The La Luna formation: chemostratigraphy and organicfacies in the Middle Magdalena Basin

A. Rangel a,*, P. Parra b, C. NinÄ o b

aECOPETROL-ICP, PBX 4185, Bucaramanga, ColombiabUniversidad Industrial de Santander, Colciencias, GEMS Ltda, PBX 4185, Bucaramanga, Colombia

Abstract

A detailed geochemical study and a sequence stratigraphic interpretation have been conducted on a sedimentary

sequence of the Upper Cretaceous La Luna Formation, in a section outcropping in the eastern ¯ank of the MiddleMagdalena Basin (MMB), Colombia. The goals were to evaluate geochemical variability related to lithofacies andorganic facies changes, characterize depositional environment and investigate the possible relationship between geo-

chemical data and sequence stratigraphic cycles. The La Luna Formation is composed of organic-rich sediments ofmonotonous appearance, with good to excellent potential for oil generation. Most of the bulk, petrographic and bio-marker parameters display a relatively narrow range of variation. However, the geochemical variations are su�cient to

di�erentiate organic facies types B, BC and C in the Salada Member, B and D in the Pujamana Member and B in theGalembo Member. Certain biomarker ratios are consistent within the La Luna Formation and are characteristic of itsdepositional environment, for example, average ratios of diasterane/sterane are lower than 1, Ts/Tm averages are less

than 0.33, the C35/C34 hopane ratio is more than 0.92, and oleanane/C30 hopane ratios range from 0.02 to 0.19.Regarding depositional condition indicators, the C35/C34 hopane ratio shows a good positive correlation with HI. Thissuggests that in carbonate environment changes in this parameter are more strongly related to redox condition than tochanges in carbonate content. Regarding the possible relationship between organic matter characteristics and sea level

changes, in regressive carbonate shelves during shallow stages, HI tends to increase and TOC tends to decrease, whilein regressive siliciclastic shelves, both TOC and HI decrease continuously. Some biomarker ratios (oleanane/C30

hopane, C20/C23 tricyclic, Ts/Tm) increase during base level falls. Regarding d 13C/12C isotope composition, the aro-

matic fraction and whole bitumen display an isotopic shift associated to the main deepening event in the section.# 2000 Elsevier Science Ltd. All rights reserved.

Keywords: La Luna Formation; Sequence stratigraphy; Molecular geochemistry; Depositional environments; Carbonates; Source rock

1. Introduction

The La Luna Formation has been considered to be

the main hydrocarbon source rock in the Middle Mag-dalena Basin (MMB) by Zumberge (1984), Rangel et al.(1996), as well as in other important basins such as

Maracaibo Basin (Talukdar et al., 1986). Nevertheless,the existing knowledge about depositional processescontrolling the Upper Cretaceous-La Luna Formation

deposit in the MMB is minimal and there are few inte-grated stratigraphic and geochemical studies on thisformation.

Zumberge (1984) addressed the hydrocarbon poten-tial of the La Luna Formation in the La Sorda Creek.Rangel et al. (1996) identi®ed four oil families, and

based on oil characteristics he suggested that two ofthem are possibly derived from the La Luna Formation.Ramon and Dzou (1999) discussed some geochemicalprocesses in the MMB, based on oil-derived parameters.

A clear understanding of geochemical characteristics ofthe La Luna Formation and associated organic facieswould increase con®dence in the oil±La Luna source

0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.

PI I : S0146-6380(00 )00127-3

Organic Geochemistry 31 (2000) 1267±1284

www.elsevier.nl/locate/orggeochem

* Corresponding author.

E-mail address: arangel@ecopetrol.com.co (A. Rangel).

rock correlation, as well as better delineate the con-tribution of each part of the sequence to oil generationin this basin. The relationship of geochemical para-meters to local depositional processes and stratigraphic

cycles would help identify the best source rock areas andallow us to better understand the spatial relationshipbetween source rock and migration conduits for oils

related to the La Luna Formation in the MMB.Bulk, petrographic, carbon-isotopic and molecular

geochemical data were used in this study, along with a

stratigraphic interpretation. The objectives were to evalu-ate geochemical changes related to existing lithofacies andorganic facies, to characterize depositional environment

and to investigate possible relationships between geo-chemical parameters and sequence stratigraphy cycles.

2. Methodology and sampling

Outcrop samples (160) were systematically collected

from an estimated 240 m vertical interval of the LaLuna Formation in La Sorda Creek, on the western¯ank of the Nuevo Mundo Syncline.

The sequence stratigraphic interpretation utilized inthis study follows the methods developed by the GeneticStratigraphy Research Group (GSRG) of Colorado

School of Mines (Cross, 1988; Cross et al. 1993; Crossand Lessenger, 1995). This methodology identi®es uni-directional trends of increasing and decreasing ratio ofaccommodation space to sediment ¯ux (A/S ratio).

