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Reservoir Characterization of the Hollín and Napo Formations, Westem Oriente Basin, Ecuador Howard J. White

Jose A. Rodas

Robert A. Skopec

Onp: Ecuador Eller¿,~{ Companu Quito, Ecuador

Felix A. Ramirez

Guido Bonilla

Oryx Ellergy Company DalIas, Texas, U.5.A.

Petroecuador Quito, Ecuador

Abstract r-rhe Oriente basin of Ecuador has produced a substantial amount of oil over the past 20 years. Nearly .1 3 billion bbl of oil have been recovered from the principal reservoirs in the Cretaceous Napo and Hollin formations. Subtle north-south structures, commonly associated with Andean-related faulting, have trapped much of the recoverable hydrocarbons in the thicker sandstones deposited within the Hollin and Napa reservoirs. East to west thinning of these reservoir units also contributes to the formation of stratigraphic traps. Both the Hollin and Napo formations comprise successíons of eastward-sourced fluvial and deltaíc sedimentary deposíts that prograded westward into shoreline and marine shelf parasequences. The Albían Hollin reservoir interval consists of a dominant alluvial plain sandstone sequence (Maín Hollín sandstone) that occupíes much of the Oriente basin. In the westem Oriente, the uppermost Hollin section grades vertically into open marine strata with isolated tidal- and storm-influenced sandstone bodies. The overlying Napo stratigraphy also consists of sand-rich fluvial and deltaic deposits in the eastem Oriente and abruptly changes to marine shales and limestones and lowstand valley-fill sandstones in the westem part of the basin. Extensíve structural and stratigraphic trap potential remains within the Napo and Hollín strata in the Oriente basin. High-resolution geophysical techníques and detailed geologic reservoir characterization facilitate successful exploitation of these remaining reserves.

Resumen

E

n los últimos veinte años la Cuenca Oriente del Ecuador ha producido una cantidad sustancial de hidrocarburos. Alrededor de tres mil millones de barriles de petroleo han sido recuperados de los reservarios principales de las formaciones cretácicas Hollín y Napo. Estructuras sutiles orientadas norte-sur, comunmente asociadas con fallamiento de edad Andina, han entrampado la mayoría de los hidrocarburos recuperables dentro de los espesos depósitos arenosos de los reservorios de Napo y Hollin. La formación de trampas estratigraficas ha estado favorecida por los adelagazamientos este-oeste de dichas unidades reservorios. Las formaciones Napo y Hollín comprenden una sucesión de sedimentos deltaicos y fluviales alimentados desde el este, los cuales progradaron hacia el oeste integrando parasecuencias de zonas de playa y marino-plataformicas, El reservorio Albense Hollin consiste de una secuencia predomínantemente arenosa de planicie aluvial (Arenisca Hollín Principal) la cual se encuentra ocupando la mayoría de la Cuenca Oriente. En el occidente del Oriente, la sección superior de Hollín grada verticalmente a sedimentos marino-abiertos con cuerpos arenosos influenciados por mareas y tormentas. La sobreyacente estratigrafía de Napo tambíen consiste, en el este del Oriente, de depósitos deltaicos y fluviales ricos en arena, los cuales cambian abruptamente 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 Hollín de la Cuenca Oriente. Las técnicas geofísicas de alta resolución y la caracterización geologica de los reservoríos facilitaran una explotación exitosa de las reservas remanentes.

Whíte, H. J., R. A. Skopec, F. A. Rarnirez, J. A. Rodas, and G. Bonilla, 1995, Reservoir characteristics of the Hollin and Napo formations, westem Oriente basín, Ecuador, in A. J. Tankard, R. Suárez S., and H. J. Welsink, Petroleum basins of South Arnerica: AAPG Memoir 62, p. 573-596.

573

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Figure 12-Correlation of Napo stratigraphy with global sea level change for the Early-Late Cretaceous. The T and U sandstone packages correspond to the significant sea levellows during the Cenomanian and Turonian, respectively.

formation of the early Andean orogenic belt. A passive margin shelf apparently received the Hollín and Napa sedirnentation. Limited exposures of phosphatic shales and cherts in the northwestem Napa uplift suggest the existence of a shelf slope break and Late Cretaceous upwelling prior to its destruction during Andean deformation.

