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

Organic Geochemistry 31 (2000) 1267±1284

www.elsevier.nl/locate/orggeochem

The La Luna formation: chemostratigraphy and organic facies in the Middle Magdalena Basin A. Rangel a,*, P. Parra b, C. NinÄo b b

a ECOPETROL-ICP, PBX 4185, Bucaramanga, Colombia Universidad 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 Middle Magdalena Basin (MMB), Colombia. The goals were to evaluate geochemical variability related to lithofacies and organic facies changes, characterize depositional environment and investigate the possible relationship between geochemical data and sequence stratigraphic cycles. The La Luna Formation is composed of organic-rich sediments of monotonous appearance, with good to excellent potential for oil generation. Most of the bulk, petrographic and biomarker 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 the Galembo Member. Certain biomarker ratios are consistent within the La Luna Formation and are characteristic of its depositional 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. This suggests that in carbonate environment changes in this parameter are more strongly related to redox condition than to changes 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, while in 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 aromatic 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 Magdalena 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 processes controlling the Upper Cretaceous-La Luna Formation

* Corresponding author. E-mail address: [email protected] (A. Rangel).

deposit in the MMB is minimal and there are few integrated stratigraphic and geochemical studies on this formation. Zumberge (1984) addressed the hydrocarbon potential 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 of them are possibly derived from the La Luna Formation. Ramon and Dzou (1999) discussed some geochemical processes in the MMB, based on oil-derived parameters. A clear understanding of geochemical characteristics of the La Luna Formation and associated organic facies would increase con®dence in the oil±La Luna source

0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0146-6380(00)00127-3

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rock correlation, as well as better delineate the contribution of each part of the sequence to oil generation in this basin. The relationship of geochemical parameters to local depositional processes and stratigraphic cycles would help identify the best source rock areas and allow us to better understand the spatial relationship between 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 evaluate geochemical changes related to existing lithofacies and organic facies, to characterize depositional environment and to investigate possible relationships between geochemical parameters and sequence stratigraphy cycles. 2. Methodology and sampling Outcrop samples (160) were systematically collected from an estimated 240 m vertical interval of the La Luna Formation in La Sorda Creek, on the western ¯ank of the Nuevo Mundo Syncline. The sequence stratigraphic interpretation utilized in this study follows the methods developed by the Genetic Stratigraphy Research Group (GSRG) of Colorado School of Mines (Cross, 1988; Cross et al. 1993; Cross and Lessenger, 1995). This methodology identi®es unidirectional trends of increasing and decreasing ratio of accommodation space to sediment ¯ux (A/S ratio). Stratigraphic cycles register the time in both rise and fall of A/S. Regarding the geochemical study, all samples were submitted for bulk geochemical analysis. Organic carbon analysis (in a Leco Carbon Analyzer) and Rock Eval pyrolysis analysis (Espitalie et al., 1977) were performed on all samples along with determination of carbonate content by acid treatment. The screening results were followed by analyses of 40 samples by gas chromatography and gas chromatography±mass spectrometry of rock extracts. Kerogen was isolated by consecutive HCl and HF treatment, and ¯oated in ZnBr. Powdered samples were Soxhlet extracted with chloroform to remove extractable organic matter. The hexane soluble material was then separated by liquid chromatography into saturate, aromatic and NSO fractions on an alumina and silica column. The whole bitumen, saturate and aromatic fractions were prepared for carbon isotope composition by a modi®ed method of Sofer (1984) and measured on a Finnigan MAT Delta S instrument The saturate fractions were subjected to GC and GC±MS analyses. Total alkane fractions 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 temperature programmed from 60 to 320 C at 4 C/min and

