Pung Araya Cu

Cretaceous Research (2002) 23, 845–859doi:10.1006/cres.2002.1028

Palynological and sequence stratigraphicanalysis of the Napo Group in the Pungarayacu30 well, Sub-Andean Zone, Ecuador

*C. Vallejo, †*P. A. Hochuli, *W. Winkler and ‡*K. von Salis

*Geological Institute, ETH Zentrum, Sonneggstrasse 5, CH-8092 Zurich, Switzerland†Palaontologisches Institut und Museum, Universitat Zurich, Karl Schmid-Str. 4, CH-8006 Zurich, Switzerland‡Glarnischstrasse 11, CH-8805 Richterswil, Switzerland

Revised manuscript accepted 20 November 2002

The study of palynomorphs and calcareous nannofossils from the Albian–Campanian Napo Group in the Pungarayacu 30well in the Subandean Zone of Ecuador has led to a new biostratigraphic framework revealing the existence of several hiatusesfor this area. The palynological and palynofacies data are used together with other fossils and lithological evidence to definea sequence stratigraphic framework. The distribution of palynomorphs and palynofacies indicates a strong terrestrial input forthe lower part of the Napo Group (Napo Basal and Lower Napo formations). In the upper part (Middle and Upper Napoformations), terrestrial input is reduced and a restricted marine environment with several dysoxic–anoxic intervals can beinferred. The hydrocarbons present in the well studied have traditionally been regarded as locally sourced. However, severallines of evidence (TAS, Tmax and VR) prove the immature stage of the source rock in this borehole as well as in a largerarea. � 2003 Published by Elsevier Science Ltd.

K W: Napo Group; Sub-Andean Zone; Ecuador; palynology; palynofacies; sequence stratigraphy.

1. Introduction

The sediments of the Late Cretaceous Napo Groupwere deposited in the epicontinental Oriente Basin ofEcuador, which occupied the position of a retro-arcforeland basin with respect to the evolving Andeanchain. Tectonic shortening has driven the uplift of thevolcanic basement and cover series in the Sub-AndeanZone (SAZ) (Figure 1). The Pungarayacu 30 wellpenetrated the Napo Group in the so-called NapoUplift of the SAZ. The Oriente Basin and Napo Uplifthave yielded in Ecuador the majority of hydrocarbonsthat have been extracted from the Putumayo-Oriente-Maranon oil province (e.g., Rivadeneira & Baby,1999) extending from Colombia over Ecuador toPeru. Palaeogeographic reconstructions (Pindell &Tabbutt, 1995), stratigraphic ages and lithologicalcorrelations suggest that contemporaneous organic-rich sediments in northwestern South America weredeposited over a larger area (Figure 1).

The Napo Group consists of organic-rich shales,limestones and sandstones, and has been subdividedinto several informal members, which can be corre-lated over large distances in the Oriente Basin ofeastern Ecuador (Tschopp, 1953). Two sandstone

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units (labelled T and U sandstones) constitute themost important hydrocarbon reservoirs, and theorganic-rich zones (e.g., Basal Shale Member) areconsidered the source of almost all hydrocarbons(Rivadeneira, 1986; Dashwood & Abbotts, 1990;Mello et al., 1995). The biostratigraphy of the NapoGroup has been previously established (e.g., Tschopp,1953; Jaillard, 1997; Pocknall et al., 1997). For thisstudy, we have selected the Pungarayacu 30 well(Figure 1), which provides the most complete recordof the Napo Group in the SAZ. The aim of this studyis to reconstruct the depositional environment andsequence stratigraphy of the Napo Group in theSubandean Zone of Ecuador and to evaluate its oilsource potential. The model presented in this paper isbased on palynofacies analyses, new biostratigraphicdeterminations, and previously published sedimento-logical and palaeontological data (Ordonez et al.,1992; Jaillard, 1997).

2. Geological setting

The Sub-Andean Zone of Ecuador (SAZ) constitutesthe westernmost and proximal part of the Oriente

� 2003 Published by Elsevier Science Ltd.

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Basin. In this area, the Palaeozoic basement,Mesozoic–Tertiary volcanic rocks and sedimentaryformations are exposed in large-scale antiforms andthrust slices (Sub-Andean Thrust Belt). The outerantiforms in the SAZ are referred to as the NapoUplift in the north and the Cutucu Uplift in the south(Figure 1). This architecture is due to the northeast-ward thrusting of the Cordillera Real over the GuyanaShield and Oriente Basin-fill series with evidence ofright-lateral faulting (Litherland et al., 1994; Rosero,1997; Rivadeneira & Baby, 1999). The Napo Groupis recognized in outcrops in the SAZ to the west, andin seismic profiles and wells in the flat-lying OrienteBasin to the east. It stratigraphically overlies theAptian–Early Albian Hollin Formation, which iscomposed of mostly quartz-sandstones that weredeposited in braided river systems and marginalmarine environments (Canfield et al., 1982; Balkwillet al., 1995; White et al., 1995; Shanmugam et al.,2000). In the Pungarayacu 30 well and surroundingarea, these sandstones contain degraded heavy oil,which also seeps from rocks in outcrops.

Continental red beds of the Maastrichtian–Palaeocene Tena Formation unconformably overliethe Napo Group. The contact between the TenaFormation and the Napo Group is either an erosionaldiscordance or a hiatus, depending on the locationwith respect to the basin axis (Tschopp, 1953;Faucher et al., 1971; Faucher & Savoyat, 1973;Baldock, 1982; Balkwill et al., 1995).

