the origin of petroleum in the oriente (ecuador)

8/9/2019 The origin of Petroleum in the Oriente (Ecuador)

1/10

The American Assoclatlon of Petroleum Geologists Bulletin

V. 59 No.

7

July

1975). P.

1166-1

175,

2 Figs.

2 Tables

Origin

of

Petroleum in the Oriente

of Ecuador

bahcI Large reserves of petroleum in the Oriente of Ec-

uador are prawnt in sedimentory racks deposited on a conti-

nental h l f during the Cretaceous. The petroleum was not

genwoted in these rocks but in the fino-grained terrigenous

c h i c sediments of a contemporaneous continental-rise

prism deposited in deeper water farther west. The rise sedi-

ments subsequently were metamorphosed and are now part

of the metamorphic rocks of the Eastern Cordillera of the

Andes. At the beginning of deformation of the continentol-

rise sediments caused by the onset of subduction during the

Maostrichtion most of the petroleum in the northern part of

the prism was driven upward ond eastward porallel with

bedding. Thus much of the petroleum entered the shelf rocks

lotorally. More complex deformation of the continental-rise

sediments in the south prevented the escape of petroleum

there. he trapped petroleum subsequently was converted to

graphii by metamorphism. Quantitative wlculations show

that

the proposed mechanism is rwronoble and thus may

have applications elsewhere and should be

considered in

planning exploration for petroleum in racks deposited an

continental shelves. A single carbon analysis of metamorphic

rocks from the south end of the Eastern Cordillera suggests

that the graphite content also diminishes southward. If true

this augurs well for the finding of oil in fields currently under

explorotion toward the east in Peru.

Tsch opp 1953) wrote a splen did summ ary of

the geology of the Oriente of Ecuador based on

12 years of detailed but unsuccessful exploration

fo r petroleum by geologists of Th e Shell Com pa-

ny of Ecu ado r, Ltd. decade later regional ex-

ploration again was undertaken, but by a dozen

companies new to the Oriente a nd each working a

smaller concession than that originally held by

Shell. On April 8, 1967, Texaco-G ulf co mpleted a

producing well at Lago Agrio and discovered the

first of many large fields now in produ ction in the

north Oriente Fig. 1). Conco mitant exploration

an d test drilling by other com panies in the south-

e m Oriente, within a nd peripheral to the area ear-

lier studied by Shell, again w ere unsuccessful.

Tschopp 1953, p. 2345) an d most petroleum

geologists currently at work in Ecuador ascribed

the source of the Oriente petroleum to bitumi-

nous shale and limestone the Napo Form ation;

see following) tha t are present east of the Andes.

Nevertheless, I shall show that these rocks are un-

likely sources, an d instead propose herein that the

oil was generated in and driven from a thick

prism of fine-grained clastic terrigenous sedi-

ments on the west, at the site of the present

Andes. These sedim ents subsequently were meta-

morphosed and now crop out as the schist, phyl-

TOM SEININQEP

Quito Ecuodor

lite, slate, and quartzite of the Eastern Cordillera

of the Ecuadorian Andes. If these metamorphic

rocks were the source of the petroleum, the out-

standing anomaly of petroleum occurrence in

eastern Ecuador can be explained successfully-

namely, the almost compleie restriction of peiro-

leum to the north Oriente. Correlative potential

reservoir rocks in the south Oriente, as thick as

those on the north-and with similar structural,

stratigraphic, and petrophysical characteristics-

contain little or no petroleum.

Geologically, eastern Ecuador consists of three

parallel, approximately north-striking belts. F rom

east to west these are I) the upper Amazon basin,

2) the Andean foothills, an d 3) the Eastern Cor-

dillera of the An des Fig. 1).

Upper

Amazon

Basin

Stretching from the eastern border of Ecuador

west to the Andean foothills, the upper Amazon

basin is a vast rain-forest-covered area of rela-

tively little local relief. Regional surface gradients

are east an d southeast, and elevations on the east-

ern bord er of Ecuad or are less than 300 m. Th e

area is draine d by large consequent rivers, such as

the Napo and Pastaza Fig. l , and countless

smaller tributarie s of th e Amazo n.

