Section I-Paper 32

GEOLOGY OF THE MAQUIA OILFIELD IN EASTERN PERU AND ITS REGIONAL SETTING

BY

EDWIN KOCH*

T. In March 1957 in the Upper Creta

,g Cia. Peruana ( I -1 .. -.

kilometers. This major feature i zone. Toward the west there is a the cover of unfolded younger

The stratigraphie sequence ranging from uppermost Creta Oligoeene age ; 2) Cretaceoiis the Ucayali basin, but containir . -- _ ^ _ . .

ABSTRAC , the wildcat well Maquia 1, located NE of Contamana, Eastern Peru, struck oil ceous Sugar sandstone. It was drilled by a Peruvian-German syndicate comprisin le Petrleo El Oriente, Deutsche Erdoel Aktiengesellschaft - Gewerkschatt Eiweratn -wintershall Aktiengesellschaft. Up to April 1958, a total of seven producer wells had been drilled. Production has not been started.

The discovery well was located by photogeologic and field surveys on an oval-shaped anticline 7 kilometers long, which is part of a long range of anticlines that can be followed for about 85

s separated from the Contamana structures on the east by a synclinal broad monoclinal band of steeper dipping beds which disappear under Sediments. is composed of the following formations: 1) continental red beds,

ceous to Miocene with a brackish-marine intercalation of probable formations in facies similar to the previously described sections of

ig two prominent sand layers within the Sugar saadstone: a sand in the marino a.hnntn 11nniarienl fnmnation and a sand to shaly sand unit which is equivalent to

Agua Caliente (Turonian-Neocomian) . The Cretaceous trans- I in Maquia and over the Ordovician farther to the east.

_-_- -__- ~ ___- -__-___- , _--- the marine Esperanza shale of the gresses over Permo-Carboniferous ( ?)

RESUME. En mars 1957, le puits Maquia no 1, une wildcat situ au N.E. de Contamana dans le Prou Oriental, atteignit une couche de ptrole et dhuile dans le grs en sucre du Crtac Suprieur. Les travaux de forage taient conduits par un syndicat germano-pruvien comprenant

iaft irs.

un ns one des

ges ile mi- hes me) :tte n). Est.

592 FIFTH WORLD PETROLEUM CONGRESS

Introduction During the last 30 years, the area of Contamana

(Fig. 1) has been studied by several oil companies be- cause of the excellent outcrop conditions. The area is folded and contains several horizons, especially the Lower Cretaceous Agua Caliente sandstones (pro- ductive in Ganso Azul, 170 kilometers SSW of Con- tamana) and the Upper Cretaceous Arenisca de AzUcar sandstones. Drilling started in 1945 with the wildcat well Santa Clara No. 1 (of the Empresa Petro- lera Fiscal) and was followed in 1947 by Rayo Well No. 1 (of the Ca. EI Oriente). These wells were lo- cated on anticlines which expose the Cretaceous and they encountered good reservoir rocks in the Lower Cretaceous but no oil indications.

Fig. 1-Index Map of the Contamana Area. i). Maquia Oil Field. 2). Ganso Azul Oil Field. 3). N.W. Peruvian Oil

Province. Shaded Belt indicates Andean Front.

Gewerkschaft Elwerath started its activity in 1954 and, after carrying out photogeological and field sur- veys, drilled the following wells: Cashiboya 1 and 1A (T.D. at 1396 mts. in Ordovician), Inuya 1 (T.D. at 1836 mts. in Agua Caliente) and the discovery well Maquia 1 (T.D. at 1623 mts. in (3) Permo-Carboni- ferous) . Later, based on an additional seismic survey, Pacaya 1 (T.D. at 1380 mts. in Agua Caliente) was drilled. These wells are located on anticlinal structures, where the prospective horizons are covered by differ- ent thicknesses of Cretaceous and Tertiary red beds.

