AmericanChemicalSociety.com

To conclude this study we used project wireline data collected from oil and gas wells to  correlate rock units, including the Ordovician bentonites and Silurian salts, across the entire Michigan Basin. We used wireline data, mainly gamma ray (GR), density (RHOB) or neutron (NEUT) logs, to trace geologic units. In addition to the bentonites, some other lithologies and formations in the basin are also characterized by very high GR values (e.g. Antrim black shales and the Salina potash salts) and criteria based on combinations with other logs, mainly RHOB, has to be developed to distinguish the bentonites. Since salts have distinctive RHOB densities, this log in conjunction with the GR can be used to identify bentonites in the basin. The final product of this research was a set of updated digital cross sections similar to the well-received report by Lilienthal (1978), updated to include more key wells and in a digital format rather than paper files.

We examined approximately 500 digitized well logs covering the basin and selected those that were deepest and contained the Ordovician principal bentonites and salts where possible. These logs were then used to obtain detailed isopach and structural maps for selected  formations in the Michigan Basin. Wells for the cross sections were selected to create a collection that followed  the original section lines used by Lilienthal; approximately 120 wells were selected which radiate out as 15 spokes from the Sparks well (permit #29739), which is the deepest well in the state and previously used to anchor Lilienthal’s lines. Graphic logs were constructed for each of these individual wells and correlations between wells were made along the section lines. Lilienthal’s report mainly used GR lines for the cross section correlations, but here we paired the GR with RHOB or NEUT logs for each well to obtain better delineation of formations.

Initial correlations were done for the Salina deposits as well as the Antrim shale above and the Utica shale below. Correlation attempts have been limited to these distinctive units as other correlations are often unclear due to mismatched formation naming. Formation names especially, seem to vary between wells of different ages, and in some cases the top picks do not align with distinctive well log signatures. It is obvious that additional effort could be used to update formation names using a standardized nomenclature (e.g. Catacosinos et al. 2001).

There are some interesting observations that can be made from these cross-sections. For example, while the Salina persists across the entire Michigan Basin in the East to Northeast with minimal changes in thickness, it thins markedly (to the point of pinch-out in some cases) to the South and Southwest. Added to this, it can also be noted that the thickness of the two correlated shales is more consistent across the basin than the Salina. For example, in the A-I line, the Salina is near 2000 ft thick in the Sparks well, but thin to less than 200 ft, while the upper and lower shales are essentially the same thickness (~300 ft) throughout.

Changes of the maximum GR response are also noted within the Antrim Shale across the Michigan Basin. While quite “hot” (GR >300) over large areas of the basin, there are distinct pockets of lower maximum-GR values in the Northwest, Southeast and as a swath in the center of Michigan (SW from Saginaw Bay). Despite these changes in absolute GR values, the overall pattern of GR values is similar between wells. With the high density of wells, the changes in pattern is small enough between the individual wells to allow for correlation.

The overall utility of these cross sections will be greatly improved once more definitive correlations can be made and the nomenclature has been standardized for all wells.