584

vVhite et al.

Upper Hollín Paleogeography Late Albian (O) Dpen Marine

(C) Shorezone

Main Hollln Paleogeography Early Albian (A) Braid Plain

(8) coastat Plain

Figure 13-Hollin paleogeography in Albian time. (A) Braided alluvial plain. (8) Initial transgression during Main Hollin coastal plain deposition. (C) Upper Hollín shore zone deposition in tidally influenced nearshore environments. (O) Open marine sedimentation ending Hollin sedimentation.

Figures 13 and 14 show a series of block diagrams that summarize the paleogeography during Hollín and Napo time. The Albian braided alluvial plain was built on the edge of the Guyana shield and eovered the Oriente basin farther west than the Napo uplift. The position of the Albian shoreline has been obliterated by Andean deformation. Inundation by a late Albian sea level rise established fluvial, deltaic, estuary, and tidal shoal environments (Figures 13b, e, d). The delta and estuarine sand aecumulations now forro exeellent hydroearbon reservoirs in addition to the Main Hollin. Sand sedimentation rates 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 of Hollin deposition. The late Albian maximum flooding event (Lower Napo Shale) essentiaily closed Hollin sedimentation. The Napo Formation eonsists of severa! transgressiveregressive paekages related to Late Cretaeeous eustatie sea level fluctuations (Figure 12) (Haq et al., 1988), including the Napo T and U (Figure 14). The sueeessive parasequenees in the Upper Hollín and Napo formations were deposited in a basin with a ramp margin (see Van Wagoner et al., 1988). This model implies that relative sea

Reservoir Characterízation, Hollin and Napo Formaiions, Oriente Basin, Ecuador

Middle Turonian

585

Late Turonian

(e) Middle Napo

(A) Lower Napa Figure 14-Napo paleogeography during Cenomanian-Turonian time. (A) Early Cenomanian marine shelf deposition dornlnating much of the Oriente. (8) Napo T sandstone deposition in the western Oriente. (e) Transgressive marine mud deposition (Middle Napo Shale). (O)Turonian marine shelf sedimentation prior to sea levellowering and deposition of the Napo U sandstone cycle.

level did not fall below the shelf break, which precludes lowstand sediments within lowstand fan or prograding wedge settings. The Hollin and Napo shore zone to shelf facies tract transgressed and regressed several times. The quartzose sandstones of the Upper Hollin, T, U, and M sequences were deposited after maximum sea level fall and within depocenters (eroded valleys) created during falling sea level. Eventually both the T and U were inundated and covered with limestone shoals and shelf muds during the subsequent sea level rise. The ramp margin model permits major shifts of the shoreline, especially where the rate of sedimentation exceeded the rate oí subsidence.

RESERVOIR CHARACTERIZATION Reservoir-Scale Heterogeneity of the Hollin Lithofacies The Coca-Payarnino and Gacela fields in the northwestern Oriente basin have sufficient well density to allow detailed stratigraphic correlation. Well spacings range up to 3 km in the Gacela field and average about 1 km along the Coca-Payamino structure. The NNW-SSE orientation of the Coca-Payamino is nearly orthogonal to the east-west depositional pattern interpreted for the Hollin strata. Figure 15 is a simplified cross section

586

W1zite et al.

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Figure 15-Stratigraphie eross section of the Hollin lithofacies in the Coca-Payamino .fi~ld, western Oriente bas!n. litho~a­ cíes have been determinad from eored intervals in the fleld, The top 01the Upper Hollín IS commonly a succession of thín, fossiliferous limestones. See Figure 1 for Joeation.

through Coca-Paya mino field and illustrates the local variation of depositional facies interpreted for each well along the structure. Overall, the Coca #4 well contains the thickest development of the coastal and shelf sequences mainly because of thicker shore zone sandstones. A relatively thin veneer of coastal deposits is present in each well, except in Coca #7, where the equivalent interval is dominated bv a braided channel. The Main Hollín remains consistent throughout the structure. Figure 16 shows the overalllithofacies variations between two wells in the Gacela area immediately south of the Coca-Payamino field. In the Gacela #1 well, both the glauconitic sandstones of the shelf and the tidal sandstones of shore zone origin are thicker than their counterparts in the Gacela #2 welL The shelf sandstones in the Gacela #2 are not as glauconite rich as those in Gacela #1 and have retained significant reservoir porosity. Finally, the coastal plain deposits in Gacela #1 appear to be absent in Gacela #2. The coastal plain facies is believed to interfinger with braided channellithologies. These two field examples suggest that the Main Hollin braided stream sandstones are remarkably consistent in character across each field. The coastal plain, shore zone, and open marine units, by comparison, show significant compartmentalization that is largely a function oí depositional environment, Optimum field development must account for this lateral and vertical heterogeneity.

