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Geochemical Constraints on Microporosity Formation

Abstract

The most abundant form of limestone microporosity is hosted among micron-sized crystals of low-magnesium calcite with magnesium contents typically between 10 and 40 mmol/mol Mg/Ca (approximately 1-4 mol% MgCO3). Published studies have documented these crystals in almost all carbonate depositional environments from terrestrial lacustrine to deep marine. These microcrystals have variably been suggested to form in marine, meteoric, shallow burial, and/or deep burial fluids, often relying on the geochemical trends and petrographic relationships obtained from other larger carbonates phases present in samples. A review of dozens of published studies on the geochemistry of microporous limestones augmented with new work on approximately a dozen other petroleum reservoirs suggests that the calcite microcrystals associated with microporosity form and undergo diagenesis under shallow burial conditions. For example, these microcrystals are usually 2 to 4‰ more negative in d18O and about 1 to 2‰ more negative in d18C than the generally accepted marine calcite composition of the period in which they were initially deposited. In addition, d18O is roughly correlated with depth of samples. The Sr/Mg ratios of these calcite microcrystals are consistent with an abiotic origin. However, depositional chalks show enrichment in Sr typical of a biological origin. Whether of biotic or abiotic origin, marine or lacustrine, the geochemistry of these microcrystals appears to document a diagenetic history very similar to what has been described for the textural changes marine carbonate oozes undergo under increasing burial.