Hollin y Napo Formaciones

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Reservoir Characterization of the Hollin and NapoFormations, Western Oriente Basin, Ecuador

Abstract

The Oriente basin of Ecuador has produced a substantial amount of oil over the past 20 years. Nearly3 billion bbl of oil have been recovered from the principal reservoirs in the Cretaceous Napo and Hollinformations. Subtle north-south structures, commonly associated with Andean-related faulting, have trappedmuch of the recoverable hydrocarbons in the thicker sandstones deposited within the Hollin and Napo reser-voirs. East to west thinning of these reservoir units also contributes to the formation of stratigraphic traps. Boththe Hollin and Napo formations comprise successions of eastward-sourced fluvial and deltaic sedimentarydeposits that prograded westward into shoreline and marine shelf parasequences. The Albian Hollin reservoirinterval consists of a dominant alluvial plain sandstone sequence (Main Hollin sandstone) that occupies muchof the Oriente basin. In the western Oriente, the uppermost Hollin section grades vertically into open marinestrata with isolated tidal- and storm-influenced sandstone bodies. The overlying Napo stratigraphy alsoconsists of sand-rich fluvial and deltaic deposits in the eastern Oriente and abruptly changes to marine shalesand limestones and lowstand valley-fill sandstones in the western part of the basin. Extensive structural andstratigraphic trap potential remains within the Napo and Hollin strata in the Oriente basin. High-resolutiongeophysical techniques and detailed geologic reservoir characterization facilitate successful exploitation of

these remaining reserves.

Resumen

En los ltimos veinte aos la Cuenca Oriente del Ecuador ha producido una cantidad sustancial dehidrocarburos. Alrededor de tres mil millones de barriles de petroleo han sido recuperados de los reser-vorios principales de las formaciones cretcicas Hollin y Napo. Estructuras sutiles orientadas norte-sur,comunmente asociadas con fallamiento de edad Andina, han entrampado la mayora de los hidrocarburosrecuperables dentro de los espesos depsitos arenosos de los reservorios de Napo y Hollin. La formacin detrampas estratigraficas ha estado favorecida por los adelagazamientos este-oeste de dichas unidades reservo-rios. Las formaciones Napo y Hollin comprenden una sucesin de sedimentos deltaicos y fluviales alimen-tados desde el este, los cuales progradaron hacia el oeste integrando parasecuencias de zonas de playa ymarino-plataformicas. El reservorio Albense Hollin consiste de una secuencia predominantemente arenosa deplanicie aluvial (Arenisca Hollin Principal) la cual se encuentra ocupando la mayora de la Cuenca Oriente. Enel occidente del Oriente, la seccin superior de Hollin grada verticalmente a sedimentos marino-abiertos concuerpos arenosos influenciados por mareas y tormentas. La sobreyacente estratigrafia de Napo tambienconsiste, en el este del Oriente, de depsitos deltaicos y fluviales ricos en arena, los cuales cambian abrupta-mente a calizas y lutitas marinas, y areniscas lowstand de relleno de valle en la parte oeste de la cuenca.Existe enorme potencial en trampas estructurales y estratigraficas dentro de los estratos Napo y Hollin de laCuenca Oriente. Las tcnicas geofsicas de alta resolucin y la caracterizacin geologica de los reservoriosfacilitaran una explotacin exitosa de las reservas remanentes.

Howard J. White

Robert A. Skopec

Felix A. Ramirez

Oryx Energy CompanyDallas, Texas, U.S.A.

Jose A. Rodas

Oryx Ecuador Energy CompanyQuito, Ecuador

Guido Bonilla

PetroecuadorQuito, Ecuador

573White, H. J., R. A. Skopec, F. A. Ramirez, J. A. Rodas, and G. Bonilla, 1995, Reservoir char-acteristics of the Hollin and Napo formations, western Oriente basin, Ecuador, in A. J.Tankard, R. Surez S., and H. J. Welsink, Petroleum basins of South America: AAPG

Memoir 62, p. 573596.


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INTRODUCTION

The Oriente basin of Ecuador produces a substantialamount of oil and provides attractive exploration oppor-tunities. The Hollin and Napo sandstone reservoirs haveproven to be consistent producers since initial produc-tion was first established in August 1972. The Naposandstone has a cumulative production (December 1992)of 1.17 billion bbl, and the Hollin Formation has a cumu-lative production of 1.70 billion bbl. Production estimatesfor the next 20 years are about 2 billion bbl, which will bederived from the currently producing fields with addi-tional reserves from fields in the process of development.

This paper presents an integrated geologic study of

the Hollin and Napo sandstone reservoirs in the greaterOriente basin, with emphasis on the western Oriente.Figure 1 illustrates the regional setting of the Orienteforeland basin in front of the Andean fold and thrust

belt, as well as the distribution of producing fields.

STRUCTURAL AND STRATIGRAPHIC

SETTING

The Oriente basin of eastern Ecuador is part of theupper Amazon River drainage basin and covers an area

greater than 80,000 km2. It is contiguous with thePuntamayo basin of Colombia and the Maraon basin ofPeru. Stratigraphically and structurally, the Orientepreserves a complex Phanerozoic geologic history

beginning with earliest Paleozoic deposition and culmi-nating with Tertiary deposits shed from the Andean foldand thrust belt. Figure 2 shows the general stratigraphyof the Oriente basin. The succession is subdivided intoseveral unconformity-bounded sequences: Paleozoic

Jurassic stratigraphy, the Cretaceous Hollin and Napoformations, and Upper CretaceousQuaternary sedimen-tary sequence.

The sedimentary fill rests on Precambrian igneousand metamorphic basement that has been intersected byseveral wells in the eastern Oriente adjacent to theGuyana shield. The SilurianJurassic interval consists ofseveral thousand meters of carbonates, shales, andsubordinate sandstones and conglomerates that have

been structurally deformed during several episodes ofuplift and extension. Examples of the pre-Hollinstructure are documented by Balkwill et al. (1995). Theuppermost strata of this interval belong to the JurassicChapiza Formation and the associated Misahuallivolcanics. In parts of the western Oriente basin, theHollinNapo interval unconformably overlies the

574 White et al.

Figure 1Overview of theOriente basin, Ecuador,showing structural features,distribution of producing fields,location of cross sectionsshown in Figures 7, 15, and 16,and specific wells referred to inthe text.