Sandstone Petrography A representative suite of sandstone samples frorn Hollin and Napa facies was examined using standard petrographic techniques. From this analysis, it is concluded that similar sandstone framework and diagenetic characteristics occur in each of the reservoir

intervals. Two sandstone types are present: quartzose sandstones occur in each facies, while glauconitic sandstones occur only in Upper Hollin and Napo intervals. Figure 17 shows the framework and diagenetic characteristics of the Hollin and Napo sandstones. Quartzose sandstones (Figure 17a) volumetrically dominate the arenaceous deposits. Grain size varies substantial1y within a single cored intervalo The coarsest detritus in cores or outcrops occurs in the braidplain depositional system of the Main Hollin succession. In the Tiguino #3 core, for example, the braided stream sandstones contain beds dominated by coarse to very coarse quartz grains, as well as local quartz granule conglomerate lag. The average grain size of the Main Hollin is medium grained. Bímodal grain size segregation in slipface laminae is typical of much of the cross bedding. The westem Oriente Pungarayacu area has the finest grained Hollin channel sandstones encountered in the Oriente area. Excellent porosity and hydrocarbon staining occur throughout the Oriente in the fine-grained to granule textured lithologies. Sandstones in the Upper Hollin and Napo suceessions also vary signifieantly in grain size, but generally within the very fine to medium-grained size range; they have locally excel1ent porosity and permeability (Figure 17b). Glaueonitic sandstones of the Upper Hollin shelf facies tract and each of the Napo intervals consist oí a framework of glaueonite and quartz grains (Figure 17c). Glauconite content varíes from trace to dominant, Typically, the glauconite grains are about 200 um larger than associated quartz grains. Whereas the quartz in the shelf sand shoal facies was reworked from deltaie and shore zone deltas, the glauconite was locally derived by diagenetie replaeement of biogenie material. Glauconite grains are easily compacted under moderate overburden

Reserooir Characterizatum, Hollin andNapa Formations, Oriente Basin, Ecuador

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

pressure and may form a pseudomatrix that occludes the original primary porosity. Where the percentage of glauconite is less than about 20% of the sandstone framework, the quartz-dominated framework retains much of the original porosity, resulting in significant reservoir potential. In contrast, the dark green laterally equivalent glauconitic sandstones are tight due to framework grain compaction. Quartz dominates the detrital framework in al1 sandstones except the glauconite-rich shelf facies. The quartz is generally monocrystalline and less cornrnonly polycrystalline; it has a strong undulose extinction. Feldspars and micas are subordinate to rare, but more abundant in the Napo T and U sandstones. Feldspar composition varíes from sodic plagiodase to potassic feldspar. Unless encased in early calcite cementation, most surviving feldspar grains exhibit moderate to extensive secondary leaching. Secondary leaching during burial diagenesis helped reduce the feldspar contento The provenance is believed to be the feldspar-rích granitic Guyana basement to the east. However, the possibility of a quartzose Paleozoic sandstone source overlying the basement is also possible. Other components of the sandstone include heavy rninerals such as zircon and coalified plant debris. f

The burial diagenetic history of the Cretaceous reservoir sandstones reflects several processes that occurred in the following order: • Limited mechanical compaction of framework grains • 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 in thin sandstone beds, especially adjacent to shale interbeds where it forms small, spherulitic concretions. These calcite-cemented sandstones show no evidence of mechanical compactíon, suggesting that protective cementation occurred at an ear1y stage. Pyrite precipitation in the forro of concretionary cements or framboids are characteristically associated with the organic debris trapped within the sandstones and shales. Early mechanical compaction is again limited to isolated grain ínterpenetrations. Organic debris and pyrite crystals are concentrated along stylolite-like surfaces. Silica overgrowths are ubiquitous throughout the

588

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et al.

