Correlating Rocks With Microfossils

Sometimes, in certain areas, the well samples that are returned to the surface from the drill bit do not consist of various alternating beds of shale, limestone and sandstone.  The offshore Texas Gulf Coast is one of these places. 

Instead of rocks that are fairly easy for the Petroleum Geologist to identify, the well may drill through thousands and thousands of feet of rocks that are made up only of shale.   This makes it very hard for the PG to tell what exactly what rock layers are being drilled, since most shale looks like most other shale.  It also makes it difficult to tell what age rocks are being penetrated.

But, the PG has an answer for this.  In these difficult areas, the rocks are identified by the tiny "microfossils" they contain.  These microfossils are the skeletons of the tiny animals or plants that have lived in the ocean for millions of years.  Eventually, they died, and were buried in the shale on the ocean floor.  The animals changed and evolved through time, and PG's have learned how to identify rock layers by looking at the types and variety of microfossils contained in them.  This particular type of work is always done with a microscope

A PG that studies microfossils through a microscope is doing very specialized work.  This type of geologist is called a micropalentologist  (my-crow pale-lee-on-tall-o-jist).

If a PG finds a certain microfossil in a rock layer (call it "Fossil A") that is 9,000 feet deep, and then finds the exact same microfossil in another well at a depth of only 8,000 feet, he may decide that the rocks containing Fossil A in each well are the same layer.  He may then name these rocks the "Fosssil A Layer".  

Other PG's can then use pictures of Fossil A to identify the Fossil A Layer in their own wells.  If enough PG's learn to recognize Fossil A, it becomes an important tool to identify certain rocks, and PG's can talk with each other about the Fossil A Zone....and other PG's will know what they are talking about.

The drawings of microfossils on this page like planularia and textularia are expanded many times!  These tiny fossils would fit on the head of a pin!

More Microfossils

Clockwise from top left: ostracode, planktic foraminifer, diatom, benthic foraminifer, and pollen grain.

(From USGS Sound Waves, Jan/Feb 2009)