Source Rock for Oil and Gas*

Mark F. Wood

Associate Professor of Geology, JCTC

Louisville, Kentucky

* Illustrations will be posted.

   Petroleum geology is a specialized field within the science of geology that studies and applies how to find oil and gas (also called hydrocarbons).  Finding oil and gas is a combination of knowledge, scientific process, intuition and often a bit of luck.

   There are several geologic factors that must occur for hydrocarbons (oil and gas) to be found in an area.  These factors are source rock, reservoir rock, burial (temperature) and a trap.  In addition the timing of the trap formation and the burial must be compatible.  This discussion will focus on the source rock.

   Source rock is any rock that will generate hydrocarbons (or has done so in the past).  Most source rock is gray or black shale.  There are some limestones, coal and other rocks that, under special circumstances, will occasionally generate hydrocarbons as well.  The material in the rock that will form oil is the organic material that is incorporated in the rock when it is formed.  The higher percentage of organic material in the rock, the more oil it is capable of forming.

   There are several types of organic material found in rocks that will generate hydrocarbons.  There are three general categories, one is algae, a second is marine planktonic organisms and bacteria and the third is terrestrial plants (like leaves and stems).  These are the major contributors for the oil and gas fields known today.  The algae and marine organisms generate oil when heated.  When temperatures rise substantially higher they will generate gas.  Coals and coaly shales are formed mainly of terrestrial plants and they generate mostly gas.

   The organic rich rocks can form in a variety of environments.  These include deep water marine (ocean), lakes, deltas and swamps.  Many environments have organic material in them, but most of this organic material is eaten by the local organisms or oxidized as they decay.  It is important that the environment have little oxygen and few organisms present, or that burial is very rapid in order to preserve the organic contents.  For instance, reef environments in the ocean are teaming with organic life but little of it is preserved (i.e. most of it is eaten or oxidized) and so most reef rock is not a source rock and will rarely form oil or gas.

   It is also important for sediments to continue to be deposited on top of the organic rich rocks.  With additional burial the temperature will rise in the rock.  The high temperature converts the organic material into oil and at much higher temperatures natural gas will form.  Some of the terrestrial source rock will not generate any oil, but form only gas at similar higher temperatures.

   So the gas we use to heat our homes and the refined oil we use in our cars and airplanes all had a natural source from living creatures millions of years ago.  If these organisms were preserved in the sediments, then heated changed to hydrocarbons then trapped and stored you are using it today, thanks to nature.

Created January 11, 2012