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Jurassic Chapiza and Misahualli volcanics, but elsewherethe Cretaceous rocks overlie Paleozoic strata andPrecambrian basement.

The HollinNapo interval consists of up to 500 m ofcontinental and marine sandstones, shales, and carbon-ates. The basin deepens toward the southwest, while its

eastern margin is masked by basin margin arches relatedto the Guyana shield (Figure 1). Structural arches shownon the regional map of Dashwood and Abbotts (1990)indicate two east-west trending arches extendingwestward from the Guyana shield. Interval isopachmaps confirm the existence of these two intrabasinhighsthe Aguarico arch to the north and the Cononacoplatform or arch to the south. The arches are believed to

have provided sediments as well as localized the Hollinand Napo fluvial systems. Reservoirs within the Hollinand Napo formations are structurally less deformed thanthe underlying strata. Nevertheless, Hollin and Napostructures are large enough to form prolific hydrocarbontraps. A complex of major reverse faults mark thewestern limit of the present-day basin adjacent to theNapo uplift. Most of the major oil fields occur east of thiscomplex in structural traps that parallel the north-southstructural grain. The source rocks for these reservoirs are

believed to be the organic-rich Napo shales which havebeen extensively tested for maturity, as reported byDashwood and Abbotts (1990).

The overlying Upper CretaceousPaleogene sedimen-

tary rocks in the Oriente basin were the first to be influ-enced by incipient Andean movement. The Tena andTiyuyacu formations (Figure 2) are the earliest strata ofthe post-Napo basin fill and consist of interbeddedshales, sandstones, and minor conglomerates. The basalsandstone of the Tena Formation in the western Orientewas probably derived by erosion and local reworking ofuppermost Napo. Late Tena and Tiyuyacu depositionconsisted of episodes of continental redbeds and limitedincursions of marine deposition. Deposition continuedwith the Orteguaza and post-Orteguaza formations andconsisted mainly of clay-rich continental strata erodedfrom the Andean volcanics. These continental sedimen-tary rocks mark the infill of the Andean foreland basin

and comprise over 1600 m of section in the westernOriente. The Andean volcanic arc controlled much of theTertiary sedimentation along the western margin of theOriente basin.

HOLLIN STRATIGRAPHY ANDDEPOSITIONAL SYSTEMS

Characterization of the Hollin and Napo reservoirsincludes data from seismic, well logs, core descriptions,and petrophysical analyses. Over 1100 m of core wereexamined. Regional mapping of the Ecuador Oriente

basin focused on depositional environments, paleoshore-line trends, facies distribution, and reservoir continuity.

The Hollin Formation occurs throughout the Orientebasin. It thickens from a zero edge along the easternmargin to nearly 200 m thick, forming a sand-rich

blanket composed of several depositional sequences.Figure 3 is an isopach map of Hollin strata from itspinchout in the eastern Oriente to the depocenter in thesouthwestern part of the basin.

In the western Oriente basin, the Hollin can be subdi-vided into the Main Hollin sandstone and the thinner

Reservoir Characterization, Hollin and Napo Formations, Oriente Basin, Ecuador 575

Figure 2Stratigraphic column for the Oriente basin.


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Upper Hollin sandstone. General Hollin stratigraphy is

described elsewhere (Wasson and Sinclair, 1927;Tschopp, 1953; Campbell, 1970; Canfield et al., 1982;Dashwood and Abbotts, 1990; Canfield, 1991). TheHollin is Albian in age, although the basal strata of theMain Hollin may date to the late Aptian. Faunal andflora taxa, although sparse, suggest that the Hollin is timetransgressive and trace the overall sea level rise duringlate Hollin and early Napo deposition. The Hollin in thewestern Oriente basin consists of five successive deposi-tional sequences: three sequences in the Main HollinSandstone and two in the Upper Hollin Formation(Figure 4).

Main Hollin SandstoneValley Fill Deposition

The initial Main Hollin sediments occupied substan-tial relief that was eroded into the underlying Jurassicstrata. Several wells penetrate this valley fill succession,including Oso #1 and Entre Rios X1. The sedimentaryrocks are interpreted to be paleovalley fluvial deposits ofchannel sandstone and flood basin shales; no cores of thisinterval have been taken. Figure 5 shows these character-istics in the Oso #1 well. This interpretation is alsosupported by Pungarayacu cores in the Napo uplift and

by isolated Hollin outcrops near Puyo. Val ley fil lthickness varies up to 40 m, reflecting the original topo-graphic relief. The resulting depositional surface was avery low relief, gently tilted surface over which braid-plain deposition occurred.

Braidplain Deposition

The dominant depositional package in the MainHollin consists of stacked cross-bedded sandstone andsubordinate intervals of interbedded mudstone andsandstone (Figures 4, 5) of Albian age. The upper part ofthis interval forms the main oil reservoirs in the westernOriente basin.

There are three lithofacies types in the braidplainsequence. The first and predominant one consists ofstacked channel sandstones that range in thickness from3 m to more than 10 m. The sandstones are quartzose incomposition and fine to very coarse grained. Granuleconglomeratic lag occasionally overlies channel scoursurfaces. Channel units generally have a uniform grainsize distribution without any obvious fining-upwardtrend. Internally, the sandstone units are structured by

576 White et al.

Figure 3Isopach map of the entire Hollin sequence.Contour interval is 25 m.

Figure 4Hollin stratigraphic column, western Orientebasin, showing the five depositional systems comprising

the complete Hollin sequence.


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planar tabular and trough cross beds in sets 30100 cm orthicker (Figure 6a). Cross bed slip faces commonlydisplay a grain size segregation typical of avalancheprocesses. Sandstone units are separated by erosionsurfaces with carbonaceous shale laminae and mudstone

beds up to 30 cm thick. Macerated plant debris oftenoccurs as concentrations along cross bed laminae. Dia-genetic kaolinite is dispersed throughout the sandstones.The resulting gamma ray signature is that of a shalysandstone rather than the high-porosity sandstones thattypify the Main Hollin.