Figure 17-Petrography 01 Hollin and Napo reservoir sandstones as seen in thin section photomicrographs and scanning electron micrographs. (a) Plane Iight view 01 Main Hollin braidplain sandstone. (b) Quartzose sandstone 01 Napo U interval. (c) Glauconitic sandstone of the Upper Hollin open marine facies. (e) Diagenetic kaolinite occupying isolated pores in thin section. (1) Scanning electron micrograph 01 secondary silica overgrowths and kaolinite clay mineral.

Reserooir Characterizaiion, Hollin and Napo Formations, Oriente Basin, Ecuador quartz arenites of the Hollin and Napo sequence and provide the framework support that has preserved porosity to reservoir depths in the Oriente basin. Although the overgrowths (Figure 17d) make up only a small percentage of the sandstones, they strengthen the highly porous sandstones while only slightly reducing overall primary porosity. Mechanical testing of these sandstones documents the high compressive strength required to break the strong silica-cemented framework. The amount of porosity attributable to framework-grain dissolution is not significant compared to the primary intergranular porosity preserved by silica overgrowth. Precipitation of kaolinite day minerals followed overgrowth formation. The kaolinite typical1y fil1s smal1 c1usters of pores, but does not seriously affect sandstone permeability (Figure 17e). Kaolinite diagenesis succeeded silica overgrowth formation (Figure 17f), but preceded oil emplacement. Such relationships are common and invariably associated with the oil-water contact where differentially stained sandstones may occur below the base of the oil-saturated sandstones.

Petrophysical Characteristics Electric Log Response The stratigraphic and sedimentologic characterization of the Napo and Hollin reservoirs has been facilitated by using core studies combined with electrical log evalúations. Many of the mineralogic characteristics observed in cores have a petrophysicallog response. Carbonaceous debris on cross bed slip faces induces a stronger shaly gamma ray response than would be expected from core examination. A clean gamma ray deflection is typical of a clean sandstone, but a higher gamma response may indicate relatively c1ean sandstones contaminated with carbonaceous laminae, shaly sandstone, or carbonaceous limestone or marl. Glauconite and pyrite reduce the resistivity. The glauconite-rich sandstones result in sorne of the lowest resistivity responses on observed logs. Pyrite is locally abundant as a disseminated replacement fabric or as concretions in all lithologies. Dolomitic shales tend to have higher resistivity than nondolomitic shales due to carbonate cementation of pore space. These shales are the most resistive clastic lithofacies in the Oriente basin. Fluid chemistry is also reflected in log response, and its effects limit the usefulness of resistivity or SP curves for facies correlation. Low salinities within the Main Hollin succession limit the reliability of the SP curve and also moderately affect the resistivity curve. The presence of oil is noticeable regardless of lithology. Porosity-Permeability Relationships, Pore Geometry, and Capillarity Multiple rock types occur in the Hollin formation because of variations in deposítional environment. The most important factors affecting porosity preservation are lithology, compaction, and diagenesis. Porosity and permeability generally correlate in the Upper Hollín despite sígnificant mineralogic differences throughout

