Presalt Reservoir Analogs: Lacustrine Microbialites Fed by Shore Zone Hot Springs, Lakeside UT

Abstract

Travertines and microbialites compete in opposing scenarios that are commonly used for interpreting reservoir facies observed in South Atlantic late sag phase presalt lacustrine carbonates. Early Great Salt Lake shoreline deposits at Lakeside provide spectacular outcrop analogs for these reservoir facies, comprising cascade and pool systems that run down slope to fracture fed domes at the then regularly flooded lake margin, and to shore-parallel lacustrine reefs of microbialite mounds. Geothermal aquifers, driven by significant falls in lake level, followed faults and fractures in the Mississippian Great Blue Limestone bedrock to feed mesothermal hot springs along the shoreline. Highly porous facies developed along the shoreline during higher frequency but lower amplitude cycles of change in lake level. They show crude to fine layering from successive cycles of growth and exposure in both shoreline travertine and bioherms. Classic microbial clotted textures both in travertine drapes and in microbialite mounds are cemented by syndepositional to early diagenetic minerals similar in composition to the clots. Mineralogy of travertine in fracture vents is dominated by aragonite, whereas lacustrine carbonates are dominated by nonstoichiometric dolomite. These minerals reflect changes in water temperature and chemistry down the depositional profile going from source to sink, possibly together with a change in microbial communities. In contrast mound-flank hot spring terracette increments comprise shrubby dropwalls of intermediate-Mg calcite, which developed during cycles of exposure of larger shore zone mounds. Prolonged submersion led to pervasive dolomitisation of micritic and grainy facies. This outcrop analog provides field-based data to show how highly porous facies, linked together within travertine and microbialites, may be composed of site-specific minerals deposited before burial diagenesis, and that the travertine and microbialite scenarios can be intimately associated rather than to be mutually exclusive for interpreting presalt reservoir facies.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018