helium carrier gas at 1.5 ml/min. Selected ion recording was 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 approximately 20 km West of Bucaramanga (Fig. 1). The sedimentary column of the MMB, consists of Jurassic sandstones of ¯uvial origin, Cretaceous limestones and shales of shallow marine to paludal origin, and Tertiary sedimentary rocks of predominantly ¯uvial origin (Fig. 2). Morales (1958) subdivided the La Luna Formation into 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 BuÈrgl, 1965; Macellari, 1988; Colleta et al., 1990; Dengo and Covey, 1993; Cooper et al., 1995). The Eastern Cordillera consists of predominantly clastic material and carbonates overlaid on a Precambrian and Paleozoic basement. During Triassic± Jurassic time, rifting and magmatic events produced by Paci®c plate subduction, was responsible for the uplift of the Central Cordillera and the deposition of continental and volcanic rocks in the backarc setting. During the Early Cretaceous, a marine transgression led to the backarc basin to be ®lled with a prograding sequence. Maximum transgression during the Turonian±Santonian period, led to the deposition of the La Luna Formation and its equivalent rocks, namely the Villeta (in the Upper Magdalena Valley) and Chipaque or Gacheta Formations (in the Llanos basin and the Eastern Cordillera), both also with excellent source rocks. During the latest Cretaceous (Maastrichtian), the beginning of marine regression allowed deposition of a transitional sequence (the Umir Formation); by accretion in the Western Cordillera. Finally, during Tertiary, the rising of the Eastern Cordillera (Andean Orogeny) was responsible for the development of a whole continental sequence. This event reached its maximum during the Miocene-Pliocene period and is still continuing at present. 4. Results and discussion 4.1. Lithofacies and organic facies in the La Luna Formation (La Sorda Creek section) The La Luna Formation is a calcareous petroleum source rock with good to excellent potential for oil. Most of the sedimentologic and geochemical parameters

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

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

of the La Luna Formation in La Sorda Creek display a relatively narrow range (Table 1). However, the sedimentologic and geochemical variations are sucient to di€erentiate several lithofacies and organic facies (Table 2 and Fig. 3).

4.1.1. Lithofacies and organic facies in the Salada Member This member consists of 102.2 m of foraminiferal wackestones interbedded with occasionally-cherty calcareous shales. The lithologies observed in this member are grouped in four sedimentary lithofacies. . Poorly laminated wackestones (plW): This lithofacies 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 dark gray, thin bedded foraminiferal wackestones and calcareous shales, both with planoparallel laminations and small nodules. . Phosphatic calcareous shales and laminated mudstones (pcSM): This lithofacies consists of calcareous, 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 crystalline limestone with laminae of organic matter.

Fig. 2. Cretaceous stratigraphic units in the Middle Magdalena Basin.

Organic facies type B, BC, and C, sensu Jones (1984), was identi®ed in the Salada Member (Table 2). Organic facies B is related to mlW and pcSM lithofacies. This facies 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

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Table 1 Average values (meanstandard deviation) of bulk geochemical parameters for each lithofacies of the La Luna Formation % TOC

Tmax ( C)

S1 (mg HC/g rock)

S2 (mg HC/g rock)

S3 (mg CO2/g rock)

Hl (mg HC/g TOC)

1.4 3.9 3.2 8.5

15 15 14 44

2.61.0 2.30.8 2.41.0 2.40.9

4344 4364 4383 4364

4.612.25 3.721.64 3.791.88 4.041.94

13.465.23 11.334.52 11.365.22 12.074.98

0.590.17 0.470.15 0.400.08 0.490.16

pcSM2b B mlW2 pcSM2, B, mlW2

49.3 0.5 1.4 51.1

63 3 3 69

3.21.1 0.30.1 3.70.9 3.11.2

4353 437 4383 4363

4.432.52 0.04 4.410.66 4.242.58

14.415.69 0.040.02 15.373.65 13.826.23

plW mlW3 pcSM3b cL plW, mlW3, pcSM3, cL

18.4 10.9 10.5 0.5 40.4

18 11 8 7 44

1.81.3 4.21.3 4.61.2 0.90.9 2.81.9

4353 4383 4344 4329 4355

1.741.33 2.950.98 5.863.47 0.810.95 2.602.41

7.065.85 15.553.18 22.509.66 3.474.33 11.288.85

Organic facies

Lithofacies

Galembo

B B B Organic facies B

pPh phPW pcSM1b pPH, phPW, pcSM1

B D B Organic facies B and D BC B B C Organic facies B, BC and C

Average Pujamana Average Salada

Average a b c

% thickness

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

OI (mg CO2/g rock)

% Carbonate

% Bitumen

52363 49162 46772 49568

259 229 2010 229

31.99.7 52.010.7 27.414.5 37.315.8

2.340.32 2.350.98 1.080.77 1.780.90

0.470.11 0.160.11 0.690.08 0.460.14

43488 1412 42251 415121

1817 5230 205 2018

22.313.1 0.30.5 49.83.7 22.614.6

1.551.15 n.d.c 1.680.76 1.571.10

0.470.12 0.560.11 0.520.05 0.310.22 0.480.15

36077 39584 474132 234190 366132

3827 56.910.2 136 5332 3026

75.112.3 0.89 28.111.3 66.211.0 59.920.7

0.640.41 3.692.83 n.d. 1.601.98

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

na

Member

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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 TOC (wt.%) HI (mg HC/g TOC) OI (mg CO2/g TOC) Tmax ( C) Carbonate (%)

