Overpressure in unconventional reservoirs can determine success or failure in an exploration play. A technique using sonic travel time (DT) logs to identify intervals of undercompaction is a standard of evaluation. It is based on the concept that shale compaction has a predictable relationship with burial depth because porosity declines by an exponential function. Application of this method in Rocky Mountain basins includes the added complexity of post-burial uplift. This paper presents a study of DT logs and pressure in Mesaverde and Mancos/Niobrara deposits of the Piceance Basin that identifies a new paradigm for estimating present-day pressure, one which accounts for both burial and uplift.
A unique step in the methods described here includes scaling the DT data from 1-to-0, ranging from a maximum at the paleosurface to a minimum at well TD. Scaling facilitates application of the exponential fit with depth, which defines the trend of normal compaction and represents a hydrostatic pressure gradient. Anomalously slow intervals indicating undercompaction are apparent where the DT curve deviates from the predicted hydrostatic trend. Paleopressure is estimated based on the magnitude of the anomaly using pressure gradients and paleodepth. The maximum overpressure indicated by undercompaction during burial appears to be around .6 psi/ft.
The change in pressure due to uplift and erosion is estimated by matching present-day pressure measurements (from DFIT data) to the paleopressure profile. Data show a linear correlation between uplift and pressure drop suggesting that no pressure relief has occurred with ‘5600’ of uplift; thereafter the pressure drop increases at a rate of .43 psi/ft. In the basin center where epeirogenic uplift is the major tectonic effect, essentially no post-depositional pressure adjustment occurs. However, pressure is reduced dramatically toward the basin margins where orogenic uplift increases cooling and dilation from unroofing. Some highly overpressured intervals not predicted by this method have been encountered in the Niobrara, probably due to additional pressure generated by volume changes associated with the cracking of liquid HC to gas.