In all of the wells oil impregnation was found in the Arenisca de Azcar sandstones, but only in Maquia was commercial production obtained. The older Cre- taceous sequence appears to be barren of oil in this area.

Stratigraphy stratigraphie Sequence

In Figs. 2 and 3 the general results of the surface mapping are shown. I t can be noticed, that between a zone of Cretaceous outcrops in the east and the Ucayali plains to the west, there is exposed a strip of Tertiary Sediments, 15 kilometers in width.

In the central part of the Maquia anticline conti- nental deposits of early Tertiary age-Paleocene and Eocene-appear at the surface. The lowermost beds are the so-called Capas Rojas (red beds)-1, which are composed of fine-grained sandstone, siltstone and mudstone of reddish and brownish colors; also some coarser sandstone, green claystone and gypsum are present. In Maquia, the total thickness is 600 meters; to the east, at the headwaters of the Cachiyacu creek, 800 meters are exposed. Upwards this unit changes gradually into a 15 meter-thick tuffaceous and iron- stained bed, the Capas Rojas-2. This bed is overlain in t u n with a sharp contact by a conglomeratic, im- pure sandstone, the basal sand of Capas Rojas-3, hav- ing a thickness of approximately 90 meters around Maquia. The basal sand of Capas Rojas-3 grades into a 200-meter-thick sequence of interfingering marls and sandstones, the upper Capas Rojas-3. Within this mem- ber, there are some horizons of better sorted sand- stone, which form persistent beds, one of them having been continuously followed between Maquia and In- uya. As shown in Fig. 2, this persistent bed delineates the structures of Pacaya and Inuya.

Above the Capas Rojas-3 we find a brackish marine intercalation of around 50 meters of sandy limestone with marine oysters and snails and sandstones with foraminifera. It is considered, that this horizon is equivalent to the Pozo formation in the northern part of eastern Peru and therefore of probable Oligocene age.

Overlying this marker the continental Sediments re- appear again, represented by another sequence of mark and sandstones, the Capas Rojas-4. The grain size, however, is much coarser than in Capas Rojas-1 and -3, the sands are arkosic and often strongly cemented

593 GEOLOGY OF THE MAQUIA OILFIELD IN EASTERN PERU AND ITS REGIONAL SETTING

by lime, and contain conglomeratic and brecciated Iayers, whereas the mudstones contain abundant irreg- ularly-shaped calcareous concretions and bands. For these reasons, the Capas Rojas-4 sequence which has a thickness of 400 meters, is easy to differentiate from the Lower Tertiary. It is covered by an additional 1,000 meters of terrestrial Sediments, the Capas Rojas-5, com- posed of clays, sands, silts and pebble layers, which can

be recognized by their light color, the near absence of carbonates and their softness. Capas Rojas-4 and-5 belong to the ( ? ) Oligocene and Miocene.

An angular unconformity indicates that the com- plete section was folded at the end of the Miocene. This was followed by peneplanation, deposition of Pliocene and Recent fluvial Sediments and by a new cycle of uplifting and erosion.

Fig. 2-Geological Map of the Contamana Area (modified after Kummel, 1948). 1). Ordovician. 2). Aqua Caliente. 3). Chonta. 4). Arenisca de Azcar. 5). Sol. 6). Capas Rojas. 7). Capas Rojas-2. ,8). A sand horizon in the Capas Rojas-3.

9). Capas Rojas-5.10). Unfolded Pliocene to Recent Sediments. 11). Oil Seepages.

594 FIFTH WORLD PETROLEUM CONGRESS

Fig. 3-Geological Map of Maquiri Oil Field. i). Formation Contacts. 2 ) . Faults. 3) . Structural contours below sea level at the top of the Arenisca de Azcar. 4 ) . Well locations.

Underlying the Tertiary, formations of Cretaceous and pre-Cretaceous age are encountered at the surface in the Contaya and Contamana Mountains and also in the wells. A description of these pre-Tertiary rocks is

based principally on the section found in wildcat well Maquia 1 (see Fig. 6).

