Often in the same locality a measure will pass from one class to another by insensible gradations. A shale may be traced along and found to begin to show particles of sand, then gradually a greater and greater sand content until it finally merges into a sand stone, with only a trace, if any, shale remaining in it.
Sands and Sandstones. Sands are the partly unconsolidated bodies while sandstone is the term applied to the same material when in a more compact, solid and hard condition. Both are shallow water deposits and the grains of quartz comprising them vary in size from extremely fine particles to the coarser varieties and to gravel. Since most of the oil produced is obtained from beds of sand, where the oil has accumulated in the space between the grains, it is evident that the porosity of the sand and its capacity for containing oil will have an important bearing on the production to be obtained from a well drilled into it. The amount of oil that comparatively dense sandstones can hold is often surprising ; it is estimated that loose sands frequently contain over 20% by volume of oil, although probably not over three fourths of this is recoverable.
The variation in texture and porosity of sand beds within short distances no doubt accounts for the noticeable differences in production capacity of wells closely situated, and which to all outward appear ances should yield equal amounts of oil. With all other factors equal, it is generally accepted as true that the relative thickness of sands will have a bearing on their productivity, and while this point fails to hold in very many cases, yet it is usually considered distinctly encouraging when a wide body of sand is found to hold the oil rather than a narrow one.
The ideal sand is that in which the grains adhere sufficiently to prevent their loosening and moving, and which, at the same time, is porous enough to permit ready passage of the oil to the opening through which it is brought to the surface. Too compact a sand may retard the flow of oil towards the opening and so allow only a small amount to reach the point from which it may be recovered. A sand that is too porous is apt to be loose and fall against the pipe, collapsing it. It may fill the inside of the pipe, 'sanding it up', re quiring that it be cleaned out and with the disadvantage of increased labor costs in its maintenance as well as the loss of production while it is being cleaned.
In this, as seems to be the case in all matters associated with the development of petroleum, conditions differ in various fields and in some localities the results of experience have shown that, as ex pressed by the driller, "The well must make the sand in order to make the oil." Wells, in which the flow of gas and oil has been great enough to keep the loose sand moving along to the surface with the fluids as fast as it reached the pipe, have often developed into immense gushers, in the course of which they would bring up surprising quantities of sand. There is no question but that, under such circumstances, the area from which the oil supply is derived becomes greatly widened, many tributary channels are opened and the well continues a good producer for a long time, while nearby wells that are sunk later and after the sand has been relieved of its great initial gas-pressure, do not get the benefit of such a strong flow of gas and sand and remain only fair producers.
Beds of sandstone are also the principal type of reservoir for the storage of underground waters, and it should be particularly noted in this connection that, except within narrow limits of local fields, sands have no marked physical characteristics by which they can be described as oil sand, gas sand, or water sand. Much harm has been done and much money needlessly squandered through the belief that a certain form of sand surely contains oil and that some other form of sand may hold only water. Sands are sands, and the only oil sand is a sand containing oil and the only water sand is one holding water. Careful microscopic study of sands is often useful in the detailed study of a local district but the application of data obtained in this way to a wider area cannot be depended upon and is more apt to be misleading and, harmful.
Heaving, or running sands, encountered when drilling, are bodies of loose sand usually carrying water, which often give much trouble by reason of their not 'standing up' on the side of the hole but con tinually falling in and filling it. Tar sands are those containing variable quantities of heavy oil and the term is generally applied to non-productive measures.