Definition of Porosity Trends for Pore Pressure Modeling in Scenarios With Erosive Unconformities

Abstract

Normal porosity trends, which in cases of variation of porosity controlled by burial and compaction are called normal compaction trends, can be defined empirically from the analysis of geophysical data and well logs. Gaps in the geological record resulting from erosive processes can be identified and quantified by this analysis. The purpose of this paper is to demonstrate the application of the analysis of porosity trends to pore pressure modeling and to discuss its inherent uncertainties. First, a mapping of the chronostratigraphic horizons and erosive unconformities from seismic volumes and sections is necessary, as well as the recognition of lithological types (in order to individualize fine-grained lithologies). Next, this same procedure is applied to the well logs to be analyzed, including the seismic interval velocity log along the well trajectories. Logs sensitive to porosity variations such as acoustic logs, resistivity, density, D-exponent are used in a same lithological type. Usually, porosity does not vary linearly with depth, so normal porosity trends can be better established graphically through a semi-logarithmic scale. When there are layer sequences with gaps of lithological sections due to erosion, the curves of the above-mentioned logs reveal breaks in their regular trends. Within each strata sequence limited by erosive unconformities, normal porosity trends are established using the above-mentioned logs. With the support of the seismic sections and of the mapping of the unconformities, trends from offset wells are extrapolated and adjusted for the trajectory of a new well location, as well as the logs required for the pore pressure modeling, in order to assist in this task. Through pore pressure monitoring, the model is adjusted, in real time, with the new data obtained during drilling. Any overpressured zones with top at erosive unconformities or below non-pelitic lithologies limited by these unconformities, are then remodeled in real time using the porosity trend extrapolated from offset wells and compatible with its continuous sequence of strata. In some cases, when the formation is overpressured, the correct definition of normal porosity trends is essential for characterization of the overpressured zones and quantification of pore pressure. The estimate of the eroded rock thickness greatly supports the well correlation work aiming at pore pressure modeling, as well as retro-analysis, real time monitoring and the assessment new locations.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018