This well was drilled through 105 meters of upper- most Cretaceous red beds, the Sol formation, which

GEOLOGY OF THE MAQUIA OILFIELD I N EASTERN PERU AND ITS REGIONAL SETTING 595

underlies practically conformably the Capas Rojas-1. It consists of lacustrine limestones and mark in the upper part, tuffaceous materials similar to the Capas Rojas-2 in the middle part, and multicolored mud- stone as well as largely fine-grained sandstone in the lower part. The same sequence was found in Pacaya 1, huya 1 and in the Cashiboya structure. But, whereas the thickness of the Sol formation over these structures is very consistent, it increases to approximately 200 meters towards synclinal areas, thus indicating in- cipient anticlinal growth during sedimentation.

The contact with the underlying Arenisca de Azcar again is conformable, showing in places, however, evi- dence of ero his formation has a very uniforu intamana area, i.e. : 190 meters in iviaquia i, 188 meters in Pacaya 1, 180

i Cashiboya 1 - 1A. :rent components nto the following

sion at the contact. T i thickness over the Cc

* L r . . .,.,. . meters in huya 1 and 184 meters ir However, the thickness of its diffi varies. The formation is divided i ..- :... . UL11 LS

Upper main sand (littoral) . Whitish, medium- to-coarse-grained, relatively pure, quartz sand- stc with thin layers of da id permeable; litho-

me; cross-bedded; pebbly; Irk gray shale; very porous ai -.:--i1 Y .... -: x r

ne-cemented sands 3 lower ones are shl

:..+-+L.a" ,

logi~aiiy v u y uriiiurrn; in iviaquia 1 it is 53 meters thick.

Multicolored beds (continental) . Predomi- nantly red, violet and green-gray marls; with 3 to 4 silty and lin tone interbeds (of which the twc own in Figs. 6 and 7 ) . The sanciJLVllL I11LLluLu3 change rapidly in thic )n and content of shal a 1 i< 51 meters.

kness, degree of cementatic e. Total thickness in Maqui; I " , L -1, " I " I I.,"" 1-:..L Y --"-- BbUbh J I L u L c i J ( " r u ' n L J I ' - r r L u r r r r e ) . Dark gray i

black shales, containing laminated layers of and fine-grained sandstone; lithologically pers ent. In Maquia 1 it is 16 meters thick.

ind silt ;ist-

Lower main sand (littoral). Whitish, medit to coarse-grained, pebbly, relatively pure qu; sandstone; cross-bedded; with thin bands

nf AqrG .h,,ln.. nn._n..C ,,,-,,i

im- irtz md

I\rllJLJ uLIII\ oIILIIL.i, v L L y yviuuJ yLiiiiL.ade; lithologically very persistent. In Maquia 1 it is ?n A ~ : - i -

the whole Contamana area, consists of marine shales, which in the upper part are occasionally limey and contain some sandy layers, whereas in the lower part it includes some prominent sandstone intercalations. Based on the evidence of ammonites the Chonta forma- tion belongs mainly to the Coniacian.

Underlying the Chonta formation conformably is the Agua Caliente formation. I t is composed of medium-grained sandstones with some thin shale inter- calations and the middle part of the section shows a somewhat shaly development. The formation was drilled completely in Maquia 1 and Cashiboya 1-1A with a thickness of 563 meters for the first well and 505 meters for the second. The section of both wells showed an almost identical facies development. Fossils are very scarce. They indicate a marine environment, and a preliminary Turonian - Neocomian age has been assigned.

