On The Prediction Of Abnormally Pressured Sedimentary Rocks From Seismic Data
Authors: Fillippone W.R. 
Published: October 2008

INTRODUCTION

Many Tertiary basins in the world have undergone rapid sedimentation of clastic rocks, including floods of volcanic ash with montmorillonite. Lenticular-shale or sand bodies that are capped by impervious montmorillonite or carbonate layers are unable to expel the connate water, oil, and gases entrapped within and, as a result, become undercompacted and overpressured when deeply buried by continued deposition of sediments. Pressures within these lenticular, entrapped sedimentary zones can range as high as 95% of the overburden load and frequently exceed 70%. Prediction of these overpressured zones from surface sources prior to drilling a well through them can aid in preventing dangerous and expensive blowouts. Prediction can aid in designing casing and drilling fluid programs to permit safe penetration of these zones and continue drilling to reservoir rocks at greater depths.

INTERPRETATION OF VELOCITY ANALYSES

In the course of a seismic reflection survey, a general practice is to use common depth point (CDP) profiling. The many closely spaced source and receiver positions produce a large number of ray paths to common reflection points that permit the calculation of root-mean-square velocities for each reflection event and time. Fig. 1 illustrates a typical CDP velocity analysis with points representing all of the time/velocity pairs determined by applying normal moveout corrections and summing the trace amplitudes. The range of velocities used in the analysis includes those associated with primaries, multiples, and other coherent events. The amplitude of these types of events is not a discriminating parameter in the interpretation of velocity analyses. It is not uncommon for multiples to be several times stronger than primary events at the same reflection time. Therefore, it is essential that all coherent events be represented by a time/velocity pair. On this display, even those pairs represented by a mere dot are necessary to a proper interpretation. The columns at the right are graphs of the polarity with amplitude and dips of the events in milliseconds per CDP spread and direction of dip. The listing shows the selected RMS velocity file, two-way time, dip and direction, and amplitude and polarity for the highest amplitude events per 100-millisec gate. It is preferable to use the complete set of values unlimited by amplitude discrimination.

COMPARISON OF SEISMIC RECORD SEDTION, WELL DATA, AND SEISMIC VELOCITY INTERPRETATION

A portion of a seismic record section is shown in Fig. 2 illustrating the location of the wellbore, the sonic log of the well on the left, and a pseudosonic log derived from an interpretation of the CDP velocity analysis near the well location on the right. One can correlate the reflection events to velocity contrasts on both sonic logs. Of particular interest is the high contrast at 2.55 seconds, which was predicted as a significant overpressured zone at 11,005 ft from the seismic data and was encountered in the well at a slightly greater depth of 11,042 ft. The predicted formation pressure was 8321 psi requiring 14.5 lb/gal drilling fluid.