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PETROLEUM AND NATURAL GAS

Nature and Distribution

Petroleum is a natural hydro-carbon, the origin of which is not definitely known. It is highly probable, however, that nearly all of the petroleum which has been produced has been derived from organic matter which was deposited with sediments laid down in water. In order that petroleum may form, the animal or vegetable matter seems to require an impervious covering, such as clay, which excludes the air and also, very probably, association with salt water. The fatty and waxy portions of the enclosed organic matter under some such conditions are slowly transformed into petroleum. In some cases the organic matter may consist largely either of plants or of animals or of both, and the character of the petroleum formed usually varies accordingly. Its character is also influenced by pressure, temperature, and filtration through porous beds.

Petroleum as thus originally formed is disseminated through petroleum-forming beds and is often absorbed by porous beds which are in contact with them, such as sand, sandstone, porous limestone, or sandy shale. This absorbed petroleum under certain conditions may become concentrated in the porous beds to such an extent that commercial "oil pools" are formed. A knowledge of the causes of such concentration is, of course, of the greatest importance to the prospector.

The concentration of petroleum in the rocks depends on the nature of the forces acting upon the disseminated petroleum. These are gravitational forces, capillarity, the pressure of the rock strata, and the pressure of ground waters and gases. The forces which cause accumulation are usually created by the structural conditions of the beds containing petroleum. These are rock structures such as folds in the beds, fractures, slopes with sudden breaks, lens-shaped porous beds which are sealed by impervious beds, etc. Concentration of petroleum in such structures further requires that the oil-bearing beds must be sealed by some impervious capping which arrests migration. Even if there are petroleum source beds present and structural conditions favorable for concentration, it frequently happens that pressure and changes in temperature in the rocks so alter them that the volatile matter in any petroleum which may be in the earth is driven off and only an asphaltic, parafine or mixed residue is left.

There are thus three fundamental conditions controlling the formation and concentration in sedimentary rocks of bodies of petroleum which, at the present time, may be found in commercial quantities:

1. Source beds must be present which contain petroleum in sufficient quantities.

2. Structural conditions must be present which are favorable for the concentration of petroleum and which affect the porous reservoir beds which are saturated with petroleum.

3. The rocks must not have been altered to any great extent.

There are two general indications of the extent of rock alteration. They are, (1) the physical and chemical character of the rocks; (2) the degree to which the beds have been folded and faulted.

Rock alterations which are usually apparent and which may signify unfavorable oil conditions are, for instance, the alteration of sandstone into quartzite, non-graphitic black shale into graphitic shale, and limestone into marble. Experience proves, however, that a more reliable index to rock alteration is to be found in the amount of volatile matter left in coals which are associated with beds of which the degree of alteration is to be determined.

David White of the U. S. Geological Survey has estimated that when the proportion of the fixed carbon in coal is to the volatile (combustible) matter approximately as 1.86 is to 1 (fixed carbon, 65%; volatile matter, 35%), most of the volatile portion of any petroleum which may be associated with the coal-bearing beds will have disappeared, and that when the ratio is as high as 2.33 to 1 (fixed carbon, 70%; volatile matter, 30%), commercial quantities of oil will not be found. This relationship has been tested a great many times and as a rule has been found to be fairly reliable. These proportions only take into consideration the fixed carbon and volatile matter and neglect the moisture, sulphur, ash, etc., which may be in coal. Natural gas, however, may be found in beds which are considerably altered, provided the structural conditions are such that the gas cannot escape. In the Arkansas Valley region east and south of Fort Smith, for instance, there are source beds of petroleum and favorable structural conditions present, but only gas has been found in the rock folds there, and the coal analyses show a comparatively large amount of fixed carbon present in the coal in comparison to the volatile matter (about 5.66 to 1 near Fort Smith).

Oil and Gas Possibilities of Arkansas

In Arkansas, favorable structural conditions can often be easily located in the Paleozoic area in the northern and west central parts of the State, as the rocks there are consolidated and strong enough to transmit the lateral pressures which bend them into folds. The Coastal Plain beds on the other hand, are made up of such soft and unconsolidated sediments that it is usually very nearly impossible to locate folds or structural conditions from the surface, which might be expected to reflect the structure of deeply seated oil bearing sands. However, a fairly accurate knowledge of the underground structural conditions of the Coastal Plain country may be gained from a careful comparison and correlation of deep well records which have been accurately compiled.

For the purpose of discussing the oil and gas possibilities of Arkansas, as for many other purposes, the state is best divided into

two general divisions, that is, the Gulf Coastal Plain area, and the Paleozoic area. The first area is shown on the accompanying map as Sections 1, 2, 3 and 4, and the second as Sections 5, 6, 7, 8 and 9.