Stratigraphic cycles register the time in both rise and fallof A/S.Regarding the geochemical study, all samples were

submitted for bulk geochemical analysis. Organic carbonanalysis (in a Leco Carbon Analyzer) and Rock Evalpyrolysis analysis (Espitalie et al., 1977) were performed

on all samples along with determination of carbonatecontent by acid treatment.The screening results were followed by analyses of 40

samples by gas chromatography and gas chromato-

graphy±mass spectrometry of rock extracts. Kerogenwas isolated by consecutive HCl and HF treatment, and¯oated in ZnBr. Powdered samples were Soxhlet

extracted with chloroform to remove extractableorganic matter. The hexane soluble material was thenseparated by liquid chromatography into saturate, aro-

matic and NSO fractions on an alumina and silica column.The whole bitumen, saturate and aromatic fractions wereprepared for carbon isotope composition by a modi®ed

method of Sofer (1984) and measured on a FinniganMAT Delta S instrument The saturate fractions weresubjected to GC and GC±MS analyses. Total alkanefractions or branched/cyclic sub-fractions were analyzed

in the selected ion recording mode on an HP 5890 GC±MS system. The GC column was a 30 m HP-5 tem-perature programmed from 60 to 320�C at 4�C/min and

helium carrier gas at 1.5 ml/min. Selected ion recordingwas performed on the 5890 MSD monitoring ions 177,191, 217, 218, 259 for saturate fractions.

3. Geological framework

The La Sorda Creek section is located approximately20 km West of Bucaramanga (Fig. 1). The sedimentarycolumn of the MMB, consists of Jurassic sandstones of

¯uvial origin, Cretaceous limestones and shales of shallowmarine to paludal origin, and Tertiary sedimentaryrocks of predominantly ¯uvial origin (Fig. 2).

Morales (1958) subdivided the La Luna Formationinto three members, which from base to top are, Salada(Turonian), Pujamana (upper Turonian-lower Coniacian)and Galembo (uppermost Turonian-Coniacian and

possibly Santonian).The tectonic evolution of the eastern edge of the MMB

is closely related to the tectonic evolution of the Eastern

Cordillera, as widely discussed by several authors(Campbell and BuÈ rgl, 1965; Macellari, 1988; Colleta etal., 1990; Dengo and Covey, 1993; Cooper et al., 1995).

The Eastern Cordillera consists of predominantlyclastic material and carbonates overlaid on a Pre-cambrian and Paleozoic basement. During Triassic±

Jurassic time, rifting and magmatic events produced byPaci®c plate subduction, was responsible for the upliftof the Central Cordillera and the deposition of con-tinental and volcanic rocks in the backarc setting. Dur-

ing the Early Cretaceous, a marine transgression led tothe backarc basin to be ®lled with a progradingsequence. Maximum transgression during the Tur-

onian±Santonian period, led to the deposition of the LaLuna Formation and its equivalent rocks, namely theVilleta (in the Upper Magdalena Valley) and Chipaque

or Gacheta Formations (in the Llanos basin and theEastern Cordillera), both also with excellent sourcerocks. During the latest Cretaceous (Maastrichtian), thebeginning of marine regression allowed deposition of a

transitional sequence (the Umir Formation); by accretionin the Western Cordillera. Finally, during Tertiary, therising of the Eastern Cordillera (Andean Orogeny) was

responsible for the development of a whole continentalsequence. This event reached its maximum during theMiocene-Pliocene period and is still continuing at present.

4. Results and discussion

4.1. Lithofacies and organic facies in the La LunaFormation (La Sorda Creek section)

The La Luna Formation is a calcareous petroleumsource rock with good to excellent potential for oil.Most of the sedimentologic and geochemical parameters

1268 A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284

of the La Luna Formation in La Sorda Creek display arelatively narrow range (Table 1). However, the sedi-

mentologic and geochemical variations are su�cient todi�erentiate several lithofacies and organic facies (Table2 and Fig. 3).

4.1.1. Lithofacies and organic facies in the SaladaMember

This member consists of 102.2 m of foraminiferalwackestones interbedded with occasionally-cherty cal-careous shales. The lithologies observed in this memberare grouped in four sedimentary lithofacies.

. Poorly laminated wackestones (plW): This litho-facies consists mainly of planktonic foraminiferal

wackestones with some vertebrae and bones of®sh, as well as pyrite traces.

. Muddy laminated wackestones and calcareous

shales (mlW): This lithofacies consists of darkgray, thin bedded foraminiferal wackestones andcalcareous shales, both with planoparallel lami-nations and small nodules.

. Phosphatic calcareous shales and laminated mud-stones (pcSM): This lithofacies consists of cal-careous, slightly phosphatic and ®nely laminated

shales and claystones, with abundant foraminifera(Fig. 4)

. Crystalline limestone (cL): This lithofacies consists

of two layers of 40 cm of greenish-gray crystallinelimestone with laminae of organic matter.

Organic facies type B, BC, and C, sensu Jones (1984),was identi®ed in the Salada Member (Table 2). Organicfacies B is related to mlW and pcSM lithofacies. Thisfacies is composed of organic matter with average values

of HI around 428 (mg HC/g TOC), TOC around 4.3(wt.%) and S2 between 15.6 and 22.2 mg HC/g sample.The saturated hydrocarbons of this organic facies is

Fig. 1. Location of the La Sorda creek section in the Middle Magdalena Basin, Colombia.

Fig. 2. Cretaceous stratigraphic units in the Middle Magdalena

Basin.