The average thickness of the Napo Group is 300 m,with a maximum of 600 m in the southern SAZ(Cutucu Uplift). Based on regional correlations oflimestone units, Jaillard (1997) and Rivadeneira &Baby (1999) subdivided the previously defined NapoFormation into four distinct formations, inter-preted to represent shallow marine sequences. In thePungarayacu 30 well, these formations are character-ized as follows (Figure 2): (1) the Napo BasalFormation, including a calcareous member at the base(C Limestone), a shaly member called Basal Shale,a calcareous member (T Limestone) and an oil-bearing member (T Sandstone); (2) the Lower NapoFormation, composed of a calcareous member at thebase (B Limestones), shales in the middle section (UShales), and an oil-bearing calcareous to sandy inter-val at the top (U Limestone-Sandstone); (3) theMiddle Napo Formation, including shallow carbonateplatform deposits (A and M2 Limestones), whichdiffer in their faunal diversity and by the presence ofsandstones. The M2 Limestone Member is morefossiliferous, rich in glauconite and contains oil-bearing sandstones (Rosero, 1997). In addition,seismic data (Rosero, 1997; Rivadeneira & Baby,

1999) reveal synsedimentary deformation in theMiddle Napo Formation; and (4) the Upper NapoFormation, which is composed of alternating shales,limestones and sandstones. In the SAZ, this formationis incomplete compared to the flat-lying OrienteBasin-fill to the east, indicating that this part of thesection was either eroded or never deposited (e.g.,Faucher et al., 1971; Balkwill et al., 1995). In thePungarayacu 30 well, the thickness of this formation isreduced to approximately 20 m of limestones andshales referred to as the M1 Limestone and the M1Shale, respectively.

3. Methods and approach

Twenty-seven shale and marlstone samples were col-lected from the Pungarayacu 30 well. In order topinpoint age and facies changes, samples were prefer-entially taken at lithologic boundaries. They wereprocessed by standard palynological techniques usingHCl (30%) and HF (70%) to remove the carbonatesand silicates, respectively. In a further step, any sili-cate gel present was eliminated by hot HCl treatment.The samples were then sieved and the material coarserthan 15 �m was used for palynofacies analysis, with-out an oxidation step. The palynofacies data wereobtained by counting at least 300 organic particles foreach sample. The particulate organic matter (POM)was classified according to morphology and stage ofpreservation. To concentrate the marine and terres-trial palynomorphs for palynological analysis, thesieved residue was oxidized with fuming HNO3(100%). In some samples rich in amorphous material,the treatment with HNO3 was combined with ultra-sonic vibration. The palynomorphs were counted toobtain the ratio of marine to continental palyno-morphs in order to refine the environmental inter-pretation. Calcareous nannofossil data were obtainedfrom smear slides prepared from the same samplesused for palynofacies and palynology.

3.1. Palynofacies

The term palynofacies was introduced by Combaz(1964) to describe the palynological study of the totalassemblage of particulate organic matter (POM) thatis recovered from sediments after removing the matrixwith hydrochloric and hydrofluoric acids. Tyson(1995) extended this concept by suggesting that paly-nofacies analysis is the palynological study of depos-itional environments and hydrocarbon source rockpotential based upon the total assemblage of POM.

Palynology and sequence stratigraphy of the Napo Group 847

Following the classification of Whitaker (1984) andSteffen & Gorin (1993), the POM of marine sedi-ments can be grouped according to their origin interrestrial (allochthonous) and marine environments(autochthonous). The terrestrially-derived organicmatter includes higher plant debris and is representedby cuticles, membranes, pieces of wood and opaque,oxidized, structured material. These particles arecommonly termed phytoclasts (Tyson, 1995).Terrestrially-derived organic matter also includessporomorphs (pollen and spores). The marine-derived

material is represented by: (1) marine phytoplank-ton, including dinoflagellate cysts, acritarchs, andprasinophytes; (2) foraminiferal test linings; and (3)amorphous organic matter (AOM) of marine origin.Dinoflagellate cysts, acritarchs, prasinophytes andforaminifera are referred to as marine palynomorphs.

Figure 1. Simplified regional geology of northwestern South America showing the distribution of Cretaceous basins and thelocation of the Andean ranges and Subandean Zone in Ecuador.

3.2. Palynofacies and sequence stratigraphy

POM analysis combined with other sedimentary faciescharacteristics may allow recognition of the sequence

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Figure 2. Stratigraphic section for Pungarayacu 30 well showing age, lithology and the main calcareous nannofossil andpalynomorph biostratigraphic events.

Palynology and sequence stratigraphy of the Napo Group 849

stratigraphic significance of a given lithological unit(Steffen & Gorin, 1993; Gotz & Feist-Burkhardt,2000). POM assemblages of lowstand systems tracts(LSTs) are characterized by abundant and often quitecoarse phytoclasts and terrestrial palynomorphs,which are often degraded and mixed with stronglyoxidized organic debris. Dinoflagellate cysts are gen-erally uncommon in such associations. The assem-blages from transgressive system tracts (TSTs) show ahigh number and diversity of marine palynomorphs,whereas terrestrial palynomorphs and phytoclasts areless common. The highstand systems tracts (HSTs)are characterized by a decrease in abundance anddiversity of marine palynomorphs, an increase in theabundance of degraded particles, and an increase inthe size of black opaque fragments (Steffen & Gorin,1993).