The second facies type consists of fining-upwardchannel units of finer grained sandstones andinterbedded mudstones generally less than 30 cm thick.These sandstones are more poorly sorted than the firstsandstone facies and are dominated by trough cross

bedding.Facies three comprises mudstones and mudstone

interbedded with thin sandstones. The sandstoneinterbeds are fine to very fine grained and often ripplelaminated. The mudstones are massive to faintlylaminated and often dolomitic. This facies is up to 13 mthick (e.g., Oso #1 well, Figure 5).

These sedimentary facies are attributed to depositionin a braided alluvial plain environment. The overallfacies architecture resembles the Platte River and BijouCreek models of Miall (1977) and Cant (1982). In thisenvironment, the planar and trough cross bed sets areinterpreted as straight and sinuous-crested mid-channel

bars. River discharge may have been seasonally variable,but the sand bedload was sufficiently high to account for

the stacked braid bars that dominate the Main Hollin. Inaddition to sediment supply, local topography, precipita-tion, and vegetation also influenced fluvial deposition.Facies two, interbedded mudstone and sandstone, areattributed to channel, overbank levee, and crevasse splaydeposition. The third facies, dominantly thickmudstones, is inferred to have originated as channelabandonment intervals and laterally equivalent flood

basin deposits.

Coastal Plain Deposition

The contact between the Main and Upper Hollinformations has historically been picked on electric logs atthe base of the thicker shales overlying the stacked sand-stones of the Main Hollin. Detailed core examination ofthis contact in the western Oriente basin demonstratesthat, instead of the stacked sandstones at the top of theMain Hollin, this sequence is frequently a fining-upwardsuccession of planar to trough cross-bedded sandstonesand thin mudstones. These sandstones are slightly finerthan the braided sandstones, thin upward, and are rhyth-mically interbedded with numerous, thin, laminatedmudstones (Figure 6c). Rooted horizons are occasionallypresent in the sandstones. The distinctive appearance ofthese lithofacies (at least in cored intervals) indicatescoastal plain deposition. The package ranges up to 15 min thickness in the western Oriente.

The coastal plain depositional sequence is interpretedto represent the overall abandonment of the Main Hollinfluvial system. As such, a lower energy, higher sinuosityfluvial and estuary depositional system is envisaged forthese capping sediments. The intervals fining upwardcharacter, the occurrence of planar and trough cross

bedding and ripple lamination, the rhythmic mudstoneinterbeds, and the occasional rooting are interpreted tohave originated in a meandering stream system thatlikely entered a coastline estuary setting.

The features observed in the coastal plain depositionof the Main Hollin are similar to the meandering fluvialto estuary profiles recognized by Smith (1987). Theinterbedded sandstones and mudstones in the top part of

the estuary profile exemplify features observed in tidallyinfluenced sediments, although the tidal reworking isminor (microtidal) in the coastal plain deposits.

Upper Hollin Formation

Shore Zone Deposition

The upper Hollin transgressively overlies the coastalplain veneer of the Main Hollin sandstone. It has beengenerally interpreted as a marine deposit (Dashwoodand Abbotts, 1990; Canfield, 1991). This transgressive


Figure 5Hollin lithofacies and depositional systemswithin the Oso #1 well, Block 7, western Oriente basin. Thedepositional systems have been interpreted in more than

100 Oriente wells.


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578 White et al.

Figure 6Core photographs from the Main and Upper Hollin formations. Photographed slabs are from conventional 10-cmwhole-diameter cores. (a) Typical planar cross bedding within the braided fluvial facies. (b) Cross-bedded sandstones andthin mudstones of the coastal plain facies. (c) Interbedded sandstones and shales of the tidally reworked Upper Hollin shorezone facies. (d) Open marine glauconitic sandstones beneath the capping limestones of the Upper Hollin.

(a) (b)

(c) (d)


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blanket occurs throughout the Oriente basin, except inthe extreme northeast. It consists of two distinctive litho-facies associations. The lower shore zone depositioncomprises a sandstone and shale complex that varies upto 15 m thick in the western Oriente. The upper openmarine sequence caps the overall abandonment of theHollin depositional system.

The shore zone lithofacies consist of fine- to medium-

grained planar to trough cross-bedded sandstones, veryfine to fine-grained ripple laminated sandstones, and

burrowed lenticular-bedded mudstones. Above the basalmudstones, the shore zone locally displays a verticalprofile of stacked cross-bedded and ripple-laminatedsandstone with minor shale interbeds. A few kilometersaway, the profile may be dominantly lenticularmudstone and isolated, thin sandstones of limitedreservoir quality. The majority of the ripple sandstonescontain abundant clay drapes within the lamination. Thecoarser sandstones occasionally exhibit strongly obliquecross-bedding orientations. Lenticular mudstones aremoderately to weakly burrowed (Chondrites, Planolites,and minor Teichichnus) with rare ripple-laminated

sandstone lenses.A variety of shoreline to shallow marine depositional

environments combined to create the shore zone litho-facies. A continued transgression of the tidally influ-enced coastal plain resulted in the formation of sand-dominated bay head deltas, estuaries, and subtidalshoals. Muddy tidal flat and shallow marine mud depo-sition locally dominate the shore zone lithofacies.

Open Marine Deposition

The open marine succession completes the trangres-sive Upper Hollin depositional sequence. It may rangeup to 15 m in thickness. The lithofacies consists of glau-conitic and quartzose sandstone, limestone, marl, and

shale. Ripple-laminated, very fine to fine-grainedquartzose sandstones are commonly thin bedded andmoderately burrowed and occur at the base of the openmarine succession. In the upper part of the open marinesequence, glauconite-rich sandstones (Figure 6d) arecapped by a thin veneer of micritic and fossiliferous lime-stones and marls. The sandstones are typically very fineto fine-grained quartz with fine- to medium-grainedglauconite which vary up to 1 m in individual bedthickness and form sharp-based tabular units. Mudstonerip-up clasts occasionally overlie scour surfaces. Sedi-mentary structures include trough cross bedding, ripplelamination, and flaser bedding. Bioturbation often oblit-erates all primary sedimentary structures. This unitcontains an open marine biota, including ammonites and

both thick- and thin-shelled bivalves. In the glauconiticsandstones, the glauconite content is locally in excess of50% of the framework grains. Capping limestones (fossil-iferous wackestones) and marl beds generally measureless than 2 m thick and are well lithified. Vuggy porositydue to shell dissolution occurs sporatically.

