ABSTRACT: Blackened Smackover: Thermal Evolution and Mass Transfer Adjacent to a Subsurface Alkalic Igneous Dike in Northern, Louisiana

DUTROW, B. L., C. CHRISTENSEN, and D. J. HENRY

The Smackover Formation is a major hydrocarbon source rock in the Gulf Coastal region. During the Cretaceous, the hydrocarbon-rich Smackover was intruded by several alkalic dikes which 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 records the presence of a 10 m dike penetrating the Smackover siltstones and carbonate mudstones at 2.1 km depth. An asymmetric contact metamorphic aureole surrounds the dike as determined by alteration mineral assemblages of the host rock and by C and O isotopic signatures.

Adjacent to the dike, 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. These mineral assemblages require increased temperatures and significant mass transport of components from the dike into the host rocks. Numerical heat transport calculations suggest temperatures in excess of 450 degrees C adjacent to the dike consistent with temperatures calculated from mineral chemical data. This study demonstrates that significant fluid infiltration, heat and mass transport accompanied dike intrusion and that the intrusive dikes had the potential to significantly effect hydrocarbon maturation and migration in northern Louisiana.

AAPG Search and Discovery Article #90941©1997 GCAGS 47th Annual Meeting, New Orleans, Louisiana