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:
http://pubs.usgs.gov/wri/1994/4013/report.pdf
http://www.slb.com/services/technical_challenges/carbonates/near_wellbore/rock_properties.aspx
http://en.wikipedia.org/wiki/Porosity
http://www.searchanddiscovery.com/documents/2004/mazzullo/images/mazzullo

        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.

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. 

Oxygen and Mcrobes and Algae and the Early Earth

Q: Hi!

Oxygen in our atmosphere was created by small creatures who had just invented a new process:  photosynthesis.

The waste product of photosynthesis is oxygen.

After an unfathomable number of  years, too much O2 built up in the atmosphere, changing the greenhouse gas, methane, into carbon dioxide, which isn’t such a strong greenhouse gas.  This caused the earth to cool off to the point where the first known ice age began, the huronic, I think.  It lasted for millions of years and enveloped the entire earth with ice.

Question 1: how did these photosynthetic creatures survive in an iced-over earth?

Question 2: what caused the end of this very long ice age?

 Just curious - Thank you!

 - Susan K

A: Your question suggests you are well along in studying this topic. I've stored a number of closely related questions and answers on this blog, and by way of a long answer, some of these may help:

http://askageologist.blogspot.com/2013/07/climate-change-is-it-real.html

http://askageologist.blogspot.com/2013/04/castastrophes-and-mass-extinctions.html

http://askageologist.blogspot.com/2012/06/snowball-earth-faint-young-sun-paradox.html

The medium-length answer: Our atmosphere passed through the oxygenation transition around 2.5 billion years ago, and it certainly involved photosynthesis - stromatolites (fossil algal clumps) have been found dating as far back as 3.3 billion years. However, there are also suggestions that mantle outgassing, tectonics, and oceanic current-shifts may have contributed. There really was a Snowball Earth episode, and there have been a series of cold-warm cycles since then. Scientists have been exploring - with limited data - what could have caused these events, and suggestions range from the fairly mundane to the exotic: asteroid impact, tectonic change that interrupted oceanic current flows, etc. The Chicxulub asteroid event 65 million years ago certainly knocked the oxygen levels in the atmosphere down dramatically, requiring millions of years to recover. This is almost certainly why bar-headed geese can easily fly over Mount Everest: birds have evolved a truly advanced respiratory system since that event. Evolution is well-documented to speed up under environmental stress.

Some short answers:

1. If there is one thing certain about microbial life, it is that it can survive almost anything. Microbes have been found kilometers deep in the Earth, and temperatures steadily increase with depth due to radio-isotope decay in the Mantle and Core of the Earth (the temperature rises to typically 60 C at 4,000 meters depth).

2. There are a lot of variables that may have been involved in the recurring cool-warming cycles, including the fact that the Sun has steadily grown in luminosity during its lifetime, as well as tectonics, and methane-emitting life forms. Likely a combination of these - and probably other factors - led to out-of-control feedback loops that dead-ended in climate extremes before the atmosphere eventually  recovered. 

Damage Earthquakes Cause

Q: What kind of damage can a(n) earthquake do?

- Carrera K

A: Earthquake damage can be very wide-ranging. In increasing order of destructiveness (roughly following magnitudes from M ~4 to M ~ 8) :

1. Small cracks form in drywall, stucco, and concrete walls of buildings.

2. Fragments of building fall into the surrounding streets - glass, or bricks from the corners of windows, etc.

3. Topographic settling, leading to serious internal structural damage in buildings, making them uninhabitable (San Francisco, Loma Prieta earthquake, 1979).

4. Large fractures form in soils and rocks. Water towers fail. Electric power is cut and gas lines rupture (White Wolf Fault, Bakersfield, CA, 1952).

5. Major fires start that are difficult or impossible to control (San Francisco, 1906 AD).

6. Collapse of concrete floors in buildings, crushing most of what lies within (Izmir, Turkey, 1999 and 2013).

7. Major infrastructure collapse, leading to water-borne diseases, and starvation happens on a wide scale (Haiti, 2010).

8. Tsunamis wreak broad damage to coastlines, killing most people within reach of the water (Aceh, Indonesia, 2004).

9. Allocthons and major landslides cover or sweep inhabited regions. Segments of coastline drop below sea level (Puget Sound, 1700 AD), and rivers temporarily reverse direction (Mississippi River, 1811).

