Diapiric shale along the Texas and Louisiana Gulf Coast is highly pressured and characteristically has low values of resistivity, density, and acoustic velocity, properties easily measurable by electrical logging. However, deep-water marine shale in normal stratigraphic positions also may be highly pressured and produce similar log characteristics. Therefore, although logs which reflect the above-mentioned parameters indicate that highly pressured shale has been penetrated, they do not indicate whether the shale is part of a diapiric mass or is in normal stratigraphic position.
Dip meter surveys provide information to establish whether or not the shale is intrusive. In addition, if the shale is found to be intrusive, dipmeter information locates the apex of the diapir with respect to the position of the well. As the shale diapir is approached from above, normal bedding-plane dips (away from the apex) increase in magnitude—just as if domal salt were being approached. After the low-resistivity shale is penetrated, the dips (cleavages?, flow surfaces?) are relatively constant in both magnitude and azimuth. These dips within the diapiric mass approximate the dip of the contact between the normally bedded strata and the diapiric shale. This consistent dip within the diapiric mass is very different from the random dips found in gouge shale adjacent to piercement salt domes.
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.