BLACKENED SMACKOVER: CONTACT METAMORPHIC AUREOLE SURROUNDING AN ALKALI IGNEOUS DIKE IN THE SUBSURFACE OF NORTHERN LOUISIANA

The Smackover Formation is a major hydrocarbon source rock in the Gulf Coastal region. During the Cretaceous, the hydrocarbon-rich Smackover was intruded by numerous alkali igneous dikes which are suspected to have transferred heat to the surrounding host rocks and caused conversion of liquid hydrocarbons to gas. A drill core recovered from the Carter Hope-Fee Well, Morehouse Parish, LA records the presence of an 11 m section of intrusive alkaline igneous rock penetrating the Smackover siltstones and carbonate mudstones at 1.9 km depth.

Surrounding the dike is an asymmetric contact metamorphic aureole as recorded, in part, by newly developed metamorphic/metasomatic minerals in the host rock six meters above and four meters below the dike. Adjacent to the dike, the siltstones and mudstones were transformed to produce metamorphic mineral assemblages rich in alkali and alkaline earth elements as well as fluorine; hydrogrossular, diopside, pectolite, apophyllite, fluorite, and feldspars. Modes of minerals change as a function of distance from the dike, thermal environment, and protolith composition. Formation of these mineral assemblages requires increased temperatures and significant mass transport of components from the dike into the host rocks.

A series of coupled heat and mass transport calculations provided thermal constraints for the aureole and insights into accompanying mass transport associated with the sedimentary rock–dike system. Calculations were done for systems with homogeneous, anisotropic and layered permeability. Transport, dissolution, and precipitation of silica were also incorporated into calculations. All systems modeled indicate that the thermal pulse waned in about 3 yr with a return to background temperatures in about 10 yr. Heat and fluid transport produce maximum temperature isotherms that are distinctly different in spatial extent and lateral variability for each numerical system. The small size of the thermal alteration zone suggests that if significant amounts of gas is to be produced by thermogenic sources such as these relatively thin dikes, that many dikes or much thicker dikes must be emplaced within close spatial and temporal proximity.