The Agua Caliente transgresses in Maquia 1 over a section of sandstone, limestone and dolomite compar- able with that of Permo-Carboniferous of the Ganso Azul oil field. Drilling was suspended 54 meters within this formation. In Cashiboya I-1A and in the out- crops of the Contaya Mountains the Agua Caliente lies directly over Ordovician shales with included grapto- lites (Newell and Tafur [ 11 in Contaya encountered the middle Ordovician zone of Didymograptus murch- isoni) . As shown in Fig. 4, it is assumed that the Permo-Carboniferous pinches out between Maquia 1 and Cashiboya 1-1A and that the Permo-Carboniferous transgresses over Middle Ordovician.

he main develo ind Andean fro nember of 150 n..-1. .l. .I.-- T< 1

Comparison of Sections The intention here is to mention only the principal

features by which the Cretaceous in this area differs from tl pment towards the west (Ucayali basin a nt) . Kummel [ 21 found a marine shale I: meters and 85 meters respectively in the busnaaaray dountains (100 kilometers WNW of Maquia) and in Ganso Azul. Rosenzweig [3] in 1950 mentions some 80-300 meters of marine and shaly Agua Caliente in the middle Huallaga River area. In the Contamana area this member has turned predomi- nantlv into sandstone. as described before bv Kummel.

I u IIICLCI LiiiLn. Similarly, the Chonta formation changes from a M ern facies of highly fossilferous marls and limestc into the eastern marginal facies described above.

The Arenisca de Azcar overlies with a sharp con- tact the Chonta formation, which in Maquia has a

rest- mes On

596 FIFTH WORLD PETROLEUM CONGRESS

A Ucqali M.q"l. Rner O i i l i ~ i c

B I I Co"l.rnUli MI. Conlaya MIL

I J Fig. &Cross-Section Through the Contamana Area (See Figure 2, Line of Section A-B). i). Unfolded Pliocene

to Recent. 2). Capas Rojas and Sol. 3). Arenisca de Azcar. 4) . Chonta. 5). Aqua Caliente. 6). Perno-Carboniferous. 7 ) . Ordovician.

also the multicolored beds of the Arenisca de Azcar are found all over the Ucayali area and the Andean front. The upper main sand, however, is a deposite restricted to the eastern part of the basin; it pinches out towards the west, where it is replaced by multi- colored beds.

Tectonics

Structures in the Contamana Area

Fig. 4 shows a section cutting the main structures between the Contaya and the Contamana Mountains. From east to west, it shows first the Contaya uplift, where Ordovician and Lower Cretaceous are broadly exposed. The steep western flank of this uplift gives way to a broad SJ Mer Tertiary sedi- ments and to the orium, which has

rncline filled with loi Contamana anticlin s at the surface in its .I- Ll--l- -5 4.L- n-..

Lower Cretaceou more deeply anti- clines. The westtril MIM UI LUC uulitamana anticlin- orium is also the steeper one and in some places it is overturned and thrusted. Rayo 1 is located at its north- ern extremity and Cashiboya 1-1A at the southern ex- tension, where the Sol formation has been preserved from erosion.

Towards the Ucayali there follows a second syn- clinal element filled by Tertiary Sediments. I t is gently undulated and the beds then rise on a third range of anticlines, which we might call the "western trend." On this trend, the wells Maquia, huya 1 and Pacaya 1 are located on different culminations. Furthermore, it is quite probable that the Santa Clara dome, approx- imately 45 kilometers NNW of Maquia, where Cre- taceous rocks are again exposed, belongs to this same western trend. In the better known area between the village of Contamana and the Ahuaya creek (see Fig.

2) , it is seen that the western and southern flanks of the separate structures of the western trend combine to form a continous and broad flexure, in which the formations are steeply dipping into a still deeper and so far unexplored level.

In 1957 and 1958, a seismograph survey was made of the western trend, the syncline and, with one seismic line, the Contamana anticlinorium (at Cashiboya 1-IA). Fig. 5 shows the contour map on the top of the Arenisca de Azcar. I t is controlled by the wells and by velocity surveys in Cashiboya 1A and Inuya 1. The map is extended towards the north according to surface geologic data. Beginning at the Cashiboya structure, where the Arenisca de Azcar has its apex at i- 200 meters (datum plane referred to is sea level), the horizon dips towards the west into the steep flank of the structure and is cut there by the thrust, which separates the Cashiboya structure from the syncline. West of the fault, the horizon remains between - 700 and -800 meters and then climbs with very gentle dips towards the structure of Pacaya.