The basal quartzitic sandstone and shale of the openmarine facies are interpreted to be of subtidal shoalorigin. These sandstones are generally thinner than theoverlying glauconitic shoal deposits and are almost

always interbedded with marine shelf shales (lenticular-bedded burrowed mudstone). The quartzitic beds origi-nated as nearshore, tidally reworked marine sands. Theglauconite-rich sands accumulated more seaward of theshoreline as storm-generated sand waves. These shoalsincorporated whatever quartz sand reached the middleshelf position, as well as the glauconite-replaced fecalmaterial derived on the shelf, and any reworked inverte-

brate shell debris. It is likely that fossil abundanceincreased away from the clastic shoreline. The shelf area

beyond the glauconitic shoals provided sites for thin butwidespread carbonate deposition.

The fossiliferous, micritic limestone and marlscapping the Upper Hollin record the final phase ofHollin deposition as the sea transgressed eastward overthe Cretaceous Oriente margin. Because of the typicalthickness of the limestones (less than 2 m), seismicamplitude contrast at the top of the Hollin is generallyminimal. The acoustic contrast does increase locallywhere limestone thickness increases.

Pungarayacu Area

The Pungarayacu concession is located on thebasinward margin of the Napo uplift. It is noted for alarge, shallow, heavy oil reservoir that has beenevaluated by Petroecuador (Almeida et al., 1983). ThePungarayacu #2 well is representative of the entireHollin section in this area west of the Oriente basin. Eight

braided channel sandstone packages are present in theMain Hollin. Significant mudstone intervals separateseveral of the upper channel sequences. Thesemudstones indicate periodic abandonment and aggrada-tion of the alluvial plain. The lower channels, above theMisahualli volcanics, are sand rich and devoid ofinterbedded mudstone. Figure 7 shows a north-south

oriented cross section through three of the Pungarayacuwells. Rapid facies changes occur within the distal orwestward fluvial components of the Main Hollin. Theabundance of mudstone suggests that the braidplaindepositional system responsible for the Main Hollin

became a mixed sand and mud system as it progradedwestward. The Upper Hollin is represented by relativelythin mud-rich beds, subordinate quartzose and glau-conitic sandstone, and capping limestone.

Hollin stratigraphy is well exposed along theHollinLoreto road in the Pungarayacu area on the Napouplift (see de Souza Cruz, 1989). A composite section ofthe Main and Upper Hollin strata exposed in the roadcutis shown in Figure 8. The Main Hollin consists princi-pally of the braidplain facies, which unconformablyoverlies the Misahualli volcanics. The braidplain succes-sion is locally saturated with oil. The outcrop showswell-developed levee and floodplain deposits (Figure 8).The lower braided channel sandstones in this section arecomparable with the Bijou Creek model. The capping

beds of the Main Hollin are correlative with the coastalplain sandstones observed in cores from the westernOriente. Shale interbeds in the coastal plain facies are

both more numerous and thicker than those observed inthe braidplain deposits.



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The Upper Hollin deposits present in this roadcut arepart of the shore zone deposition. Individual channelshave a lenticular geometry 30150 m or more wide and

13 m thick. The strata are interpreted to be of tidal flatand tidal channel origin. No exposures of the openmarine facies of the Upper Hollin were observed here,such as the glauconitic sandstones or carbonates. DeSouza Cruz (1989) interpreted the Main Hollin as origi-nating from braided fluvial and eolian paleoenviron-ments. We see little evidence for eolian deposition. Weagree with de Souza Cruz that the Upper Hollin in thisoutcrop is estuarine, although the tidal range need nothave been macrotidal.

NAPO STRATIGRAPHY AND

DEPOSITIONAL SYSTEMS

The Napo Formation consists of organic-rich shales,bioclastic grainstones and packstones, and terrigenoussandstones believed to have been deposited in fluvial,deltaic, marginal marine, and marine shelf environmentsduring the Late Cretaceous. The Napo Formationconformably overlies the Upper Hollin Formation and isin turn overlain unconformably by the Tena Formation(MaastrichtianPaleocene). Total Napo thickness exceeds275 m over much of the western Oriente basin. Figure 9

shows the lithologies and stratigraphic relationships ofthe Napo in the western Oriente basin. The Napo T, U,and M sandstone units are related to a series of regres-

sive-transgressive cycles that built the Napo stratigraphy.There were at least four such cycles in the westernOriente basin. Only the T and U intervals depositedsands in the western Oriente basin. To the east in thecentral Oriente (e.g., Shushufindi field), these sequencesare indistinguishable because of their stacked, sand-richcharacter which resulted in their amalgamation.Mapping of the Napo transgressive shales (referred to asthe lower, middle, and upper Napo shales) define anortheast-southwest Napo shoreline trend within thewestern Oriente basin.

Seismic reflection data show that Napo stratigraphyhas substantial acoustic contrasts that can be resolved,depending on data quality and signal processing. The

most conspicuous acoustic change is at shale-limestoneinterfaces. The sandstones generally have gradationalcontacts. The least resolvable acoustic contrasts occurwithin the U and T sandstones where they thinwestward and are difficult to distinguish seismically.Geophysical modeling of the U sandstone shows a subtleamplitude increase where the sandstone is welldeveloped. A marked amplitude increase also occurswhere the U sandstone is replaced laterally by limestone.Seismic models for the T sandstone indicate subtleamplitude decrease where the sandstone is well

580 White et al.

Figure 7Hollin sandstone cross section, Pungarayacu concession, eastern Napo uplift. Inferred correlations demonstratethe more frequent facies changes in the western Oriente. See Figure 1 for location.


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developed; however, the overlying B limestone is astrong reflector that tends to mask the T sandstone. Onthis basis, subtle amplitude anomalies in the westernOriente basin are attributed to the U sandstone.Amplitude versus offset (AVO) analysis of the Usandstone indicates that there is no significant offset dueto hydrocarbon-bearing lithologies.

