Thermochronological and Petroleum Generation Studies next to Dikes in the Upper Jurassic-Lower Cretaceous Strzelecki Group, Onshore Gippsland Basin, Australia
Charles E. Barker, Michael D. Lewan, Yvonne Bone, and Susan
Fluid inclusion, apatite fission track analysis (AFTAR®) annealing, mean random vitrinite reflectance (Rv-r) data and heat flow modeling studies of nine basalt dikes ranging from 0.06 m to 40 m thick in the Strzelecki Group were used to constrain the conditions of contact metamorphism of coaly sedimentary organic matter (SOM) to investigate it as an extended-duration natural analog to hydrous pyrolysis experiments.
Thermal history reconstruction suggests that at the time of dike intrusion the host rock was at a paleotemperature of 100 to 120 °C. Re-equilibrated fluid inclusions in the host rocks next to thin dikes (< 3.4 m thick) suggest maximum temperature (Tpeak) systematically increases towards the dike margin to around 550 °C. These Tpeak data are in fair agreement with those predicted by a conductive heat flow model and measure Rv-r. AFTA indicates the time of uplift and cooling as mid to Late Cretaceous and that paleotemperatures were too high for heating duration information to be recorded by annealing of tracks in apatite near the dikes.
The estimated Tpeak next to some larger dikes (20 to 40 m thick) as indicated by an extended zone of elevated Rv-r did not fit the pattern seen next to the thin dikes. Heat flow modeling along with whole rock elemental and isotopic data suggest that the extended zone of increased Rv-r is caused by convection of heated fluids. This is in contrast to the apparent advection effects found adjacent to thin dikes.
Laboratory hydrous pyrolysis compares well with contact metamorphism next to thin dikes with respect to the composition of the free hydrocarbons except in samples very close to the dike. Close to the dike, fluid inclusion evidence indicates a water vapor super-critical phase in the rock making the organic geochemistry results diverge from those found in hydrous pyrolysis experiments which maintains a liquid water phase.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California