For our reconstruction of the sequence stratigraphicsuccession, we have used, in addition to the palyno-facies data, primary sedimentary structures andfaunal data (Figure 3) previously described from thePungarayacu 30 well by Jaillard (1997). The followingparameters were selected: (1) karst surfaces assumedto form during subaerial exposure, hence related tosequence boundaries; (2) the presence of oysters,indicating a brackish to inner neritic depositionalenvironment; (3) bioturbation, which, in the presentcontext of a restricted basin setting, is characteristic ofshallow marine conditions with abundant benthicactivity; (4) fine sedimentary laminations interpretedas indicators of low energy levels prevailing duringdeposition; and (5) regular occurrence of ammonitesand planktonic foraminifera, indicating open marineconditions.

3.3. Preservation and oil potential

The preservation of the organic matter depends onseveral factors such as the distance over which thisfraction has been transported, the primary grain size,the availability of oxygen and the degree of bioturba-tion at the site of deposition. Preservation maybe enhanced when the overlying water column isdepleted in oxygen, and if deposition occurs in shal-low water (Batten, 1996a). In our samples, the stageof preservation of the organic matter has been deter-mined by fluorescence under ultraviolet light. In criti-cal cases, the fluorescence is also useful to distinguishbetween aquatic and land-derived components. Wellpreserved AOM with a predominantly aquatic algaland bacterial origin shows a more intense fluorescencethan material composed of land-derived aromatic(woody) structures (Bertrand, 1986; Tyson, 1995).The non-fluorescent AOM is interpreted as a com-

pletely degraded fraction that might be of terrestrialorigin.

Tyson (1995) designed a qualitative scale of pres-ervation based on fluorescence and type of POM.High values on this scale (5–6) correspond to theorganic matter with strongly fluorescent AOM yields.Lowest values (1–2) characterize non-fluorescentinert material, generally oxidized or carbonized phyto-clasts with few fluorescent palynomorphs. In thepresent study, we have adapted this scale (Figure 3) tocomplement the palynofacies data.

Following Tyson (1995) for the evaluation of thesource rock potential, the POM is classified into fourcategories: inert, gas-prone, oil-prone and very oil-prone. The inert material corresponds to the non-fluorescent, opaque black material derived fromoxidized wood. Gas-prone material includes the non-fluorescent and translucent structured phytoclasts,woody fragments and partially oxidized palyno-morphs. Fluorescent (well preserved) AOM and paly-nomorphs represent oil-prone material. This groupalso includes cuticles and membraneous debris. Thevery oil-prone category consists of the strongly fluor-escent organic matter corresponding to structured andamorphous material of algal origin. Some cuticles arealso included in this group. In the classification ofsource potential of kerogen, four types are differen-tiated: kerogen type I corresponds to the very oil-prone material, type II to the oil-prone organic matter,type III to the gas-prone and type IV to the inertmaterial (strongly oxidized organic matter) (Tyson,1995; Batten, 1996b).

3.4. Other analyses

Other important parameters for the interpretation ofrelative sea-level changes are the ratio of marinepalynomorphs (dinoflagellate cysts and foraminiferallinings) to land derived palynomorphs (pollen andspores) (Figure 4) and the diversity and abundance ofcalcareous nannofossils (Figure 3). Abundant anddiverse assemblages of the calcareous nannofossilsare usually found during highstands (Eshet &Almogi-Labin, 1996).

Combined with the palynofacies data, measurementof the total organic carbon content (TOC) in thesamples is a very useful parameter for evaluating thedepositional environment and source rock potential(Tyson, 1995; Batten, 1996b). High TOC values aregenerally associated with a high abundance of AOM,accumulated under anoxic conditions (Tribovillard &Gorin, 1991; Tyson, 1995; Batten, 1996b). However,as pointed out by Batten (1996b), TOC alone cannot

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be used to determine source rock richness, in particu-lar for sediments where most of the carbon consists ofcharcoal or graphite with low hydrogen contents.

4. Palynomorph and calcareous nannofossilbiostratigraphy

4.1. General observations

Several biostratigraphic schemes have been publishedfor the Napo Group (Tschopp, 1953; Ordonez et al.,1992; Jaillard, 1997; Pocknall et al., 1997). Most ofthese reconstructions include a large number of oilwells, including the Pungarayacu 30 well (seecompilation by Jaillard, 1997). Together with a reviewof previous results, we present a new biostratigraphicdating for the Pungarayacu 30 well based on paly-nomorphs and calcareous nannofossils (Figure 2). Ingeneral, palynomorphs provide a reasonable resolu-tion in the lower part of the section, including theBasal and Lower Napo formations (340–215 m). Inthis part of the section, calcareous nannofossils arescarce and non-diagnostic. In the Middle and UpperNapo formations, we obtained good coveragewith calcareous nannofossils and a few additionalpalynological events.

4.2. Napo Basal Formation

For constraining the biostratigraphy of the Napo BasalFormation, we use several first appearance eventsat the base of this formation (330 m depth). Forinstance, the dinoflagellate cysts Xenascus ceratioidesand Dinopterygium cladoides are both known to havetheir first appearance datum (FAD) in the Late Albian(cf., Helenes & Somoza, 1999). A characteristicassemblage of angiosperm pollen, with the polyporateforms Cretaceiporites polygonalis and C. mulleri as wellas the zonoaperturate pollen Dichastopollenites spp., isfound in this interval. The regular occurrences of theelater-bearing pollen Elaterosporites protensus and theconsistent presence of the Afropollis group confirm aLate Albian age. However, most of these pollen formsare present in the interval above.