A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284 1269

Table 1

Average values (mean�standard deviation) of bulk geochemical parameters for each lithofacies of the La Luna Formation

Member Organic

facies

Lithofacies %

thickness

na %

TOC

Tmax

(�C)S1

(mg HC/g

rock)

S2

(mg HC/g

rock)

S3

(mg CO2/g

rock)

Hl

(mg HC/g

TOC)

OI

(mg CO2/g

rock)

%

Carbonate

%

Bitumen

Galembo B pPh 1.4 15 2.6�1.0 434�4 4.61�2.25 13.46�5.23 0.59�0.17 523�63 25�9 31.9�9.7 2.34�0.32B phPW 3.9 15 2.3�0.8 436�4 3.72�1.64 11.33�4.52 0.47�0.15 491�62 22�9 52.0�10.7 2.35�0.98B pcSM1b 3.2 14 2.4�1.0 438�3 3.79�1.88 11.36�5.22 0.40�0.08 467�72 20�10 27.4�14.5 1.08�0.77

Average Organic

facies B

pPH, phPW,

pcSM1

8.5 44 2.4�0.9 436�4 4.04�1.94 12.07�4.98 0.49�0.16 495�68 22�9 37.3�15.8 1.78�0.90

Pujamana B pcSM2b 49.3 63 3.2�1.1 435�3 4.43�2.52 14.41�5.69 0.47�0.11 434�88 18�17 22.3�13.1 1.55�1.15

D B 0.5 3 0.3�0.1 437� 0.04 0.04�0.02 0.16�0.11 14�12 52�30 0.3�0.5 n.d.c

B mlW2 1.4 3 3.7�0.9 438�3 4.41�0.66 15.37�3.65 0.69�0.08 422�51 20�5 49.8�3.7 1.68�0.76Average Organic facies

B and D

pcSM2,

B, mlW2

51.1 69 3.1�1.2 436�3 4.24�2.58 13.82�6.23 0.46�0.14 415�121 20�18 22.6�14.6 1.57�1.10

Salada BC plW 18.4 18 1.8�1.3 435�3 1.74�1.33 7.06�5.85 0.47�0.12 360�77 38�27 75.1�12.3 0.64�0.41B mlW3 10.9 11 4.2�1.3 438�3 2.95�0.98 15.55�3.18 0.56�0.11 395�84 56.9�10.2 0.89

B pcSM3b 10.5 8 4.6�1.2 434�4 5.86�3.47 22.50�9.66 0.52�0.05 474�132 13�6 28.1�11.3 3.69�2.83

C cL 0.5 7 0.9�0.9 432�9 0.81�0.95 3.47�4.33 0.31�0.22 234�190 53�32 66.2�11.0 n.d.

Average Organic facies

B, BC and C

plW, mlW3,

pcSM3, cL

40.4 44 2.8�1.9 435�5 2.60�2.41 11.28�8.85 0.48�0.15 366�132 30�26 59.9�20.7 1.60�1.98

a n samples analyzed.b Total lithofacies pcSM=pcSM1+pcSM2+pcSM3=62.96%.c n.d., no data

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istry31(2000)1267±1284

characterized by the high ratios of C30/C29 sterane andrelatively low ratios of C29/C30 hopane and C23 tricyclic/C24 tetracyclic (Fig. 5; Tables 2 and 3).

The organic facies BC is related to the plW litho-facies. This facies is characterized by average values ofHI around 360, TOC around 1.8%, and an average of

S2 of 7 mg HC/g of rock. This facies has the highestvalues of oleanane/C30 hopane and tricyclic C20/C23

ratios, and relatively low values of diasterane/sterane

(0.46). The other geochemical parameters are withinthe range of the La Luna Formation (Tables 2 and 3;Fig. 5).

Table 2

Characteristics of the organic facies in the La Luna Formation (La Sorda Creek section)

Member Galembo Pujamana Salada

Organic facies B B D B BC C

% in the section 8.48 50.62 0.51 21.42 18.44 0.53

TOC (wt.%) 2.4 (�0.9) 3.2 (�1.1) 0.3 (�0.1) 4.3 (�1.3) 1.8 (�1.3) 0.9 (�0.9)HI (mg HC/g TOC) 495 (�68) 434 (�87) 14 (�12) 428 (�111) 360 (�77) 234 (�190)OI (mg CO2/g TOC) 22 (�9) 18 (�16) 52 (�30) 15 (�9) 38 (�27) 53 (�32)Tmax (

�C) 436 (�4) 436 (�3) 437 436 (�4) 435 (�3) 432 (�9)Carbonate (%) 37.3 (�15.8) 23.6 (�14.1) 0.3 (�0.5) 44.8 (�17.9) 75.1 (�12.3) 66.2 (�10.7)

Fig. 3. Stratigraphic pro®le, cycles interpretation and lithofacies and organic facies substitution diagrams of the La Sorda Creek

section. The percentages of each lithofacies are shown in the diagram.

A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284 1271

An organic Facies C associated with the cL litho-facies, displays values of HI around 234 and TOCaround 0.9%. This organic facies has the lowest average

ratios of oleanane/C30 hopane (0.02), C30/C29 sterane(0.17), gammacerane/C30 sterane (0.09), diasterane/sterane(0.18) and C35/C34 hopane (0.92) (Tables 2 and 3; Fig. 5).

4.1.2. Lithofacies and organic facies in the PujamanaMember

This member consists mainly of calcareous phospha-tic shales with abundant foraminifera, phosphatic cal-careous mudstones, cherts and bentonites. Calcareousnodules as large as 1 m in diameter are observed. The

abundance of pyrite is greater than in Salada Member.Three lithofacies were identi®ed in this member (Fig. 3).

