GEOLOGY AND OIL RESOURCES OF COLOMBIA; TIlE COASTAL PLAIN.

ELFRED BECK.

INTRODUCTION. The increasingdemand for petroleum and its productshas directedattention to the unexploredregions of South America. Becauseof its proximity to the United States, Colombia has beenscrutinized and a numberof developmentschemes have been 15utinto operationduring the last few years. Geologicalinvesti- gationshave beenmade by nearly all the large oil companiesand interestseems to begrowing steadily. The writer spent six months in Colombia exploring a large. tract of land near the Caribbean coast. The work was carried on from March to August, I919, for the purposeof making a geologicalmap and report on the so-calledSan Andres conces- sion. This paper includesa detailed accountof the geology in the vicinity of the concession,a reconnaissance report of the remain- ing area, and comparisonswith the geologyof Colombiaas a whole. This was a pioneerexploration, as no previouswork had been done. Only a generaloutline map was availableand little or no information has ever beenpublished concerning the geologyand geographyof this territory. Methods of l•ork.•The country is exceedinglydifficult to investigateonaccount of thetropical climate and densely woode•l regions. H.orses and mules afford the only meansof transpor- tation. Althoughprovision should always be madefor "camp- ing out," it is rarely necessary,for small towns are numerous and shelterunder a palm-leafroof can readily be secured. Hard- shipsare many, and specialcare must be taken in the prevention of disease and sickness.

457 458 ELFRED BECK.

In swampyregions or in otherimpassable country. distances to certainpoints were calculatedby first determiningthe distancea crew of canoe-mencould push a canoe in an hour, and then recordingthe dis/•ancetraveled by the watch. The Brunton compasswas usedto determinethe direction. The mule-pacing methodwas advantageousfor generalmaps, where only approxi- mate locationswere called for. A plane table and telescopic alidade were used where accuracywas required and detailed results necessary. Climate and People.--Geographically,Colomb!a is a tropical country,but it has great climatic diversity. The climate is tropi- cal on the coast and low-lying river basins,but owing to the great relief and the curiousconfiguration of the mountainchains; .theclimate becomes a matterof locality. 'At SantaMarta one can go from a hot climateto a cold one in a few hours. It is possibleto reachan elevationof •8,ooo feet on the Sierra Nevada Mountains. Regardlessof the intenseheat of the day, the even- ings are always cool. In general there are two rainy seasons; the so-calledwinters from April to June and Septemberto De- cember;sei•arated by'two periods of dry weather. The average day temperatureof the Magdalenaand Sinu countryis about95 degreesand at night it.may fall to 70 degrees. The total area of Colombia is 46•,ooo squaremiles, and ac- cordingto the censusof •9•2 the populationwas 5,500,000. The peopleare clusteredtogether in small towns; rural districtsare relativelyfew. The coastalinhabitants are a mixed race,prob- ably of Spanish,Indian, and Negro blood. They are a law- abidingpeople with a greatrespect for foreigners.

GENERAL FEATURES. Location.--The area discussedin this report lies on the Carib- beancoast of Colombia,South America, and extend• in a strip about 40 to 5ø miles wide from the mouth of the Magdalena River southwestto PuertasEscondido. (See Fig. 30.) Culture.•Roads in this part of Colombiaare narrow and re- semblecow-paths. The only improvementsmade on themcon- GEOLOGY AND OIL RESOURCES OF COLOMBIA. 459 sist in cutting the brush twice a year. If this were not done, the road w.ouldbe closedin lessthan a year'stime. Occasionally a bridge is constructed. The main roads follow either divides or valleys and connectinglines are few. It is interestingto note that town sitesare invariably located' on the outcrop of a sandstoneformation or on the bank of a

ranges, drain- streamwhere there is a sandysoil. The larger towns,such as Cartagena,Barranquilla, Sincelejo, Monteria, Tolu, CarmenSan O'nofre,Palmito, Lorica, and SanAndres, have telegraph serv- ice,post offices, and some have electric-lighting plants. The first two mentionedcities are up to date in nearly all modernattain- mentsand havea populationof about50,00o. Topo#raphy.--Theregion may be dividedinto lowlands, along the coastand river valleys, where dismal swamps, lakes and lagoonsare numerous;and into an uplandcountry, which is ruggedand rocky. The altituderanges from sealevel to nearly 460 ELFRED BECK.

