the origin of petroleum in the oriente (ecuador)
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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.
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Origin
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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.
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68 Tom s Feininger
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Origin
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Petroleum in the Oriente of cuador
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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-
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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
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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.
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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
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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.
-
8/9/2019 The origin of Petroleum in the Oriente (Ecuador)
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.
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