Density/Neutron Cross-Plotting; Its Application as a Qualitative Basinal Tool for Pore Pressure Prediction: Case Study North Carnarvon Basin

Abstract

Density/Neutron cross-plot analysis has for long been a petrophysical technique. The approach is used to differentiate between sands and shales, as well as being used to identify gas in a reservoir. A further use, and the focus of this paper, is to calculate shale volume. Simplified, the more clay there is in a sequence, the higher the neutron porosity. Amount of clay in the shale will also be differentiated. In this paper we highlight how this tool, sometimes used in conjunction with compressional velocity/density cross-plots, can be used in multiple ways when understanding formation pressures in shales. Some of these ways include understanding seal capacity and leakage, identifying mechanisms of pressure generation, and determining the need for multiple compaction models. We present results from a recent regional analysis of the North Carnarvon Basin in which logs were quality assessed and several corrections applied such as density log correction and caliper out of gauge zones and wash out zones were also noted. In the resulting analysis, we looked at 170 compressional velocity/density and 70 density/neutron cross-plots from wells in Exmouth Plateau, Exmouth, Barrow and Dampier sub-basins and the Rankin Platform. The conclusions are that: (i) There are two compaction models for the Muderong shales based on a strong lithological effect in this unit; this enables a map to be produced over the entire basin showing clay-rich and silty shales. A key point is how variable shales can be, even locally, which has implications for uncertainty in pressure even at acreage scale. (ii) Many of the trends we see in compressional velocity/density space are lithological effects rather than pressure mechanism responses.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015