A Review of Geopressure Evaluation From Well Logs - Louisiana Gulf Coast
Authors: Lane R.A. , Macpherson L.A. 
Published: December 2007

Recent Gulf Coast drilling experience and log data reveal irregularities in resistivity trends. Anomalies caused by age boundaries, younger sediments, and other phenomena may make log relationships difficult to apply. The geographic distribution and interpretation techniques for some of these anomalies are presented. Resistivity-trend departure/pressure relationships are examined.

Introduction

Since the beginning of geopressure drilling in the Louisiana Gulf Coast, attempts have been made to quantify log parameters as an aid in pressure prediction.In 1965, Hottman and Johnson presented an empirical correlation relating abnormal formation pressures to departures from normal shale velocity and resistivity trends observed in Gulf Coast formations. These relationships have been used widely for predicting younger Tertiary abnormal pressures, although both sets predicting younger Tertiary abnormal pressures, although both sets of data were obtained from Miocene-Oligocene sediments.In recent years, other empirical but largely undocumented resistivity relationships, based chiefly on mud-weight observations, have been established and are commonly used offshore. These account for local trend anomalies wherein the Hottman and Johnson resistivity relationship is not suitably accurate. Some of these erratic trends have been found to be systematic either in kind or areal extent and, once recognized, can be interpreted. In 1972, while this study was in progress, Eaton suggested that variations in overburden gradient might be responsible for irregularities in departure trends.Since 1965, drilling activity has moved farther offshore into younger Pleisto-Pliocene sediments. With the onset of production in these newer fields, some 50 additional pressure measurements in virgin geopressured reservoirs have become available. The density log has become the primary porosity log offshore, and the prevalence of density data provides a means to calculate overburden prevalence of density data provides a means to calculate overburden gradients in these fields. It is considered timely to include the new data with those of Hottman and Johnson. Resistivity data are emphasized because the resistivity device often is the only log run over sufficient intervals of borehole.

Pressure Estimation Pressure Estimation To estimate formation pressures from logs in the Gulf Coast, the following information is necessary:an established normal log response trend in hydropressured shales,an observed departure from the normal trend, andan empirical relationship between this trend departure and formation pressure gradient.

Hydropressured Trends

The first trends of sonic and resistivity data for the offshore Miocene-Oligocene were presented by Hottman and Johnson. These trends are averages of early observed data in the Louisiana Gulf Coast. However, since compaction trends probably depend not only on depth but also on rate of compaction, cementation, and overburden, these Miocene-Oligocene data should not necessarily apply to the Pleisto-Pliocene sediments presently being explored. Fig. 1 shows the observed normal pressure resistivity trends superimposed on an age-correlation dip section from Atchafalaya Bay through Vermilion Block 321. Because of sediment age, the Hottman and Johnson trends apply to Atchafalaya Bay and Eugene Island Block 100.

JPT

P. 963