At the well Inuya 1, there is only a minor undulation recognizable at the top of the Arenisca de Azcar. This undulation develops upwards according to surface mapping into an anticline with 50 meters closure, as indicated in Fig. 2. The decrease of structural relief is caused by thickening of various Tertiary horizons in the direction of the well, i.e.: between Pacaya 1 and Inuya 1 the Capas Rojas 1 thickens from 578 to 592 meters, the Capas Rojas 2 has 35 meters in both wells, the basal sand of Capas Rojas 3 thickens from 62 to 78 meters. Variation of thickness in continental sedi- ments is very common and also occurs very frequently in eastern Peru. But, as has already been mentioned in the case of the Sol formation, there is thinning

GEOLOGY OF THE MAQUIA OILFIELD I N EASTERN PERU AND ITS REGIONAL SETTING 597

towards structural crests and in consequence there is more structural relief with depth.

In the Pacaya structure the de Azcar top rises to a level slightly above - 650 meters. The closure amounts to approximately 50 meters. In the Maquia field the formation is found at -315 to - 320 meters.

100 meters decreasing towards the surface, where the diminishing displacement is compensated by steeper dips in combination with minor faults (see also Fig. 3) . Therefort : Arenisca de Azicar, the

i (higher) and an eastern structure e, at the depth of thc is split into a westerr

dipping thrust fault. The fault h .as a displacement of block.

Fig. 5-Structural Map at the Top of the Arenisca Azcar, Based on Seismic Survey. 1). Cretaceous outcrops. 2) . Structural contours below sea level at the top of the Arenisca de Azcar, according to the seismic survey. 3 ) . Structural

contours according to field geologic data. 4) . Thrust faults. 5 ) . Normal fault.

598 FIFTH WORLD PETROLEUM CONGRESS

Maquia Structure

The axis of the anticline is found on i r l__ . . L 1.1--1 A 1 m. 0 . I .

the higher up- LIIIUSK DIOCK. AS mown in rig. 3, tne structure is oval- shaped and encloses an area of some 7 x 3 square kilo- meters within the outcropping Capas Rojas 2. The northern flank is gently dipping. The western flank dips gradually and later steeply towards the flexure. The southern flank is relatively flat close to the crest, dipping more steeply outside of the Capas Rojas 2 ring and finally e main flexure immediately the attitude is iiaLLci. IL IS IICIC, WIICIC tut: rIiiusr rauir: cuts the structure.

tttains dips of 30" as it merges into the . The eastern flank has dips of up to 30" adjacent to the axis, but in the subsurface, ' ^ LI-&*-- T I :- LA-- -..La-- *L- A1 -..... I l - - - l L

The oil field itself is situated in the very central part

of the structure, where dips are almost flat, as the con- tour map on the top of Arenisca de Azcar shows (Fig. 3) . No fault has been revealed by drilling.

Oil Accumulation n:i ...Pa-nnel ---

8

e Arenisca de A .e prospective res :rite, Chonta and .ll _ _ AL- -----11 1

wIl a c i y a g u air; known in the south flank of the Cashiboya anticline, where they appear in the Sol formation (the oil being derived from the Arenisca de Azcar), and also in the "Sol 7" anticline at the top of th .zcar. Of course, drilling tested all th ervoirs, i.e., the Cretaceous Agua Calie i Arenisca de Azcar sandstones, as wtii as u t : unaeriying pre-Cretaceous.

The oil impregnation was found to be limited to the Arenisca de Azcar. Cashiboya 1-1A had minor oil

c U

O K U

.-

a o - .o Lo

N W S E U 6 M 1 u 2

. . . .

