Subaerial Exposure and Diagenesis of the Miocene Kardiva Platform, Maldives

Abstract

Subaerial exposure and meteoric fluids have the ability to improve reservoir quality by increasing both porosity and permeability beneath exposure surfaces. The Kardiva platform ultimately drowned, but the amplitude and frequency of sea-level changes in the Miocene make it likely that subaerial exposure occurred during its evolution. Abundant moldic porosity has been interpreted as meteoric diagenesis, but stable isotope evidence to support this has not been reported.

Using bulk stable isotope analyses and petrographic methods, we sought to identify evidence of meteoric diagenesis by investigating the variations in grains, cements, porosity, δ¹³C, and δ¹⁸O at IODP Sites U1645, U1469, and U1470.

Within the platform, grain distribution is variable with benthic foraminifera, algae and corals representing the most abundant grain types. Cement abundance generally increases while porosity decreases with depth, with some variability. δ¹⁸O and δ¹³C range from -7.0‰ to 3.2‰ and -7‰ to 2.5‰, respectively. Petrography and isotope values show evidence for subaerial exposure and alteration by meteoric fluids, with a cross-plot of δ¹³C and δ¹⁸O showing the characteristic inverted “J” trend associated with dissolution and precipitation reactions mediated by meteoric fluids, resulting in values that are more negative.

The dissolution of Mg-calcite benthic foraminifera suggest that flow rates were relatively high beneath the subaerial unconformity. This evidence for meteoric diagenesis of the Kardiva Platform indicates that potential high-amplitude sea-level fluctuations have positively contributed to the reservoir quality of carbonate reservoirs during the Miocene in the Indo-Pacific region. Dating this event will enable the prediction of favorable reservoir conditions in similar platforms.

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