Napo T and U Sandstones

Stacked fluvial and deltaic sandstones comprise theNapo T and U reservoirs of Shushufindi and Libratadorfields in the central Oriente (Canfield et al., 1982). Theseintervals quickly thin and become separated by thickermarine shales in Sacha and Auca fields (Canfield, 1991).From these fields westward, both the T and U sand-stones exhibit different lithofacies (and depositional envi-ronments) than in the central Oriente. Quartzose sand-stones occur in each of the western Oriente Napo cores


Figure 8Roadcut stratigraphy exposed along the Hollin-Loreto road, eastern Napo uplift. See Figure 1 for location.

Figure 9Composite log of Hollin and Napo formations,western Oriente basin.


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examined. Glauconitic sandstones, laminated toburrowed mudstones, and fossiliferous limestones arecommon components of both T and U sequences. Thequartzose sandstones exhibit the following characteris-tics: (1) abrupt basal contacts, (2) bed thicknesses from 30cm to 1 m, (3) medium to very fine grain size, (4) largescale planar and trough cross bedding to ripple lamina-tion, (5) abundant clay drapes along laminations, and (6)occasional disruption due to burrowing. The glauconiticsandstones are similar to those of the Upper Hollin and

typically occur in the upper parts of both the T and Usuccessions. Cross beds commonly occur in obliqueorientations and less often in strongly oblique, orherringbone, orientation.

Figure 10 shows a typical vertical profile through theNapo T and U sequences. A varying complex of thinfossiliferous limestones, burrowed silty mudstones, andthin quartzose sandstones form an interval less than 5 mthick that commonly occurs above the laminated shales

below both the T and U. The sandstones abruptly overliethis complex. Stacked, fine- to medium-grained, cross-

bedded sandstones dominate the lower half of the NapoT and U sandstone packages. In the upper half of theprofile, the sandstone beds are thinner, finer grained,ripple laminated, and generally glauconitic. Mudstoneinterbeds are intermittent within the sandstones.Capping the T and U intervals are more burrowedmudstone, minor sandstone (locally thicker and mediumgrained), and limestone interbeds. Laminated shales

separate the sandstone intervals from thick carbonatewackestone and mudstone (B and A limestones, respec-tively).

Situated between thick intervals of marine limestonesand mudstones, the Napo T and U sandstone packagesshow a channel-like development, locally exceeding 10m thick, that is attributable to fluvial channel, shorelineestuary, and subtidal shoal origin. Channels and shoalswithin this setting were probably controlled by the rangeof tidal energy, the shoreline configuration (embaymentsand estuaries), and the physiography of the marine shelf(de Boer et al., 1988; Terwindt, 1988). Westward-flowingstreams delivered sediment to these Napo shorelines.The initial stacked sandstones of these prograding

channels is inferred to be of fluvial point bar or deltaicorigin.

Overlying the channel sandstones, the beds exhibitreworking by tidal currents that progressively controlledsand distribution seaward of the fluvial-dominateddeposition. As in the Upper Hollin, the variety of tidalenvironments recognized includes tidal flat, tidal creek,and subaqueous tidal shoals. Dimensions of these shoalscan range up to hundreds of meters in width, hundredsof meters to several kilometers in length, and more than5 m in thickness. Positioning of these sandstone bodieswas probably influenced by paleotopographic highs onthe Napo marine shelf. Glauconitic sandstone shoals aremixed with quartzose shoals in the upper parts of the

Napo T and U sequences. The cycle of progradationdominated by channel and tidal shoal sedimentation wasreplaced vertically by mud-dominated marine conditionsfollowed by transgressive bioclastic and micriticlimestone deposition (Figure 11).

CRETACEOUS PALEOGEOGRAPHY

The four principal sandstone packages deposited inthe western part of the Oriente basin duringAptianMaastrichtian time were the Main Hollin, UpperHollin, Napo T, and Napo U intervals. The Main Hollin

is the thickest and most widespread of these intervals. Itwas deposited initially on an irregular erosional surface.The valley fill deposits smoothed this relief and createdan alluvial plain that was dominated by braided rivers.The provenance for the Main Hollin sandstones is

believed to have been the Guyana shield and itsPaleozoic cover. Grain size decreases from east to west.However, isolated outcrops of Hollin valley fill depositsin the uplifts west of Puyo contain gravel- to cobble-sizedclasts of locally derived igneous basement demonstratingthe influence of possible local source areas during early

582 White et al.

Figure 10Idealized Napo depositional package resultingfrom sedimentation following sea level drop on the Napomarine shelf.


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Figures 13 and 14 show a series of block diagrams that

summarize the paleogeography during Hollin and Napotime. The Albian braided alluvial plain was built on theedge of the Guyana shield and covered the Oriente basinfarther west than the Napo uplift. The position of theAlbian shoreline has been obliterated by Andean defor-mation. Inundation by a late Albian sea level rise estab-lished fluvial, deltaic, estuary, and tidal shoal environ-ments (Figures 13b, c, d). The delta and estuarine sandaccumulations now form excellent hydrocarbon reser-voirs in addition to the Main Hollin. Sand sedimentationrates are inferred to have been very rapid within the

Hollin depositional systems. Wells suggest that the

shoreline was close to the Guyana shield at the end ofHollin deposition. The late Albian maximum floodingevent (Lower Napo Shale) essentially closed Hollin sedi-mentation.

The Napo Formation consists of several transgressive-regressive packages related to Late Cretaceous eustaticsea level fluctuations (Figure 12) (Haq et al., 1988),including the Napo T and U (Figure 14). The successiveparasequences in the Upper Hollin and Napo formationswere deposited in a basin with a ramp margin (see VanWagoner et al., 1988). This model implies that relative sea

584 White et al.

Figure 13Hollin paleogeography in Albian time. (A) Braided alluvial plain. (B) Initial transgression during Main Hollincoastal plain deposition. (C) Upper Hollin shore zone deposition in tidally influenced nearshore environments. (D) Openmarine sedimentation ending Hollin sedimentation.


