St. Clair County Oil and Gas
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This Earthcache will explain about oil and gas found in Michigan, especially in St. Clair County.
ORIGIN AND SOURCE
It is known that oil and gas, although they vary in composition, are chemical mixtures. Oil is chemically more complex than gas. The mixtures, in turn, consist of a variable number of chemical compounds called HYDROCARBONS. These consist primarily of the elements carbon (C) and hydrogen (H), put together in various ways.Some of the simplest, natural hydrocarbons, such as methane ("marsh gas," CH4) have been reported from the vapors of volcanoes and thus in some cases might be of inorganic origin, but geochemists believe that the more complex hydrocarbons that occur in nature came from the dead bodies of plants and animals, just as methane and other hydrocarbons are released from dead organic matter today. Significantly, oil and gas nearly always occur in or near sedimentary rocks that originally were deposited in water.
In some cases the sediments were fresh water deposits; more often they were of marine origin. Modern sediments, of types which could form sedimentary rocks, are accumulating today and many such deposits are known to be rich in dead organic matter of both plant and animal origin.
There probably never were enough large animals and plants living in lakes and seas of the past to have produced even a small part of all the known petroleum and natural gas in the world, but even today there are tremendous numbers of tiny microorganisms, one-celled plants and animals and some higher forms of life, living in the seas and lakes.
Oil and gas are chemically and physically different from the types of hydrocarbons that may have been their source. Thus, many complex changes must have occurred to transform the raw organic oils, gases, and solids into liquid petroleum or natural gas.
Natural gas is a common associate of oil and it is reasonable to believe that it is a product derived from organic materials similar to those that produced petroleum. Gas may be produced both during the evolution of petroleum and as an end product of that process. Although oil and gas may occur separately, this is probably because gas, being a little more mobile, can escape and migrate through rocks that are impermeable to the passage of oil.
RESERVOIR ROCKS
It is a common misconception taht oil and gas occur in open "pools" underground. This is rarely true, the holding capacity of the reservoir normally being the result of the presence of myriads of small interconnecting pores and cavities. If a cavern dissolved out of carbonate rocks later comes to contain oil and gas, this is an exception rather than the rule.
On a worldwide scale the commonest and best resorvoir rocks are sandstones. Carbonate rocks, such as limestones and dolomite, rank second as reservoirs, although in Michigan the oil reservoirs are dominantly carbonates. In any case the reservoir rock must be porous.
Openings and spaces, no matter what size, make a rock porous. Most sedimentary rocks, at least in the upper part of the earth's crust, have some porosity, but the percentage varies from a maximum of nearly 50 percent of the volume occupied by the rock to practically zero.
Porosity may be PRIMARY, that is produced at the time of deposition of the sediments.
Organic reefs, especially those formed by the tiny skeletons of corals, commonly have primary porosity.
SEALS AND TRAPS
Even in the best source rocks oil and gas apparently are not generated in sufficent quantities to provide economically valuable amouts "in place." Even if present there, they are not producible because the source rocks normally are to tight for the hydrocarbons to be gotten out. They must have migrated and become concentrated in local pockets, "fields" or "pools." And there must be something to stop the movement so that the oil will collect. This brings us to the terms "seal" or "seal rock" and "trap."
SEAL ROCKS generally are nonporous ar at least relatively impermeable rock formations. Shale and anhrdrite are common seal rocks, but dense limestones and even thoroughly cemented sandstones may also serve as seals. The best seal rocks, especially in Michigan, are anhydrites and anhydritic shales.
A TRAP is a geometric codition, involving the shape of rock formations or their composition or both, which collects and holds oil or gas in quantities sufficent to make their recovery economically worthwhile.
OIL AND GAS IN MICHIGAN
In 1962 Michigan accounted for about 0.5 percent of the total annual petroleum and natural gas production of the United States. Obviously, this state is not a major producer. Oil and gas ranks third in the state after iron and cement in mineral importance.
HISTORY OF MICHIGAN OIL AND GAS PRODUCTION
Oil was first discovered in Michigan at Port Huron in 1886, but extensive production did not begin until after the discovery of oil in the Saginaw area in 1925. Oil was found in the important Mount Pleasant field in 1928.
The most recent developments have been those of the prolific reef accumulation in St. Clair County.
OIL AND GAS FIELDS IN MICHIGAN
In 1967, oil was being produced from 44 of the 83 counties of the state, and natural gas from 25 all in the Lower Peninsula. Just over half of the gas was produced from wells that also yielded oil. The organic reefs of St. Clair County provided 45 percent of the gas, and together with the Albion-Scipio Trend and gas fields in Macomb, Missaukee, and Rosecommon counties accounted for 87 percent of the total gas produced in the state.
In Silurian rocks in St. Clair County, oil and gas are trapped in porous organic reefs which are overlain by seal rocks consisting of Salina evaporites. The reef structures occur as mounds, resembling anticlines, and serving a simple function as traps. Although reef production now is mainly in St. Clair County, similar reefs are known to occur in many other places throughout the state. In the present area of reef production, 9 out of 10 reefs have been found to contain oil, gas, or both.
SUMMARY
The origin of oil and gas in Michigan was a direct consequence of the Paleozoic marine history of the state. The source, reservoir, and seal rocks were deposited as marine sediments. Carbonates (limestone and dolomite) are the most productive reservoir rocks. Traps are of both the structural and stratigraphic type.
NOW THAT YOU'VE LEARNED SOMETHING ABOUT OIL AND GAS IN ST. CLAIR COUNTY, LET'S LOG THIS EARTHCACHE.
DO NOT POST YOUR ANSWERS WITH YOUR LOG!!!! E-MAIL ME THE ANSWERS TO QUESTIONS 1 AND 2.
1) Go to the posted coord's and e-mail me the name of this facility and what you believe they are pumping out of the ground. What does the air smell like?
2) Now go to N42* 43.274 and W082* 43.251. Look a little to the northwest. E-mail me how many oil rigs are here and if operating or not. Now look northeast and e-mail me how many green tanks you see.
3) Now post a picture with the two gray tanks to the northwest in the background of your picture.
Additional Hints
(Decrypt)
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