The weathering of existing rocks is responsible for producing the sediments that will form the sedimentary rocks we find today. The mechanical (physical) weathering processes responsible for deriving these sediments include frost wedging, crystallization of salt, pressure release, the action of organisms (tree roots), and thermal expansion. These processes work to break rocks into smaller particles (sediments) that have the same chemical compositions as their parent rock. Rocks may also be weathered by chemical processes as minerals in a rock react with its new environment. Water and oxygen play a significant role in the chemical weathering of rocks by dissolution, oxidation, hydrolysis, and spheroidal weathering. The rate at which rocks begin to break down is a function of the rock types (mineralogy) resistance to weathering and the environment (temperature/rainfall). Sediments are carried off and deposited in a new location by running water, wind, glaciers, and gravity.
By interpreting today’s sediment depositional environments geologists are able to infer what past environments may have looked like, this geological principle is referred to as uniformitarianism.
Understanding the depositional environments from which the sedimentary rocks have formed, geologists are able to infer the history of a sedimentary rock including; sediments methods of transport, origins of its particles, and the environment of deposition. Most importantly to the hydrocarbon industry are the environments associated with the movement of carbon from the atmosphere, biosphere, hydrosphere and ultimately to the geosphere.
Most of Ohio’s bedrock reflects depositional environments associated with marine type environments. This is the result of periods of marine transgressions and regressions over the state of Ohio. Marine Transgressions occur when sea level rises relative to the land. This is reflected in the rock record as deeper sea sediments (shales and limestones) being deposited on top of continentally derived beach sediments (sandstones). Marine regressions occur when sea level falls relative to the land. Marine regressions are reflected in the rock record as continentally derived beach sediments (sandstones) being deposited on top of deeper sea sediments (shales and limestones). It was these periods of sea level change over the course of millions of years ago in Ohio that is responsible for setting the stage for the oil and gas production we see today. During the Devonian period when most of Ohio was under water, sediments were shed from the Acadian mountains to the East of Ohio and deposited in the deep ocean over Ohio. It was the fine grained, rich organic sediments that were deposited during the Devonian that are responsible for the gray and black Devonian shales (Marcellus shales) present in Ohio today.
The major contributors for the production of hydrocarbons (petroleum, natural gas, and coal) in Ohio are directly associated with the organic rich environments in which those source rocks were deposited. A source rock is any rock type that will generate hydrocarbons and is typically found in gray or black shales, particularly the Utica and Marcellus shales plays in Ohio. Some limestones, coal and other rock types can on occasion generate hydrocarbons as well. The organic material responsible for the production of hydrocarbons in the source rocks includes marine algae, marine planktonic organism and bacteria, and terrestrial plants (leaves and stems). This organic material is incorporated into the source rock at the time of deposition. The greater the total organic carbon in a source rock the more hydrocarbons it is capable of producing. Organic rich rocks are associated with a variety of depositional environments including deep water marine (ocean), lakes, deltas, and swamps. Many environments have organic rich sediments but not all of those deposits will produce hydrocarbons. This is partly due to most of the organic material being consumed by local organisms prior to burial or the organic material being oxidized as it decays. For an environment to have the potential to produce hydrocarbons, burial of the organic material must happen rapidly (geologically speaking) to preserve the organic material, be deprived of oxygen to decrease the rate of oxidation, and have very few organisms present to reduce consumption of the organic material. As sediments are deposited rapidly on top of the organic rich rocks temperature and pressure increases with depth. This increase in temperature and pressure heats the organic material and generates the hydrocarbons in the source rocks.
It is the result of living organisms from millions of years ago that we have the many hydrocarbon resources available to us today. The coal we use to generate electricity and the crude oil we refine are the products of these organisms being deposited, buried, heated, changed into hydrocarbons, trapped and stored in the Earth’s crust.