Friday, June 12, 2015

Porosity in Sandstone and Carbonate Rocks

Q: Dear Sir, I would like to ask what are the classification of porosity in carbonate rocks?
- Tarek M

A: Carbonate rock porosity is a very big issue in the oil and gas world – perhaps THE issue. Water, gas, and oil must be able to move through the rocks or there is no oil and gas business. The nature of porosity in a source or reservoir rock can make all the difference in whether the hydrocarbons can be formed in the first place, and later economically extracted or not. Basically, can the oil mature and be concentrated in the first place – and if so, can it later then be extracted?

Here are some links that might help answer this for you:

        By comparison, sandstone porosity is pretty simple: it consists primarily of inter-grain spaces.

For carbonates (e.g., limestones and dolomites) however, porosity evolves with time and can be quite complex. Porosity in carbonate rocks is thus usually classified on the basis of the timing of the porosity: how the porosity changes and develops over time. Perhaps surprisingly, porosity often evolves over time. First, there is primary or depositional porosity, in which the pores are inherent in the newly deposited sediments and the particles that make them up. These pore types include inter-particle pores in carbonate sands (muddy or otherwise), and intra-particle pores (i.e., small passageways within tiny fossils such as foraminifera). Another pore type is fenestral: these are pores formed by gas bubbles and sediment shrinkage in tidal-flat carbonates, and also the growth-framework or structural pores common in reef buildups.

        In carbonate rocks (as opposed to sandstone) you can also have secondary pores – in other words pores that form as a result of later, generally post-depositional dissolution of certain constituent minerals. You can also have vugs: large pores that cut across the rock fabric. This means that the dissolution has not been controlled or driven by the fabric of the original deposition. Incidentally, you can also have pores that are basically dissolution-enlarged fractures. The natural tendency in most carbonate sediments is to for groundwater to cement in and overlying rock pressure to compact the pores during post-depositional burial. Most geologists believe that the bulk of the porosity in limestone and dolomite reservoirs is secondary in origin: post depositional, in other words.

Keep in mind in this discussion that carbonates and sandstones are discrete members of what is usually a continuum of rocks types in a stack of sediments. One can encounter sandstones, limestones, anhydrites, shales, and dolomite in relatively close proximity in a sedimentary package.

Saturday, June 6, 2015

Hey! A Sedimentary Rock Looks Like an Igneous Rock!

Q: I encountered a sedimentary rock in lab called Greywacke and it reminded me of an igneous rock, Gabbro. How does one differentiate these 2 apart? Or basically igneous from sedimentary and vice versa.
- Feiruz R

A: Igneous rocks were formed when the material was a melt, so the individual mineral grains are tightly fused and intergrown, and the porosity in the rock is very low. A greywacke was formed by accumulation of cold, weathered detrital material. It might superficially LOOK like an igneous rock, but a porosity test would give it away immediately. A closer examination with a hand-lens will show angular grains in a greywacke that do not interlock seamlessly; the word "greywacke" means that this rock also includes very fine silt, so this tends to fill those inter-grain boundaries between the larger crystals - but with a hand-lens you can see this. A petrographic microscope makes it even more obvious that a greywacke is really a garbage can term, representing an accumulation of weathered material of all different grain sizes.