I,OOO feet at several places on a divide, which trends north- easterly close to the center of the area. The ridges are com- paratively uniform in height and are of prime importancein locatingroads. The characterof the underlyingformations has influencedthe shapeof the hills, the more resistantrocks forming conspicuousoutcrops or "Cerros" (from the Spanish,meaning hills), but the dip of bedsis the determiningfeature. The out- cropping ledges of sandstoneand limestoneincline at sharp angles,forming a seriesof escarpmentswhich trend in a north- easterly direction. Some of the most prominent of these topo- graphic features are: Cerro Tofeme, south of Chima; Cerro Bombo,near Huertas; Cerro Chiquito,north of Sabaneta;Cerro Pato, south of Sabaneta;Cerro Sapo, south of T.oluviejo. In this ruggedterritory erosionis active and deepgulches and valleysare characteristic. The regioneast of the Sincelejoanti- cline or east of the outcrop of the San Antonio sandstonesis characterizedby soft sandstonesand shalesdipping at low angles to the east, which producea gentle rolling plain. The Palmito limestoneforms a seriesof ridges or "Cerros" for a long dis- tanceacross the area. This formation is inclinedat an angle of about 9ø degrees,and on accountof its resistanceto erosionthe adjacent shalebeds have beencut away, leaving the limestonein the form of an outlier. Caves formed by percolating surface waters are common in this limestone. The San Antoni.o sand- stonescœp the major part of the Sincelejoanticline and syncline and form the divide betweenthe Magdalenaand coastaldrainage systems. The tributariesof the Sinu and 'Magdalenarivers drain most of the area. The streamsare intermittent and flow only in the wet season. The water ordinarily standsin pools along a dry bed or in artificial reservoirs,made by throwing dirt damsacross creeks. Sometimesafter heavy rains suchstreams as the arroyo Mucha, arroyo Pechelin,and arroyo San Antonio.become dan- gerousraging t.orrents which are practicallyimpassable. The Sinu and Magdalenarivers are navigablefor severalhun- dredmiles. A gasolinelaunch runs from Cartagena up the Sinu GEOLOGY AND OIL RESOURCES OF COLOMBIA. 461 as far as Monteria, making the trip twice a week. Accommo- dation for passengersis providedin additionto freight facilities. Transportationon the Magdalena is done entirely by large flat- bottom boats. Semi-weeklyservice is maintained betweenall pointsas far as Honda on the Upper Magdalena. Under•7roundI/Vater Resources.--The topography of the coun- try, the amount of vegetation,.and the porosity of the surface are factors that contr. ol the amount of rainfall that enters the surface and gives rise to artesian and spring water. In this area the annual rainfall averages90 inchesand is unequallydis- tributed throughoutthe year. The region is characterizedby sandysoil and. dense forest growth, consequently a large portion of the rainfall entersthe ground. The ground water table is constantlyfluctuating; i.t rises in the wet seasonand sinks during the dry season. Several at- tempts have been made to secure fresh-water w,ells in the cattle regions near the Caribbean coast. Most of-these have been failures. In all the casesexamined by the writer it was found that the wells were dug in clays and shales,therefore yielding little or no water.

ß

Fro. 3L Sketch showing the occurrenceof water on coast'alplain, near Madre de Dios.

If the wellswere deepened,the chancesof strikinga porous or perviouslimestone, a sandstone,or a gravel bed would be good. Fig. 3t illustrateswhy wells near 'Madrede Dios were failures,and shows a possiblemethod for completinga good well. At Sincelejothere occurs an ideal arrangementof the strata for obtainingan artesianwell. Fl.owingwater shouldbe ob- tainedat a depthof •5o-2oo feet, but deepersands would de- velopa strongerflowing well, sincethe hydraulichead increases with depth. A detailedillustration is shownin Fig. 32. 4.62 ELFRED BECK.

