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OIL AND GAS ACCUMULATIONS

This brief circular is in response to frequent inquiriesconcerning the nature of oil and gas accumulations.Accumulations (or pools) of oil and gas, like those ofgold, silver and many other minerals, are rare. Andthose that have commercial significance are even moreso. The geology of oil and gas deposits is complicatedand cannot be fully told herein. Pools form over millionsof years when a certain set of geological conditions

exists or are fulfilled. Among others, important factorsare: 1) source rocks, primarily marine, wherein oil andgas are formed from organic materials such asmicroscopic plant and animal remains; 2) porous and,permeable rocks to provide a route for oil and gasmigration from source rock to a place of entrapment oraccumulation; 3) a suitable trap containing sufficientporosity and permeability to allow migration andaccumulation or storage, and an impervious cap rock toprevent escape; 4) an energy source such as gas orwater under pressure sufficient to move the fluids frompore space to pore space and out into the well bore oncethe trap, or reservoir, has been penetrated by the drill.

There are different kinds of oil and gas traps. InMichigan the most common kind is related to relativelysmall areas where the rocks have been folded intodomes or anticlines. Another type of trap found inMichigan is related to small reefs. These are found atmany places in Michigan besides the northern part of theSouthern Peninsula. The sketch, not drawn to scale,shows a reef trap and how it relates to overlying rocklayers.

As well as in northern Michigan, many productive reefs

have been found in St. Clair, Macomb, Oakland, Wayne,Ingham, Calhoun and other counties of the state. Thereefs found in these areas were formed during Silurian-Niagaran time, about 400 million years ago whenMichigan and much of the continent was covered byshallow seas. These ancient, buried reefs, formed fromthe hard parts of numerous kinds of sea life, are similarin most respects to reefs found in present-day warmseas. Geologic data from hundreds of wells show thatmost Michigan reefs are small in areal extent, seldomcovering more than a few hundred acres of productive

reservoir. Individual reefs may be several hundred feetin height above the surrounding Niagaran sea floorsurface. In this respect they resemble mounds or smallhills on the ancient sea floor. The preceding sketchshows how the reefs relate to the rocks that overly them.

The general land area overlying an oil or gas filled reeftrap (or some other type) is called a field. The wordpool means an underground reservoir containing acommon accumulation of oil or gas or both. A field maybe underlain by more than one pool, each being a

separate reservoir with its own characteristics and notconnected to reservoirs above or below, or in somecases laterally. The preceding sketch shows that wellsA and E were drilled into Niagaran rocks but did notencounter any reef build-up because they were drilledoff-reef. No reservoirs were found in the rocks overlyingthe Niagaran in well A, thus it would be plugged to thesurface and abandoned. Well E, however, did drillthrough a shallower pool on the way to the Niagaranobjective. It could be plugged back to this shallowerreservoir and completed as a gas well in the same poolas well D. Wells B and C found no gas in the shallowerformation but did penetrate an oil reservoir in a reef trap.

Well B was drilled on the flank of the reef, so it has athinner interval of reservoir rock. Well C was drilled onthe higher part of the reef and thus penetrated a full reefsection.

Renewed interest in drilling for new oil and gas fields inthe northern part of Michigan's Southern Peninsula hasled to misconceptions among some laymen as to thenature of oil and gas entrapment in this region. Localand nation-wide publicity following the discovery of newfields in Otsego, Kalkaska and Grand Traverse countiesseems to have created the impression that this northernarea is one vast oil field covering several thousandsquare miles. It has been compared to Alaska's north

slopea comparison which has little basis in fact.Though some fields do cover many square miles, this isapt not to be the case in Michigan where most fields,especially the reef type, are of very small size in terms ofland area.

Another misconception commonly held by many peopleis that oil and gas are contained in underground cavernslakes, veins or some other kind of very large, open voidin the rocks beneath the surface. This idea probablystems from the long-time use of the word pool inreference to oil and gas accumulations. Geologic datafrom hundreds of thousands of wells scattered all overthe world do not support the view. In reality, gas, oil andassociated water is held in pore spaces, most of whichare very small, within otherwise solid rock. The size andshape of the voids can, and do, vary considerably butcertainly they are not of Mammoth Cave magnitude.

Pore or void space in a reservoir rock may be comparedto the pore space or holes in a sponge. The amount ofpore space per unit volume of rock is referred to asporosity. Thus, if 20 per cent of a given unit volume ofrock, say one cubic foot, is pore space, the rock is saidto have 20 per cent porosity. But fluids or gases cannot

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move from pore space to pore space unless the poresare interconnected. If the pore spaces are connectedthe rock is said to be permeable. However, the quality ofpore space connection may vary considerably. The rateor ease with which fluids or gases can move through therock is referred to as permeability. Permeability isgenerally stated in terms of a unit called a millidarcy.Sketches A and B illustrate the relationship of porosityand permeability. In sketch A, fluids and gases couldnot escape into a well bore because there is no

connection between pore spaces. In sketch B, the porespaces are connected and gases and fluids could moveout into the well bore. Virtually all rocks that are directlyassociated with oil and gas have some porosity andpermeability. The quality and quantity of these factorsgenerally varies, sometimes considerably, within areservoir. The quality and distribution of these factors,among others, has a bearing on how efficiently areservoir can be drained. Also, they may have a bearingon the size of the drilling unit or area that can beefficiently drained by a single well.

Prepared by the Petroleum Geology UnitGeological Survey Division, Department of Natural ResourcesGarland D. EllsJ anuary 21, 1971

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