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Fissility of Shale and Its Relations to Petroleum1 BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA FISSILITY OF SHALE AND ITS RELATIONS TO PETROLEUM 1 BY J. VOLXEY LEWIS (Presented before the Society December 29, 1922) CONTENTS Page Introduction................................................................................................................... 558 Nature of clay.............................................................................................................. 558 Definition................................................................................................................ 558 Mineral constituents........................................................................................... 559 Condensation of clay.......................................................................................... 562 Condensation in Hudson River silt.............................................................. 564 Induration of clay.............................................................................................. 566 Meaning of shale and shaly...................................................................................... 567 Vague and confusing usage....................................................................... • • 567 Definition of shale.............................................................................................. 568 Mineral constituents of shale.......................................................................... 568 Fissility of shale........................................................................................................ 570 Nature and origin................................................................................................ 570 Gravitational compression.................................................................................. 571 Plastic flow....................................................................................................... • 572 Accessory processes..................................................................................................... 573 Kneading and jigging movements..................................................................573 Tides........................................................................................................................ 574 Variations in atmospheric pressure.............................................................. 575 Waves...................................................................................................................... 575 Earthquakes........................................................................................................... 575 Seasonal movements........................................................................................... 576 Slumping................................................................................................................ 576 Development of fissility............................................................................................ 577 Suggestions of secondary origin...................................................................... 577 Effects of compression and flowflge................................................................ 577 Observation and experiment............................................................................ 579 Primary fissility............................................. .................................................... 580 Dominance of shale in the older strata.......................................................... 581 Petroleum and natural gas...................................................................................... 581 The problem of origin...................................................................................... 581 Migration and concentration.......................................................................... 583 1 Manuscript received by the Secretary of the Society June 12, 1924. XXXVI— B u l l . G e o l . S o c . A m ., V o l . 35, 1923 (557) Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/35/3/557/3414617/BUL35_3-0557.pdf by guest on 28 September 2021 5 5 8 J. V. LEWIS---- FISSILITY OF SHALE AND RELATIONS TO PETROLEUM ' Page Further research needed............................................................................................ 586 The pitfalls of deduction.................................................................................... 586 Exact data required............................................................................................ 587 Summary of conclusions............................................................................................ 587 Acknowledgments......................................................................................................... 590 I ntroduction The purpose of this paper is to present a discussion of the nature and origin of shaly structure and its relations to petroleum, together with brief reference to such observational and experimental data as seem per­ tinent to these problems. In view of the paucity of knowledge and the importance of this struc­ ture from both scientific and practical points of view, the necessity for more extended, and particularly for more critical, field observations and for experimentation under more rigid control, is strongly urged. In the meantime, however, certain, tentative conclusions seem to be war­ ranted, and these are set forth here in the hope that they may elicit dis­ cussion and thus aid in mapping out the most profitable lines of research. N a t u r e o f C l a y DEFINITION Clay, which may tentatively be regarded as the chief antecedent of shale, has been defined2 as: “A natural substance or soft rock which, when finely ground and mixed with water, forms a pasty, moldable mass that preserves its shape when air-dried; the particles soften and coalesce on being highly heated and form a stony mass upon cooling. Clays differ greatly mineralogically and chemically and consequently in their physical properties. Most of them contain many im­ purities, but ordinarily their base is hydrous aluminum silicate.” Following the usage of the Bureau of Soils, the term clay has also been widely applied to those fine particles of soil and sediment that meas­ ure below .005 millimeters in diameter, without regard to mineral or chemical constitution. It is generally assumed, however, that such ma­ terials contain a large proportion of hydrous aluminum silicates, includ- ing, perhaps, a wide range of crystalline and amorphous substances. Thus Leith and Mead3 say: “Clays represent the fine-grained products of rock decay and disintegration, which have been separated from the coarser-grained materials by the sorting 2 A. H. Fay : U. S. Bur. Mines Bull. 95, 1920, p. 159. 3 Metamorphie Geology, 1915, p. 101. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/35/3/557/3414617/BUL35_3-0557.pdf by guest on 28 September 2021 NATURE OF CLAY 559 which accompanies erosion, transportation, and deposition. They also include certain substances deposited from solution.” Mechanically the clays grade into the coarser silts and sands, and many shales have been derived from silt, silty clay, sandy clay, and the more heterogeneous mixtures that may be termed mud. Chemically the average clay and shale are more closely related to the marine clays (“green mud,” “blue mud,” and “red clay”) than to river silts. In content of silica, alumina, and the alkalies, the silts of the Mis­ sissippi delta and the lower Hudson Eiver channel are intermediate be­ tween average shale and average sandstone.4 Hence it may be inferred that many shales were originally deposited as clays in open water at some distance from the shore. MINERAL CONSTITUENTS Leith and Mead5 enumerate among the fine-grained products of rock decomposition: . “the clay minerals of the kaolin group, hydrous iron oxides, such as limonite, and fine-grained hydrous aluminum silicates of ferrous iron and magnesia of the chlorite type. Sericite is known to develop from feldspars during katamorphism, and so far as this mineral escapes further alteration to kaolin it may be contributed to the clays and muds. In the clays also an impure white mica, sericite or muscovite, may develop secondarily.” The mineral constitution of the average sedimentary clay, silt, and mud from which shales have been formed may be approximated from the composition of average shale6 by recalculating and substituting 7.67 per cent combined water (the average of 105 soils and clays) for the 3.68 per cent found in the shales. The results are given in the following table: Mineral Composition of average Clay calculated from composite Analysis of Shale Kaolins and hydromicas................................................................ 43.9 Quartz ................................................................................................ 31.3 Feldspars .......................................................................................... 12.3 Oalcite .............................................................................................. 5.4 Limonite ............................................................................................ 4.2 G ypsum .............................................................................................. 1.3 4 F. W. Clarke: Data of geochemistry, 1920, pp. 29, 501, 510. S. D. V. B u rr: Tunneling under the Hudson River. New York, 1885, p. 49. Several unpublished analyses of the Hudson River silt have been supplied through
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