Summary Formation compaction in unconsolidated geopressured turbidite reservoirs in the Gulf of Mexico (GOM) is an issue of great uncertainty and concern. When compaction occurs, it changes the porosity and permeability properties of the reservoir rock and can affect recovery efficiency and well productivity. It can deform well tubulars, creating operational problems and shortening well life. If compaction is significant, especially when multiple stacked reservoirs are involved (as in the case in this field example), then compaction can create a subsidence bowl at the ocean floor. In GOM offshore operations, this could cause platforms to subside deeper into the water and create potentially severe safety problems; therefore, failure to properly address issues of compaction and subsequent subsidence during the design and development phases of these capital intensive, deepwater projects could lead to severe financial setbacks. It is crucial to have a compaction monitoring program in place because of the broad impact of compaction and seafloor subsidence. As a result, we developed a new formation-compaction monitoring tool (FCMT) and new methods of measurement and interpretation. The FCMT is a wireline device that uses multiple gamma ray detectors to determine locations of and precise distance between radioactive (RA) markers planted in the formation or casing. Compaction of the formation can be measured by changes in the distance between the markers. For precise estimation of the vertical distance between a pair of markers, the new method uses an array of three or four detectors. By examining the tool response to a marker, we developed a new method to determine the exact vertical and lateral location of the marker by using a Lorentzian response model; consequently, not only the vertical compaction but also lateral displacement of markers can be monitored with the new method. The accuracy of the tool was established in the test facility where gamma ray sources were placed at precisely known intervals. The tools were run centralized at three logging speeds (5, 10, and 15 ft/min.), and data were collected at 0.1-in. intervals. The vertical distances between a pair of RA markers spaced 30 ft apart were measured accurately to within 0.1 in. The first baseline logs were collected successfully in the four wells in a GOM deepwater development. High consistency among measurements from different logging passes proved that the FCMT can provide precise distance measurements with newly developed methods.