Basement rocks of the eastern upper Amazon

basin are granulite-facies metamorphic rocks of

the Guyana shield. Nearer the Andean foothills

wells have penetrated younger basement rocks:

fossiliferous limestone and terrigenous clastic sed-

imentary rocks of the Permian-Carboniferous

~ a c u m a ormation , and volcan ic and terr ige-

nous clastic chiefly continental) sedimentary

Q Copyright 1975. The American A ssocia tion of Petroleum

Geologists. All rights reserved.

IManuscript rece~v ed. une 17, 1974; accepted, October 31,

1974.

2Escuela Politecnica Nacional. Contribution no.

I

Depart-

ment of Geology, Escuela Politecnica Nacional.

My warm thanks go to Eugene Jarosewich, chief chemist, and

analysts

J.

Norberg and P. Brenner, of the Department of Min-

eral Sciences, Smithsonian Institution, for the chemical analy-

ses. Geologic information helpful to me was graciously provided

by Ben Fassett of Cayman del Ecuador, Robert Canfield of

Texaco, and Britton Wherry. The lnstituto Geog rafico Militar,

Quito, provided cartographic help.


2/10

Origin

of

Petroleum in the Oriente of Ecuador

67

rocks of the Jurassic ?) Chapiza Formation

TSC~OPP,953, p. 23 10-2316).

The basement rocks in turn are overlain uncon-

formably by shallow-water marine sedimentary

rocks of Cretaceous age, the basal Hollin Forma-

tion and the overlying N a p Formation. The Hol-

lin of Albian-Aptian age Tschopp, 1953, p. 2316-

23 17; Campbell, 1970, p. 38) is composed of med-

ium-grained, porous, white orthoquartzite with a

few shale partings. The thickness of the Hollin

shows little change, and through most of the Ori

ente ranges only from 84 to 136 m. The conform-

ably overlying Napo Formation is of Albian to

Coniacian or Santonian age Tschopp, 1953, p.

2317-2324; Campbell, 1970, p. 15). The middle

Napo consists of a uniform fossiliferous lime-

stone with shale partings) whose thickness ranges

from 78 to 91 m Tschopp, 1953, Table 11); it is

overlain and underlain by shale and glauconitic

sandstone. Thickness of the entire Napo Forma-

tion ranges from 200 to 400 m. In the eastern

Oriente, the Napo thins and grades laterally into

a sandy facies indistinguishable from the underly-

ing Hollin Campbell, 1970, p. 15-16).

Fold structures in the upper Amazon basin are

broad warps. Basement faulting, chiefly Miocene

or younger, and related to the final uplift of the

modem Andes Campbell, 1970, p. 22), has juxta-

posed the Hollin and N a p Formations in many

places Tschopp, 1953, Fig. 7).

The N a p Formation is overlain unconform-

ably by a thick sequence of clastic, in part tuffa-

ceous, poorly lithified, brackish-water to conti-

nental sedimentary rocks of Maestrichtian and

Tertiary ages. The basal part of this sequence is

the Tena Formation. Post-Tena rocks have been

given a profusion of local formation names. Tena

and post-Tena rocks thin from a maximum of

several kilometers

in

the west to 1,000 m or less at

the eastern border of Ecuador Tschopp, 1953, p.

2325-2342).

Andean oothills

belt of uplifts, in part with complex struc-

tures, forms prominent but discontinuous ranges

that separate the upper Amazon basin from the

high Andes on the west Campbell, 1970, p. 27-

29). These uplifts bring to the surface older rocks

that range from the pre-Macuma lower Paleo-

zoic ?) Pumbuiza Formation in the Cutucu uplift

Fig. 1) to the Napo Formation throughout the

foothills.

The uplifts which produced the Andean foot-

hills are young structures and formed at the end

of the Miocene Campbell, 1970, p. 22). Many are

bordered on the east by large reverse faults. Ex-

tinct volcanoes stand atop parts of the Napo

uplift Fig. l), culminating in Sumaco,a spectacu-

lar fresh composite cone 3,900 m high, 55 km

north-northeast of Puerto Napo.

Eastern

Cordillera

The high Andes rise abruptly west of the foot-

hills belt. The loftiest chain of the Ecuadorian

Andes is the Eastern Cordillera with summit ele-

vations commonly in excess of 4,000 m. Most of

this cordillera is composed of pelitic and quartz-

ose metamorphic rocks with steeply dipping folia-

tion. These rocks are cut locally by intermediate

to silicic stocks and small batholiths. Parts of the

cordillera are crowned by active or dormant an-

desite volcanoes more than 5,000 m high.