589

this interval. In the quartz-dominated Main Hollín, sediment texture is the primary factor controlling pore geometry and connectivity. Figure 18a shows the porosity-permeability data for the entire Upper Hollín in a single Coca-Payamino well. Permeability ranges over SLX orders of magnitude, and no distinct trends are discernible in the overall data set. The poorest permeabilities are associated with glauconitic sandstones and clay-rich interbeds. Figures 18b and 18c ilIustrate the wide range of measured porosity and permeability in this highly heterogeneous Iorrnation. Quartz-rich zones are of high reservoir quality and comparable to those found in the Main Hollin. Median values for porosity and permeability are 8.6% and 1.67 md, respectively. A histogram of grain density (Figure 18d) further demonstrates the diversity of minerals present in this interval, Mercury injection extended range capilIary pressure data were generated to examine reservoir rock quality, determine size and sorting of pore throats, and evaluate seal capacity. Shales within the Upper Hollín (Figure 19) are microporous and considered to be effective seals. Because of inhíbiting diagenetic effects, glauconitic sandstones have bimodal pore throat size distributions and complex pore geometries (Figure 20). Further reduction in reservoir quality can result from extensive diagenetic pyrite and the abundance of detrital cIay drapes and coalified plant debris. Figure 21a shows the porosity-permeability data for the Main Hollín Formation in a single Coca-Payamino well. A clear cluster of data in the 15-20% porosity range and greater than 100 rnd permeability demonstrates excellent reservoir quality. The Main Hollin is a "clean" uniform sandstone, although thin, impermeable c1ay-rich interbeds are not uncommon. Figures 21b, e, and d illustrate the guartz-dominated nature of the Main Hollin. Medían values for porosity and permeability are 18.6% and 1013 md, respectively. Mercury injection data (Figure 22) show unimodal well-sorted and wellconnected pores, further substantíating hígh reservoir guality. Most pore throat radii are larger than 1 um, with most pores greater than 10 um in width. Anisotropy within the Maín Hollin causing directional preferences in permeability is mínimal. Horizontal and vertical permeabilities were rneasured on fulldiameter core to determine the potential for reservoír fluid coning. In the quartz-rich zones of the Main Hollin, horizontal and vertical permeabilities are almost egual (Figure 23). This indicates that cross bedding and other sedimentologíc features do not create anisotropy in this sand body. Rock Mechanics Uniaxial and triaxial compression testing was performed on four lithologiesfrom the Hollin formation: shale, limestone, glauconític sandstone, and quartzose sandstone. These data were critical in the assessment of borehole stability and other engineering evaluations useful for horizontal drilling parameters (Ramirez and Rodas, 1992). Mohr-Coulomb failure criterla were established under triaxial load on four samples for ea eh

590

W1zíte et al. Permeability Histogram _

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Figure 18-Porosity-permeability relationships 01 the Upper Hollin Forrnation. (a) Porosity versus perrneability (to nitrogen at an estirnated net ef1ective reservoir pressure 012250 psi). (b) Permeability histogram 01 alllitholaeies 01 the Upper Hollin. (e) Porosity histogram 01 alllithologies. (d) Grain density histogram for the Upper Hollin.

lithology. Compressive strengths were measured at 9520-25)70 psi for shale, 16,700-28,040 psi for limestone, 8370-27,550 psi for low-percentage glauconitic sandstone, and 5100-16,870 psi for clean sandstone (Figure 24). Tensile strengths ranged from 1760 psi for shale to 660 psi for c1eansandstone.

Wettability Both the Upper and Main Hollin demonstrate intermediate to oil-wetting tendencies based on qualitative and quantitative índicators, Localized development of mixed wettability or preferentially oil-wet characteristics can be mineralogy specific, that is, glauconite-rich rocks tend to show stronger oil-wet conditions. Complex pore geometries formed by small, irregular pore throats lead to high immobile saturation of the wetting phase. Irreducible water saturation tends to be low, with an average of 15%, and residual oil saturation ranges from 25 to 40%

based on "fresh state" water-oil relative permeability measurements. Wettability índices in the Upper Hollin support the theory of intermediate to slightly oil-wet conditions. Asphaltinic oils (up to 15.2% asphaltene by weight) are cornmon near the oil-water contacto Hollin wetting tendencies could have significant impact on production (fluid flow characteristics) and reservoir development scenarios, such as water flood potential.

CONCLUSIONS Core descriptions have shown that four depositional systems comprise Hollin stratigraphy: braidplain and coastal deposits of the Main Hollin Sandstone, and shore zone and open marine shelf facies in the Upper Hollin Formation. This reconstruction enlarges on previous interpretations of marine-influenced Hollin fluvial depo-

Reserooir Characteriauion. Hollin andNapo Formaiions, Oriente Basin, Ecuador

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sition. Sandstones in overlying Napo strata in the western Oriente basin are al so divided into two sequences (T and U intervals). The Hollin braidplain depositional system is a sandstone-dominated unit that comprises most of the Hollin succession. It is also themost prolific reservoir zone in the western part of the basin. The braided fluvial sandstone units have excellent continuity and connectivity, as shown by analysis of closely spaced wells. However, shale interbeds and thicker channel abandonment mudstones adversely influence local perrneability. It is believed that the braidplain deposits are most productive in structural traps where there is limited stratigraphic trapping potential. The coastal plain depositional system consists of braided and meandering river sediments, overbank floodplain strata, and deltaic-estuarine deposits. Even between closely spaced wells, sandstone-shale ratios may be variable. Similarly, the overlying shore zone

