Shale masses are here defined as large bodies of shale at least several hundred feet in thickness. These may be formed either as diapiric masses or as depositional masses. The shale masses are like salt masses and the two are many times combined to form domal masses; they both may form the updip seal for stratigraphic accumulation of oil. The shale masses exhibit the following properties by comparison to the normal section: (1) low velocities—in the range of 6,500 to 8,500 ft/sec with very little increase of velocity with depth, (2) low densities—estimated to be in the range [Formula: see text] to [Formula: see text], (3) low resistivities—approximately 0.5 ohm‐m, and (4) high fluid pressures—about 0.9 overburden pressure. These properties all seem to be caused by the high porosity and low permeability of these large shale masses. Maps and cross sections of an example area block 113, Ship Shoal Area are shown. The low shale velocities were measured by acoustic logs and verified by refraction shooting. The low densities were deduced from gravity maps. The low resistivities are shown on electric logs, and high pressure is evidenced by the drilling difficulties with heaving shales. These physical properties allow the outlining of the shale mass by one or more of the following ways: the gravity method is used to outline the low density material, the seismic reflection method is used to outline the lack of reflection contrast and in some cases map the velocity configuration, the seismic refraction method is used to indicate the velocity of the anomalous mass, thereby differentiating between shale and salt.
No abstract
A computer is required to calculate the complex wave‐front charts which are needed in many areas. On a medium size computer wave‐front charts can be constructed using up to 40 layers. Each layer can be a constant velocity or can start with any velocity and have an increase in velocity with vertical time. These wave‐front charts may be automatically plotted for use in migration in a vertical plane. At the same time that the wave‐front chart is being obtained, a list may be made which shows the depth and offset for each reflection time and stepout value. This migration list may be used to migrate values from time maps in three dimensions. Before migrating, these time maps should have contours of all time values even though overlapping occurs as on buried foci of sharp synclines. Thus, it is a simple matter to make a migrated depth map from any time map regardless of the crookedness and discontinuity of the profiles or the lack of cross‐line control.
Techniques have been developed for the usefulness of the expanding reflection profile whose basic elements were described by Dix in 1955. Procedures have been established which make the shooting and interpretation of these expanding reflection spreads simpler and more reliable. Special presentations have been developed for the displaying of these profiles on record sections, and nonlinear paper has been designed for plotting the time‐squared versus distance‐squared graphs. The expanding spread is a valuable seismic tool and has numerous applications. It may be used for the identification of reflections and multiples. From the true reflections, calculations can be made to present the normal velocity survey information, i.e., time, average velocity, and interval velocity versus depth. Among other uses is the determination of the normal moveout curve. Various types of presentations are used to display expanding spreads in record section form. Also, various noise and multiple problems are exhibited.
Short surface‐to‐surface refraction lines define the top of a shallow salt dome previously located by reflection methods. A map is made from the results of a number of longer refraction lines radiating from the center of the dome. The increased accuracy of this system is primarily dependent upon the accurate determination of velocities and distances. Flank wells are used for further refraction shooting which yield more accurate velocity information and more detailed salt profiling. A map from this integrated information permits exploitation at a minimum risk, even though every location is essentially a wildcat.
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