Depositional and Diagenetic Investigation of a Tertiary Isolated Carbonate Platform, Evidence for Significant Gas-Charge-Related Dolomitization

Abstract

A Tertiary offshore isolated carbonate platform formed on a horst block during a late rifting phase in an oceanic basin. The platform subsequently drowned and was buried in >1000m of fine-grained clastics. The carbonate edifice is gas-bearing, with significant quantities of methane and CO₂. Rocks that make up the reservoir are of two units, each representing a geologic stage of greater than 2 million years. Rocks of the lower unit are composed of a photozoan assemblage including hermatypic corals, and foraminifera interpreted to have flourished in shallow marine grass beds. They are capped by a brecciated and iron-stained interval that may represent an exposure surface. The overlying unit (∼100 m thick) is comprised of rocks of the heterozoan association, decimeter-scale fining-upward cycles bearing innumerable rhodoliths and large benthic foraminifers. Analogs point to deposition of these rocks near the base of the photic zone. Limestones at the top of the platform, and immediately below the fine-grained clastics, are iron-stained and fractured, with fractures filled by planktonic-foraminifer-bearing packstone and glauconitic sand. Dolomitization has a significant positive effect on the reservoir properties. XRD analyses show these enhanced rocks to be between 30% and 85% dolomite. The dolomite is not stoichiometric (Ca-rich) and occurs predominantly as rhombs less than 50 um and most commonly 10-20 um in width. The dolomite is interpreted to preferentially replace carbonate mud. Whereas the absolute percentage of dolomite is variable, the dolomitized intervals are continuous and have only been observed in the lower part of the upper unit. Depositional history, petrographic relationships, and geochemical investigations all point to texture inversion and dolomitization predominantly occurring after burial and gas charge. The evolution of reservoir properties is regional, occurring over at least 10s of km², and with no evidence of fault or fracture control. A simple model of the dolomitization process involves magnesium-calcite dissolution in the presence of CO₂. Whatever the driver, large-scale and significant evolution of reservoir matrix properties through dolomitization in the burial realm should remain on the table of carbonate diagenetic paradigms.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016