Geologic terms and concepts


Q.    What is petroleum and how does it form in rocks?

The term petroleum refers to all naturally occurring mixtures of hydrocarbon compounds. Solid forms include asphalt. Crude oil is liquid petroleum. Natural gas is the gaseous form of petroleum. The hydrocarbon molecules in petroleum are made up of chains and rings of carbon and hydrogen atoms. The simplest hydrocarbon is methane, the principal component of natural gas. Petroleum is formed when sedimentary organic matter is deeply buried in an oxygen-deficient environment. As these organic-rich sediments are buried, heat and pressure transform the biological precursors into oil and gas.

Additional Resource(s):
Educational Series No. 8 – Oil and Gas in Pennsylvania
See, “The origin of oil” beginning on page 9 in the Spring 1998 issue of Pennsylvania Geology, Vol. 29, No. 1

 

Q.    What constitutes a “source rock”, and what is meant by the term “reservoir”?

A petroleum source rock is a fine-grained organic-rich rock in which oil or gas is generated. A petroleum reservoir is comprised of subsurface rocks with sufficient porosity and permeability to hold and ultimately produce commercial amounts of oil or gas.

Additional Resource(s):
Educational Series No. 8 – Oil and Gas in Pennsylvania
 

Q.    What’s the difference between a “conventional” and “unconventional” reservoir?

Conventional reservoirs typically consist of porous and permeable sandstones or carbonate rocks that yield oil and/or gas by draining interconnected pore spaces. Unconventional reservoirs include not only “tight” (i.e., low porosity) sandstones, but also self-sourcing reservoirs rich in organic matter such as gas shales and coalbed methane formations. These reservoirs lack the porosity and permeability of conventional reservoirs, and must be stimulated in some manner to enhance the interconnectedness of pore spaces. The Marcellus shale is an unconventional reservoir.

Additional Resource(s):
Educational Series No. 8 – Oil and Gas in Pennsylvania
 

Q.    What is meant by the terms “pool” and “field”?

A pool is a subsurface deposit of oil and/or gas that occurs in a separate reservoir under a single pressure system. A field is a group of pools related to a single stratigraphic feature, structural feature, or geographic area.

Additional Resource(s):
Educational Series No. 8 – Oil and Gas in Pennsylvania
 

Q.    What is a geologic structure map?

A geologic structure map illustrates the attitude and relative positions of rock units using a series of contour lines to connect points of equal elevation on a particular rock horizon.

Additional Resource(s):
Educational Series No. 8 – Oil and Gas in Pennsylvania
 

Q.    What is an isopach map?

An isopach map illustrates the thickness of a particular rock unit using a series of contour lines that connect points of equal thickness.

Additional Resource(s):
Educational Series No. 8 – Oil and Gas in Pennsylvania
 

Q.    What is an isopleth map?

An isopleth map illustrates contours that connect points of equal value of any numerically measurable characteristic (including, but not limited to, thickness). The Survey has prepared an isopleth map showing the net thickness of organic-rich shale in the Marcellus Formation based on data collected as part of the Eastern Gas Shales Project (EGSP).

Additional Resource(s):
Full-color Marcellus isopleth map

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