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through Coca-Payamino field and illustrates the localvariation of depositional facies interpreted for each wellalong the structure. Overall, the Coca #4 well containsthe thickest development of the coastal and shelfsequences mainly because of thicker shore zone sand-stones. A relatively thin veneer of coastal deposits ispresent in each well, except in Coca #7, where the equiv-alent interval is dominated by a braided channel. TheMain Hollin remains consistent throughout the structure.

Figure 16 shows the overall lithofacies variations

between two wells in the Gacela area immediately southof the Coca-Payamino field. In the Gacela #1 well, boththe glauconitic sandstones of the shelf and the tidal sand-stones of shore zone origin are thicker than their counter-parts in the Gacela #2 well. The shelf sandstones in theGacela #2 are not as glauconite rich as those in Gacela #1and have retained significant reservoir porosity. Finally,the coastal plain deposits in Gacela #1 appear to beabsent in Gacela #2. The coastal plain facies is believed tointerfinger with braided channel lithologies.

These two field examples suggest that the Main Hollinbraided stream sandstones are remarkably consistent incharacter across each field. The coastal plain, shore zone,and open marine units, by comparison, show significant

compartmentalization that is largely a function of deposi-tional environment. Optimum field development mustaccount for this lateral and vertical heterogeneity.

Sandstone Petrography

A representative suite of sandstone samples fromHollin and Napo facies was examined using standardpetrographic techniques. From this analysis, it isconcluded that similar sandstone framework and diage-netic characteristics occur in each of the reservoir

intervals. Two sandstone types are present: quartzosesandstones occur in each facies, while glauconitic sand-stones occur only in Upper Hollin and Napo intervals.Figure 17 shows the framework and diagenetic charac-teristics of the Hollin and Napo sandstones.

Quartzose sandstones (Figure 17a) volumetricallydominate the arenaceous deposits. Grain size variessubstantially within a single cored interval. The coarsestdetritus in cores or outcrops occurs in the braidplaindepositional system of the Main Hollin succession. In the

Tiguino #3 core, for example, the braided stream sand-stones contain beds dominated by coarse to very coarsequartz grains, as well as local quartz granule conglom-erate lag. The average grain size of the Main Hollin ismedium grained. Bimodal grain size segregation in slip-face laminae is typical of much of the cross bedding. Thewestern Oriente Pungarayacu area has the finest grainedHollin channel sandstones encountered in the Orientearea. Excellent porosity and hydrocarbon staining occurthroughout the Oriente in the fine-grained to granuletextured lithologies. Sandstones in the Upper Hollin andNapo successions also vary significantly in grain size, butgenerally within the very fine to medium-grained sizerange; they have locally excellent porosity and perme-

ability (Figure 17b).Glauconitic sandstones of the Upper Hollin shelf

facies tract and each of the Napo intervals consist of aframework of glauconite and quartz grains (Figure 17c).Glauconite content varies from trace to dominant.Typically, the glauconite grains are about 200 m largerthan associated quartz grains. Whereas the quartz in theshelf sand shoal facies was reworked from deltaic andshore zone deltas, the glauconite was locally derived bydiagenetic replacement of biogenic material. Glauconitegrains are easily compacted under moderate overburden

586 White et al.

Figure 15Stratigraphic cross section of the Hollin lithofacies in the Coca-Payamino field, western Oriente basin. Lithofa-cies have been determined from cored intervals in the field. The top of the Upper Hollin is commonly a succession of thin,fossiliferous limestones. See Figure 1 for location.


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pressure and may form a pseudomatrix that occludes theoriginal primary porosity. Where the percentage of glau-conite is less than about 20% of the sandstoneframework, the quartz-dominated framework retainsmuch of the original porosity, resulting in significantreservoir potential. In contrast, the dark green, laterallyequivalent glauconitic sandstones are tight due toframework grain compaction.

Quartz dominates the detrital framework in all sand-stones except the glauconite-rich shelf facies. The quartzis generally monocrystalline and less commonly poly-crystalline; it has a strong undulose extinction. Feldsparsand micas are subordinate to rare, but more abundant inthe Napo T and U sandstones. Feldspar compositionvaries from sodic plagioclase to potassic feldspar. Unlessencased in early calcite cementation, most survivingfeldspar grains exhibit moderate to extensive secondaryleaching. Secondary leaching during burial diagenesishelped reduce the feldspar content. The provenance is

believed to be the feldspar-rich granitic Guyanabasement to the east. However, the possibility of aquartzose Paleozoic sandstone source overlying the

basement is also possible. Other components of thesandstone include heavy minerals such as zircon andcoalified plant debris.

The burial diagenetic history of the Cretaceousreservoir sandstones reflects several processes thatoccurred in the following order:

Limited mechanical compaction of frameworkgrains

Early calcite and pyrite precipitation Dissolution of unstable framework grains

(feldspars) Precipitation of silica overgrowths Precipitation of kaolinite clay minerals

Calcite precipitation occludes the initial porosity inthin sandstone beds, especially adjacent to shaleinterbeds where it forms small, spherulitic concretions.These calcite-cemented sandstones show no evidence ofmechanical compaction, suggesting that protectivecementation occurred at an early stage. Pyrite precipita-tion in the form of concretionary cements or framboidsare characteristically associated with the organic debristrapped within the sandstones and shales. Early mechan-ical compaction is again limited to isolated grain inter-penetrations. Organic debris and pyrite crystals areconcentrated along stylolite-like surfaces.

Silica overgrowths are ubiquitous throughout the


Figure 16Stratigraphic correlation in Gacela field 5 km southwest of Coca-Payamino. Thickness and lithofacies variationsare especially noticeable in the coastal plain and open marine facies.


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588 White et al.

Figure 17Petrography of Hollin and Napo reservoir sandstones as seen in thin section photomicrographs and scanningelectron micrographs. (a) Plane light view of Main Hollin braidplain sandstone. (b) Quartzose sandstone of Napo U interval.(c) Glauconitic sandstone of the Upper Hollin open marine facies. (e) Diagenetic kaolinite occupying isolated pores in thinsection. (f) Scanning electron micrograph of secondary silica overgrowths and kaolinite clay mineral.