For the upper part of the Napo Basal Formation,the first appearance datum (FAD) of the dino-flagellate cyst Palaeohystrichophora infusorioides at307 m, and the last appearance datum (LAD) ofOdontochitina rhakodes and O. ancala, as well as thepresence of the pollen Elaterosporites protensus at277 m, indicate a latest Albian age for the TLimestone-Sandstone Member (308–277 m). Thefew calcareous nannofossils existing in this interval,

such as Calculites anfractus, also confirm a Late Albianage.

The above-described intervals assigned to the LateAlbian and latest Albian correspond to spore-pollenzone 7 in Venezuela (Elaterosporites protensus/E.verrucatus-Afropollis Zone of Muller et al., 1987).

4.3. Lower Napo Formation

No new biostratigraphic data have been obtained forthe B Limestone Member (276–255 m). Jaillard(1997) indicated a Late Albian age for this interval. Inour interpretation, considering the latest Albian age ofthe underlying member and the Cenomanian ageabove, a latest Albian–Cenomanian age is suggestedfor this interval.

In the U Shale Member, the FAD of the pollenspecies Triorites africaensis is located at 254 m and theCorollina group occurs regularly. LADs within thisinterval include the dinoflagellate cysts Cyclonepheliumpaucimarginatum, Dapsilidinium warrenii, Florentiniaberran, F. radiculata, F. verdieri and Palaeoperidiniumcretaceum. At 225 m, the LAD of the elater-bearingpollen Elateroplicites africaensis occurs together withrepresentatives of the Corollina group and the sporadicpresence of the spore Crybelosporites pannuceus. Gneta-ceaepollenites diversus, a marker for the Cenomanian(Pocknall et al., 1997) is found only at 228 m. At215 m, in the uppermost part of the U Limestone-Sandstone, the Ephedripites group is regularly to fre-quently observed. According to some authors (e.g.,Jardine & Magloire, 1965; Regali, 1989), the FAD ofTriorites africaensis is typical for the middle and upperpart of the Cenomanian, and the regular occurrence ofthe Corollina group has been observed up to the top ofthe Cenomanian (Jardine & Magloire, 1965). Mulleret al. (1987) postulated a continuation of theAlbian zone 7 (see above) and zone 8 (Trioritesafricaensis Zone), which are supposed to cover theentire Cenomanian. Thus, a Cenomanian age canbe interpreted for the U Shale and U Limestone-Sandstone Members (255–213 m).

4.4. Middle Napo Formation

In the Middle Napo Formation, a Turonian age issuggested for the Lower A Limestone and the lowerpart of the Upper A Limestone Member (213–187 m). Typical features include the FAD of severalspecies of calcareous nannofossils such as Eprolithusmoratus, Quadrum gartneri and Reinhardites antophorusat 212 m. The first occurrence of the nannofossilEiffellithus eximius, also recorded by Jaillard (1997),indicates a Middle Turonian age (zone CC12 of

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Perch-Nielsen, 1985 and zone UC8 of Burnett, 1998)for the base of the Lower A Limestone. These dataimply that the Early Turonian section is missing. Apossible hiatus is also suggested by the presence of akarst surface at the top of the U Limestone-SandstoneMember (Figure 3). A distinct break in the paly-nomorph assemblages is observed in the intervalbetween the top of the U Limestone-Sandstone(215 m) and the base of the lower A Limestone(212 m). In the latter unit, the palynomorph assem-blages are characterized by a high dominance ofdinoflagellate cysts and relatively poor diversification.However, most of the recorded forms are smooth andnot age diagnostic. The spore-pollen assemblages areessentially composed of tricolpate, tricolporate and afew polyporate angiosperm pollen grains. A few speci-mens of the Corollina group are found up to the top ofthis interval, which is in agreement with the interpret-ation of Muller et al. (1987) who assigned a Turonianage to the top range of the Corollina group in theirzone 9.

It must to be emphasized that the pollen speciesDroseridites senonicus has not been found in the wellstudied. According to records from West Africa andVenezuela, this species characterizes the intervalbetween the Late Turonian and the Coniacian (Lawal& Moullade 1986) and according to Muller et al.(1987) defines palyno-zone 10 in the lower part of theSenonian. The palynological evidence is comple-mented by the LAD of the calcareous nannofossilEprolithus octopetalus at 187 m at the base of the UpperA Limestone, a typical feature of the UC8b zone ofBurnett (1998) and dated as Middle Turonian.Therefore, we assume that only the middle part of theTuronian is preserved in the Pungarayacu 30 well.

A clear turn-around in the calcareous nanno-fossils can be recognized at the base of the Upper ALimestone Member (between 187 and 185 m). Thisincludes the last occurrence of Eprolithus octopetalus at187 m and the first record of Micula decussata near thebase of the Upper A Limestone Member at 185 m.The presence of the latter nannofossil species suggestsa Late Coniacian or younger age for this unit. In thepalynomorph association of the Upper A LimestoneMember, the most diagnostic events are the FAD ofseveral representatives of the Dinogymnium grouptogether with several types of syncolporate pollen. TheFADs of Dinogymnium eucalense and D. nelsonenseobserved at 185 m depth are typical for Santonian oryounger sections (Helenes & Somoza, 1999). Thisevidence combined with the presence of the calcare-ous nannofossil species Eprolithus moratus (177 m)restricts the age of the interval between 185–177 m tothe Santonian. The consistent occurrence of the

calcareous nannofossil Lithastrinus septenarius at139 m (top of the upper M2 Limestone) extends theSantonian succession up to 139 m and suggests thepresence of an important hiatus spanning the LateTuronian and Coniacian. The biostratigraphic evi-dence is corroborated by the presence of a karstsurface at 188 m (Figure 3).

