Time To Depth: Where Geologists and Geophysicists Meet

Abstract

Accurate depth conversions are critical in any exploration area to avoid time-consuming and costly errors in a drilling program. Onshore and offshore, detailed well prognoses from the explorationist are key for the drilling engineer to develop an economical and safe well plan. ln addition, depth conversion is used to calculate the burial depths required for determining geothermal history, provide a depth model for accurate geosteering, and provide for fluid flow simulation. This article explores the necessary steps an interpreter uses to derive and qualify the depth conversion of the seismic interpretation within the workstation environment. Geophysical interpretation begins by identifying regional seismic markers, basinal unconformities, and sequence boundaries to develop a general structural framework. Once the basic framework is established, the task of identifying the seismically represented geologic zones begins. To accurately match geologic tops to seismic reflectors, the interpreter needs a high-quality sonic Jog from which a synthetic seismogram is generated. Check-shot or vertical seismic profile (VSP) data are used to calibrate the sonic curve; the final calibration will define the time and depth relationship along the borehole path. Often the interpreter must rely on other sources for velocity information, i.e., apparent velocities from depth markers tied to regional seismic markers, processing velocities, or a known regional velocity function. Better Synthetic Seismograms Sonic Log Calibration. The sonic log data is critical for accurately creating a synthetic seismogram used in the initial time-to-depth correlations. A density Jog is also important, although standard density values can be applied when no Jog is available. ln the absence of sonic information, a pseudo-sonic Jog may be computed from other logs, such as resistivity. Several activities are commonly performed before the synthetic seismogram is created: editing of the check-shot or velocity information for sonic calibration, generation of a reflectivity series (RC curve), and selection of analytical and extracted wavelets for synthetic convolution.