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The AAPG/Datapages Combined Publications Database

GCAGS Transactions

Abstract


Gulf Coast Association of Geological Societies Transactions
Vol. 28 (1978), Pages 7-8

A Radiometric Investigation Relative to a Gamma-Ray Log, and its Geologic Implications (1)

E. G. Anderson, (2) Robert C. McIlhenny, (3), David E. Pope (4)

ABSTRACT

The Cotton Valley Operators Committee (CVOC) No. 1 Hunt-Hope et al. well was drilled in 1973-74 in the Cotton Valley Field, Webster Parish, Louisiana to 20,395 feet. The object was to drill through the Louann-Werner sequence (Early-Middle Jurassic) and to an additional depth sufficient to determine the age, lithology and structure of the underlying strata. The Louann Salt was found to be 3,589 feet thick; the Werner, approximately 200 feet; the remaining nearly 5,000 feet is considered to be Eagle Mills (Late Triassic). One 810 foot-thick section of the Eagle Mills (17,875^prime-18,685^prime) is especially noteworthy. Its appearance on the gamma-ray log is that of a massive quartzose sand body, whereas microscopic examination of well samples reveals basic igneous detritus. The massiveness and thickness indicated by the log, and, the volume of igneous detritus thus implied, are anomalous in the Eagle Mills; therefore, a radiometric verification of the log seemed necessary.

Gross laboratory gamma counting verified the log, and refined gamma-ray spectrum measurements suggested the presence of thorium and uranium, and potassium-40. The former are present above and below the sand body, but only K40 appears within the sand body.

Data from this and other wells reaching sub-salt strata in the northern coastal plain are interpreted as follows: during the Late Triassic these areas were emergent, and deposition was controlled by Eagle Mills taphrogeny. Upfaulted features provided the detritus and downfaulted features trapped continental deposits. Relief diminished away from scarps maintained by recurrent faulting: thus, fanglomerates were deposited along piedmonts formed near the scarps; these graded distally into silts and muds laid down on floodplains. Before the drilling of the subject well, only a broad Eagle Mills graben in Arkansas was identifiable. A few scattered deep wells -- supplemented by salt-tectonic indications and by facies patterns in overlying Late Jurassic strata -- suggest a complex series of grabens and intervening horst structures lying gulfward from the "South Arkansas Graben."


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