4.5. Upper Napo Formation

In the Upper Napo Formation, the continuous recordof the dinoflagellate cysts Palaeohystrichophora infuso-rioides and Odontochitina spp. indicates an age notyounger than Campanian. Jaillard (1997) reportedthe presence of the foraminifera Heterohelix striataand the nannofossil Lucianorhabdus cayeuxii in thePungarayacu 30 well and the nearby Misahuallisection. In the present M1 Shale Member at 127and 126 m, the calcareous nannofossil Marthasteritesfurcatus occurs sporadically. Considering the generallyaccepted LAD of the latter fossil in the EarlyCampanian (top of the CC18 zone of Perch-Nielsen,1985), a Santonian–Early Campanian age can beassigned to the Upper Napo Formation in thePungarayacu 30 well.

4.6. Comparison with previous biostratigraphic studies

Comparison with the stratigraphic framework pro-posed by Jaillard (1997) shows quite good correlationin the lower part of the Napo Group. Differencesappear concerning the recognition of an uppermostAlbian interval and the lack of arguments for deter-mining the Lower Cenomanian. There is also agree-ment about the interpretation of a hiatus between theCenomanian and the Turonian. However, in theupper part of the section, the stratigraphic frameworksdiffer considerably. Our evidence suggests that onlythe Middle Turonian section is present and that it isseparated by a hiatus from the overlying section datedas Santonian. It must be pointed out that this hiatuslies within the Upper A Limestone Member.

Based on palynological analyses of many wells,Pocknall et al. (1997) published a stratigraphic frame-work for the Napo Group. These authors divided theNapo Basal Formation into a Late Albian and anEarly Cenomanian part. Similar to our results, thelower part of the Lower Napo Formation is assignedto the Cenomanian and the lower part of the MiddleNapo Formation is distinguished as a Turonianwedge. For Pocknall et al. (1997), the upper part ofthe Middle Napo Formation and the lower part of theUpper Napo Formation are interpreted to be of LateTuronian age, whereas the upper part of the Upper

Figure 3. Pungarayacu 30 well fauna and sedimentary structures (simplified from Jaillard, 1997), TOC, preservation,palynofacies and sequence stratigraphic interpretation. These data show two different stages. The first, which includesthe Napo Basal and the Lower Napo formations, is characterized by a major input of sand and terrestrial organicmatter during well-developed LSTs. The second (Middle and Upper Napo formations) reflects dominant carbonatesedimentation in a restricted marine environment, including several dysoxic–anoxic intervals. For the key to lithologies,see Figure 2.

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Napo Formation is considered to be Coniacian. In ourview, the Turonian section only encompasses theMiddle Turonian. This interval is bound by a hiatus atthe base and at the top. Our palynological data suggesta distinct break between 187 and 185 m, with indica-tions of a Santonian age in the latter sample. Thisimplies a hiatus comprising the Late Turonian and theConiacian. The sedimentological evidence (Jaillard,1997) (Figure 3) suggests the presence of severaladditional erosional phases within the Santonianinterval. However, the biostratigraphic resolution isinsufficient to determine the extent of the missingsections.

5. Palynofacies

The distribution of POM (Figure 3) indicates strongterrestrial input in the lower part of the sectionstudied (Napo Basal and Lower Napo formations),dated as Late Albian to Cenomanian. This inputis clearly reduced in the Middle and Upper Napoformations where the POM is dominated by marine-derived organic matter deposited in a restrictedmarine environment including several dysoxic–anoxicintervals.

5.1. Palynofacies of the Upper Albian–Cenomaniansuccession (Napo Basal and Lower Napo formations)

The Upper Albian POM assemblages in thePungarayacu 30 well are dominated by grey to blackamorphous organic matter (up to 85%) presumably ofmarine origin. The terrestrial fraction is representedby pieces of wood and relatively rare membranes.Among the palynomorphs (Figure 4), dinoflagellatecysts represent the dominant group (80%), indicatinga high sea level during this period with maximumflooding during the deposition of the Basal ShaleMember.

The uppermost Albian to Cenomanian section(307–215 m) is characterized by a decrease of AOMvalues and an increase in the terrestrially derivedmaterial (up to 80%). The latter is dominated byrelatively coarse debris of wood, membranes andcuticles. The particular abundance of wood impliesthat the deposition of the T Limestone and TSandstone members took place in near-shore (coastal)environments (see also White et al., 1995;Shanmugam et al., 2000). Compared to the intervalbelow, dinoflagellate cyst abundance decreases,whereas terrestrial palynomorphs become moreimportant, representing up to 86% of the palyno-morph assemblage and, as expected for near-shore

conditions, they are dominated by thick-walledspores.

5.2. Palynofacies of the Middle Turonian–LowerCampanian succession (Middle and Upper Napoformations)

The Middle Turonian section is dominated by AOMof marine origin (up to 95%). The terrestrial fractiondecreases (less than 20% of the POM), suggestinga shift towards a restricted environment (innershelf). Marine palynomorphs are dominant (60–80%)(Figure 4); however, the dinoflagellate cyst assem-blages are poorly diversified. The terrestrial paly-nomorphs are represented by a few small and thintricolporate, polyporate and Corollina pollen. Thecomposition of the assemblage suggests a shallow,low-energy marine environment (Tyson, 1993;Ibrahim, 1996). At the top of this interval, the highabundance of well-preserved AOM is in line with highTOC values (10%), indicating bottom water anoxia.

