Geologic time is extremely long, and has been broken down into smaller intervals. Upon realizing that rock layers record geologic time, those layers were initially ordered by age relative to each other and given names according to their location. As scientific techniques improved, specific (absolute) ages were given to the layers. The result of these two approaches is the geologic time scale, which combines period and era names with absolute ages.
Although our understanding of geologic time is fairly complete on Earth, in different parts of the planet, the rock record is not complete, and gaps (called unconformities) exist. For example, in a particular location a rock layer with an absolute age of 10,000 years may be directly overlying a rock layer with an absolute age of 40,000 years. This relationship demonstrates a 30,000-year gap, or unconformity, in the rock record at that particular geographic location. Because the rock record is incomplete at various locations, the construction of the geologic time scale has relied upon the integration of stratigraphic work from many different locations on Earth.
This knowledge is crucial for locating oil and natural gas in the subsurface. Throughout Earth’s history, specific environments have yielded organic material that became oil and natural gas deposits, trapped in subsurface rock. Being able to determine the relative and absolute age of subsurface rock is important for finding these deposits. For example, during the Mesozoic (252 million years ago – 66 million years ago), the climate was warm, and life was abundant in the oceans. However, the oceans lacked oxygen (hypoxic and this decreased the rate of decomposition in the ocean. As marine organisms died and their organic matter settled on the ocean floor and built up since the rate of decomposition was lower than normal. The volume of this matter was significantly higher compared to previous times in Earth’s history. This provided abundant source material to form oil and natural gas as the organic matter was buried, compressed, and heated over 100’s of millions of years.
By studying rock outcrops and subsurface core samples, scientists and engineers gather critical information that helps them identify potential hydrocarbon resources. Advances in scientific understanding and technology have made it possible to safely and efficiently produce these resources from deep within the Earth. Successful exploration requires an integrated approach that combines insights from both surface observations and subsurface data. In essence, we use clues preserved in the geologic record to locate the valuable energy resources that power our society today and will continue to support our needs in the future.
Image Credits
- oilNaturalGasFormation: NEED Project