All of the metamorphic rocks belong to the

greenschist facies. The presence of abundant gar-

net and chloritoid, the local occurrence of kyan-

ite, and the absence of andalusite show that the

rocks belong to the Barrovian, or medium-pres-

sure-facies series of regional metamorphism.

Rocks in the northern half of the Eastern Cor-

dillera belong mainly to the upper greenschist fa-

cies. They consist principally of thoroughly re-

crystallized medium-grained mica schist. Rocks

of lower grade, such as weakly recrystallized

phyllite and slate, are present in a narrow belt

adjacent to the Andean foothills. A few kilome-

ters south of the Banos-Puyo road Fig. l), the

grade of the metamorphic rocks of the Eastern

Cordillera drops sharply and most are phyllite,

slate, and fine-grained quartzite not unlike those

adjacent to the foothills belt farther north.

n outstanding feature of the low-grade rocks

in the south is their abundance of graphite. The

dominant dark-gray phyllite and slate are sooty,

even coally; where weathered, they readily soil

the hands. Correlative rocks of higher metamor-

phic grade in the north are far less graphitic, light

colored, and with little obvious graphite. The con-

trast in graphite content of metamorphic rocks

from the northern and southern partsof the East-

ern Cordillera is brought out in chemical analyses

Table 1).

Ceologic Synthesis of Cretaceous-Tertiary of

astern

cuador

The Hollin and Napo Formations were depos-

ited in a shallow sea which transgressed from the

west. The source of the clastic sedifnents was the

deeply weathered Guyana shield on the east. The

limestone beds of the Napo Formation are chiefly

bioclastic rocks deposited during periods of di-

minished influx of terrigenous clastic sediments.

The shoreline zone of this sea is indicated by the

sandy facies of the Napo Formation in eastern

Oriente.


3/10

68 Tom s Feininger


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Origin

of

Petroleum in the Oriente of cuador


5/10

Torn

Feininger

Table 1. Noncarbonate Carbon Content weight-percent) of

Metamorphic Rocks from Eastern Cordillera, Ecuador*

Sample Location Percent Carbon

1. Saraurcu

2. Papallacta-Baeza road

3. Banos-Puyo road

0.13

0.33 average 0.25

0.30

4.

East of Cuenca

5. San Lucas verage 0.65

*Analysts: J. Norberg (1. 2. 4. 5) and P. Brenner 3). Department of

Mineral Sciences. Smithsonian Institution. Washington, D.C.

Samples listed from north to south see Fig. 1): 1. Composite of 10

chips of schist from south and west of Saraurcu, Pichincha Province.

2.

Composite of

35

chips of schist and phyllite from cuts in road

from Papallacta to Baeza.

Napo Province. 3.

Composite of 20 chips of

schist and phyllite from cuts

in Banos-Puyo road between Banos and the

Rio Zunac, Tungurahua Province. 4. Representative sample of phyllite

from east of Cuenca, Morona-Santiago Province. 5. Composite of 20

chipa of phyllite from the San Lucas area, Loja Province.

The metamorphic rocks of the Eastern Cordil-

lera are traditionally interpreted to be Paleozoic

and F'rcuunbrian(?) (Tschopp, 1953; Sauer, 1965,

p. 26; W c i o Nacional de Geologia y Mineria,

1969; Campbell, 1970, p. 25). The arguments

mustere

to

support the proposed antiquity of

these rocks are that they are metamorphic, struc-

turally

complex, and not in any way correlative

li thologidy with the relatively orderly sequence

of supposedly younger rocks of the upper Ama-

zon basin farther east. This age interpretation is

here considered erroneous. Rather, I intend to

show that the parent sediments of the metamor-

phic rocks are contemporaneous with the Hollin

and Napo Formations. Similar suggestions have

been advanced by Liddle

in

Liddle and Palmer,

1941, p. 14) and Faucher et al (1968, p. 46 .

