Monroe, John N.1
(1) The Monroe Company, Waco, TX
ABSTRACT: The Origin of Shale Diapirs, Salt Dome Initiation, and Sedimentary Volcanism
The petroleum maturation process appears responsible for genesis of shale diapirs - from the generation of methane gas within organic shales.
For example, an organic carbon content of 1.1% converted to methane gas results in a 15% forced expansion of shale at a depth of 5500 meters (by Van der Waals' Equation). This expansion reduces shale density to that of salt, and the forced expansion of that shale lifts overburden for diapir initiation.
A salt bed contiguous to the gas-expanded shale joins that shale in diapirism; the expanded shale furnishes the initiation for both - and the consistency of salt at depth is similar to that of an expanded shale. The low density of salt, however, has greater persistence than that of expanded shale; therefore, it passes through the shale that was responsible for diapir initiation. The shale, however, often remains as a "sheath" about the salt dome. The salt dome in its continuation upwardly creates fractures and other channels for gas escape, thereby inhibiting further progess of the gas-expanded shale body.
When salt is not involved in the process, the shale diapir continues in upward motion until it creates its own channels for gas escape by disruption of overburden. An associated outburst or eruption is recognized as sedimentary volcanism.
Other effects of methane generation involve changes in consistency and density of sediments, friction reduction in faults and bedding planes, plus immense horizontal and vertical forces - some creating traps and all supplying exploration leads.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.