The Santonian POM assemblage is still dominatedby AOM of marine origin (80–95%), suggesting thatshallow marine conditions prevailed. The marinepalynomorphs are abundant and show an increaseddiversity compared to the section below. Palaeohystri-chophora infusorioides and the Dinogymnium group aremore abundant. The comparatively rare terrestrialpalynomorphs are dominated by the Syncolporitesgroup.

The Santonian–Early Campanian POM assem-blages show a dominance of AOM of marine origin(80–90%) while terrestrial material is still relativelyscarce (9–18%). Nevertheless, the ratio between themarine and the terrestrial palynomorphs (Figure 4)indicates a shift towards more terrestrial conditionsprior to the unconformable deposition of the overlyingred beds of the continental Tena Formation.

6. Sequence stratigraphy of the Napo Group

In the C Limestone Member at the base of the NapoGroup (339–331 m), bioturbation is well developedand the strata generally contain abundant plant andcoal debris. Erosional surfaces and reworking due torepeated emersion is evident (Jaillard, 1997). Thesample at 332 m contains common land-derivedphytoclasts (21% wood, 14% membranes andcuticles), suggesting the presence of late LSTconditions.

The POM in the Basal Shale Member (331–308 m)is dominated by AOM of marine origin (up to 88%).Fine-scale laminations are ubiquitous, whereasbioturbation is lacking. The observed increase in the

Figure 4. Ratio of marine to continental palynomorphs and sequence stratigraphic interpretation. The changes in this ratioare related to relative changes in sea level. The Napo Basal and Lower Napo formations show two main regressionsrepresented by the T Sandstone and U Limestone-Sandstone members, whereas transgressions occur during depositionof the Basal Shale, B Limestone and U Shale. The Middle and Upper Napo formations show a relative sea-level high.Below the unconformably-overlying continental Tena Formation, an increase in terrestrial palynomorphs is observed.Dashed parts of the relative sea-level curve represent sequence boundaries, which we cannot depict in detail. For the keyto lithologies, see Figure 2.

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abundance of dinoflagellate cysts and increase incalcareous nannofossil diversity point to a marineenvironment in a TST with a maximum transgressionsurface occurring in the middle part of the member.

The T Limestone Member (308–296 m) is inter-preted as a prograding environment, following theTST of the Basal Shale. An HST is indicated bythe presence of oysters, abundant bioturbation(Jaillard, 1997) and the high abundance of terrestrialpalynomorphs and phytoclasts.

The sandstones of the Napo Group have beenstudied by several authors. White et al. (1995) gave ageneral interpretation of these sandy units, commenc-ing with estuary channels followed by deepening-upwards, shallow-marine shelf deposits withintercalated storm shoals. Shanmugam et al. (2000)found tide-dominated estuarine depositional environ-ments also with a clear deepening-upward trend.

The T Sandstone Member (296–276 m) wasdeposited during a sea-level fall, which is highlightedby a change to a terrestrially dominated association inan LST. At the base of this member (296 m), asequence boundary is inferred. Samples from this levelcontain abundant land-derived material (67% mem-branes and cuticles) and the ratio of marine to conti-nental palynomorphs confirms an important drop insea level (Figure 4).

The base of the B Limestone Member (276–255 m)corresponds to a relative sea-level rise. A TST settingis supported by the presence of laminated shales andan increase in planktonic foraminifera at 272 m(Jaillard, 1997). The palynofacies at the top of themember (at 256 m) again show an increase interrestrially-derived organic matter (up to 15%) that,together with the presence of bioturbation, points toan LST.

According to sedimentological evidence, the UShale Member (255–230 m) is interpreted to com-prise two sequences. At the base, a TST is indicatedby the presence of laminations and pyrite, as well as bythe absence of benthic activity (Jaillard, 1997), sug-gesting low energy and anoxic conditions. In themiddle part of this member (240–245 m), lithologicaldata include the presence of bioclasts and detrital finesand layers, which we interpret as deposition duringan LST. In the upper part of the U Shale Member, aTST environment is suggested based on the presenceof laminated shales and the absence of benthic activity(Jaillard, 1997).

The U Limestone-Sandstone Member (230–213m) is characterized by intense bioturbation and thepresence of oysters (Jaillard, 1997), indicating a near-shore environment. The samples at 228 and 225 myield significant proportions of terrestrial material

(80–95%) and the palynomorph assemblages arecharacterized by abundant pteridophytes spores(c. 75%). Most spores are partially biodegraded,reflecting intense biological activity during an LST.The subsequent decrease in land-derived material(c. 32%) and the increase in marine AOM and dino-flagellate cyst abundance at 215 m, indicate a pro-gradational environment (HST) for the upper part ofthe U Limestone-Sandstone Member.

