The chemical analysis of water found in drilling will enable one to determine the presence of oil or dissolved gas in it, and therefore the presence of either one may be expected further up the dip if the analysis shows their traces. If a well is drilled in an old abandoned territory and water found, the water may be from the sand itself or leaked in from other improperly plugged wells, such a possibility may be determined by a chemical analysis of the water found in the sand and the water that has been cased off above the sand at higher levels. This may also be used in connection with pumping wells, to determine whether the water that is pumped with the oil is from the oil sand or from above it.
Water conditions may determine the extent of deeper pools, and if it is borne in mind that of two sands of equal porosity and equal thickness, the upper or younger one will be more thoroughly saturated with water than the lower or older stratum, therefore it may be expected that the edge of the underlying pool will not be directly under the upper one, but will extend further down the dip.
Interesting and valuable information is pointed out by Carl H. Beal in Bulletin 658 of the United States Geological Survey, wherein he states: "The evidence indicates that in general the oil and gas areas in an elongated dome, where folding is simple, extend farther down on the long axis of the anticline or dome than on the steeper sides. In other words, the area that contains water only, occurs at a higher structural position on the steeper sides of an elongated dome than it does on its plunging axis." (Fig. III.) Whether two pools in the same sand may or may not be connected some information may be had by the comparison of the pressure and gravity of the oil. If it is found that the prop erties of the oil and gas are similar it may be expected that the two pools may be connected. Care must be taken that the samples of the oils are a true representation of the field, as it is possible that the gravity of the oil in a pool may have undergone some change; wells have been known to produce oils of different gravities at various times, and the rock pressure in one pool might have undergone a greater decline than the others. Arti ficial methods of production, such as the use of compressed air, • may lower the gravity of the oil, and lessen its gasoline value, and therefore will lower the value of the casinghead gas for such a purpose. It is necessary, therefore, that when a com parison is made between two pools, they should be studied under similar conditions.
David White's studies have shown a valuable relation that exists between the overlying coals and their extent of alteration, and oil and gas deposits, and he states that "In regions where the progressive devolatilization of the organic deposits in any formation has passed a certain point, marked in most provinces by 65 to 7o per cent of fixed carbon (pure coal basis) in the associated or overlying coals, commercial oil pools are not present in that formation nor in any other formation normally underlying it, though commercial gas pools may occur." Fur
ther on he states: "The lowest rank oils of each type arc found in regions and formations in which carbonaceous deposits are least altered, . . . the highest rank oils being, on the whole, found in regions where the carbonaceous deposits . . . have been brought to correspondingly higher ranks." Spacing of Wells. The distance between wells in most cases is determined by the various usages or customs in the field. In a consolidated sandstone or magnesian limestone field, closely spaced wells cannot be expected to bring good results, and for that reason all town-lot developments are to be discouraged, as it may safely be stated, that out of ten producers, it is unlikely that more than one will make a financial success of his holdings, and past history will bear out this statement in all instances. Wells may come in with a large initial or flush production, but with many wells draining a small area, the number of barrels of oil per well must necessarily be small. Although such develop ments may be a Mecca for the drilling contractors, they are far from being good prospects for the oil man. The rule in all such cases should always be: KEEP AWAY FROM TOWN-LOT EXCITEMENTS.
In general practice it is customary to off-set wells, that is, the opposing producers either by mutual agreement or through -custom, will stay equal distances away from each other's prop erty line. If a well is being drilled 30o feet from the property line, the owner of the adjoining lease will off-set this well if it proves to be a good producer, and stay 30o feet from the line, making the entire distance between the two wells 600 feet. Under ordinary conditions the following rule of thumb may be applied to spacing of wells: in a shallow field where the depth of wells does not exceed Soo feet, the wells should not be less than 40o feet apart, and about 15o or 200 feet from the property lines. Where the wells are deeper they should be at least 600 feet apart, 30o feet away from the property lines. This rule may be applied to oil wells, while for gas wells the distances should be greater, and it is good practice to locate one gas well to forty acres, in a gas belt where the wells are 2000 feet or deeper.