Spectrum of Micropore Types and Their Origin in Limestones
Loucks, Robert G.; Lucia, F. Jerry
Microporous limestones form tight carbonate reservoirs; with the advent of horizontal drilling and fracking these reservoirs are now important exploration targets. Micropores have pore diameters of <10 microns and pore throats ~1 to 3 microns in diameter. In a reservoir, however, arrangement and connection of pores form the basis of reservoir quality. Three main types of micropores in limestones are: (1) micropores associated with transformation of aragonite mud to calcite, (2) micropores associated with transformation of Mg-calcite allochems to calcite, and (3) original pores in coccolith-rich sediments (chalks).
Transformation of aragonite to microcrystalline calcite shows that calcite microspar forms by dissolution of aragonite mud to form microcrystalline calcite with no change in porosity of the mud. Lower Miocene Java Sea limestones have micropore systems of this type.
Transformation of Mg-calcite to microcrystalline calcite and associated micropores has been documented for former Mg-calcite allochems such as red algae, foraminifera, bryozoans, stromatoporoids, ooids, Lithocodium, and micrite rims. The first stage is documented by Mg-calcite rods in living organisms changing in morphology. The rods appear to precipitate nanoballs along their surface and then separate into individual nanoballs. These Mg-calcite nanoballs dissolve and reprecipitate as microcrystalline calcite. Evidence of this process is found in relic nanoballs observed in ancient microcrystalline calcite. Significantly, no special fluids are needed to transform Mg-calcite to microcrystalline calcite; the Mg-calcite simply needs to become chemically unstable, which can happen in any number of fluids. This transformation probably is completed in the first several thousand feet of burial or, most likely, shallower. The Lower Cretaceous Stuart City and Sligo are examples of formations dominated by Mg-calcite stabilization micropores.
Chalks contain a micropore system, which is a function of original interparticle pores between coccoliths and segmented coccolith platelets. Through time, the coccolith hash compacts (from ~75% to 50% pore space) and cements to form a tighter limestone. The Upper Cretaceous Austin and North Sea Chalks are examples.
Micropores in limestone take three different and distinct forms. They have different origins and distributions at the thin-section level. Understanding these differences is important to the exploitation of microporous limestone reservoirs.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013