The traditional paleogeologic reconstruction of

eastem Ecuador during the time of Hollin-Napo

deposition has been to show ,the area of the pre-

sent

ndean

foothills and upper Amazon basin as

a broad shallow seaway, bounded on the east by

the emergent Guyana shield, and on the west by

the similarly emergent Eastern Cordillera (Sauer,

1965, p. 66-61; Campbell, 1970, p. 14 . A strong

argument against this interpretation is the ab-

sence o evidence indicating the proximity of land

on the west. In fact, the westernmost outcrops of

the Hollin and Napo Formations suggest that

they were deposited in deeper water than were

rocks of the same formations farther east. The

Hollin, for example, contains increasingly abun-

dant shale partings westward, and the sandstone

bodies in the Napo Formation on the east are

replaced

toward the west by shale and limestone

(Campbell, 1970, p. 15-16).

A more likely paleogeologic reconstruction

(Fig. 2A) is that the Hollin and Napo Formations

were deposited in a shallow continental-shelf sea

with a shoreline only on the east, next to

an

emer-

gent Guyana shield. On the west, roughly coinci-

dent with the base of the present Eastern Cordill-

era, lay a shelf edge beyond which, in the deeper

waters of an open ocean, was deposited an enor-

mous volume of fine-grained temgenous clastic

sediment as a continental rise prism, or miogeo-

cline in the sense af Dietz and Holden (1966).

During the time of Hollin and N a p deposition,

the coast of this part of the South American con-

tinent was of the Atlantic type, in which the

continental plate underlying the shelf was cou-

pled to and moving with the adjacent oceanic

plate. Stable shelf-rise conditions were terminated

abruptly during the Maestrichtian by the decou-

pling of the continental and oceanic plates, the

onset of subduction, and the creation of

a

Benioff

zone dipping eastward from a trench in the west,

at the site of the present Western Cordillera of the

Ecuadorian Andes. The sediments of the conti-

nental-rise prism overlying the Benioff zone were

deformed and subsequently metamorphosed in

response to the rise of isogeothermal surfaces

(Fig. 2B). In early Tertiary time the location of

the trench shifted westward, probably to its pre-

sent offshore location. In the Oriente of Ecuador,

the depositional record of this orogeny is the east-

ward-thinning wedge of Maestrichtian and Ter-

tiary tuffaceous, brackish-water and continental,

terrigenous clastic sedimentary rocks beginning

with the basal Tena Formation. These rocks are

an exogeosynclinal wedge, or back-arc deposit

shed eastward from the erosion of a rising volca-


6/10

Origin of Petroleum in the Oriente of Ecuador 1171

FIG. 2--Schematic cross sections along line A-B of Fig. 1: A) at close of Napo deposition;

B)

at beginning

of Tena deposition;

C)

at close of Tena deposition. Symbols: hachures, continental basement; x s, oceanic

basement; ruled and dotted, sediments and metamorphic rocks (in C) of continental-rise prism; black, Napo

Formation excluding sandy facies; blank, Hollin Formation and sandy facies of Napo; crosses, intrusive rocks;

checks, volcanic rocks; dotted, Tena Formation and equivalent sediments shed westward; upper fine line, sea

level; heavy line,

25 C

isogeotherm. Note: Benioff zone greatly oversteepened because of vertical exaggeration

of sections.

nic orogen-the first vestige of the modem

Andes-at the site of the present Eastern Cordil-

lera (Fig. 2C).

Recent findings from the Cuenca basin in the

high Andes of southern Ecuador strongly support

this proposed series of events. Here, Bristow

(1973, p. 1 ) traced without interruption the lat-

eral passage of fine-grained schist and phyllite of

the Eastern Cordillera into pelitic and sandy, tur-

biditic, sediments of the fossiliferous Upper Cre-

taceous Yunguilla Formation. The Yunguilla here

is interpreted as the distal western part of the con-

tinental-rise prism that was pushed eastward dur-

ing telescoping of the prism that accompanied de-

formation and metamorphism (Fig. 2C).

Geologic mapping farther west is less detailed.

East of Cuenca, however, passage of the Hollin

and Napo Formations into low-grade metamor-

phic rocks t

the base of the Eastern Cordillera

appears gradational (Faucher et al, 1968,

p.

46;

Rudolph Trouw, written commun., 1974). Signifi-

cantly, no geologist during more than one-half

century of exploration for oil has reported rocks

of either the Hollin or Napo Formations lying

unconformably on metamorphic rocks of the

Eastern Cordillera.

ORIGIN

F

P TROL UM

N TH

ORI NT

The petroleum fields of the Ecuadorian Oriente

are entirely in the north (Fig. 1). Preliminary esti-

mates of in situ oil (Table 2) show that 98 percent

are north of lat. 1

S.

