ECONOMIC GEOLOGY WITH WHICH IS INCORPORATED THE AMERICAN GEOLOGIST Vox..V SEPTEMBER, I9•O No. 6 A PROPOSED CLASSIFICATION OF PETROLEUM AND NATURAL GAS FIELDS BASED ON STRUCTURE2 FREDERICK G. CLAPP. Although not necessaryto take time here to give the history of the "anticlinal theory" in full; it seemsdesirable to point out severalprominent features of that theory and of its history and practical application, before giving some of its limitations and stating the standingof the theory to-day. For over fifty years various geologistsand others have publishedpapers attempting to solve the problem of the distributionof oil and gas fields. Among others T. Sterry Hunt (I859 and I863), E. B. An- drews (I86X) and Hans Hoefer (I876) long ago recognized certain general relations of oil and gas pools to the anticlinal structureof a region. It remainedfor Edward Orton and I. C. 5¾hite, however, to bring the theory before the oil and gas world in sucha way as to force a measureof belief in it. Later investigationshave expandedand limited the theory and have made its applicationmore practicalto the operators,but much of the credit belongsto the early investigators. The anticlinaltheory was, in brief, that oil and gas were orig- inally widely disseminatedthroughout the formationsin which they are found, or in contiguousformations, and their segrega- tion was believedto be due to the different specificgravities of oil, gas and water. If a porous stratum contains these sub- 'Read before the Geological Society of Washington, March 9, •9xo. 504 FREDERICK G. CL.4PP. stances,when it is tilted by geologiccauses they will arrange themselvesaccording to specificgravity; the gas, being lighter, will be driven into the higher parts of the stratum (towards the crestof the anticline), the oil will be floatedon top of the water, •vhile the water occupiesthe lower portions of the stratum (those nearest the syncline). Limitations of the .4nticlinal Theory.--In practice, however, the anticlinal theory frequently did not meet expectations. Operators, after making careful locationsof .wells based upon this theory, were frequentlyrewarded only by dry holes. The successes,overshadowed by the fancied failure of the theory, were lost sight of by practicaloil men, and thus the theory fell largely into disreputefor a time. The reports of the United States GeologicalSurvey and the various state geologicalsur- veysthroughout the countrycontained references to the theory, and eachwriter tried to show the geologicalrelation of fieldsin the particularterritory covered. In many casesgeologists were successfulin finding sucha relation, and in all casesthe major axes of poolswere found to correspondin a generalway with the main anticlinal and synclinalaxes. Many of the gas pools correspondedclosely with the crestsof anticlinesand seemedto provethe theory. Other casesprevailed, however, in whichthe relationwas lessstriking and there were somein which no rela- tion could be determined. To explainthe defectsof the theory,various so-called "lim- itations" were formulated. The limitations, like the original theory,were imperfectin their application.They have been added to and rehashedby various geologists,until now little seemsto be left of the original "anticlinal theory." However, greatadvances have been made consequent upon detailed map- pingof geologicalstructure by governmentsurveys and private geologists.While we have not yet reached,and may never reach, the state where a productioncan be located correctly everytime, we may truthfully assertthat geologycan now save a considerableproportion of dry holes. Moreover,much of the territory held for yearsby oil and gas companieshas been shown to be not worth paying rentals on. CL,'ISSIFIC,'ITION OF PETROLEUM FIELDS. 505 State•nentof the Structural Theory.--The writer of thispaper prefersto usehere the term "structuraltheory" instead of "anticlinal theory" to explain the relations which accumula- tions of oil and gas hold to geology within certain limitations, evenwhere n.o definite anticline or synclineexists. The theory, as understood,is as follows: Through some means or other, by organic or inorganicagencies, or by both, the petroleumand gas have got into the sandstoneor limestoneformations in which they are found. The deposits may have originated through the decompositionof plant and animal remains on an ancient sea bottom, as the adherentsof the organic theory claim. Or they may be the product of chemicalaction on carbidesof iron or other substancesexisting under a stateof potential fusion deep in the earth, as the adherentsof the inorganictheory claim. Or certainpetroleum deposits may be of organicand certain deposits of inorganicorigin, which seemsa conservativeopinion. Whichever theory be true, the oil, gas