An unusualoccurrence of a fresh-waterspring is found near Palmito, where the water penetratesa porous limestone (per- colatingdown through bedding planes and crevices)and a sandy shaleand producesthe springat the baseof the hill.

FIG. 32. Sketch of the Sincelojo syncline,.illustrating an ideal condition for an artesian well.

ß Goodwater wells may readily be obtainedby drilling porous strata. If drilled beneath the lowest fluctuatio• of the water tablethe. supply will bepermanent, thereby relieving the scarcity whichoccurs every year, and at the sametime improve the health conditions.

STRATIGRAPHY. A rough sketchof the mountainranges and drainagesystems of Colombiais shownin F•g. 3¸. It will be notedt. hat the moun- tain chainsrun in a northeasterlydirection, producing lines of folding which range from north •5 degreeseast to north 5ø degreeseast. The mountainmasses whose development have probablybeen the mostactive in determiningthe natureof the minor structural featuresin the coastalterritory are the WesternCordillera Moun- tains to the south. The uplift which producedthese mountains wasso great that the foundation upon which the thousands of feet of Paleozoic and Mesozoic rocks were laid down is now exposedalong the crestof this range.• The stratigraphiccolumn in the area shownon Fig. 33 is made up of rocks of early Miocene age. They are predominantly ßsandstones and shales,but somelimestone is present. They have • A generalizedgeological may of Colombiais given in U.S. Geol. Surv. Prof. Paper No. 7I. The location and correlation of the formations are not absolutelycorrect, but the map will give someidea of the distributionof the rocks of different geological age. GEOLOGYAND OIL RESOURCESOF COLOMBIA. 463

Light grey sandstone,locally conglomeritic. Massivelight grey to dark'brown sandstorre, forming ruggedtopography.

Thick sandstones. Light grey t'o dark brown. Alternating layers of shale and calcareoussand- stone.

Thin limestone beds profusely fossiliferous. Many beds of soft friable sandstones.

A thinly bedded 30' limestone. Contains few fossils.

A hard compact light grey shale. Color varies from blue to brown.

) Red shales and light grey sandstones.

A massivesandstone 2oo-4oo' t'hick.

Upper limestonebed is hard, snow-white,mas- sive, without beddingplanes. Forms conspicu- ous "Cerros." Lower limestone,friable thinly bedded.

Fro. 33. Generalizedvertical section of the rocksexposed near Sincelejo, Department of Bolivar, Colombia, South America. 464 ELFRED BECK. an aggregatethickness of more than 5,000 feet. It is believed that the positionof theserocks in the 'Mioceneseries is not far &bovethe horizonof the Eocenegroup. The exposedrocks are all sedimentaryand nameshave beengiven to formations,ac- cordingto the typelocality in whichthey outcrop. This is the only attempt,to the writer's knowledge,that has beenmade to namethe rocksin this region. Consequentlythe classificationis subjectto revisionupon the selectionof a bettergrouping. Fossilsare exceedinglyscarce in therocks in thisvicinity. The Huertas LimestoneSeries contains the only known fossils. At San Andres three large sea-urchinswere found and small marine brachiopodswere numerous. From Buena Vista north, in the same formation,a large quantity of gastropodsoccur. The names of these fossils have not been ascertained. On the countryroads about 35 miles from Barranquilla,near Usiacarii, a great number of well-preservedbrachiopods were found. Theseare fancycorrugated shells probably of Plioceneage. At Tenerife, on the Magdalenariver, just above Calamar, remains of 'a huge mammal have been collected. This specimenwas identifiedby the AmericanMuseum of Natural History as the giant slothMegatherium, but othershave pronouncedit a rela- tive of the huge Mastodonwhich is found so abundantlyin the Pleistocene of western Nebraska. The fossil is without doubt of Pleistoceneage.

