Abstract: Radiometric Investigation Relative to a Gamma-Ray Log, and its Geologic Implications
E. G. Anderson, Robert C. McIlhenny, David E. Pope
The Cotton Valley Operators Committee (CVOC) 1 Hunt-Hope et al was drilled in 1973-74 in the Cotton Valley field, Webster Parish, Louisiana, to 20,395 ft. The object was to drill through the Louann-Werner sequence (Early to 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 ft thick; the Werner, approximately 200 ft; the remaining nearly 5,000 ft of penetrated sediments is considered to be Eagle Mills (Late Triassic). One section of 810 ft (17,875 to 18,685 ft) is especially noteworthy. Its appearance on the gamma-ray log suggests that it is a massive quartzose sand body, whereas microscopic examination of well samples reveals basic igneous detritus. The mass veness 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 indicated.
Gross laboratory gamma counting verified the log, and refined spectrum measurements suggested the presence of thorium and uranium, and of 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, Eagle Mills sediments were known to occur only in a broad graben in Arkansas. 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."
AAPG Search and Discovery Article #90965©1978 GCAGS and GC Section SEPM, New Orleans, Louisiana