. Phosphatic calcareous shales and laminated mud-stones (pcSM): This lithofacies consists of shalesand calcareous mudstones that are ®nely lami-

nated and slightly phosphatic, with abundant for-aminifera and bones of ®sh.

. Bentonites (B): This lithofacies is composed ofyellowish-gray, greenish-gray and grayish-orange

clays. (smectite±illite). The lithofacies generallyappears in tabular layers thinner than 35 cm.

. Muddy laminated wackestones and calcareous

shales (mlW): This lithofacies consists of darkgray, thin-bedded foraminiferal wackestones andcalcareous shales.

Organic facies type B and D were observed in thismember. The organic facies B, the most abundant, is

associated with the pcSM and mlW lithofacies. Averagesvalues of HI and TOC are around 434 and 3.2, respec-tively, and S2 varies between 14,41 and 15,37. Thisorganic facies shows very little variation in the average

values of Ts/Tm (0.16±0.19), C35/C34 hopane (1.10±1.32),C24 tetracyclic/C26 tricyclic (1.08±1.09) (Tables 1, 2 and3; Fig. 5).

Organic facies C, associated with the bentonite litho-facies, has HI values of about 14, and TOC of about0.3%. This is a minor organic facies (0.5% of the geo-

logical column by volume) (Tables 1 and 2).

4.1.3. Lithofacies and organic facies in the Galembo

MemberThe lower part of this member (19 m) was studied.

The section consists of a series of packstone phosphor-

ites (sensu Greensmith, 1989), wackestones and phos-phatic packstones, chert and calcareous and phosphaticshale. Layers are tabular, with a thickness rangebetween 5 and 30 cm and concretions up to 2 m in dia-

meter are present toward the base of this member. Threelithofacies were identi®ed in this member (Fig. 3).

. Phosphatic calcareous shales and laminated mud-stones (pcSM): This lithofacies consists of cal-careous, slightly phosphatic and ®nely laminated

shales and claystones, with abundant for-aminifera.

. Packstone phosphorites (pPh): This facies consistsof packstone phosphorites with abundant for-

aminifera, pellets, ®sh bone fragments, andoolites. Wavy and lenticular lamination is com-mon and in some samples ¯aser lamination can be

observed.. Phosphatic packstones and wackestones (phPW):

This lithofacies was observed towards the top of

the Galembo member. It consists mainly ofslightly phosphatic foraminiferal packstones, withlenticular and wavy lamination. This lithofacies

also contain ®sh fragments, pellets, oolites andPlanktonic fossils.

The organic matter of Galembo Member exhibits the

greatest HI around 495, OI average of 22, TOC of about2.4% and S2 of 12. These geochemical parameters aretypical of organic facies B. This organic facies displays

Fig. 4. Photomicograph of a thin section from mlW (a) and pcSM2 (b) lithofacies. Sample CD-152 and sample CD-90. Lamination

corresponds to alternation of foraminifer tests, organic matter and clay minerals. Magni®cation 2.5�. Plane light.

1272 A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284

Table 3

Average values (mean�standard deviation) of biomarker parameters for each lithofacies and organic facies in La Luna Formation

Member Organic

facies

Lithofacies na Ts/Tm Diasterane/

regular

steraneb

C35/C34

extended

hopanes

Tricyclic

terpanes/

hopanesc

%C27

sterane

%C29

sterane

C30/C29

sterane

C24 tetracyclic/

C26 tricyclic

C23 tricyclic/

C24 tetracyclic

C29 norhopane/

C30 hopane

Oleanane/

C30 hopane

Gammacerane/

C30 hopane

Steranes/

hopanesdC20/C23

tricyclic

Pr/Ph

Galembo B phPW 2 0.28 0.71 n.d. 3.13�2.06 28.67�8.40 24.42�7.98 0.26 0.98�0.40 8.22�5.59 2.13�0.58 0.05 n.d.e 0.15�0.07 0.61�0.45 1.01

B pPh 3 0.25 0.35�0.07 1.13�0.16 2.02�0.08 38.52�9.59 29.10�7.28 0.23�0.03 0.59�0.07 14.55�1.17 1.40�0.66 0.04�0.03 0.10�0.04 0.34�0.08 0.20�0.02 n.d.

B pcSM1 4 0.26�0.01 0.59�0.49 1.35�0.32 2.85�0.53 38.97�6.40 26.88�1.98 0.27�0.02 1.12�0.57 10.17�5.92 0.95�0.33 0.13�0.11 0.33�0.37 0.29�0.09 0.52�0.51 0.60

Average Organic

facies B

pPH,

phPW,

pcSM1

7 0.26�0.02 0.51�0.36 1.28�0.28 2.63�0.93 36.53�8.19 27.07�5.10 0.26�0.03 0.91�0.45 11.20�4.93 1.36�0.65 0.08�0.09 0.25�0.31 0.28�0.10 0.42�0.37 0.81

Pujamana B pcSM2 23 0.16�0.05 0.45�0.44 1.10�0.18 2.33�1.06 44.57�6.12 23.46�5.71 0.29�0.08 1.09�0.56 11.86�5.01 1.27�0.49 0.08�0.11 0.32�0.28 0.20�0.09 0.28�0.16 0.85

B mlW2 8 0.19�0.02 1.20�0.60 1.32�0.34 2.60�1.23 39.12�2.67 26.20�4.50 0.35�0.11 1.08�0.50 11.00�5.47 0.94�0.65 0.03�0.02 0.30�0.28 0.10�0.04 0.22�0.02 n.d.

