Correcting Density/Sonic Logs for Total Organic Carbon to Reduce Uncertainty in Pore Pressure Prediction

Pore pressure prediction in shales undergoing compaction, including mechanical and chemical diagenesis, is customarily related to the mechanism referred to disequilibrium compaction. However, even when this mechanism is established and the normal compaction trend in sonic velocity, as a proxy for shale porosity, is well constrained, the pore pressure prediction may be in error because of the lithological variation in shale composition. Presence of high levels of organic matter in shales that are immature for hydrocarbon generation is an example, causing marked overprediction of pore pressure unless properly accounted for. All published datasets involving TOC and wireline data record a similar relationship between TOC and the bulk density and P-wave velocity log response, in the sense that the measured wireline data shows a decrease (which implies an increase in porosity) as the TOC content increases. In this paper it has been shown that a rock physics model that links TOC and bulk density can be utilised to correct the measured bulk density in immature shales, and, when limited to immature shale, the correction can be extended to velocity data using simple industry-standard models.