Diagenetic Origins of the Calcite Microcrystals That Host Microporosity In Limestone Reservoirs

Micron-size pores are common in limestones, and they host significant hydrocarbon reserves in conventional and unconventional plays worldwide. Previous studies have shown that the vast majority of micropores in limestones occur in the interstices between micron-size calcite crystals that are present in nearly all depositional environments and burial depths throughout the Phanerozoic. In an effort to elucidate the genetic history of these calcite microcrystals, a geochemical study was undertaken to investigate a global dataset collected from seven new subsurface microporous reservoirs as well as data from 21 previous studies.

Bulk elemental analyses indicate that the vast majority of microcrystals are composed of low-Mg calcite (average 14 mmol/mol, range 1.6–90, Mg/Ca). Microcrystal Sr/Mg ratios are consistent with abiotic calcite cements, though overall Mg/Ca and Sr/Ca are lower than modern abiotic calcite cements reported in the literature. Higher Sr/Ca observed in depositional chalks implies a biotic, though probably not aragonitic, precursor. Most calcite microcrystals have oxygen isotope compositions that are 0 to 4‰ more negative, and carbon isotope compositions just slightly more negative (< 1‰), than contemporaneous abiotic marine calcite.

Most stable oxygen and carbon isotopic data plot either along a burial trend anchored at its most positive end in the age-equivalent marine-calcite composition or parallel to that trend with an offset in δ¹³C. Assuming that δ¹³C offsets are due to marine carbon isotopic excursions, the isotopic data are consistent with shallow-burial diagenesis as the dominant diagenetic pathway for most calcite microcrystals. For samples with information on porosity and depth, more negative δ¹⁸O (ca. 1–4‰) is associated with increased depth and reduced porosity, implying that progressive overgrowth of calcite microcrystals is modifying bulk chemistry and rock properties of the reservoir during burial diagenesis. A meteoric calcite line was evident only in two datasets. This demonstrates that evidence of meteoric diagenesis can be preserved in the chemistry of calcite microcrystals, but that its occurrence is rare in comparison with evidence for diagenesis during shallow-burial.

Geochemical data from calcite microcrystals are bulk samples, i.e., measurements averaged over many, many calcite microcrystals which themselves may have zoned chemistries. As such, microcrystal data require additional interpretation compared to samples from calcite macrocrystals where superposition puts samples into clear chronological order and produces diagenetic trends on, for example, δ¹⁸O–δ¹³C plots. Bulk samples of calcite microcrystals should show mixing between phases on diagenetic trends. With this model in mind, the assembled database suggests that most samples are mixes of primary marine and burial calcite.