STRUCTURE. Sincelejo Anticline.--The dominant structural feature in this territory is the Sincelejoanticline and it is bestdeveloped near the town of Sincelejo. (Fig. 34.) It is a narrow elongated anticlinethat extendsin a general northeasterlydirection for morethan fifty miles. Practicallythe entire length of the fold is cappedby the San Antoniosandstones. On the easternslope of the anticlinethe bedsdip to the east 5 to 3ø degrees,while thoseon the westside incline at an angleof 45 degrees. (See Fig. 35.) West from the axis of the anticlinethe dip gradually increases,until at a pointabout ten mileswest it is nearlyverti- GEOLOGY AND OIL R'ESOURCES OF COLOMBIA. 465 cal. Becausethe San Antonio sandstonesdiverge both north and south from this point, it is thought the form of the bedsis that of a dome. Although not definitelydetermined, this anti-

Fro. 34. Sketch map of the structural features in t'he vicinity of the San Andres Concession. cline undoubtedlyextends farther north in the direction of Car- men. oCapoFold.--The Sapo anticlinal fold or flexure occursin the

Fro. 35. Generalized horizontal cross-sectionmeasured. along line A-B. 466 ELFRED BECK.

Tofeme formation. It outcropsin the road two miles east of Palmito, and can be traced north a few miles, where it appears to merge into a fault. The west limb of the structure dips sharply at anglesvarying from 60 to 75 degrees,while the east- ern flank is inclined 35 to 5ø degrees. Becauseof its narrowness and undeterminedlimits the fold is of no interest to the pros- pector. San.,'IndresDome.--A well-defineddome is developedabout two miles and a half southwest of the town of San Andres. It covers an area of one square mile. Outcrops of the Huertas LimestoneSeries completely surround the dome, and dips vary from t to to degrees. The beds are easily correlated and .a detail structural map could be made. Lorica Structure.---A few miles north of the town of Lorica theso-called "Lorica structure" extends in a northeaste.rlydirec- tion for a distanceof abouteight miles. The outcroppingrocks probablybelong to the TofemeFormation. The anticlinal'fold is symmetricaland the strata are inclinedfrom xo to 30 degrees. The northend of the elongatedstructure has the'form of a plunginganticline, but evidenceof a similar closureis not dis- cernibleon the south. The StandardOil Companydrilled a well on the easternflank of this fold in x9x5 to a depth of 300 feet. It is reportedthat a smallshowing of oil and gaswas encountered. Monteria,'lnticline.--An anticlinal fold has been•nterpreted to be presentin the valley of the Sinu river, just north of the town of Montefla. This conclusion has been reached because the rocks dip to the west on the west side of the river, and to the east on the east side. Both sidesof the hypotheticalfold are upheldby outcropsof the samesandstones. Consequentlysome folding must have taken place. The anticline is paralleled on the westby a narrow syncline. Along t.he coastnorth of Puertas Escondidoa prominentoutcrop of calcareoussandstone outlines the synclinefor a distanceof ten miles. Minor Folds.--There are several other folds or anticlines on the coastalplains of Colombiawhich the writer did not have an opportunityto visit. The combinationof faulting and folding GEOLOGY AND OIL RESOURCES OF COLOMBIA. 467

which surroundsthe Bolivar fault producessome complicated features. On accountof thesesharp inclinations very little work was done on this territory. 'However, with more detailedwork, anticlinesor geologicalstructures of value to the oil prospector might be revealed. This would especiallyapply to the region eastof Cartagena,where the dip of the bedsis not so sharp,and where there are a large numberof oil seeps. An anticlinal fold which occursin the Pleistoceneformation, a few miles southeast of Puerto'Colombia, has beenprospected for severalyears with- out any successfulresults. It is a long, narrow structure,cover- ing about IO square'miles,and is similar to the one found at Lorica. Its outline is not well defined because the surface rocks are loosefriable sandstonesand shales,whose imperfect bedding planesyield unreliabledips and strikes. There are many other structural forms in Colombia of economic interest to the oil industry, particularly in the Santanderdistrict, where the Cre- taceousrocks. are exposed. Thesewill be discussedfurther under the following heading.