depositional system of the Upper Hollín succession has variable sandstone distribution, with local good quality reservoir development. The capping open marinesandstones are moderately prospective, especial1y where glauconite content is low. Stratigraphic trapping potential is implied by the heterogeneity of these lithofacies. Fluviodeltaic Napo sandstones are prolific producers of oil from fields in the central part of the Oriente basin. These stacked channel and shore zone sandstones have reservoir characteristics similar to the underlying Hollín fluvial sandstone reservoirs, albeit with local heterogeneities. Toward the west, the Napo sandstones occupy valley-like, topographic lows; these sandstones have locally significant reservoir potential. A better understanding of the Hollin and Napo stratigraphy and distribution of reservoir quality sandstones will help to optimize wellbore placement during field development. This understanding has been further

594

Wllite ei al.

(a)

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enhanced by detailed petrophysical analysis of the reservoir sandstones, which has provided the appropriate data for accurate reservoir simulation. The Oriente basin of Ecuador is a proven oil province tha t has tremendous potential for future production.

Aeknowledgments The authors ioould like to thank ihe Direccion Nacional de Hidrocarburos (DNH) and Peiroecuador for permission to publish this paper and for iheir inualuable assistance in making Hollin and Napo eores available. The core examinaiion in Ecuador (Quito and Lago Agrio) was undertaken by the principal author, Ed Robbs and Felix Ramirez (Oryx Energy, Dalias), and Mariana Lascano (Petroecuador). Acknowledgment is given for their assistanee in collecting the iniiial eore data for the projeet. Harold Illich (OnJx Energy, Dalias) contriouted substantially to the Hollin outcrop study and our understanding of the Oriente basin burial history. Furiher acknowledgment is given to Tim Martín (Oryx Energy, Daüae), Clíff Thomson (Oryx Ecuador), Oryx Energy (Dallas), and our partners for permission to publish this paper, and to the Oryx Graphie group for preparation of theillustrations.

REFERENCES CITED Almeida, J. P., R. Campania, M. Rivadeneira, F. A. Ramirez, H. Poveda, H. Gutierrez, e Cordero, and S. Guevara, 1983, El campo de crudos Pesados Pungarayacu: Paper presented at the Congreso Ecuatoriano de Ceologia, Guayaquil/ Ecuador. Balkwill, H. R, G. Rodrigue, F. 1. Paredes/ and J. P. Almeida, 1995/ N orthern part of Oriente basin, Ecuador: reflection seismic expression of structures, in A. J. Tankard, R