10. Entire civilizations collapse and do not recover (the Minoan civilization, 1,500 BC, though how much was explosive volcanism and how much was caused by earthquake damage is unclear). 

Ethics & Conflicts of Interest - No Private Work in Minerals

This Q&A actually raises an important issue, which at its core involves ethics, and conflicts of interest in science. The US Geological Survey was established in 1879, but an important event in 1871-72 led to its creation. An unscrupulous Kentucky con man tried to sell a worthless mining property to some gullible investors in San Francisco. After he staked a claim on federal land, he “salted” the property with rough diamonds that he had apparently stolen from a drill-bit manufacturer that had employed him. He apparently thought that he could fool some greedy investors who would not look beyond the few diamonds and other rough gems that they saw at the surface. The investors were nearly taken to the cleaners, and the con man eventually made off with what in today’s money would be $8 million.

However, a young, Yale-educated geologist named Clarence King had been funded by Congress to assess the huge federal lands that Congress had responsibility for in the Western United States. King was aware of the news of the “great discovery” – actually one of a long list of scams and get-rich schemes common at the time in what was truly a Wild Wild West. Putting two and two together from the partial information the con man had released, King and his party located the claims in a remote part of Colorado. It didn’t take long to tick off the obvious evidence (diamonds were only found at the surface where there were already footprints, and none were found below in trenches that they dug to test the local soils, etc.). As their field investigation concluded, King and his party discovered that they had been spied on for days by a New York diamond dealer who had secretly followed them and who had been watching with a telescope from a nearby butte. When someone from the party blurted out that the discovery was a giant hoax, this man exclaimed “What a chance to sell short on the stock!” Stung at hearing of this obviously unethical plan, King raced back to San Francisco to beat him, and informed the wealthy investors about what had been going on. He was greeted as a savior. The San Francisco Bulletin wrote “Fortunately for the good name of San Francisco and the State, there was one cool-headed man of scientific education who esteemed it his duty to investigate the matter in the only right way.”

Read more here: http://www.smithsonianmag.com/history/the-great-diamond-hoax-of-1872-2630188/#wt4lHMrIiH1XwdM7.99

In part because of this success, in 1879 King became the first Director of the newly-formed US Geological Survey. From the very beginning, ethics and integrity were built into its policy and operation. Today a USGS employee and even family members are not permitted to hold stocks or any other interest in a natural resource or a related company. It’s not a great reach beyond this to ensure that no federal employee uses skills and facilities to help a private individual while on the tax payers payroll. At the core of the USGS Fundamental Science Practices policy we find these words “The scientific reputation of the U.S. Geological Survey (USGS) for excellence, integrity, and objectivity is one of the Bureau’s most important assets. This reputation for reliable science brings authority to data and findings, creates and protects long-term credibility, and ensures that the public trust is met.” (May 24, 2006). I find great personal satisfaction in belonging to an organization that treats integrity and excellence as its most valuable assets.

Q: I know u probably get this alot. My name is arial. I am sure i found a precious gem deposit in georgia. I know that if it is this is an incredible find. I cannot find anyone who can or will test the stones. I have done all the research and i do have possion of the stones. I removed them feom the rock myself. If u are interested in the least bit i can show u the samples and the location from where they came. Thank you
- Arial L

A: I am so sorry, but my volunteer work for Ask-a-Geologist prohibits me from getting involved in anything like a private gemstone or mine evaluation. It's actually built into the Organic Law which started the US Geological Survey in 1879 - I would be breaking that federal law. 

Our work here is closely guided by specific Congressional mandates (for instance in my office we monitor potentially active volcanoes), and we are not supposed to do anything outside our mandate. We are very much aware that the US taxpayer is paying for what we do, and we try very hard to be as efficient and careful as possible with that trust. 

We are ESPECIALLY prohibited from doing ANYTHING that could possibly be construed at helping a single individual or group of individuals with a mineral deposit, or in a way that could financially benefit an individual or group of individuals. This is viewed as using taxpayer funds to promote something for someone other than the country as a whole - a form of corruption. 