Average Organic

facies B

pcSM2,

mlW2

31 0.17�0.05 0.52�0.49 1.13�0.22 2.36�1.05 43.94�6.06 23.77�5.58 0.30�0.08 1.09�0.54 11.76�4.96 1.23�0.51 0.08�0.11 0.31�0.27 0.19�0.09 0.28�0.16 0.85

Salada BC plW 6 0.31�0.17 0.46�0.28 1.03�0.09 3.01�0.78 38.99�4.16 27.36�4.89 0.31�0.04 0.98�0.36 9.54�7.68 1.38�0.25 0.19�0.15 0.26�0.15 0.24�0.05 0.79�0.71 0.52

B mlW3 8 0.19�0.04 0.31�0.19 1.10�0.09 2.73�0.93 41.74�5.29 26.12�2.88 0.21�0.05 0.93�0.24 12.32�5.90 1.43�0.32 0.05�0.06 0.23�0.23 0.28�0.11 0.38�0.23 0.98

B pcSM3 6 0.33�0.22 0.64�0.49 1.24�0.38 1.78�0.72 38.91�4.98 24.23�5.81 0.76�0.66 2.97�2.31 5.70�6.60 0.78�0.43 0.03�0.01 0.15�0.06 0.21�0.12 0.25�0.05 0.49

Average Organic

facies B

mlW3,

pcSM3

14 0.25�0.16 0.46�0.38 1.17�0.28 2.32�0.95 40.53�5.17 25.31�4.29 0.46�0.51 1.80�1.78 9.49�6.86 1.15�0.49 0.04�0.05 0.20�0.19 0.26�0.11 0.34�0.21 0.73

C cL 1 0.16 0.18 0.92 1.90 41.22 26.63 0.17 0.65 17.00 1.31 0.02 0.09 0.27�0.00 0.21 n.d.

a n, Samples analyzed.b C27 ba diasterane (20S)/C27 aaa sterane (20R).c Sum tri/sum hopanes.d Sum sterane/sum hopanes.e n.d., no data.

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1273

Fig. 5. Histograms showing the average values and standard deviation of some geochemical bulk parameters and representative bio-

marker ratios for each lithofacies in the La Luna Formation.

1274 A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284

Fig. 6. Schematic evolution of the carbonate ramp.

Table 4

Carbon isotopes ratios in whole bitumen, saturates and aromatic fractions for di�erent samples of the La Luna formation

Member Organic facies Lithofacies Sample ID Cummulative thickness 13C Bitumen d13C saturates 13C Aromatics

Galembo B pcSM1 CD07 3.0 ÿ27.40 ÿ28.45 ÿ27.38B phPW CD15 9.0 ÿ27.43 ÿ27.42 ÿ27.37B pPh CD22 14.0 ÿ27.24 ÿ27.35 ÿ27.21B pcSM1 CD41 18.2 ÿ27.33 ÿ27.32 ÿ27.36

Pujamana B pcSM2 CD50 25.9 ÿ27.76 ÿ27.91 ÿ27.40B pcSM2 CD59 34.0 ÿ27.27 ÿ27.57 ÿ27.28B pcSM2 CD60 34.3 ÿ27.33 ÿ27.27 ÿ27.31B pcSM2 CD82 58.2 ÿ27.90 n.d. ÿ27.94B pcSM2 CD98 74.4 ÿ26.91 ÿ27.52 ÿ26.99B pcSM2 CD104 102.5 ÿ26.08 ÿ28.18 ÿ26.78B mlW2 CD110 112.0 ÿ26.61 ÿ27.78 ÿ26.34B pcSM2 CD114 130.0 ÿ26.47 ÿ27.41 ÿ26.49

Salada B pcSM3 CD119 142.0 ÿ26.13 ÿ27.98 ÿ26.56BC plW CD129 179.5 ÿ27.51 ÿ27.99 ÿ27.47BC plW CD135 192.5 ÿ27.62 ÿ28.43 ÿ27.84B mlW3 CD143 204.0 ÿ27.61 ÿ27.55 ÿ27.67BC plW CD154 221.0 ÿ27.46 ÿ28.31 ÿ27.60B pcSM3 CD157 228.0 ÿ27.41 ÿ28.31 ÿ27.43BC plW CD158 233.0 ÿ27.39 ÿ27.56 ÿ27.79B pcSM3 CD160 235.0 ÿ27.96 ÿ28.33 ÿ27.86

Average ÿ27.24 ÿ27.82 ÿ27.30

A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284 1275

little variation of geochemical parameters such as Ts/Tm

(0.25±0.28), C35/C34 hopane (1.13±1.35) and C30/C29

sterane (0.23±0.27). The oleanane/C30 hopane ratio ishigh, varying between 0.04±0.13 (Tables 2 and 3; Fig. 5).

The phosphatic packstone lithofacies of the Galembomember shows the highest values of HI, the highestratios of sterane/hopane (0.34) and C23 tricyclic/C24

tetracyclic (14.55) and the lowest ratios of C20/C23 tri-cyclic (0.20) and C24 tetracyclic/C26 tricyclic (0.59) of theLa Luna Formation. This parameter could correlate

with increased algal material within this lithofacies.These characteristics are consistent with the observedabundance of phosphates indicating an upwelling event.