Most production is from the

top of the Hollin Formation, although the sandy

facies of the Napo is the chief producer in the


7/10

Tom Feininger

Table

2 .

Estimated in s it u

i l i n

Oriente

of

Ecuador

( b i l l ions of barre l s ) *

Proved Probable Tot al of Total

North of l a t . 1S. 3.948

2.744 6.692 98

South of l a t . 1s.

0 .013

0.128 0.141

Total 3.961 2.872 6.833 100

. .

Direm ion General

e

Hidrocarburos, Quito.

eastern fields. Rocks older than the Hollin and

younger than the Napo are barren.

Inadequacy of

Napo Formation s Source

ock

Th e Napo Forma tion is cited repeatedly as the

source of the Oriente petroleum. I t is a n attractive

candidate, being composed of bituminous shale,

sandstone, a nd richly fossiliferous dark limestone.

Most freshly broken samples of limestone or

shale from the N ap o emit a strong smell of petro-

leum, an d nodules of asphalt are widespread. Im -

pregnation of Na po sandstone by asphaltic oil is

common. Moreover, the Napo is present in all

fields of the Oriente of Ecuador.

Nevertheless, the Napo is an unlikely source

rock. In the north O riente, nearest the large petro-

leum reserves, Na po limestone an d shale beds ar e

especially rich in oil. They are so saturated that it

is unlikely that they ever could have held more

oil. Rather than being a source rock, they consti-

tute an impermeable and unexploitable reservoir

rock. Moreover, if the Napo was the source rock,

migration of oil into the underlying Hollin For-

mation would have been downward. This would

be very unlikely in these water-saturated form a-

tions. Normal upward migration of oil would

have produced saturation in at least the lower

part of the Tena Formation. The Tena, however,

is nearly barren throughou t the Oriente Robert

Canfield, personal comm un., 1974). critical

evaluation of these observations leads one to dis-

card the Napo Formation and look elsewhere for

the sou rce of the Oriente petroleum.

Graphite in

Metamorphic

ocks

of Eastern

Cordillera

The low-grade metamorphic rocks in the south

of the Ea stern Cordillera a re richly graphitic. The

origin of the graphite is enigmatic. vegetal ori-

gin is doubtful, because a continental rise far

from land is a n unlikely environment for the ac-

cumulation of plant remains. Also, my own

searches during the past six years have failed to

reveal a single plant fossil. The graphite in these

rocks is uniformly distributed a s microscopic dus t

along foliation and parallel bedding planes.

Graphite content varies markedly only across

bedding.

Th e absence of plant fossils an d the distribu-

tion of the exceedingly fine-grained graph ite lead

me to the conclusion that the graphite represents

a petroleum residue once contained in the sedi-

mentary precursors of these rocks. Metamor-

phism destructively distilled the petroleum, leav-

ing only a graphite residue. The ability of

metamorphism to convert coal to graphite is well

docum ented Quinn and Glass, 1958).

Spatia l Distribution of Graphitic Metamorphic

ocks and Petroleum in the Oriente

Known oil reserves Table 2) and producing

fields Fig. 1) are present only in north Oriente,

almost entirely north of lat.

1 s.

From the Co-

lomb ian border to somew here south of the Ba-

nos-Puyo road lat. 1

25'S),

the metamorphic

rocks of the Easte rn Cordillera are relatively poor

in graphite. Far ther so uth, the metamorphic rocks

are markedly graphitic and on the average con-

tain two and a half times more graphite than

those in the north Table 1). Th e petroleum re-

serves of the O riente a re exclusively in the region

east of graphite-poor metamorphic rocks.

Proposed Mechanism

Petroleum was generated contemporaneously

with or closely following deposition throughout

the entire fine-grained sequence that constituted

the continental-rise prism. Th is is the basic prem-

ise of my p roposed mechanism of origin,3 and it is

strongly corroborated by observations made in

the Cuenca basin. Here, the distal western part of

the continental-rise prism, which escaped meta-

morphism, is exposed as the Yunguilla Forma-

tion. Many seeps of heavy oil are known in the

Yunguilla Bristow , 1973, p. 36-37). It indeed

)In a paper that postdates the writing of the present manu-

script, Dickinson (1974) proposed a somewhat similar mecha-

nism to accou nt for the accumu lation of oil elsew here. He sug-

gested that late Tertiary continental collision in part induced

lateral migration of petroleum from distant source rocks to form

the proli fic reserves of the Persian Gulf area.


