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The term "shale masses," as used herein, refers to large bodies of shale at least several hundred feet thick. They may be either diapiric masses or depositional masses. The shale masses act like salt masses, and the two may combine to form domal masses; either or both may form the updip seal for a stratigraphic accumulation of oil.
The shale masses have four properties: (1) low-velocity sound transmission, in the range of 6,500-8,500 ft/sec with very little increase in velocity with depth; (2) low density, estimated to be in the range of 2.1-2.3 g/cm3; (3) low resistivity, approximately 0.5 ohm-m; and (4) high fluid pressure, about 0.9 of the overburden pressure. These properties seem to be caused by the high porosity and low permeability of these masses.
Maps and cross sections of an example area, Block 113, Ship Shoal Area, are included. The low velocity values were measured by acoustic logs and verified by refraction shooting. The low density values were deduced from gravity measurements. The low resistivity levels are shown on electric logs, and high pressure is indicated by the drilling difficulties with heaving shales.
These physical properties allow the outlining of the shale mass in one or more of three 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 to map the velocity configuration; and the seismic-refraction method is used to indicate the velocity within the anomalous mass, thereby differentiating between shale and salt.
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