The HI versus OI plot, the modi®ed Van Krevelendiagram (Fig. 7), shows that organic matter is pre-dominantly Type II, amorphous (algal and bacterial),and Type I organic matter. A very small proportion of

Type II/III and III/IV kerogen is also observed. Talukdaret al. (1986) also noted, based on microscopic analysesand molecular geochemistry, that the bulk of the

organic matter is algal and bacterial in origin.Summarizing, Organic Facies B is the most repre-

sentative organic facies of the La Sorda Creek section

with a total percentage of 80.5% in volume. This faciesis related to phPW, pPh, pcSM, and mlW lithofacies.The organic facies BC represent 18.4% of the section by

volume. Organic facies C and D represent 0.5 and 0.5%of the stratigraphic column, respectively.

4.2. Sedimentary cycles

Based on lithologic and sedimentological character-istics, a facies substitution diagrams (Fig. 3) for the La

Luna formation in La Sorda Creek was interpreted.This diagram provides information about the naturalsuccession and substitution of lithofacies under increasingor decreasing accommodation conditions. The area of

each lithofacies is proportional to its abundance withinthe section. This diagram helps to identify stratigraphiccycles of high, intermediate, and low frequency, for

sequence stratigraphic analysis.The intermediate and low frequency cycles for the La

Luna Formation in La Sorda Creek section de®ned in

this work, using lithologic and stratigraphic attributes,correlate with cycles described for this formation byReyes et al. (1998), using well logs and cores.

From base to top, the transition of the argillaceousshales of the SimitõÂ Formation to calcareous shales, andthe appearance of thin layers of foraminiferal wack-estones (Salada Member), indicates a generalized low

frequency hemicycle of base level fall. This hemicyclereaches a maximum fall (minimum in A/S) where thelimestone layers have the greatest thickness, up to 170 m

of measured thickness (Fig. 6).From this point of maximum progradation, a deepening

event began, evidenced by the decrease in thickness of the

wackestones layers, and the increase in thickness ofshales and calcareous mudstones (pcSM lithofacies).This increase in A/S ratio ends in a surface of maximum

¯ooding up to 75 m thick, with high contents of organicmatter and low percentages of carbonates.After the maximum deepening within the Pujamana

Member, the amount of phosphates progressively

increases. Packstone phosphorites (lithofacies pPh) andphosphatic packstones (lithofacies phPW) appear. Thisdemonstrates a base level fall in the basin (Fig. 5). The

shallowing is not completely recorded in the column ofthe La Sorda Creek, because of a fault at the top of theLa Sorda Creek section. Stratigraphic events in the

basin are shown in detail by the intermediate frequencycycles (third order cycles, sensu Vail et al., 1977) (Fig. 3).

4.3. Paleoceanographic events and environmental

considerations

The ®ne lamination and the calcareous character of

Salada Member and the presence of very ®ne planktonicforaminifera arranged in laminae, demonstrate a lowenergy marine environment of deposition.

The shale sequence and the minor calcareous char-acter, which characterize the Pujamana Member, evi-dence deeper conditions in the carbonate platform than

those during the deposition of the Salada Member (Fig.6). The increase of phosphates towards the top is con-sistent with the occurrence of upwelling currents. Thehigh contents of organic matter, as well as the presence

of pyrite are evidence of high productivity and suboxicto anoxic condition, which favored accumulation andpreservation of organic matter. The bentonites represent

Fig. 7. Modi®ed van Krevelen diagram showing lithofacies and

organic facies.

1276 A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284

sedimentary input from volcanic activity during thisperiod.The packstone phosphorites and phosphatic pack-

stones that characterize the Galembo Member indicate a

¯ow regime greater than the one during the sedimenta-tion of the Salada Member and the Pujamana Member.The presence of wavy and lenticular lamination, ripples,

¯aser laminations, reworking of ®sh fragments and pre-sence of quartz demonstrates a shallow water environ-ment within wave base. These lithofacies correspond to

the shallowest level within the carbonate platform inwhich the La Luna Formation was deposited (Fig. 6).The increase of phosphates indicates upwelling and high

primary productivity in a suboxic environment.The accumulation of organic rich sediments in this

area, during Late Cretaceous, was favored by interactionof important regional and global paleogeographic events,

such as Ekman water transport (closely related toupwelling regimens), and oceanic anoxic events describedby Macellari (1998), Martinez and Herna ndez (1992) and

Villamil (1998) for the north corner of South America.Taking into account the data of this study, and the

regional paleogeographic framework, the La Luna For-

mation was deposited on a large carbonate platform. Onthis platform, sedimentation took place in a through,limited to the West by the submerged Central Cordil-

lera, which restricted water circulation and contributedto an anoxic environment of deposition. During someless restricted periods, upwelling currents favored highprimary productivity, and organic matter accumulation

and preservation that characterize the sediments of theLa Luna Formation. According to Martinez and Her-na ndez (1992), the deepest parts of the ramp were loca-

ted near the present area of Maracaibo, where amaximum depth of approximately 600 m was attainedduring the Campanian.

