chat a g~eat deal of the argon is. coming from extraneous sources. ~: The textural relationships seen in chin section, most no~ably the abundance of acicular plagioclase and the presence of glass, are indicative of very rapid cooling (quenching). It is likely that the basalts were either extruded at the surface or emplaced at a very shallow level. Given the size of GMC Data Report No. 195 Page 2/71 the samples, it is di££icult to determine vher_her ext~lon vas on ~e sea£1oor or on land. Previously determined K/Ar ages are ~eliable and due co the altered nature of the basalts no further age dating is ~ecommended. 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