PROSPECTING FOR OIL IN COLOMBIA. Although petroleumhas been known in Colombia for over a hundredyears, no attempthas been made to developthis natural resburceuntil recently,chiefly becausethe governmentwas un- friendly toward foreign capital. This uncertain feeling has .graduallychanged, and now the oil industryis progressingsuc- cessfully. Drilling hasbeen done at the followingplaces: (I) PuertoColombia district, five milessoutheast, near town of Turbara. (2) The Turbacodistrict, I II• milessoutheast of Cartagena. (3) Sinu district, near Lorica and Monteria. (4) BarancaBermeja district, 450 milesup theMagdalena river in the departmentof Santander. (5) Pamplonadistrict, near the Venezuelanboundary. Puerto Colombia District.--The Puerto Colombia district has 468 ELFRED BECK. undergonespasmodic development since •9o7. Oil seepagesand gas volcanoesare numerous,a fact that keepsoperators inces- santly at work. A Canadiancompany started operations by drilling3 .or.4 welis,ranging in depthfrom 60o to 3,00ofeet. One of the wells yielded 7 to 8 barrels of oil daily and encoun- tered several showings of gas. Later, in •9•2 or •9x3, the Kelly Oil Companycommenced .operations, and they are still at work in anticipation of finding oil in commercialquantities. Ten or more wells have beendrilled with meagerresults. Infor- mationregarding the developmentof thesewells is not givenout, but it is certain that there is no startling production. Drilling has beendone near oil seepagesand on a small,narrow, poorly defined anticlinal fold. The softness of the Pleistocene forma- tions has caused many difficulties. The churn drill was first used, but later, after several wells had to be abandoned on ac- countof crookedholes, the rotary methodwas introduced. The companyhas a fully equippedup-to-date camp on their property, and they are hopeful of reward. Turbaco D[strict.--This district lies about •2 miles southeast •)f .Cartagena.The writer has no informationconcerning the 'structuralconditions in this vicinity. The surface evidencethat 'has attracted attention is the large number of mud volcanoes. The Martinez Oil Company drilled the first well in •9o8. In x9•o they combinedwith the Standard Oil Company of New York and drilled three more shallow wells, none of which were over 500 feet in depth. A new locationwas made in I9•2 and a hole was drilled to a depth of 2,200 feet. Small quantitiesof oil and gas were encounteredat variousdepths. Although these attemptswere failures, they do not imply that oil doesnot exist in the underlyingrocks. None of the wells were deepenough to be considered fair tests and the work was carried on with little geologicdata. It is, therefore,improbable that thesewells were drilled on favorablegeological structure. The Sinu District.--The StandardOil Company,together with their Colombianassociates, began exploration in the Sinu coun- try in •9•4. Two wells were drilled on the flanks of an anti- GEOLOGY ,4ND OIL RESOURCES OF COLOMBL,4. 469

clinal fold. The location seems to have been selected because of severaloil seeps. Not only was the drilling doneon an unfavor- able structure, but the holes were sunk to a depth of only 50o feet. Therefore these wells can not be classed as fair tests. A year or two following the samecompany drilled a well near the town of 'Monteria, forty miles southof Lorica. Its depth has been reported at 2,0oo feet. A small showing of oil a.nd gas was encountered. This well was locatednear an oil seepand it is possiblethat it was placedon an anticlinal fold, but the inter- pretationof a hyi•otheticalstructure is still doubtful. Baranca Bermeja District.--In the fall of I9I 5 prominentoil operatorsfrom Pittsburgh,Pennsylvania, visited Colombiaand securedoption on 2,000,000 acresof land near Baranca Bermeja, They organizedthe Tropical Oil Company,and operationswere well under way in I916. The first well was drilled near an oil seep at Infanta, 20 miles up the Colorado river from Baranca Bermeja. Fortunately, this location was made by a reliable geologist,and in the winter of I918 the secondwell drilled came in with an estimateddaily productionof 5,ooo barrels. The oil was found in Cretaceousrocks a• a depth of about 1,5ooto 2,0o0 feet on a well-defined anticline. Oil covered the Colorado river for miles before the well could be placed under control. The oil is light greenin color and of a high grade, 42 to 45 degrees Beaumd. Its charactermay be seenfrom the following analysis:

AN'ALYSIS OF SEEPAGE OIL TAKEN' FROM THE TROPICAL OIL COMPANY CON'CESSION'.2

Sample Number One. Per Cent. Gasoline ...... I.oo Kerosene ...... 41.oo Lubricating ...... 53.0o Non-Volatile Residue ...... 5.00 Sulphur ...... 55

