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Fabrics and Interpretations of “Microbialites” Can Both Be Full of Holes


The discovery of prolific hydrocarbon reservoirs formed of small, branching, digitate structures in carbonates of the pre-salt play of the south Atlantic has raised awareness of the potential economic importance of microbial processes in carbonate rock formation. Historically this question has been viewed as largely academic, initially involving studies of Precambrian and early Paleozoic stromatolite and thrombolite facies, complemented by modern analog studies of mostly restricted marine environments. Important questions focused mostly on the interactions between microbes and environment, but also the evidence for the oldest remnants of life on Earth. In the past few decades the academic community has focused on the rapidly growing field of metagenomics (“who's there”), and measurements of biogeochemical processes including metabolic pathways (“and what are they doing”). That microbes are everywhere (surviving nearly all extreme environments), genetically diverse (more so than all higher organisms put together), and capable of a broad range of processes operating at the endmember spectrum of pH and Eh has lead to the conclusion that if metazoans and metaphytes don't make the carbonate then microbes must. However, to be confident of the role of microbial processes then the null hypothesis of abiotic processes must be falsified. This could come in the form of textural/morphologic criteria, or geochemical and in particular isotopic criteria. Unfortunately, neither provide strong evidence for microbial processes in the formation of digitate or “shruby” “microbialites”. Thus, the name “microbialite” suggests more confidence in genesis than can be demonstrated. The issue may seem academic but it is not. The growth of abiotic mineralized structures can result in the incorporation of significant pore space between fabric elements, such as the Cretaceous “shrubs” described from south Atlantic carbonate reservoirs and is the consequence of generic growth processes that may include, but not require, microbial influences. Understanding the water chemistry and carbonate growth mechanisms is essential to the elucidation of reservoir fairways. Analysis of the fundamental processes (abiotic vs. microbial, lake vs. marine, saline lake vs. alkaline lake) that underpin this novel hydrocarbon play can lead to enhanced reservoir predication.