8/10

Origin of Petroleum in the Oriente of Ecuador

73

would be unlikely that the only petroleum in a

continental-rise prism would be at its oceanward

distal end. On the contrary, the presence of petro-

leum in the distal part of the prism is considered

here as compelling evidence that petroleum oc-

curred throughout the prism prior to its meta-

morphism.

During the Maestrichtian the onset of subduc-

tion initiated deformation and raised pressures of

the pore fluids (oil and connate water) in the con-

tinental-rise sediments. Where the initial defor-

mation produced relatively simple open folds, the

fluids were driven upward and eastward along

bedding, away from the trench. At the former

shelf edge, the fluids present in the lower part of

the continental-rise prism found easy access into

the correlative Hollin Formation. They entered

this porous formation laterally from the west and

progressively displaced eastward the connate wa-

ter in the Hollin. Upward escape of fluids was

hindered by the largely impervious shale and

limestone of the overlying Napo Formation. Oil

saturation of the Napo occurred later, caused by

slow upward permeation of oil from the underly-

ing Hollin. Fluids in the stratigraphically higher

part of the continental-rise prism were not able to

enter the correlative Napo because of the im-

permeability of that formation. Here fluids were

concentrated in the continental-rise sediments

just west of the shelf edge.

As deformation of the continental rise prism

proceeded, folds became tighter and beds increas-

ingly were crumpled and broken by faults. Fur-

ther migration of fluids was now impossible. Si-

multaneously, isogeothermal surfaces over the

subjacent

Benioff zone rose, and increasingly de-

formation was accompaliied by metamorphism.

Petroleum in the fluids which were unable to es-

c pe was distilled destructively to leave a graphite

residue. The concentration of oil in the continen-

tal-rise sediments that abutted the impermeable

N a p Formation left particularly abundant resi-

dues of graphite upon metamorphism. One such

remarkable graphite muck has been noted by

Britton Wherry (oral commun., 1974) between the

westernmost outcrops of the N a p Formation

and schists of the Eastern Cordillera in the Rio

Antisana, located

30

km northwest of Puerto

N a p (Fig. 1 .

Where the onset of deformation of the conti-

nental-rise sediments produced not simple open

folds, but complex folds and myriads of small

faults, migration of fluids was not possible. With

the onset of metamorphism, the oil contained in

the fluids was destroyed and left a uniformly dis-

persed graphite residue in the rocks.

Clearly, it is no coincidence that the prolific

petroleum reserves of the Oriente are related spa-

tially

so

directly to the graphite-poor metamor-

phic rocks of the Eastern Cordillera. The pore

fluids including the contained oil were in large

part expelled eastward from the continental-rise

sediments into the Hollin Formation in the north,

where the metamorphic rocks of the Eastern Cor-

dillera are relatively improverished in graphite. In

the south, fluids were unable to escape and the

entrapped oil was converted to graphite upon

metamorphism.

A question to be answered is why the low-grade

rocks in the south are graphitic. Why, in this pro-

posed mechanism, were pore fluids driven prefer-

entially from the sediments that today are the rel-

atively higher grade metamorphic rocks in the

north?

The metamorphic rocks in the south are decid-

edly more intensely deformed than are those in

the north. From the Banos-Puyo road north, foli-

ation and bedding generally are planar, whereas

in the south the rocks are so thoroughly crumpled

that structural attitudes other than axes of minor

folds can be measured in very few places. It is the

relatively greater degree of deformation of the

rocks in the south that there impeded the escape

of pore fluids. Two possible causes for this

inequality of deformation north and south come

to mind.

One may have been that subduction and defor-

mation began in the north, to quicken later in

pace and migrate southward. Rocks in the north

thus would have had a longer metamorphic histo-

ry and achieved a higher grade than those in the

south prior to the seaward jump of the trench in

the early Tertiary. Initial deformation of rocks in

the south therefore would have been more intense

and accompanied by more faulting than would

have been true for rocks in the north.

Another cause of the more intense deformation

of the metamorphic rocks in the south could have

been the influence of the Canonaco arch (Camp-

bell,

1970,

p.

8).

This broad, deeply buried, base-

ment swell protrudes westward from the Guyana

shield to impinge on the Eastern Cordillera a few

kilometers south of Puyo (Fig.