A sequence boundary at 213 m separates theMiddle Napo Formation (213–138 m) from theunderlying Lower Napo Formation. This sequenceboundary is indicated by the biostratigraphic gapsuggested here and the presence of a karst surface(Jaillard, 1997) (Figure 3). Compared to the sectionbelow, the Middle Napo Formation records differenttrends in depositional environment. It is characterizedby a general decrease in land-derived material, a highpercentage of fluorescent amorphous organic matterand several intervals with high TOC values (10–12%).This section is interpreted as a repetitive succession ofHSTs and TSTs with poorly developed LSTs. TheLower A Limestone Member (213–189 m) is charac-terized by a high abundance of well-preserved marinederived AOM (up to 76%), prevalent laminations,and the greatest abundance of dinoflagellate cysts inthe entire Napo Group. The well-preserved AOM andhigh TOC values (10%) suggest prevailing dysoxic–anoxic conditions during TST deposition. The UpperA Limestone Member (189–175 m) is separated fromthe Lower A Limestone Member by a karst surfaceindicative of a hiatus encompassing the Late Turonianand the Coniacian as revealed by palynology andcalcareous nannofossil data. Oxic conditions are sug-gested by a drop in the TOC values. The presence ofoysters and pervasive bioturbation (Jaillard, 1997)indicates a shallow marine environment, which weinterpret as late HST to LST.

The Lower M2 Limestone Member (175–157 m)comprises two small sequences. The palynofacies datareveal high values of AOM and reduced terrestrialinput (less than 15%). At the top of this member(157 m), another dysoxic–anoxic interval can beinferred from the increase in the TOC value (12%)and the well-preserved AOM. Estimated from theratio of marine to continental palynomorphs (Figure4), the relative sea level seems to be high during thisinterval. The Upper M2 Limestone Member (157–139 m) can be divided into two sequences that areseparated by karst surfaces (Jaillard, 1997), indicatingrepeated short emersions. The palynofacies do notreveal any significant change from the underlyingmember, although there is a drop in TOC valuesand degree of preservation in the upper part. The

856 C. Vallejo et al.

palynomorph data indicate a relatively high sea level,with a maximum transgression occurring in the upperpart of the member (147 m).

Following the sea-level rise recorded in the UpperM2 Limestone Member, the Upper Napo Formation(139–125 m) represents a progradational sequence inthe Pungarayacu 30 well. Here the POM is againdominated by AOM of marine origin. Bioturbationand a progressive decrease in the amount of dino-flagellate cysts and calcareous nannofossils suggeststhat sediments were deposited during an HST.Dysoxic–anoxic marine conditions occur at the top,where well-preserved AOM of marine origin and highTOC values (10%) are found. The ratio of marine tocontinental palynomorphs suggests a sea-level drop.The Tena Formation unconformably overlies theNapo Formation. In proximal (western) parts of thebasin, prior to deposition of the Tena Formation,considerable erosion has in places removed youngerparts of the Napo Group (Faucher & Savoyat, 1973;Balkwill et al., 1995; Jaillard, 1997).

7. Maturity and oil potential

The colour of palynomorphs has been widely used asan indicator of thermal maturation (Staplin, 1977;Batten, 1982; Collins, 1990). In the Pungarayacu 30well, the colour of palynomorphs varies between paleyellow to yellow, corresponding to a value of 2 on theThermal Alteration Scale (TAS of Batten, 1982).This correlates well with average Tmax values of428�C as determined by pyrolysis. Vitrinite reflectancevalues of 0.4–0.5% from the same area (Dashwood& Abbotts, 1990; Bernal, 1998) are also in linewith these features. These data indicate thermallyimmature conditions for oil generation with anequivalent temperature being below 60�C. In the wellstudied, very oil-prone intervals are found in the upperpart of the Lower A Limestone Member and theLower M2 Limestone Member. They are character-ized by a high abundance of well-preserved AOM (upto 70%) together with a general decrease in terres-trially derived material and faunal diversity, as well ashigh TOC values ranging between 9.8% and 12%.This suggests an anoxic–dysoxic environment thatenhanced the preservation of the organic matter.

8. Palaeogeography and palaeoenvironmentalinterpretation

During the Cretaceous Period, the Oriente Basin ofEcuador developed on a slowly subsiding epicontinen-tal platform (Pindell & Tabbutt, 1995; Jaillard, 1997)

to the east of the Guyana Shield (Figure 1). However,the presence of a primordial Cordillera Real is estab-lished by fission-track-aided provenance analyses. Theradiometric ages of detrital zircon grains in the NapoGroup (Ruiz et al., 2002) confirm that both the veryslowly exhuming Guyana Shield and moderately torapidly exhuming rocks of a primordial CordilleraReal supplied siliciclastic material to the Napo Basin.Consequently, the basin was surrounded by large landsurfaces that supplied ample terrestrial plant materialto the basin, in particular during the Albian–Cenomanian.

According to palaeogeographic reconstructions(Pindell & Tabbutt, 1995), the northwestern SouthAmerican basins (Figure 1) were connected duringthe Late Cretaceous. During most of the LateCretaceous, the Ecuadorian segment appears to havebeen separated from the Pacific Ocean by the pri-mordial Cordillera Real (Litherland et al., 1994; Ruizet al., 2002) that isolated the basin from open marineconditions, enhancing the preservation of organicmaterial by restricting water circulation. Such barriershave also been recognized for contemporaneousformations in Colombia (Villeta Formation) and inVenezuela (La Luna Formation), where they areassumed to play a role in the development of anoxicbottom waters (Erlich et al., 1999; Rangel et al., 2000;Ramon et al., 2001). The palynofacies and palaeonto-logical data (e.g., oysters) described and the highaccumulation of organic matter suggest a prevailingshallow water column (c. 20–40 m deep), where oxi-dative and respiratory losses were minimal (Pedersen& Calvert, 1990).