. . . .

. . . . . . . . . . . . .

. . . . . . . . . . . . . . . .

Om-ZQOm

bpiriw IY.l . iO

100

zoo

300

LOO

5W

600

700

800

900

1030 m

L E G E N D

@ } produclove horizons ag

Fig. &North-South Section Through the Maquia Oil Field. (For line of section, see well locations on Figure 3.)

GEOLOGY OF T H E MAQUIA OILFIELD IN EASTERN PERU AND ITS REGIONAL SETTING 599

- - - --

. . . . . . . .

S

c

U o O .- t U O rn-200 m below surface -100 -200 -300

-400

-500

I Fig. 7-East-West Section Througl

snows in the upper main sand at appro mn+nn .-.*A :" +Ln :"cp..mpA:ntp ,,"A

intermediate sands showed ir - 705 meters resulting in non-c A thin oil cap was discovered i at - 705 meters. In Maquia thi noted at the top of both main stones, which are intercalated

LSOO rn below sea level

LEGEND

o} productive horizons I .

I the Maquia Oil Field. (For line of section, see well locations on Figure 3.)

ximately + 190 IIILLLIJ IIILLIIIILUIaLLi Jaiiu~ of the multi- colored beds at + 125 meters. In Inuya 1 oil shows were observed at the top of the upper main sand and considerable oil impregnations were noted within the intermediate sands at - 900 meters. In the structur- ally higher Pacaya anticline oil impregnations were found down to a depth of -650 meters, whereas the

npregnations down to ommercial production. n the lower main sand e oil accumulations are sands and in the sand- with the multicolored

beds. Therefore, three separate reservoirs can be dif- ferentiated (see Figs. 6 and 7) . Reservoir in the Lower Main Sand

In the structurally higher wells the oil impregnation has a thickness of 20 meters and ends at - 455 meters. Maquia 3, 5 and 7, located at the western, northern and eastern flanks, are water-bearing or are in the con-

tact zone. The grain size of the productive sands aver- ages 0.25 mm., porosity is around 20-25%, permeabil- ity exceeds 1,000 md. Some fine-grained and imperme- able layers are also present.

The bottom water is fresh and has a salt content of 0.050/, only, whereas the formation water of the oil saturated parts has 2% salt approximately.

In production tests the wells yielded 40-50 m3/d through a 4 mm. choke and with a differential pressure of 1-2 atm. The oil has a specific gravity of 0.83 (39" API). Gas-oil ratio is very low.

Reservoir in the Sand Intercalations of the Multicolored Beds

The different sands change rapidly in their grain size and cementation as well as in thickness and there- fore average figures hardly can be given. Consequently, the production varies considerably; for example, 7 m3 were produced in Maquia through open tubing and Maquia 3 gave 33 m3/d through a 4 mm. choke and with a pressure differential of 5 atm.

600 FIFTH WORLD PETROLEUM CONGRESS

The water contact was not reached in any of the wells.

Reservoir in the Upper Main Sand In the center of the structure, in wells Maquia 1 and

6, a maximum thickness of 12 meters of oil sand was encountered down to -- 330 meters. The impregna- tion is restricted to the innermost part of the structure, as all other wells are fully or partially water-bearing. Reservoir rock characteristics are similar to those given in the lower main sand. The section has not been thoroughly tested so far.

Acknowledgments

The writer is indebted to the group of companies which generously permitted the publication of this re- port. Special thanks are extended to Dr. A. Roll and Mr. D. A. Fyfe, Chief Geologists of Gewerkschaft El- werath and EI Oriente, respectively. The work was carried out by Dr. E. Blissenbach who did the well geology, by Dr. J. Stiefel and by the author who did the surface geology. Dr. H. Fuechtbauer and Dr. W. Tunn made the reservoir-rock studies. All are with Gewerkschaft Elwerath. The seismograph survey was contracted to Geophysical Service Incorporated, Dal- las, Texas, under the management of Mr. D. W. Rockwell.