This oil is either from the surfaceor elsethe origin of the oil is very 2 From certificateof analysisNo. I7648 by the Stillwell Laboratories,New York. 470 ELFRED BECK.

ancient as practic'allyall the light benzine and naphtha has been driven out of it. It is a good grade of oil for the manufacture of kerosene and lubricating oils, and should also be well suited for the purpose of a fuel oil. Sample Number Two. Distillate from Temperature Volume Specific Centigrade. Per Cent. Gravity. Water. 150ø to 24øo ...... 10.6 o.7996-415 Be. ' O.2% 260ø ,to 280ø ...... 15•5 o.8254-4o• Be. 300.ø to up ...... 8.2 o.84o9-3,21 Be. Over 300ø lubricating oil 46.5% o.8441-36• Be. Crackings ...... Xl.5 o.8441 Residuum ...... 7.7

Sample Number Three. Distillate up to 15oø C ...... 1% Water. Distillate from 14o to 3oo (Burning Oils) ..... 22.5 Distillate 3oo up (Heavy lubricating oil) ..... 6o. Residue (Tar) ...... 16.5 Specific gravity of burning oils at 6oø F ...... o.8629 Specific gravity of burning oils at 6oø F ...... o.8961 No naphthas present.

Althoughpetroleum has been discovered in commercialquanti- ties, the problemof transportationhas halted developmenttem- porarily. The Tropical Oil Companyhas beenmaking attempts to sell their intereststo somelarge companywho would be able to financethe buildingof a pipe line to the coast. It is stateds that the Standard Oil Companyhas purchasedthe interestsof the Tropical Oil Companyfor $40,000,000,most of which com- prisedan exchangeof Tropicalstock for InternationalPetroleum Companystock. It is predictedthat the new organizationwill begin extensivedevelopment and will also make arrangementsto market the petroleumin Colombiaand the United Statesin the. near future. lPamplonaDistrict.--According to Ralph Arnold,• oil both of high and low grade has been found at Pamplona near the a Oil and Gas Journal, Feb. 6, 192o. •" Conservationof the Oil and Gas Resourcesof the Americas," Ecom GEox•.,vol. XI.i p. 3o6, 1916. GEOLOGY AND OIL RESOURCES OF COLOMBIA. 471

Venezuelan frontier. The oil in this district occurs in Creta- ceous limestones and sandstonesand in. the lower Tertiary. Long, well-defined anticlines in the Cretaceousrocks are the advantageousstructures f.or accumulation. Faulted zonesand overturned anticlinespresent other possibilities. The yield of the producingwells, which are all shallow,.is small, but a larger productionwill probablybe encounteredat greater depth. The inaccessibilityof this regionis the causefor its slow development, even though the prospectsat the surface are most encouraging. MiscellaneousDistricts.--At the presentwriting there 'are 'a numberof companiesoperating or planningto operatein various