4.4. Maturity level

Because of the predominance of amorphous organic

matter, the samples contain little vitrinite, and thereforethe vitrinite re¯ectance measures are unreliable. Thematuration level in this study was determined using Rock-

Eval pyrolysis.Tmax values (average 436�C) listed in Table

1. This suggests the section is early mature to mature.Average Tmax and S1 values suggest that the section is

not greatly a�ected by oil migration, and that geo-chemical variations can largely be considered as indica-tive of depositional conditions and associated variations

in the type of organic matter.

4.5. Relationship between geochemical parameters andsequence stratigraphy

Several authors have described the relationshipbetween the characteristics of organic matter and sea

level changes (e.g. Middleburg et al., 1991; Pasley et al.,1993). The relationship between some geochemicalparameters commonly used to characterize petroleumsource rocks and interpreted sequence stratigraphy

cycles in the La Sorda Creek section are graphicallyshown in Fig. 8. The trends in variations of geochemicalparameters in this ®gure are generalized. In the regres-

sive carbonate shelf (Salada Member and Galembomembers) an increase in the relative values of HI and adecrease in TOC contents is observed, while in the

regressive siliciclastic shelf (Pujamana-Salada Member),both TOC and HI values decrease continuously duringthe shallowing stage. Some biomarker parameters such

as oleanane/C30hopane and C20/C23 tricyclic ratiosusually used to re¯ect relative contribution of con-tinental organic matter, increase during regressioncycles. This occurs on both carbonate and siliciclastic

shelves. Other parameters suggested by some authors(e.g. Waples and Machihara, 1990) as sensitive tolithology, such as Ts/Tm ratios, increase in carbonate

levels. In the carbonate shelves, HI values increasedespite the higher oleanane/C30 hopane and C20/C23

tricyclic ratios, probably suggesting that algal pro-

ductivity and preservation are predominant processes inthis depositional environment. Based on these results,organic geochemistry could be considered as an important

tool to support the sequence stratigraphy architecture ofa sedimentary sequence. Conversely, sequence strati-graphy is a useful tool to follow oil prone strata.

4.6. Sedimentological controls on geochemical composition

Biomarker fragmentograms appear similar for the

di�erent organic facies upon initial inspection (Fig. 9).The average of certain biomarker ratios (Table 4 andFig. 5), show a narrow range. Therefore, these values

can be considered typical for the La Luna Formationand its depositional conditions. This is useful for oil-source rock correlation in the MMB.The pristane/phytane (Pr/Ph) ratio is lower than 1.01

con®rming an anoxic/reducing depositional environ-ment for the La Luna Formation. As was noted byZumberge (1984), the most abundant cyclic compounds

throughout the La Luna formation are the tricyclic ter-panes and hopanes while steranes abundance are relativelylow. The sterane distributions display a predominance

of C27 steranes. Low concentration of rearranged C27

steranes relative to regular steranes are also character-istic (Fig. 9).

Most extracts display diasterane/sterane ratios lessthan 1, except those from the mlW lithofacies (Table 3).According to Rubinstein et al. (1975), Mello et al.(1988), and Moldowan et al. (1986), the relatively low

abundance of diasteranes over regular steranes shouldbe related to a carbonate/anoxic environment, typical ofthe La Luna Formation.

A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284 1277

Fig.8.Variationofgeochem

icalbulk

parametersandsometypicalbiomarkerscorrelatedwithstratigraphic

sequence

intheLaLunaForm

ation,LaSordacreek.

1278 A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284

The extended hopanes have relative abundance of C35

hopanes (C35/C34 hopane ratios higher than 0.92). HighC35 hopane are commonly associated with marine car-bonate environment (Mello et al., 1988; Clark and

Philp, 1989). Additionally, Peters and Moldowan (1993)interpret this phenomenon as a general indicator of ahighly reducing marine condition during deposition.

The high relative abundance of C35 over C34 hopane inthe La Luna Formation con®rms its association to car-bonate environment.The C35/C34 hopane ratios show a correlation with HI

(Fig. 10a) indicating that in a carbonate environment,changes in these parameters correlate with a redox condi-tions rather than with changes in the carbonate content.

Fig. 9. Hopanes and tricyclic terpanes (m/z 191) and steranes (m/z 217) in typical samples from organic facies B and C.

A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284 1279

The lower abundance of the C35 hopanes in the bitumenof the cL lithofacies might indicate more oxic conditionsduring deposition.The abundance of C24 tetracyclic terpane/C26 tricyclic

ranges from an average of 0.59 in the pPh lithofacies to

2.97 in the pcSM lithofacies. The C24 tetracyclic/C26

tryciclic ratio is in general less than 1, except in the moresiliciclastic lithofacies (pcSM). Ekweozor et al. (1981),reported abundance of C24 tetracyclic terpanes in oils of

deltaic origin. According to Mello et al. (1988) and

Fig. 10. Crossplots and some typical geochemical parameters. The symbols correspond to average values for each lithofacies.

1280 A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284

Fig.11.Isotopecorrelations:(a)relativesealevel

changes

accordingto

stratigraphic

andsedim

entologic

analyses;(b)pro®le

ofcarbonisotopic

compositionofwhole

bitumen

and

aromatichydrocarbonfraction;(c)d

13C

saturatesversusd

13C

aromatichydrocarbons.Sofer(1984)cross-plot.

