Using Multiple Attributes To Calculate Pore Pressure

Abstract

Seismic velocities are a key attribute for seismic-based pore pressure prediction. There are numerous reasons why seismic velocities alone are not good enough for the level of accuracy needed to appropriately design wells and reduce risk in prospects. The velocity method works because velocity responds to porosity and porosity can be a good proxy for pore pressure. When this is true then some confidence can be placed in the approach. However there are numerous sources of errors in the previous premise. First of all velocity also responds to lithology and fluid as well as porosity which induces errors in the approach. In well log based PPP we have the luxury of discarding the non-shale velocity points to alleviate the problem but this isn’t the case with seismic velocities. Another source of error is in the assumption that porosity is a good proxy for PPP. Due to geological and structural complexity as well as the source of the overpressure the relationship between porosity and pore pressure can be complex and can’t be described by a simple compaction trend. Clearly there is a lot of room for improvement when doing site specific PPP from seismic because it is a highly underdetermined problem. Different seismic attributes can be used to augment the traditional approach to make the problem a little less underdetermined. For example AVO, amplitude variations with offset, can be used to get a handle on the lithology and to observe the non-shale to shale reflectivity trend. Knowing lithology is useful because the velocity PP technique works best in shales. The non-shale to shale reflectivity trend can be used as a proxy for PP and therefore can provide another piece of information into the system that is independent from seismic velocities. A seismic attribute called depositional rates can be used to better understand the structural and depositional complexities. This understanding translates to a refined picture of the normal compaction trend which translates to a better understanding of the relationship between porosity and pore pressure. Seismic frequency is another very useful attribute that provides an independent way of arriving at PP. Working the PP problem from many angles provides ultimately a better solution. This presentation will explore ways of getting more information from the seismic allowing for better PPP. There are attributes other than velocity that can be used to predict PP. There are also attributes that can be used in combination with velocity to better refine the method.

AAPG Datapages/Search and Discovery Article #90324 © 2018 AAPG Asia Pacific Region GTW, Pore Pressure & Geomechanics: From Exploration to Abandonment, Perth, Australia, June 6-7, 2018