Outlining Shale Masses by Geophysical Methods1
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.
Figures & Tables
“Diapir” and “diapirism” come from the Greek diapeirein, which means “to pierce.” Diapirism sensu lato is a process by which earth materials from deeper levels have pierced, or appear to have pierced, shallower materials; it is divided into magmatic intrusion and diapirism sensu stricto on the basis of the temperature at which piercement occurs. Diapirs s.s. are composed of evaporites, argillaceous sediments, coal, peat, ice, serpentine, or other earth materials which have the critical characteristics of low equivalent viscosity and low density. These materials range in age from Precambrian to Recent. Diapirs are found in all parts of the world except the shield areas. They have many forms, ranging from smoothly rounded pillows to complexly injected laminae, are either connected with or disconnected from the “mother” bed, and are present either at the surface, where they form distinctive features, or at considerable depth. Diapirs have well-developed internal structures indicative of an origin by flow. Strata around a diapir may be strongly affected structurally and/or stratigraphically by the diapir, or they may be unaffected. Field and model studies indicate that diapirs have developed as a result of horizontal compression, gravitational instability, or both. Diapiric structures of various types contain large quantities of oil and gas, sulfur, salt, and potash and are important for underground storage and nuclear testing.