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Well Log Tomography (WLT) can be used to examine large amounts of wireline data visually which can be used to examine depositional patterns within a basin. In particular, WLT can be used with gamma-ray (GR) and density (RHOB) logs to visualize basin-wide changes, as certain lithologies are heavy GR producers, and some lithologies have distinctive densities. Siliciclastic particles produce a strong GR signal in sediments such as carbonates and salts (anhydrite, halite, sylvanite) that is easily detected by wireline logs. These particles may be distal muds from fluvial systems or wind borne volcanic ash, which remain in the record as bentonite beds. By examining the distribution of such siliciclastic material, ideas regarding the main influx directions and position of depositional basins can be made. The primary purposes of this research are: (1) to use WLT to identify and map the distribution of the dispersed siliciclastics and main lithologies in the Silurian sequence of Michigan and Ohio and (2) create structure and isopach maps for individual salt-bearing units in Michigan using available well data and top picks.

WLT can provide a much improved, semi-quantitative picture of the wider Silurian depositional environments that include details regarding the fluvial systems and their interbasinal relationships.

Previous work included digitization of approximately 500 well logs and identification of the principal bentonites and salts in the Paleozoic of Michigan. More recently, RHOB and GR data from the digitized well logs were used for WLT to create slice maps for the Silurian deposits between the tops of Cabot Head and Bass Island Formations. Some problems are encountered when data from different states are combined, as each state may have differently named formations for the same Period and thus correlation becomes an issue. The top of the Cabot Head was used as the base for the WLT slicing interval as this represented an unconformity which could be seen in Ohio at the base of the Packer Shell. The top of the Bass Island is also an unconformity, which is recognized in both states. Correlating unconformities is much easier given the available well log data than trying to slice individual formations across states lines. The unconformities can also be considered time synchronous which is useful when attempting proportional WLT slicing.

Isopach and structural maps for the Salina Formation were constructed for the state of Michigan using well log top-picks. The only problematic unit was the Potash (PTS) Bed that occurs within the A1 evaporite. The PTS bed is given a top-pick in only a handful of the wells we have data for, but because it is within another larger unit, the next top-pick does not correspond to the base of the PTS layer. To get the thickness of the PTS layer, the original wireline log data would have to be consulted in order to determine the elevation of the PTS base.

Most of the Isopach maps created shows a thickening towards the center of Michigan's lower peninsula, however, the size and East/West position of the area of maximum accumulation shifts between the deposition of the different salt units which would indicate a change in how the basin was subsiding over time. One particular exception is the D unit, which shows no thickening pattern in the area studied. This may indicate that over the span of time that the D unit was deposited, little to no basin subsidence occurred, leading to a relatively equal thickness of sediment across lower Michigan.

A set of log curve amplitude visualizations (avi animations) has been completed for the top of the Cabot Head through the Bass Island using GR and RHOB wireline log data. The results show linear bands of high GR values developing in southwest Michigan which migrate northward and then to eastern Michigan before dropping off to low GR values for the later half of the period. After this drop off and continued low in Michigan GR values, a strong and persistent increase in GR values can be seen in southeastern Ohio, which probably represents increasing siliciclastic input resulting from the Caledonian Orogeny. From the RHOB curves we can track the relative positions of halite, anhydrite and carbonate. The halite seems to start off in the Northern part of the basin and later is subdivided by linear stretches of carbonate that appear to start in the west and rotate to the east, after which the halite is surrounded by carbonate to the south, west and north. At this stage there is a fairly persistent low density region surrounded by the halite, which may represent the center of the restricted basin. While some low density readings may be the result of washouts, in this case there are also accompanying GR highs which may represent the actual presence of siliciclastics settling out in the center of a saline body. This salt zone then shrinks with its center near present day Saginaw Bay until it is disappears near the top of the section. Based on the RHOB curves it looks like Ohio deposition was dominated by limestone and dolomite during most of this time span followed by a lowering of density near the top of the section in the southeast, which is also the locus of high GR values during the same time. Unfortunately, due to the general lack of digitized well data from Ohio with both RHOB and GR curves available to us, it is difficult to see trends in the halite deposition that should be seen in the upper Silurian. Based on published maps of halite thickness in Ohio, it appears that our available wells may miss the zone of maximum halite deposition in Ohio.

Overall, the amount of data available through wireline logs is tremendous, and WLT is a vital way of being able to synthesize and visualize that amount of data. By examining both GR and RHOB maps a reasonable idea of the rock distribution can be made which can then lead to enhanced understanding of past depositional environments and how they change over time.