595

Suarez, and H. J. Welsink, Petroleurn basins of South America: AAPG Memoir 62, this volurne. Bluck, B. J., 1974, Structure and directional properties of sorne valley sandur deposits in southem Iceland: Sedirnentology, v. 21, p. 533-554. Boothroyd, J. C, 1972, Coarse-grained sedimentation on a braided outwash fan, northeast Culf of Alaska: Coastal Research División, Universitv of South Carolina Technical Report No. 6, 127 p. . Brown, L. J., and D. D. Wilson, 1988, Stratigraphy of the late Quatemary deposits of the northern Canterbury plains, New Zealand: New Zealand [ournal of Geology, v, 31, p.305-335. Campbell, C. J., 1970, Guide to the Puerto Napo area, eastem Ecuador, with notes on the regional geology of the Oriente basin: Ecuador Society of Geology and Geophysics, 40 p. Canfield, R. Vv., 1991, Sacha field, Ecuador: Oriente basin, in N. H. Foster and E. A Beaumont, eds., AAPC Treatise of Petroleum Geology, Atlas of Oil and Gas Fields, Structural Traps V, p. 285-305. Canfield, R. ve.. G. Bonilla, and R. K. Robbins, 1982, Sacha oil field of Ecuadorian Oriente: AAPG Bulletin, v. 66, p.1076-1090. Cant, D. J., 1982, Fluvial facies models and their application, in P. A. Scholle and D. Spearing, eds., Sandstone depositional environments: AAPG Memoir 31, p. 115-137. Dashwood, M. F., and I. L. Abbotts, 1990, Aspects of the petroleum geology of the Oriente basin, Ecuador, in J. Brooks, ed., Classic petroleum provinces: Geological Society of London, Special Publication 50, p. 89-117. de Boer, P. L., A. van Gelder, and S. D. Nío, eds., 1988, Tideinfluenced sedirnentary environments and facies: Dordrecht, The Netherlands, D. Reidel, 530 p. de Souza Cruz, e E., 1989, Cretaceous sedimentarv facies and depositional environments, Oriente basin, Ecuador-s-a field trip guide: Tercer Congreso Andino de la Industria del Petroleo, Petrobras Research Center, Brazil, 65 p. Haq, B. U., J. Hardenbol, and P. R. Vail, 1988, Mesozoic and Cenozoic chronostratigraphy ami cycles of sea-level change, in e K. Wilgus, B. S. Hastings, C. A. Ross, H. Posamentier, J. Van Wagoner, and C. G. Sto e Kendall, eds., Sea-level change: an integrated approach: SEPM Special Publication 42, p. 71-108. Macellari, e E' 1988¡ Cretaceous paleogeography and deposítional cycles of western South Arnerica: [ournal South American Earth Sciences, v. 1, p. 373-418. Miall, A. D., 1977, A review of the braided river depositional environment: Earth-Science Reviews, v. 13, p. 1-62. Rarnirez, F. A., and J. A. Rodas, 1992, Geosdence aspects in the first experiences with horizontal wells in the Ecuadorian Oriente basin: Proceedings, V Congreso Colombiano del Petrolero, Memorias, p. 91-100. Short, N. M., and R. W. Blair, [r., 1986, Geomorphology from space-a global overview of regionallandforms: Washington' D.e, National Aeronautics Space Administration, 715p. Smith, D. G., 1987, Meandering river point bar lithofacies models: modern and ancient examples compared, in F. G. Ethridge, R M. Flores, and M. D. Harvey, eds., Recent developments in fluvial sedirnentology: SEPM Special Publication 39, p. 83-91. Terwindt, J. H. J., 1988, Palaeo-tidal reconstructions on inshore tidal depositional environments, in P. L. de Boer, A. van Gelder, S. D. Nio, eds., Tide-influenced sedimentary environments andfacíes: Dordrecht, The Netherlands, D. Reidel, p. 233-263. J

596

vVlzite ei al.

(b)

(a)

Linear Coheaion = 4750 psi Angle o, Intemal Friction = 32.9 degrees

Linear Cohnion '" 2460 psi Angle o, Intemal Friction = 40.4 degrees

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en

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Normal Stress, PSI

(d)

(e) 30000 25000

16000

zs Q..

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Figure 24-Mohr-Coulomb failure eriteria for the Main and Upper Hollin lithologies. (a) Upper Hollin shale. (b) Upper Hollin Iimestone. (e) Glaueonitie sandstone (high quartz content, Upper Hollin). (d) Main Hollin reservoir sandstone. Tschopp, H. H. , 1953, Oil explorations in the Oriente of Ecuador: AAPG Bulletin, v. 27, p. 2303-2347. Van Wagoner.]. e, H. W. Posmentier, R. M. Mitchum, P. R. Vail, J. F. Sarg, T. S. Loutit, and J. Hardenbol, 1988, An overview of sequence stratigraphy and key definitions, in C.W. Wilgus, B. S. Hastings, C. A. Ross, H. Posamentier, J. Van Wagoner, and C. G. Sto C. Kendall, eds., Sea-level changes: an integrated approach: SEPM Special Publication 42, p. 39-45. Wasson, T., and J. H. Sinc1air, 1927, Geological explorations east of the Andes in Ecuador: AAPG Bulletin, v. 11, p. 1253-1281.

Authors' Mailing Addresses Howard J. White Felix A. Ramirez Oryx Energy Company 13155 Noel Road Dalias, Texas 75240-5067 U.5.A.

Robert A. Skopee Department of Petroleum Geology University of Aberdeen Aberdeen AB9 2UE Seotland

[ose A. Rodas Oryx Ecuador Energy Company A venue de Amazonas Quito Ecuador Guido Bonilla Petroecuador J. Lean M y Av. Orellana Quito Ecuador