061319b

Relict and dissolution porosity (blue) in mixed-particle grainstone (peloids [arrow], ooids, intraclasts), Jurassic Smackover Formation, Alabama, thin section photomicrograph, field-of-view 2.5 mm wide.

041919

Two examples of dedolomite (dolomite that has been dissolved) from the Jurassic Smackover Formation in Alabama. In the first image, some dolomite crystals are hollow. Blue indicates open pore space. In the second image, hollow dolomite crystals have been filled by calcite cement (pink, stained by Alizarin red S). These samples come from rocks buried several miles below the surface. Thin section photomicrographs, 1 mm across.

041219b

Bimodal pellet dolograinstone. Dolomitized pellets (gray) in matrix of altered calcite cement (red) with fine porosity within and among pellets (blue), Large pellet in the center of image (arrow) is Parafavreina, a shrimp fecal pellet. Red color is the result of staining with Alizarin red S. Jurassic Smackover Formation, Alabama, thin-section photomicrograph.

032419

Cross-laminated oolitic dolomitized grainstone, Jurassic, Smackover Formation, porous oil reservoir, North Choctaw Ridge Field, Alabama, five-centavo piece for scale.

A very old skeleton

What used to be a hale and healthy feldspar sand grain is now hardly a shadow of its former self. Blue is open space. Note the scale.

Holy sandstone, Batman!

Arrows point to fresh crystal faces on quartz sand grain, which is growing in place. The blue is open space; large holes are where unstable sand grains have been dissolving away.

Oil reservoir

Blue is open space, dark Brown to black is dried oil residue. White is sand grains. Field of view about 1 mm wide. From more than 2 miles below the surface.

Slide Set on Carbonate Porosity

http://www.gsa.state.al.us/online_pubs.aspx

"Educational Series 1"

A very large file, but lots of cool photos. Also, the photos are all from the Smackover Formation in Alabama.

So, um, let's get small

Sometimes, limestone is replaced by crystals of dolomite. Dolomite crystals can later dissolve, or partly dissolve....

Blue is open space filled with epoxy. These images are about 1 millimeter across.

A little pressure can crush these hollow crystals.

Or, calcite cement can fill the holes, which were calcite to begin with, before the dolomite crystals grew! (The calcite is stained red with a chemical that doesn't stain dolomite.)

Dolomite crystal molds actually play a significant role in moving oil or water here and there in the subsurface, but mostly they look cool.