Increasing Rock Strength and Reservoir Quality With Early Microbial Cements

Abstract

Modern marine cemented carbonates and microbialites, such as beach rocks, stromatolites, hardgrounds and coquina pavements, often display strong rock strength despite their high porosity. In this study, we document how microbially mediated precipitates or precipitates from endolithic borers at grain contacts act to dramatically increase the stiffness of the grain aggregate, which produces a high impedance rock with excellent reservoir qualities.

Samples for this study consist of intraclasts from active portions of the ooid shoals of Schooners Cay, Joulters Cay and submarine dunes and beach rock from Schooners Cay. In addition, hardground samples from areas between the oolitic tidal bar belt in the Tongue of the Ocean and stromatolites and cemented seafloor pavements from Hamelin Pool in the Shark Bay area complement the data set.

In all samples, grain to grain contacts are infested with microbial coatings or are plastered by EPS. Within the EPS, nanograins develop and act as the precursor of a fine mesh of aragonite needles. Direct signs of microbiologically mediated precipitation in the interstices of the grains are largely supported by the occurrence of EPS organic remains that decay very rapidly and the occurrence of various microbial forms. A second path of producing micritic bridging cement is with precipitation along filamentous fungi and bacteria. The thereby produced micritic cements fuse the grains together to form the template for subsequent marine cements like acicular aragonite needles.

These early micritic cements also produce a stiff rock framework with the ability resist compaction and fast acoustic velocity transmission. In laboratory experiments the high initial velocity remains largely constant when acoustic velocity is measured under increasing pressure. These samples will end up as fast, high impedance rocks with a high amount of primary porosity and high permeability. If subsequent diagenesis does not fill the pore space with aragonite or calcite spar, these rocks have excellent reservoir quality to great burial depth. Examples of such reservoirs are the rudist grainstone and rudstones of the Shuaiba Formation in the Middle East or the Cretaceous oolitic reservoirs offshore Brazil. In addition, the coquina units in the pre-salt are likely to be similarly cemented to the pavements in the Hamelin Pool that are high impedance rocks yet have both high porosity and permeability.

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