1). The arch may

have offered greater mechanical resistance during

deformation than was offered by basement rocks

under the continental-rise prism sediments far-

ther north. Forced against a relatively unyielding

basement, the continental-rise sediments in the

south would have been more intensely deformed

than those farther north. Also, if the arch is re-

flected at depth by a root, the greater thickness of

basement under the arch could have partly insu-

lated the overlying rocks from the rise of iso-

geothermal surfaces during subduction. This


9/10

1174 Tom Feininger

would account for the relatively low grade of the

metamorphic rocks of the Eastern Cordillera in

the south.

Quantitative

Calculations

To be considered seriously, the hypothesis pro-

posed here must be quantitatively reasonable. A

few simple calculations show that it is.

The proved and probable oil reserves of the

Oriente are 6.833 109 bbl (Table 2). The aver-

age API gravity of Oriente oil is

30 (Direccion

General de Hidrocarburos, @to, oral commun.).

At 100F, the density of the Oriente oil is 0.8618

g/cc (Levorsen, 1954, Tables A-3, 8-14). The

weight of the reserves is therefore 9.36 x I@ me-

tric tons.

Little or no petroleum is known in south Ori-

ente.

I

assume therefore that the graphite content

of the metamorphic rocks south of the Banos-

Puyo road, 0.65 percent by weight (Table I , is

the residue of n initial petroleum content repre-

sentative of the entire continental-rise prism

north to the Colombian border. The average car-

bon content of the metamorphic rocks from the

Banos-Puyo road north is only 0.25 percent by

weight (Table I), a difference of 0.40 percent.

Each cubic kilometer of metamorphic rock in the

north thus contains 1.076

X

107 metric tons less

carbon than in the south (average measured rock

density 2.69 g/cc). If one assigns the difference to

oil with 85 percent carbon (Mason, 1966, p. 238)

that was lost by being driven out toward the east

during deformation of the continental-rise sedi-

ments prior to metamorphism, each cubic kilome-

ter of carbon-depleted metamorphic rock is the

source of 1.266 107 metric tons of oil. The oil

reserves of the Oriente thus can have come from

only 73.9 cu

km

of metamorphic rock as now ex-

posed in the Eastern Cordillera from the Banos-

Puyo road north.

Metamorphic rocks underlie 720 sq km of the

Eastern Cordillera between the Banos-Puyo road

and the Colombian border (Servicio Nacional de

Geologia

y

Mineria, 1969). At first glance, the

source rock here proposed appears overly prolif-

ic; the volume of rock required, integrated over

the area of outcrop, is a layer only 103 m thick. It

must

be

kept in mind, however, that the reserve

figures (Table 2) are but a fraction of the total

amount of petroleum involved. For example, the

figures exclude the tens of millions of tons of as-

phalt that impregnate the Hollin as economic de-

posits in outcrops on the Napo uplift in the vicini-

ty of Puerto N a p (Britton Wherry, unpub. rept.).

The figures also exclude the rich but noneconom-

ic oil impregnation of the Napo Formation that

underlies 20,000 sq km of the western Oriente

north of

lat. 1S. If the petroleum content of this

250-m-thick formation is only 0.5 percent by

weight, a conservative figure based on field obser-

vations, its petroleum content would be 67.25

109 metric tons (using the same densities for rock

and oil as above). This is more than 70 times the

reserves given in Table 2. Furthermore, a s igdi-

cant part of the oil driven eastward in the conti-

nental-rise prism was blocked by the impermea-

ble Napo Formation. Much of this oil was

destroyed by metamorphism to leave graphite

masses like that exposed in outcrops along the

Rio Antisana, but perhaps the greatest part es-

caped upward along faults and fractures to be

lost at the surface.

The total amount of oil yielded by the conti-

nental-rise prism in the north is probably more

than 100 and may exceed 250 times the calculated

reserves of oil in the northern Oriente. The thick-

ness of the metamorphic rocks required as a

source for these quantities of oil, integrated over

the area of outcrop in the northern Eastern Cor-

dillera, is between 10.3 and 25.5

km

The thick-

ness of the metamorphosed continental-rise prism

prior to erosion probably was between these val-

ues. The absence of andalusite in the metamor-

phic rocks of the Eastern Cordillera and the local

presence of kyanite show that at least 20 km of

rock have been removed by erosion subsequent to

metamorphism (Miyashiro, 1973, p. 72). Part of

that cover was composed of volcanic-arc rocks,

but the great thickness of the metamorphic rocks

themselves is evident. The possibility that they

are the source rocks of the petroleum in the Ori-

ente of Ecuador is firmly established.