(a) (b)

(c)

(e) (f)

(d)


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quartz arenites of the Hollin and Napo sequence andprovide the framework support that has preservedporosity to reservoir depths in the Oriente basin.Although the overgrowths (Figure 17d) make up only asmall percentage of the sandstones, they strengthen thehighly porous sandstones while only slightly reducingoverall primary porosity. Mechanical testing of thesesandstones documents the high compressive strength

required to break the strong silica-cemented framework.The amount of porosity attributable to framework-graindissolution is not significant compared to the primaryintergranular porosity preserved by silica overgrowth.

Precipitation of kaolinite clay minerals followed over-growth formation. The kaolinite typically fills smallclusters of pores, but does not seriously affect sandstonepermeability (Figure 17e). Kaolinite diagenesissucceeded silica overgrowth formation (Figure 17f), butpreceded oil emplacement. Such relationships arecommon and invariably associated with the oilwatercontact where differentially stained sandstones mayoccur below the base of the oil-saturated sandstones.

Petrophysical Characteristics

Electric Log Response

The stratigraphic and sedimentologic characterizationof the Napo and Hollin reservoirs has been facilitated byusing core studies combined with electrical log evalua-tions. Many of the mineralogic characteristics observedin cores have a petrophysical log response. Carbona-ceous debris on cross bed slip faces induces a strongershaly gamma ray response than would be expected fromcore examination. A clean gamma ray deflection istypical of a clean sandstone, but a higher gammaresponse may indicate relatively clean sandstones conta-

minated with carbonaceous laminae, shaly sandstone, orcarbonaceous limestone or marl. Glauconite and pyritereduce the resistivity. The glauconite-rich sandstonesresult in some of the lowest resistivity responses onobserved logs. Pyrite is locally abundant as a dissemi-nated replacement fabric or as concretions in all litholo-gies. Dolomitic shales tend to have higher resistivity thannondolomitic shales due to carbonate cementation ofpore space. These shales are the most resistive clasticlithofacies in the Oriente basin.

Fluid chemistry is also reflected in log response, andits effects limit the usefulness of resistivity or SP curvesfor facies correlation. Low salinities within the MainHollin succession limit the reliability of the SP curve and

also moderately affect the resistivity curve. The presenceof oil is noticeable regardless of lithology.

PorosityPermeability Relationships, PoreGeometry, and Capillarity

Multiple rock types occur in the Hollin formationbecause of variations in depositional environment. Themost important factors affecting porosity preservationare lithology, compaction, and diagenesis. Porosity andpermeability generally correlate in the Upper Hollindespite significant mineralogic differences throughout

this interval. In the quartz-dominated Main Hollin,sediment texture is the primary factor controlling poregeometry and connectivity.

Figure 18a shows the porositypermeability data forthe entire Upper Hollin in a single Coca-Payamino well.Permeability ranges over six orders of magnitude, andno distinct trends are discernible in the overall data set.The poorest permeabilities are associated with glau-

conitic sandstones and clay-rich interbeds. Figures 18band 18c illustrate the wide range of measured porosityand permeability in this highly heterogeneousformation. Quartz-rich zones are of high reservoirquality and comparable to those found in the MainHollin. Median values for porosity and permeability are8.6% and 1.67 md, respectively. A histogram of graindensity (Figure 18d) further demonstrates the diversityof minerals present in this interval.

Mercury injection extended range capillary pressuredata were generated to examine reservoir rock quality,determine size and sorting of pore throats, and evaluateseal capacity. Shales within the Upper Hollin (Figure 19)are microporous and considered to be effective seals.

Because of inhibiting diagenetic effects, glauconitic sand-stones have bimodal pore throat size distributions andcomplex pore geometries (Figure 20). Further reductionin reservoir quality can result from extensive diageneticpyrite and the abundance of detrital clay drapes andcoalified plant debris.

Figure 21a shows the porosity-permeability data forthe Main Hollin Formation in a single Coca-Payaminowell. A clear cluster of data in the 1520% porosity rangeand greater than 100 md permeability demonstratesexcellent reservoir quality. The Main Hollin is a cleanuniform sandstone, although thin, impermeable clay-richinterbeds are not uncommon. Figures 21b, c, and d illus-trate the quartz-dominated nature of the Main Hollin.

Median values for porosity and permeability are 18.6%and 1013 md, respectively. Mercury injection data(Figure 22) show unimodal well-sorted and well-connected pores, further substantiating high reservoirquality. Most pore throat radii are larger than 1 m, withmost pores greater than 10 m in width.

Anisotropy within the Main Hollin causing direc-tional preferences in permeability is minimal. Horizontaland vertical permeabilities were measured on full-diameter core to determine the potential for reservoirfluid coning. In the quartz-rich zones of the Main Hollin,horizontal and vertical permeabilities are almost equal(Figure 23). This indicates that cross bedding and othersedimentologic features do not create anisotropy in this

sand body.

Rock Mechanics

Uniaxial and triaxial compression testing wasperformed on four lithologies from the Hollin formation:shale, limestone, glauconitic sandstone, and quartzosesandstone. These data were critical in the assessment of

borehole stability and other engineering evaluationsuseful for horizontal drilling parameters (Ramirez andRodas, 1992). Mohr-Coulomb failure criteria were estab-lished under triaxial load on four samples for each



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lithology. Compressive strengths were measured at952025,170 psi for shale, 16,70028,040 psi for limestone,837027,550 psi for low-percentage glauconiticsandstone, and 510016,870 psi for clean sandstone(Figure 24). Tensile strengths ranged from 1760 psi forshale to 660 psi for clean sandstone.

WettabilityBoth the Upper and Main Hollin demonstrate inter-

mediate to oil-wetting tendencies based on qualitativeand quantitative indicators. Localized development ofmixed wettability or preferentially oil-wet characteristicscan be mineralogy specific, that is, glauconite-rich rockstend to show stronger oil-wet conditions. Complex poregeometries formed by small, irregular pore throats leadto high immobile saturation of the wetting phase. Irre-ducible water saturation tends to be low, with an averageof 15%, and residual oil saturation ranges from 25 to 40%

based on fresh state water-oil relative permeabilitymeasurements. Wettability indices in the Upper Hollinsupport the theory of intermediate to slightly oil-wetconditions. Asphaltinic oils (up to 15.2% asphaltene byweight) are common near the oilwater contact. Hollinwetting tendencies could have significant impact onproduction (fluid flow characteristics) and reservoir

development scenarios, such as water flood potential.

