A POPULAR STRATIGRAPHY EXERCISE FOR UNDERGRADUATE MAJORS BASED ON WELL-LOGGING FROM ARTIFICIAL CUTTINGS

The interpretation of well cuttings is problematic, especially for the novice, because of the mixing of lithologies during transport in the drilling mud and contamination due to caving. To sidestep these problems and provide a realistic learning experience, a suite of typical rock types from various Paleozoic formations in the southern Appalachians was collected, passed through a crusher and sieved to produce artificial cuttings. Stratigraphic column diagrams were then prepared based on measured sections in the literature and the appropriate cuttings spooned into bags and stored in standard sample boxes. Standard strip logs (Kraftbilt 186) with a scale of 1 inch to 100 feet were used and sample bags separated by 10 foot intervals.

Most samples include a single lithology (sandstone of different grain size, shale of different color, limestone of various types, dolostone) but some include two (e.g. coal or ironstone in shale, chert in carbonates, bentonite in limestone). Students are asked to make note of unusual lithologies as possible key beds for correlation. Logging is expedited by limiting the number of lithologies in each sample suite (‘drill-hole’) and repeating lithologies consecutively or in alternation. Some thick, monotonous formations are abbreviated by specifying intervals with missing samples. Each student then logs a ~1000 foot section (2-4 hours).

As a preliminary correlation exercise, stratigraphic sections are made to overlap, and students are asked to circulate and compare columns in order to identify intervals of identical stratigraphy in the drill-holes. When complete the drill-holes fall into groups representing two to four different localities (depending on the number of students, 7-15). Working as a committee each group of students identifies formations and contacts based on a table of definitions, and then records thickness. As a final stage of the exercise students pool thickness data, construct two stratigraphic cross-sections, one emphasizing unconformities (Lower Ordovician-Devonian), the other facies changes (Mississippian), and answer pertinent questions. The entire exercise is completed in three labs (6 hours plus some work outside class).