The geologic events leading to the accumula-

tion of petroleum in the Oriente of Ecuador

in

all

likelihood occurred elsewhere. In the search for

petroleum in rocks deposited on continental

shelves, the nature and composition of the associ-

ated continental-rise sediments, be they meta-

morphosed or not, must be evaluated critically in

planning exploration.

Southernmost Ecuador and neighboring Peru

may yield an interesting test of the ideas set forth

here. Somewhat southeast of San Lucas (Fig. 1)

the metamorphic rocks of the Eastern Cordillera

increase in grade and become less graphitic, like

those from the Banos-Puyo road northward. A

chemical analysis of a composite sample of phyl-

lite and schist from the road between Loja and

Zamora shows

a

carbon content of only 0.19 per-

cent by weight.4 This value is similar to the car-

bon content of the metamorphic rocks in the

4 ~ n a l y s t : . Norberg, Smithsonian Institution.


10/10

Origin

of

Petroleum in the Oriente of cuador

north Table 1). South of the Loja-Zamora road

the metamorphic rocks of the Eastern Cordillera

are not very accessible and have not been studied.

Should they maintain a low graphite content, it

would favor the hypothesis that it was the influ-

ence of the Conanaco arch that prevented the es-

cape of pore fluids east of Cuenca and at San

Lucas.

East of Zamora the international border is in

the Andean foothills, and the upper Amazon ba-

sin is in Peru, south of the Ecuadorian border.

Here in the basin, the Capahuari, Shiviyacu, and

Trompeteros fields are currently under explora-

tion. The diminished graphite content of the

metamorphic rocks on the west, a speculative ob-

servation based on a single analysis, augurs well

for the discovery of economic reserves in these

fields.

Bristow, C. R., 1973, Guide to the geology of the Cuen-

c basin, southern Ecuador: Ecuador Geol. Geophys.

Soc.

54 p.

Campbell, C.

J.,

1970, Guide to the Puerto Napo area,

eastern Ecuador with notes on the regional geology of

the Oriente basin: Ecuador Geol. Geophys. Soc.,

40

P.

Dickinson, W. R., 1974, Subduction and oil migration:

Geology, v. 2, p. 42 1-424.

Dietz, R. S., and J. C. Holden, 1966, Miogeoclines mio-

geosynlines) in space and time: Jour. Geology, v. 74,

p. 566-583.

Faucher, B., R. Joyes, F. Magne, J Sigal, R. Vernet, J.

C. Granja V., J. C. Granja B., R. Castro, and G.

Guevara, 1968, Estudio preliminar sobre 10s princi-

pales problemas geologicos concernientes a la explo-

ration petrolera del Oriente Equatoriano: Ecuador

Min. de Indust. y Com., 53 p.

Levorsen, A. I. 1954, Geology of petroleum: San Fran-

cisco,W. H. Freeman, 703 p.

Liddle, R. A. and K. V. W. Palmer, 1941, The geology

and

paleontology of the Cuenca-Azogues-Biblian re-

gion, Provinces of Canar and Azuay, Ecuador: Am.

Paleontology Bull.,

v.

26, p. 360-421.

Mason, B. 1966, Principles of geochemistry, 3d ed.:

New York, John Wiley, 329 p.

Miyashiro, A. 1973, Metamorphism and metamorphic

belts: New York, Halsted Press, 492

p.

Quinn, A. W., and H. D. Glass, 1958, Rank of coal and

metamorphic grade of rocks of the Narragansett ba-

sin of Rhode Island: Econ. Geology, v. 53, p. 563-

576.

Sauer, W., 1965, Geologia del Ecuador: Ecuador Edit.

Min. de Educacion, 383 p.

Servicio Nacional de Geologia y Mineria, 1969, Mapa

geologico de la Republica del Ecuador, scale I:l,

000,000: Quito.

Tschopp, H. J., 1953, Oil explorations in the Oriente of

Ecuador, 1938-1950: AAPG Bull., v. 37, p. 2303-2347.

the origin of petroleum in the oriente (ecuador)

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