The palynological assemblages of the UpperAlbian–Cenomanian interval of the Napo Group inthe Pungarayacu 30 well are characterized bythe presence of the elater-bearing pollen species(Elateroplicites africaensis, Elaterosporites klaszii, E.protensus, E. verrucatus) as well as by the record ofTriorites africaensis and the Afropollis group. Theseassemblages are comparable to those described fromwestern Africa and Brazil, which have been referred toas the African–South American phytogeographic(ASA) province by Herngreen (1974), and theNorthern Gondwana province by Brenner (1976).Dino et al. (1999) considered these assemblages astypical for warm dry climates. From the Turonianupwards, pollen assemblages from the Pungarayacu30 well are strongly dominated by syncolporate pollenand thus, differ considerably from the assemblagesassigned to the African Palmae province by Herngreen& Chlonova (1981) and Herngreen et al. (1996),indicating a different palynofloral evolution forSouth America and Africa. The opening of the South

Palynology and sequence stratigraphy of the Napo Group 857

Atlantic acted as a barrier and also induced a decreasein average temperature (Dino et al., 1999).

9. Conclusions

Based on palaeontological, sedimentological andpalynofacies evidence 12 stratigraphic sequenceshave been recognized within the Napo Group inPungarayacu 30 in the SAZ of Ecuador. Thesesequences are composed of two large-scale sedi-mentary stages: the first, including the Napo BasalFormation and the Lower Napo Formation, is char-acterized by a major input of sand and terrestrialorganic matter during well-developed LSTs withreservoir-rock characteristics, and the second (Middleand Upper Napo formations) is characterized by areduction in terrestrial input. During this latter stage arestricted marine environment prevailed. Here, LSTsare not preserved or are of minor importance andseveral periods of dysoxic–anoxic conditions producedhigh quality source rocks.

In general, the Upper Cenomanian and Turonian ofthe Napo Group is assumed to represent the mostfavourable interval for hydrocarbon generation (e.g.,Mello et al., 1995). In Colombia and Venezuela ithas been related to maximum flooding events thatenhanced the preservation of organic matter in anoxicbottom-water conditions (Erlich et al., 1999; Helenes& Somoza, 1999; Ramon et al., 2001). According tothe present data, the richest source rocks are identifiedin the uppermost Cenomanian and in the middleTuronian interval. This section could partly corre-spond to organic-rich intervals in the Luna Formationin Venezuela and the Magdalena Formation inColombia. However, our biostratigraphic constraintsdo not allow us to correlate our source rocks withOceanic Anoxic Event 2 (OAE2 of Schlanger &Jenkyns, 1976), because it seems to coincide with theEarly Turonian hiatus recognized in the Pungarayacu30 well. In addition, our analysis reveals severalpotential source rocks in the Santonian–Campaniansequence (Figure 3). However, these beds do notcontain similar amounts of fresh, fluorescent amor-phous organic matter as in the Middle Turonian. Thismeans that the depositional environment was dysoxicrather than anoxic.

The origin of oil accumulation in the Pungarayacuarea remains to be explained. The thermally immatureconditions of the sediments indicate that the liquidhydrocarbons were not generated there. It is possiblethat the oil was generated in, and migrated from, anequivalent source in a neighbouring area. Based on abiomarker analysis of several oils and source rocks,Mello et al. (1995) stated that oil of the Napo Group

correlates with some of the source rocks of the sameformation, especially with the richest source at theCenomanian/Turonian boundary. According to theseauthors the composition of the oil reveals a complexgeneration/migration history and in some reservoirs,severe biodegradation. If the reservoirs of the NapoGroup are in fact sourced from the same formation,a deep and probably very distant source has tobe considered. However, as already indicated byDashwood & Abbotts (1990) and Mello et al. (1995)the potential source rocks of the Napo Group in theOriente and SAZ are immature. The maturity dataof one well in the Oriente (well A) reported by Melloet al. (1995) indicate an immature source down to3352.8 m. These data confirm the problem of theorigin of the oil, which according to Mello et al. hasbeen produced around the peak generation stage.Alternatively, the oil may have been sourced frompre-Cretaceous rocks (e.g., the Upper JurassicSantiago Formation; Rivadeneira, 1986) for which theoil potential remains to be proven.

As recognized in seismic lines in the Oriente ofEcuador (Rosero, 1997; Rivadeneira & Baby, 1999),large-scale transpressive tectonic inversion affectedthe area during the Late Cretaceous (Turonian–Maastrichtian) giving rise to discrete steep thrusts andrelated unconformities in the Middle and Upper Napoformations. We assume that some of the unconformi-ties and hiatuses observed in the Pungarayacu 30 wellwere controlled by tectonic movements. The tectonicactivity was presumably driven by the accretion ofoceanic allochthonous terranes to northwestern SouthAmerica (Spikings et al., 2001; Hughes & Pilatasig,2002), which had a primary effect on the SAZbetween the Cordillera Real to the west and the rigidGuyana Shield to the east. Based on the immatureconditions of the sediments, we can also conclude thatthe Napo Uplift is an old structure that has not beendeeply buried.

Acknowledgements

This work was supported by a Swiss GovernmentGrant to CV. We thank Ing. Stalin Salgado fromPetroproduction Ecuador for providing access to thePungarayacu 30 cores and samples. The manuscripthas profited from critical comments of two anony-mous journal reviewers and Richard Spikings. DavidBatten’s advice is much appreciated.

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