Geologists Start With Rocks

And, the main type of rocks they study are called sedimentary (sed-a-mentree) rocks.  Most sedimentary  rocks are formed in lakes, rivers, or oceans.

Rivers and streams carve out tiny bits of rock and carry them downstream.     If the rock bits are fairly coarse (about the size of salt grains, or larger), they are called "sand".  If they are a little finer, they are called "silt".  If the rock bits are really fine (like flour) they are called "mud".  Remember, "sand", "silt", and "mud" refer to the size of the grains, not what they are made of.

At some point in their travel, the rivers slow down.  This may be because the surrounding land is very flat, or the river may enter a lake, or (most commonly), the river enters the ocean.  Then, the grains of sand, silt, or mud being carried by the river drop to the bottom and form layers of sediment.  Usually a layer will be mostly sand, mostly silt, or mostly mud, but they may be mixed up.  

Take a look at the satellite photo of the Mississippi River Delta.
 


A "delta" is a fancy word for a big pile of sand that forms in an ocean or lake at the end of a stream or river.  In this case, the Mississippi River is bringing down a HUGE amount of sediment that has been scoured from North America. and is forming a new delta (many miles long) right before our eyes, south of the City of New Orleans.  Note the main channel of the Mississippi River snaking down through the delta.  The darker areas on the picture show where land sticks up (just barely) above the surface of the ocean and allows plant life to grow.  The lighter, whitish areas show sediments (sand, silt, and shale) that are just under the surface of the water.  The darker blue to the left shows deeper water without much sedimentation from the delta.

A Natural Sandbox

If you have been to a beach, you probably stood on a pile of sand that was torn from rocks many hundreds of miles away, was transported by the stream or river for a long distance, was broken into TINIER and TINIER bits,  then spread out in a long bar (shaped kind of like of a ....Butterfinger!) by the work of waves, tides, and wind.  A beach is just one type of many sand deposits that may become deeply buried and later become an oil field.

As the sediments pile up, the oldest ones are buried deeper and deeper.  When they are buried deep enough, heat and pressure and other workings of the earth make the soft sediments hard, and turn them into rock.  This is called "lithification", and sediments that have become hard are said to be "lithified".
 
SANDSTONE is formed when sand-sized sediments are turned into rock, SILTSTONE is formed when silt-sized sediments are turned into rock, and SHALE is formed when the tiniest mud-sized sediments are turned into rock.

Sandstone is a rock made up of grains that are 1/16 millimeter to 2 millimeters in size.  The largest sand grains would be about 1/2 the size of a grain of rice.  Grains larger than this are called PEBBLES or even COBBLES.

Even though sandstone is hard, and appears very solid, it is really like a sponge.  Between the grains of sand, enough space exists to trap fluids like oil or natural gas!  The "holes" in sandstone are called "porosity" (from the word "porous").

The picture at the right shows a very thin slice of actual sandstone as seen through a microscope.  The larger brown and yellow pieces are grains of "quartz", a common mineral.  Between the grains, you can see the "holes", or porosity, in the rock; it shows up as black.

Look at the picture below.  This is is a drawing of a typical sandstone under a 


microscope.The porosity is shown as black.  Oil or gas could fill these holes in the rock.  What percentage of this sample could be filled with oil or gas?  I would estimate at least 15%.  What do you think?

The ocean, lake, and river environments were ideal for the formation of sedimentary rocks like sandstone, but they also supported a very abundant and diverse collection of living things. 

These animals, whether smaller than a grain of sand or larger than the biggest whale alive today, eventually died and contributed their bodies to the sediments forming below their habitat.  When these animals were buried beneath thousands of feet of sediments, heat and pressure in the earth "cooked" their bodies into oil and natural gas.  These hydrocarbons either became trapped in the sediment layers that the animal died in, or moved some distance and ended up in a hydrocarbon trap.

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