A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284 1281

Philp and Gilbert (1986), the abundance of the C24 tet-racyclic may indicate higher plant marker. In extracts ofthe La Luna Formation (calcareous platform), varia-tions in the abundance of this compound could not be

clearly related to higher plant input. Instead, it seems toincrease in more siliciclastic facies.The tricyclic terpanes (C19±C30) display a high relative

abundance over hopanes (Table 3). Mello et al. (1988)indicated that samples of lacustrine saline environmentsand marine carbonate related environments are char-

acterized by high relative abundance of tricyclic ter-panes. Fig. 10c shows a relatively good correlationbetween carbonate content and tricyclic terpanes/

hopanes ratio.The Ts/Tm ratios are less than 0.33 (Table 3), and the

relative abundance of Ts increases systematically duringthe regressive cycles (Fig. 8). According to Peters and

Moldowan (1993), many authors have associatedanomalously low (Ts/Ts+Tm) ratios to carbonatesource rock, which is also observed in this study.

Oleanane (triterpane of higher plant origin; Ekweozoret al., 1979) is present in low abundance. The oleanane/C30 hopane ratio range from 0.02 to 0.19 (Table 3).

Most of the extracts derived from sediments depositedin deeper environments such as the pcSM Pujamana-Salada lithofacies, have lower oleanane/hopane ratios

than those derived from carbonate shallow facies. Basedon the sequence stratigraphy cycles and geochemicallogs (Fig. 8), oleanane/C30 hopane can be proposed as agood indicator of rise and fall of sea level in third order

sequence stratigraphy cycles.The cross plots of oleanane/C30 hopane,%C29 steranes,

and C20/C23 tricyclic versus TOC show inverse correla-

tion (Figs. 10d±f). This inverse correlation is consistentwith the opposite trends displayed for this parameter inFig. 8. Higher oleanane/C30 hopane,%C29 sterane and

C20/C23 tricyclic are considered to re¯ect relativelygreater contribution of higher plant material, in thiscase associated with a shallow platform environment.These plots help to delineate the three organic facies

grouped according to Jones' (1987) criteria (Fig. 10).The organic facies C, related to cL lithofacies, alwayspresents an anomalous trend.

4.7. Isotope composition

The average values of the saturate fractions isÿ27.82% PDB and the range of variation is 1.18% inthe saturate fractions (Table 4). In the aromatic fraction

and whole bitumen, the range of variation is 1.6 and1.88%, respectively, greater than observed in saturatefractions. The isotope log of the aromatic fraction andwhole bitumen displays an isotopic shift associated with

the main sea level fall in the section (Fig. 11a).Perez-Infante et al. (1996) also observed a marked d

13C Corg isotopic excursion in the middle part of a

Maraca Creek section of the La Luna Formation, whichwas interpreted as a global depletion in 12C around theCenomanian /Turonian and the Coniacian/Santonianboundary.

Using the plot of Sofer (1984) (Fig. 11c), isotopicallyheavier samples correspond to extracts from pcSMlithofacies (more siliciclastic) and isotopically lighter

extracts are related to more calcareous lithofacies. Allextracts plot in the marine area of Sofer (1984).

5. Conclusions

The La Luna Formation is a petroleum source rockwith good to excellent potential for oil. About 63% ofthe volume of this formation is composed of phosphaticcalcareous shales and laminated mudstones with abun-

dant foraminifers (pcSM lithofacies).Three low frequency hemicycles were identi®ed in the

La Luna Formation: A generalized base level fall, during

the deposition of the Salada Member; a base level rise ora deepening of the basin during the sedimentation of thePujamana Member, and a second base level fall that

permitted the deposition of the calcareous and phos-phatic lithologies of the Galembo member. Regardingthe relationship between organicmatter characteristics and

sea level changes, during shallowing stages in carbonateshelves (Salada Member and Galembo Member), HItends to increase and TOC to decrease. In the siliciclasticshelf, during shallowing stages, (Pujamana-Salada

Member), both TOC and HI decrease continuously.Certain biomarker ratios such as oleanane/C30 hopane,C20/C23 tricyclic, Ts/Tm show an increasing trend during

base level falls and could be proposed as a good indi-cator of rise and fall of sea level in third order sequencestratigraphy cycles.

Sedimentation of the La Luna Formation occurredon a large carbonate ramp, with restricted water circu-lation and anoxity. During certain periods, upwellingfavored high primary productivity and accumulation

and preservation of organic matter. Some biomarkerratios can be considered typical values for the deposi-tional environment of the La Luna Formation (e.g.

diasterane/sterane ratios < 1, Ts/Tm average < 0.33,C35/C34 hopane>0.92, and oleanane/C30 hopane rangingfrom 0.02 to 0.19). The C35/C34 hopane ratio correlates

with HI; suggesting that in carbonate environments,changes in this parameter are more strongly related toredox condition rather than to changes in carbonate

content.It is possible to di�erentiate organic facies type B, BC

and C in the Salada Member, organic facies type B andD in the Pujamana Member and organic facies type B in

the Galembo Member. The d 13C isotope composition ofaromatic fractions and whole bitumens display an isotopicshift associated with the main deepening event in the

1282 A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284

section. Based on these results, organic geochemistrycould be considered as an important tool to support thesequence stratigraphy architecture of a sedimentarysuccession.

Acknowledgements

We are grateful to Drs. R. Sassen and S. Talukdar forhelpful suggestions and constructive reviews and Drs.M.N. YalcË y n and S. Y nan for major linguistic and edi-

torial corrections.

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