Calibration of Rock Properties for Deepwater Seismic
Authors: Hilterman Fred J. , Verm Richard J. , Wilson Mark , Liang Luh 
Published: October 2008

Are seismic amplitudes for GOM deepwater plays restricted to bright spot technology? Or does AVO benefit the interpretation? Likewise, will very large source-receiver offsets increase our ability to differentiate the deepwater lithologies? Are there anomalous lithologies that would yield false bright spots or AVO responses? Questions similar to these, provided the incentive to conduct a deepwater petrophysical calibration study.

In the study, digital well-log curves from 447 GOM deepwater wells were analyzed. A foot-by-foot mineral and pore fluid volumetric was conducted for each well. Trends for the sediment velocity and density were classified as a function of depth beneath mud-line, pressure, end-member lithologies, mixed lithologies, pore-fluid properties, geographic location, and depositional environment. Additionally, by incorporating shear-wave dipole sonics, empirical transforms were developed to predict shear-wave velocity for different lithologies and pore fluid conditions such as API gravity, GOR, gas density, pore pressure and temperature. These velocity-density trend curves and empirical petrophysical transforms provided a quantitative basis for estimating lithology from near trace and AVO measurements. The prediction technique can be carried from the conventional loganalyst's crossplots to AVO synthetics.

The results are directly applicable for interpreting seismic amplitudes and AVO responses; for predicting salt thickness from gravity data; for predicting pre-drill pressure profiles; and, for time-depth conversion.

Numerous rules-of-thumb have evolved after the calibration study. As an example, reflections from clean wet sands against clay-rich shales are dominantly controlled by density contrasts. This is for reflections above the top of abnormal pressure. On the other hand, in abnormal pressure, velocity contrasts control the reflectivity. Very large offset AVO analysis does offer additional benefits in identifying lithology. Again the study shows some simple guidelines for interpreting these amplitudes. Of course rules-of-thumb come with a "buyer beware" label, as reflections from anomalous lithologies can skew the interpretation.

By providing a petrophysical framework for the deepwater, this work helps assign a risk factor to an interpretation. Most significantly, it points out the effects pore-fluid variation or mixed lithologies can have on the seismic response.

Introduction

This study was based on digital well-log curves from 447 GOM wells located in water depths greater than 500 ft. (Fig. 1). The digital curves included, when they were available, the sonic, density, neutron, shallow and deep-resistivity, gamma, SP, caliper and dipole shear-wave logs. For each well, data files containing well-log header information, questionable logging intervals, and hydrocarbon zones along with their pore fluid properties were established. The depth to the top of abnormal pressure (referred to as geopressure) was interpreted and entered into the database. Paleontological data such as forminiferia, nanno-fossils, and bathematric environment were introduced as possible classification attributes.