Scolecodonts from well samples of the Dundee, Devonian of Michigan

Abstract

[From introduction:] Through the courtesy of the late Dr. W. B. Wilson of the Gulf Oil Corporation, a small cut from well samples at Gulf No. 1 Sam, Section 27-15 N-4E, Kawkawlin Field, Michigan, was made available for study. There were 32 samples in the suite and they were taken from 2860½ feet to 2937½ feet below the surface. The object of this examination was to determine if scolecodonts, fossil polychaete annelid jaws, were present in oil and gas well samples, and if so, could they be used for correlation purposes. Scolecodonts were found in nearly every sample of this suite, although in many cases they were too damaged to be of use. It is remarkable, when the broken-up nature of the well sample is considered and that often only a teaspoonful of material was available, that any usable specimens were recovered. The jaws were obtained from the limestone cuttings by dissolving the material in a five per cent, solution of hydrochloric acid and collecting the insoluble residue in sieves up to 100 mesh. Most of the specimens were found in the samples between 2889 and 2898½ feet below the surface. In addition to scolecodonts, clear and iron-stained quartz grains, pyrite, and considerable selenite appeared as residue in certain samples. At the 2937½ foot level a teaspoonful of material produced over a hundred specimens of very small size. They averaged from one-third to one-tenth smaller than those found in other samples. Most specimens appeared to be adults yet were less than one- tenth of a millimeter in length. The size of the jaws may be due to an adverse physical-chemical condition of the environment. Considerable more selenite and pyrite were present in the residue than in other samples. The samples are recorded in the log as Dundee limestone, the top being at 2865 feet. Since the scolecodonts of the Dundee limestone have not been studied it was necessary to examine various outcrops of Dundee and determine the fauna for comparison. Preliminary studies show only one species, Lumbriconereites cooperi, a long-range form, to be common to the well samples and to the outcrops of the Dundee. Until further studies of other horizons are made, a definite geologic determination from evidence presented by the scolecodonts can not be given to the well samples. It would seem, however, from these studies that scolecodonts, similar to other micro-fossils, could be used to identify the rocks penetrated by the drilling bit.