References

1. Newell, N. D. and Tafur, I., Ordovcico fosilifero en la Selva Oriental del Per.-Bol. Soc. Geol. del Per, Tomo XIV-XV, 1943.

2. Kummel, B., Geological Reconnaissance of the Contamana Region Per.-Bull. Geol. Soc. of America, Vol. 59, 1948.

This paper was presented on June 5, 1959, by C. H. GREEN (Geophysical Service Inc., Dallas, Texas, U . S . A . ) .

Discussion B. KUMMEL* (Harvard University, Cambridge,

Mass., U.S .A . ) . I t is now approximately 40 years since the first geologists tramped the jungles of eastern Peru as agents of an organized exploration program. In the intervening years few streams, hills, or outcrops in this region have escaped the attention of at least one geologist. The fruits of these four decades of labor in terms of locating new oil fields have been small indeed. The first concrete result was the discovery of the Ganzo Azul oil field in 1937 by Robert Moran and Douglas Fyfe. Twenty odd years later the same Douglas Fyfe was instru- mental in bringing in the second discovery for eastern Peru, the Maquia field.

For those of us who are interested in or concerned with the geology and petroleum Potentials of eastern Peru the paper by Mr. Koch was most welcome and instructive. For me the significant features of this paper are the additional data on the stratigraphy and structural history of the region. The Contamana region is jungle rain forest, a most difficult region for surface geological studies. The author and his col- leagues are to be congratulated on the job they have done. I t is a combination of patient field work with photogeology that enabled the degree of success they attained.

The pertinent new stratigraphic data are briefly as follows: The total thickness of the Tertiary red bed formation (Capas Rojas) was given as roughly 2300 m., a figure considerably less than previous mea- surements. This is a trend that has been reflected in several other regions of eastern Peru, in that more

#

detailed studies tend to reduce the thickness of these red beds in terms of measurements made previously in reconnaissance surveys. The recognition of a marine tongue in the Tertiary red bed sequence, pre-

3. Rosenzweig, A., Reconocimiento geolgico en el curso medio del ro Hua1laga.-Bol. Geol. Soc. del Per, Tomo XXV, 1953.

sumably of Oligocene age and equivalent to the Pozo formation, this far south is of great interest. The base of the red bed sequence is the Sol formation of Upper Cretaceous age. The establishment of thin-

*Read by C. H. GREEN.

Bibliography

Bellido, E. and Simmons, F. S., Memoria Explicativa del Mapa Geolgico del Per. Bol. Soc. Geol. del Per, Tomo X X X I , 1957.

(1EOLOGY OF THE MAQUIA OILFIELD IN EASTERN PERU AND ITS REGIONAL SETTINO 60 1

ning of the Sol formation on the top of the Maquia structure is of considerable interest.

The differentiation of various facies units within the Arenisca de Azcar has been recognized on the outcrop at various places in eastern Peru. Outcrop conditions being what they are in rain-forest regions always left the picture incomplete; the new wells at Maquia, Pacaya, Inuya, and Cashiboya have given

much better perspective of the facies character of the formation. I t appears that the Arenisca de Azcar reflects oscillation of the depositional environ- ment on a reduced scale compared to what is seen on a large scale in the Aqua Caliente sandstone. I t remains to be seen just how far these particular facies

characters and sequence of the Arenisca de Azca extend.

Finally, a short comment on stratigraphic nomen- clature. Geologists are people; you cannot have the accumulated results of four decades of labor in a region as difficult as eastern Peru without ending up with a complex of stratigraphic names which have meaning only to the specialist. Today we are faced with an incredible array of homonyms and synonyms. I plead with the author, his colleagues, and to all the geologists actively working in this region to get together, establish a code of nomen- clature, and publish a recommended list of accept- able names in accordance with their code.