CONCLUSIONS

Core descriptions have shown that four depositionalsystems comprise Hollin stratigraphy: braidplain andcoastal deposits of the Main Hollin Sandstone, and shorezone and open marine shelf facies in the Upper HollinFormation. This reconstruction enlarges on previousinterpretations of marine-influenced Hollin fluvial depo-

590 White et al.

Figure 18Porositypermeability relationships of the Upper Hollin Formation. (a) Porosity versus permeability (to nitrogenat an estimated net effective reservoir pressure of 2250 psi). (b) Permeability histogram of all lithofacies of the Upper Hollin.(c) Porosity histogram of all lithologies. (d) Grain density histogram for the Upper Hollin.


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592 White et al.

Figure 20Petrophysical properties of Upper Hollin glauconitic sandstones. (a) Mercury saturation versus injectionpressure. (b) Pore size distribution.


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sition. Sandstones in overlying Napo strata in thewestern Oriente basin are also divided into twosequences (T and U intervals).

The Hollin braidplain depositional system is asandstone-dominated unit that comprises most of theHollin succession. It is also the most prolific reservoirzone in the western part of the basin. The braided fluvialsandstone units have excellent continuity and connec-tivity, as shown by analysis of closely spaced wells.However, shale interbeds and thicker channel abandon-ment mudstones adversely influence local permeability.It is believed that the braidplain deposits are mostproductive in structural traps where there is limitedstratigraphic trapping potential.

The coastal plain depositional system consists ofbraided and meandering river sediments, overbankfloodplain strata, and deltaic-estuarine deposits. Even

between closely spaced wells, sandstoneshale ratiosmay be variable. Similarly, the overlying shore zone

depositional system of the Upper Hollin succession hasvariable sandstone distribution, with local good qualityreservoir development. The capping open marine sand-stones are moderately prospective, especially whereglauconite content is low. Stratigraphic trappingpotential is implied by the heterogeneity of these litho-facies.

Fluviodeltaic Napo sandstones are prolific producersof oil from fields in the central part of the Oriente basin.These stacked channel and shore zone sandstones havereservoir characteristics similar to the underlying Hollinfluvial sandstone reservoirs, albeit with local hetero-geneities. Toward the west, the Napo sandstones occupyvalley-like, topographic lows; these sandstones havelocally significant reservoir potential.

A better understanding of the Hollin and Napostratigraphy and distribution of reservoir quality sand-stones will help to optimize wellbore placement duringfield development. This understanding has been further


Figure 21Porositypermeability relationships of the Main Hollin quartzose sandstones. (a) Porosity versus permeability (tonitrogen at an estimated net effective reservoir pressure of 2250 psi). (b) Permeability histogram of quartzose sandstone. (c)Porosity histogram for sandstones. (d) Grain density histogram for the principal reservoir sandstones.


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Figure 19Petrophysical properties of Upper Hollin shales. (a) Mercury saturation versus injection pressure. (b) Pore sizedistribution of reservoir seals in the Upper Hollin.


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594 White et al.

Figure 22Petrophysical relationships of the Main Hollin sandstones. (a) Mercury saturation versus injection pressure. (b)Pore size distribution from mercury injection data.


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enhanced by detailed petrophysical analysis of thereservoir sandstones, which has provided the appro-priate data for accurate reservoir simulation. The Oriente

basin of Ecuador is a proven oil province that hastremendous potential for future production.

Acknowledgments The authors would like to thank theDireccion Nacional de Hidrocarburos (DNH) and Petroe-cuador for permission to publish this paper and for theirinvaluable assistance in making Hollin and Napo coresavailable. The core examination in Ecuador (Quito and Lago

Agrio) was undertaken by the principal author, Ed Robbs andFelix Ramirez (Oryx Energy, Dallas), and Mariana Lascano(Petroecuador). Acknowledgment is given for their assistancein collecting the initial core data for the project. Harold Illich(Oryx Energy, Dallas) contributed substantially to the Hollinoutcrop study and our understanding of the Oriente basinburial history. Further acknowledgment is given to Tim

Martin (Oryx Energy, Dallas), Cl iff Thomson (OryxEcuador), Oryx Energy (Dallas), and our partners for permis-sion to publish this paper, and to the Oryx Graphic group for

preparation of the illustrations.

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Balkwill, H. R., G. Rodrigue, F. I. Paredes, and J. P. Almeida,1995, Northern part of Oriente basin, Ecuador: reflectionseismic expression of structures, in A. J. Tankard, R.

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N. H. Foster and E. A Beaumont, eds., AAPG Treatise ofPetroleum Geology, Atlas of Oil and Gas Fields, StructuralTraps V, p. 285305.

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de Boer, P. L., A. van Gelder, and S. D. Nio, eds., 1988, Tide-influenced sedimentary environments and facies:Dordrecht, The Netherlands, D. Reidel, 530 p.

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Figure 23Full diameter core permeability measurements(horizontal and vertical) for the Main Hollin sandstones.


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Authors Mailing Addresses

Howard J. WhiteFelix A. RamirezOryx Energy Company13155 Noel RoadDallas, Texas 75240-5067U.S.A.

Robert A. SkopecDepartment of Petroleum GeologyUniversity of AberdeenAberdeen AB9 2UEScotland

Jose A. Rodas

Oryx Ecuador Energy CompanyAvenue de AmazonasQuitoEcuador

Guido BonillaPetroecuador

J. Leon M y Av. OrellanaQuitoEcuador

596 White et al.

Figure 24Mohr-Coulomb failure criteria for the Main and Upper Hollin lithologies. (a) Upper Hollin shale. (b) Upper Hollinlimestone. (c) Glauconitic sandstone (high quartz content, Upper Hollin). (d) Main Hollin reservoir sandstone.

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Hollin y Napo Formaciones