June 25, 1886.] 



SCIENCE. 



563 



same point. The flow of the well seems to depend 

 on the porosity of its immediate reservoir. Free 

 communication is also shown in adjacent portions 

 of the Berea grit, but there is nothing to indicate 

 an indefinite or universal permeability. The 

 changes in the grain and thickness of the stratum 

 would naturally divide it into basins approximate- 

 ly distinct from each other. 



9. Every oil-rock has a more or less impervious 

 cover, generally fine-grained shale. To constitute 

 an oil-group, three elements are essential; viz., a 

 source, a reservoir, and a cover. The first and 

 second may coalesce, as has been already shown, 

 but the third must be distinct and well-character- 

 ized. First in order of importance, as a matter of 

 course, is the source, but so generally is petroleum 

 distributed through the rocks of our scale, that its 

 presence may almost be taken for granted. Prac- 

 tically, the character of the overlying mass is a 

 chief factor. Almost any rock of the Ohio series, 

 if covered by a heavy mass of shale, shows oil or 

 gas when reached by the drill. The Utica, Hudson 

 River, and Medina shales cover the oil-bearing 

 Trenton limestone : the Berea and Cuyahoga 

 shales overlie the petroliferous Berea grit. The 

 -comiferous limestone, which is covered by the 

 heavy deposit of the Ohio shale, ought by this 

 order to be also a source of oil. It has been found 

 to be so in Canada, but not yet in Ohio. 



10. One other factor is found to be of prime im- 

 portance in oil and gas production ; viz. . geologi- 

 cal structure. Source, reservoir, and cover may 

 each be complete in itself , and yet no accumula- 

 tion of either product may result. Illustrations 

 are found in both of the main Ohio horizons. 



For many thousand square miles, the relations 

 of the several elements of the series that has 

 proved petroliferous in north-western Ohio are ab- 

 solutely identical. A hundred wells have now 

 been drilled in this field, and the records of the 

 series traversed are monotonous repetitions of one 

 another. From one you can learn all. Not only 

 is there the same order, the same thickness, the 

 same color, but there is substantially the same 

 chemical constitution of each stratum throughout 

 its entire extent. In all cases there is some accu- 

 mulation of gas and oil, but generally slight, at 

 the top of the Trenton limestone. 



But at one point, as the drill has now shown, in 

 a drift-covered plain, where all the facts were 

 hopelessly obscured to other reading, the steepest 

 dip known in Ohio rocks has been brought to light. 

 Two terraces of Trenton limestone, with their 

 superincumbent strata, are made known to us, 

 one of which is about 310 feet (306, 312, 314) below 

 tide, and the other of which is about 475 feet be- 

 low. The slope of 165 feet that connects them 



occupies a little more than a half-mile in breadth. 

 What effect does this marked structural feature 

 seem to have on oil or gas accumulation ? On the 

 upper terrace, every well that has been drilled has 

 found a fair supply of gas without oil. The wells 

 of the lower terrace are all oil-wells, though con- 

 taining considerable gas also. And what of the 

 wells on the slope ? That depends on what part 

 of the slope they occupy. On the upper edge, 

 from 330 feet below tide to 350 below, there is a 

 belt of the most remarkable and valuable gas- 

 wells ever struck in the state. The famous Karg 

 well produces, by the lowest measurement, 12,- 

 000,000 cubic feet per day. The Trenton limestone 

 was found in it 347 feet below tide. In the next 

 well in order of production, the surface of the 

 limestone was 350 feet, and in the third well 330 

 feet, below the sea. 



Seven wells have been drilled on the slope in 

 which the limestone is between 330 and 350 feet 

 below tide. One of the number is a small pro- 

 ducer, but the smallest of the six remaining wells 

 yields more than 1,000,000 cubic feet of gas per 

 day. 



Descending the slope still farther, we come to a 

 group of three wells, in which the Trenton lime- 

 stone lies respectively 394, 403, and 405 feet be- 

 low tide. All of them were vigorous gas-wells 

 when first drilled, but they also yielded more or 

 less oil from the first. Little by little, however, 

 their character has been changed, oil and salt 

 water overpowering the gas, until now almost 

 their sole value is found in the oil that they pro- 

 duce. 



The facts above given come from the Findlay 

 field. Similar facts are found at other centres of 

 production of Trenton oil and gas. 



Equally satisfactory testimony as to the all-im- 

 portant influence of structure on gas and oil pro- 

 duction is supplied by the facts of the Berea grit. 

 This remarkable stratum, the first persistent sand- 

 stone to be reached in ascending the geological 

 scale of the state, has a bold outcrop from the 

 Ohio valley to Lake Erie, and thence eastwards 

 toward Pennsylvania. Scores of quarries are lo- 

 cated along this outcrop, from which is derived 

 some of the most valuable building-stone of the 

 country. The stratum dips gently down from its 

 outcrops at the rate of from fifteen to thirty feet 

 to the mile. It holds its continuity underneath 

 the whole of eastern Ohio. Its area in this state, 

 therefore, is not less than twenty thousand square 

 miles. Slight rolls traverse it. breaking up the 

 monotony of its descent. These rolls, or interrup- 

 tions of dip, connect themselves at once with gas 

 and oil accumulation. A single example, and the 

 one most carefully worked out, must suffice. 



