An Engineering Interpretation of Seismic Data

Abstract

ABSTRACT The reflection seismograph has long been the most important means for obtaining subsurface information prior to drilling. Interpretation of the seismic data is normally made by geophysicists to define subsurface structure. However, considerable information of great value to the drilling engineer is also contained in the seismic information. This paper discusses how the drilling engineer can use these data to estimate depth and magnitude of subsurface formation pressures, predict gross changes in lithology, and warn of possible drilling problems. Techniques are also presented for predicting relative drillability and fracture gradients in rank wildcat areas. The predictive techniques described have been used with good results over a wide area along the Texas Gulf Coast including Continental Shelf locations and might possibly be applicable to any sedimentary basin. INTRODUCTION The only means of obtaining subsurface information other than from drilling is by geophysical prospecting. Although the seismograph has been used by geophysicists and geologists for many years to help define subsurface structure, it is only recently that engineers have begun to discover useful applications. One of these applications is the use of routine seismic field data to predict both the depths to abnormal pressure formations and approximate pressure magnitudes.1 The predictive method results from the fact that velocities between subsurface reflecting layers can be obtained from seismic field data using well known geophysical techniques. Differences in interval velocities between these layers can be used to develop an average interval travel time (reciprocal of interval velocity) profile, which is in effect an acoustic log averaged over fairly long (500' - 1000') vertical intervals. Analysis of numerous well velocity surveys revealed a close correlation between interval velocity (interval travel time), and factors such as lithology and degree of rock compaction and that interval travel time varies exponentially and predictably with depth. Departure from this normal trend signifies abnormal pressure or gross lithologic changes.