parts of the country. The Carib Syndicate o.f Newß York has holdings in nearly every department in northern Colombia. They are now drilling at Honda on the Upper Magdalena. The Doherty interestshave acquiredone half interestin their Barco Concession,which propertyis now beingdeveloped. The Carib Companyalso has property in the Atrato river district, but this region is handicappedon accountot? its intolerableclimate. In the Department of Tolima, The Tolima Oil and Refining Companyof New York are operatingon the Montelier property, a tract of land comprisingabout 3,200 acres. The oil-bearing rocks are probably of Cretaceousor early Tertiary age and are a continuation of those in the Santander belt to the north. Oil -seepsand petroliferous shales are found in abundance. There is an anticlinal fold on or near the property which might be an extensionof the Tropical structure. A seriesof parallel faults characterizesthis area for a distanceof overthirty miles, and it is alongthese fault planes that seepages occur. Theselocal dis- turbancesand plicationsof the strataare factorsthat might hin- der the accumulation of oil. However, the retention of oil in normal dippingbeds, due to faulting, is a commonoccurrence, and thispossibility might present itself in this particularregion. The Gulf Producti.onCompany,.under the nameof The South AmericanGulf Oil Company,has purchasedthe so-calledSan AndresConcession, a 200,000-acretract, whichlies in and around the townof SanAndres. The onlystructure on the propertyis 472 ELFRED BECK. a small circular dome,one squaremile in extent. There are two oil seepageson the land, thought to occur along a fault plane about one mile east of the fold. A prospectiveoil sand, esti- mated at about 3,500 feet below the surface, and several other shallower possiblyoil-bearing formations are the hopes of the operatingcompany. They are now bringing in machineryand tools, building roads, establishinga camp, and making prepara- tions for developmentof the CoastalRegion of Colombia. A group of promotersfrom Kentuckyhave secureda leaseon 1,5oo,oooacres of land near Carmen,Depa. vtment of Bolivar. There are a few oil seepageson the property. The writer is not definitelyinformed as to the structuralconditions, but he believes that the Sincelejoanticline extends north acrossthis area. This presumptionhas beenupheld by statementsreceived from other geologist•who havevisited the country. Under the nameof the Caribbean Petroleum Company, this company either expectsto do somedrilling or to turn their holdings over to a larger com- pany. This companyalso owns an optionon a large tract of coal land in the Department of Antioquia. The Emerald Oil Companyof Winfield, Kansas,have acquired prospectiveoil and gas leases from The American-Colombian Corporationof Utah. The propertylies up the Magdalenariver near Magangue and comprisesabout 1,5oo,oooacres. There are many oil seepageson the property and it is the writer's belief that a large anticline, developed from the Central Cordillera Mountain Range, passesthrough the area a short distancewest of the town of E1 Banco. The companyexpects to begin drilling immediatelysucceeding 'the.close of geologicaland operation expeditions. OIL-BEARING FORMATIONS,. The rocksin which oil has beenfound are of Upper and Lower Cretaceousand Lower Tertiary age. The oil has accumulated in sandstonesin most all cases,but it has also been found in the porous limestonesof the Cretaceousin the Department of San- tander. The mostpromising oil-bearing formations are the sand- stone members of the Tofeme Formation. This formation is GEOLOGY .4ND OIL RESOURCES OF COLOMBL4. 473

cappedby the thick imperviousBombo Shales,which provides an ideal reservoir for the retention of oil and gas in water- saturated rocks. The Huertas Limestone Group also presents somepossibilities, in which alternatinglayers of sandstonesand porous limestonesshould be suitablehorizons for oil and gas to accumulate.

OIL SEEPAGES AI•D MUD VOLCAI•OES. In the foregoing review of the Colombian Petroleum fields only a few of the known oil seepageswere mentioned. To at- tempt to enter into an exhaustive discussionof locations and descriptionsof the numerousoil seepswould be beyondthe scope of this paper. Therefore only the more important occurrences are given. The seepagesoccur in perfect uniformity, always parallelingsome major uplift, which invariably trendsin a north- east-southwestdirection. This parallelismis the most s.triking characteristicof Colombiangeology. It is observedin faults, anticlines,mountain ranges,oil seeps,and naturally in the drain- age .systemsand topography. It acts as a guide many times in mapping and interpreting structural forms. A prominent mud and gas volcano is located near Puertas Escondidoon the Caribbeancoast. It has five craters, and one coneis •5 feet high and •oo feet wide. A small amount of non- inflammablegas constantlyissues from this "volcano." The forceof the gasbrings a grayishblack mud to the surface, spreadingand piling up the materialin the form of a cone. This particular peculiarcase is locatedon 'the flank and near the axis of a synclinalfold, contraryt.o the usualoccurrences along fault plane•and brokenanticlines. In many of the oil seepsand mud volcanoesa long.stick • 5 to 2o feet in lengthcan be thrustout of sight into the crater. These mud volcanoesin themselvesshould not induceoperators to drill for economicsupplies of oil or gas, as theyare notproof nor an indicationthat petroleum is present in the vicinity. Unjustifiedlocations near mud volcanoeshave already met with failure. 516 ELECTRICBUILDING, BILLINGS,MONT.