and water in the for- mations (assumed to have been approximately horizontal at the time the substancesentered them) were at first widely diffused in the oil sandsor contiguousstrata. Such is true at the present time in somesections of the world, notably in severalcounties in western West Virginia fronting on the Ohio River, where only small quantitiesof oil and gas, too slight for profitabledevelop- ment, have been found. The dip of the rocks in that sectionis very slight. Where the beds have been folded, however, as is the case throughoutthe greater part of the Appalachianregion and in most of the other oil fieldsof the world, the slopeand changesin dip have enabledthe oil, gas,and salt water to separateout accordingto their relative specificgravities. This separation and concentrationmay have been assistedby rock pressure,hy- draulicpressure, seepage, capillarity, molecular attraction, inter- nal heat and other causes;but zvhatever causesprevailed, the law of gravitation,being ever operative,must be consideredof most importance,and the separationand subsequentaccumula- tion was in the order of the densities of the substances. Thus on a stated anticlinal, monoclinal or quaquaversalstructure, the 506 FREDERICK G. CL•tPP. gas is nearest the top, the oil lower down, and still lower is the salt water, when that is present. All this is accordingto the orig- inal "anticlinal theory." Sometimesthe poolsoccur at the top of the anticlines, sometimes lower down on them, sometimes in synclines,sometimes merely on monoclines. Conver•7enceof Formations.•Oneof the mistakesof the earlier geologistsand oil men was the assumptionthat the sur- face formations were universally parallel with the oil "sands.TM This is true in many fields,but in othersit is far from being so. For example, the Clinton sand in southern Ohio is not even ap- proximatelyparallel with any surfaceformation; but the interval between the Clinton and the surface strata increases toward the east at a rate ranging from 3¸ to zoo feet per mile, thus bring- ing the Clinton rapidly to a great depth. This convergencewas illustratedby Orton,Volume VIII. ot•the "Geologyof Ohio," but it is now known to be lessuniform and in placesmore marked than Orton supposed. With the assistance of a few •vell records within a few miles of any locality, where developmenthas been commencedor test- ing done, it is possibleto work out the structureof this sand to a gooddegree of accuracy.This is doneby firstmaking a contoured structure map of some surface formation; then the exact intervals from this stratum to the Clinton sand are de- termined in whatever wells have been drilled to this sand in the vicinity, and a convergencemap is preparedon the same scale. This was first done, so far as I know, by the writer of this paper, for the Amity, geologicalfolio, and for Bulletin 3oo of the United StatesGeological Survey, in •9o7, and the nameisochore lines (meaning lines of equal interval) xvasgiven to the con- tour-like linesdenoting the convergence. Fig. 52 is an example of sucha map from Bulletin 3oo. In the Amity region,how- ever, the interval representedwas that betweenthe Pittsburgcoal and the Gantz (or Hundred foot) sand,the Clinton sand never havingbeen reached so far east. In Bulletin3•8 of the United StatesGeological Survey the samemethod of representingcon- xIn this paper the word "sands" is used, being the well-drillers' term for the productive formations. CLASSIFICATIONOF PETROLEUMFI•ELDS. 507 vergencewas usedby Griswoldand Munn, who appliedthe sim- pler, and perhapspreferable, name "convergencesheet" to maps of this type. In preparing a map of the structure of the Clinton sand, of Fro. 52. Convergence map of the Amity quadrangle, in Washington County, Pa. The isochorelines (lines of equal interval) show the changesin interval between the Pitts.burg coal bed and the Gantz sand throughout the quadrangle. (From Bull. 300, U.S. Geol. Survey, •9o7.) the formations below the Greenbriar limestone in southwestern Pennsylvania,and of certain other formations, a convergence , map must be prepared. The necessityof this is due to uncon- formities. The convergencemap is superposedon the contour map which showsstructure of the surface formation, and the intervals representedby isochorelines are subtractedat every point, the resulting elevationsof the sand being noted on an overlyingtracing. The sand is then contoured. In practisethe finishedmap always showsthat the structurebears some deft- 508 FREDERICK G. CLAPP. nite relation to the occurrenceof gas and oil. The direction of the extension of the field can therefrom be predicted witha good degree of certainty; and, if there has been no deep sand development,the locationsof producingwells can be predicted with fair probability. Fig. 53 is