My recommendation is that you contact the Georgia Geological Survey (note the mineral resource maps in this link: http://epd.georgia.gov/georgia-geologic-survey-maps), or talk with a member of the department of geology at the University of Georgia or Georgia State University. 

Sorry I can't be of more help than this.

Q: Avtually uve helped alot by letting me know who to contact thank you.  Ariel

Will The Plate We Live On Sink? More Tectonics Questions.

Q: Dear Geologist, we are students from the Schiller Gynasium in Berlin and we would like to ask you some questions about platetectonics.
- Emma and Lili K

A: Hi, Emma and Lili,
My grandmother and her family came from Bavaria (Goggingen/Augsburg), so I have a soft place in my heart for Germany. I'll try to answer your questions in order below: 

Q: Is it possible that the plates we live on are going to sink?

A: The plates we live on will not sink. They have a higher silica content than the Earth as a whole and are thus less dense (average density of 2.67 grams/cc), so they float on the denser Mantle material. The plates that DO sink during subduction tectonics are ocean floor segments. These plates are made of  injected Mantle material that comes in at ocean floor spreading centers like the Mid-Atlantic Ridge. This material has more iron, magnesium, and calcium, so it is usually more dense (average density about 3.2 grams/cc or even higher). When these ocean floor segments meet continental crust in normal plate tectonic collisions, the ocean floor segment is usually - but not always – over-ridden. There are rare instances where segments of oceanic crust are rafted onto continental margins by this messy collision process, and these are called ophiolites. Examples are in Cyprus (the Troodos ophiolite), Oman (the Semail Ophiolite) and northern California, USA (the Josephine Complex). These ophioloites are fascinating to study, and often host pods of dense chromite (more than 7 grams/cc) in them. A chromite pod the size of a large room may be worth over a million dollars/euros.

Q: Can the mantle and the outer core mix (as they are both liquid)?

A: The Mantle and outer Core are indeed liquid according to seismic refraction studies, but they have already been largely segregated by gravity over the last 4 billion years or so. If there is mixing, it is local in nature. Some laboratory studies and modeling suggest that the outer core is actually growing, as the gravity segregation apparently is still continuing. 

Q: Would it be possible that the pangea forms again?

A: Yes, it is possible to have another super-continent like Pangaea, as the Pacific continues to narrow on almost all margins. However, it will take hundreds of millions of years to accomplish this. It will likely not be a clean recombination, but more likely a complex amalgamation of crustal segments and fragments. The Atlantic opened up once before, then closed again, and then opened again a second time. It is now widening at the Mid-Atlantic Ridge, so the Atlantic grows while the Pacific narrows. As the history of the Atlantic Ocean shows, this may change at any time in the future.

Q: Thank you for taking time to read our e-mail!

A: I'm glad to help. You will perhaps be the next generation of Earth scientists, and discover things that we don't know about today. 

Ever Been in an Earthquake?

Q:  Have you ever been in a earthquake
--Gretchen H

A:  Yes, I have "been in a earthquake". As a 6-year-old I was thrown out of my bed by a M=7.2 earthquake in Bakersfield, CA, that hit at 3:30 in the morning. My Mom told me later that she called to me to come to her bedroom down the hallway, and that I answered her "I can't. The walls keep hitting me." These were probably the manifestation of Raleigh or surface waves rolling through the ground beneath the house. I do remember standing in the grass outside the house later and listening to sirens until dawn, as the adults talked about what had happened. That's where I first heard the word "earthquake."
    I also felt several earthquakes when I lived for three years in southern Venezuela. One event frightened the other residents of our apartment building so much that they all ran down the stairwell and stood in the street outside.
    You have to be physically in contact with the ground to feel small earthquakes. If you are driving a car or sleeping in a soft bed you may not feel events up to a magnitude of 3 or 4. 

Q: Thanks so much for answering my question.  I bet it felt very scary in the earthquake.

A: The experience wasn't so scary as it was a sleepy mind trying to figure out what was going on. That part took me awhile as a six-year-old. Then it was just sort of being amazed and listening to adults try to explain it as THEY were figuring it out. I remember my Mom, being very frightened, going down into a darkened basement to shut off the natural gas. I felt her fear then, my first fear, mainly for her.