All of the petroleum and more than half of the gas which has been produced in Arkansas up to the present time has been found in the Coastal Plain area. The beds making up this region have been the source of immense quantities of oil and gas in Arkansas, Louisiana and southern Texas. Paleozoic rocks, on the other hand, have produced all of the oil which has been found in Oklahoma, Kansas, north Texas and eastern United States. Up to the present time, however, the Paleozoic rocks of Arkansas have only produced gas.

The different divisions which have been made with reference to oil and gas possibilities are discussed separately. These divisions are intended to represent the broad geologic divisions which have been subjected to general regional influences. With the exception of the Coastal Plain and the Paleozoic regions the areas are not sharply demarked, but merge gradually into one another...

The Coastal Plain

The beds in the Gulf Coastal Plain of southern United States are comparatively young, geologically, and are little altered. The coal (lignite) in the Arkansas area has an average ratio of fixed carbon to volatile combustible matter of about .56 to 1.0 (fixed carbon

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36%, volatile matter 64%) and the folds found in the region are, as a rule of small aplitude. Consequently the element of rock alteration may be overlooked in considering the oil possibilities of different portions of the Coastal Plain region of Arkansas.

The Coastal Plain series is made up of (1) deposits of the Quaternary Period (fresh water); (2) the Tertiary Period (Eocene Series) sedimentary deposits usually classified as the Jackson, Claiborne, Wilcox and Midway formations; (3) the Upper Cretaceous Period, usually classified as the Arkadelphia, Nacatoch, Saratoga, Marlbrook, Annona, Ozan, Brownstown, Tokio formation and Woodbine sand and (4) the Lower Cretaceous Period (Washita group, Goodland limestone and Trinity group). See diagram and table on Geologic Formations.

The Coastal Plain area of Arkansas has been divided into four classes, as there appear to be areas more favorable than others for oil and gas accumulation.

AREA NO. 1

Area No 1 on the map includes the relatively thick and more centrally located portions of the Coastal Plain beds of Arkansas. In northern Union county the Tertiary sediments have a thickness of about 1,300 feet, the Upper Cretaceous are probably about 1,800 feet thick, and the Lower Cretaceous beds have an estimated thickness of 1,000 feet or more.

The Upper Cretaceous beds, which, up to the present time, have proved to be the only petroleum producing beds in Arkansas, are present in Area No. 1. The Lower Cretaceous beds, which have produced some oil in Louisiana and Texas, are absent in the northeastern portion. The Coastal Plain series (see diagram) thicken to the south and east and attain a thickness of over 4,100 feet along the southern border of Union county and thicken to the east approximately as far as the Mississippi River. The thickness decreases along the eastern border in passing northward up the Mississippi River and at Memphis the series probably has a thickness of 3,500 feet. A well drilled in northern St. Francis county, Arkansas, encountered the basement rocks at about 3,020 feet. The Coastal Plain beds lie uncomformably over the Paleozoic rocks and within about twenty miles of the edge of the central and southern portion of the Coastal Plain shore line in Area No. 1 have an inclination to the southeast and south of from 70 to 115 feet or more to the mile, the inclination increasing along the shore line of the Coastal Plain toward the southwestern part of the embayment. The inclination beneath Clay County is about forty feet to the mile. The surface of these Paleozoic basement rocks probably flattens out along the eastern border of the State. The surface inclination of the Coastal Plain to the south and southeast averages between one and three feet to the mile.

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Faults are

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found in this section of the Coastal Plain which have doubtless been important factors in the creation of the known oil pools of Arkansas. The El Dorado, Smackover, Stephens and Irma fields are probably all faulted structures, and faulted conditions are known to exist in other parts of the area. addition the Paleozoic floor, at least in some parts of Area No. 1, may exercise a definitely favorable structural influence for oil accumulations on the younger beds above it through variations in the topography of the floor.

Area No. 1 is, from information now available, the most favorable area in the State for prospecting for oil. It contains the oil and gas-producing areas of southern Arkansas and within the limits shown contains (1) thick petroleum-forming beds, and (2) the possibility of structural conditions favorable for the concentration of petroleum.

AREA NO. 2

Area No. 2 lies in the relatively shallower portion of the Coastal Plain and may not contain petroleum-forming beds in sufficient thicknesses to create commercial oil pools, although this has by no means been definitely determined. The Lower Cretaceous beds are very probably absent in this area. This, however, may not be of any particular significance so far as source beds of petroleum are concerned, as the Lower Cretaceous beds have not as yet been found productive in Arkansas. The chances for the existence of favorable structural conditions appear to be somewhat less in this area than in No. 1, as folding and faulting in the Arkansas Coastal Plain are probably due to deep seated causes which up to the present time have not been known to affect Area No. 2. The top of the Paleozoic basement rocks, on which the Coastal Plain series rest, incline to the southeast

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