8.48 2.4 (0.9) 495 (68) 22 (9) 436 (4) 37.3 (15.8)

50.62 3.2 (1.1) 434 (87) 18 (16) 436 (3) 23.6 (14.1)

0.51 0.3 (0.1) 14 (12) 52 (30) 437 0.3 (0.5)

21.42 4.3 (1.3) 428 (111) 15 (9) 436 (4) 44.8 (17.9)

18.44 1.8 (1.3) 360 (77) 38 (27) 435 (3) 75.1 (12.3)

0.53 0.9 (0.9) 234 (190) 53 (32) 432 (9) 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.

characterized by the high ratios of C30/C29 sterane and relatively 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 lithofacies. This facies is characterized by average values of HI around 360, TOC around 1.8%, and an average of

S2 of 7 mg HC/g of rock. This facies has the highest values of oleanane/C30 hopane and tricyclic C20/C23 ratios, and relatively low values of diasterane/sterane (0.46). The other geochemical parameters are within the range of the La Luna Formation (Tables 2 and 3; Fig. 5).

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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.

An organic Facies C associated with the cL lithofacies, displays values of HI around 234 and TOC around 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 Pujamana Member This member consists mainly of calcareous phosphatic shales with abundant foraminifera, phosphatic calcareous mudstones, cherts and bentonites. Calcareous nodules 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 mudstones (pcSM): This lithofacies consists of shales and calcareous mudstones that are ®nely laminated and slightly phosphatic, with abundant foraminifera and bones of ®sh. . Bentonites (B): This lithofacies is composed of yellowish-gray, greenish-gray and grayish-orange clays. (smectite±illite). The lithofacies generally appears in tabular layers thinner than 35 cm. . Muddy laminated wackestones and calcareous shales (mlW): This lithofacies consists of dark gray, thin-bedded foraminiferal wackestones and calcareous shales. Organic facies type B and D were observed in this member. The organic facies B, the most abundant, is associated with the pcSM and mlW lithofacies. Averages values of HI and TOC are around 434 and 3.2, respectively, and S2 varies between 14,41 and 15,37. This organic 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 and 3; Fig. 5).

Organic facies C, associated with the bentonite lithofacies, has HI values of about 14, and TOC of about 0.3%. This is a minor organic facies (0.5% of the geological column by volume) (Tables 1 and 2). 4.1.3. Lithofacies and organic facies in the Galembo Member The lower part of this member (19 m) was studied. The section consists of a series of packstone phosphorites (sensu Greensmith, 1989), wackestones and phosphatic packstones, chert and calcareous and phosphatic shale. Layers are tabular, with a thickness range between 5 and 30 cm and concretions up to 2 m in diameter are present toward the base of this member. Three lithofacies were identi®ed in this member (Fig. 3). . Phosphatic calcareous shales and laminated mudstones (pcSM): This lithofacies consists of calcareous, slightly phosphatic and ®nely laminated shales and claystones, with abundant foraminifera. . Packstone phosphorites (pPh): This facies consists of packstone phosphorites with abundant foraminifera, pellets, ®sh bone fragments, and oolites. Wavy and lenticular lamination is common 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 of slightly phosphatic foraminiferal packstones, with lenticular and wavy lamination. This lithofacies also contain ®sh fragments, pellets, oolites and Planktonic fossils. The organic matter of Galembo Member exhibits the greatest HI around 495, OI average of 22, TOC of about 2.4% and S2 of 12. These geochemical parameters are typical of organic facies B. This organic facies displays

Table 3 Average values (meanstandard deviation) of biomarker parameters for each lithofacies and organic facies in La Luna Formation Organic Lithofacies na Ts/Tm facies

Diasterane/ C35/C34 regular extended steraneb hopanes

Tricyclic terpanes/ hopanesc

%C27 sterane

%C29 sterane

C30/C29 sterane

C24 tetracyclic/ C23 tricyclic/ C29 norhopane/ Oleanane/ Gammacerane/ Steranes/ C26 tricyclic C24 tetracyclic C30 hopane C30 hopane C30 hopane hopanesd

Galembo

B B B Organic facies B

phPW pPh pcSM1 pPH, phPW, pcSM1

0.71 0.350.07 0.590.49 0.510.36

3.132.06 2.020.08 2.850.53 2.630.93

28.678.40 38.529.59 38.976.40 36.538.19

24.427.98 29.107.28 26.881.98 27.075.10

0.26 0.230.03 0.270.02 0.260.03

0.980.40 0.590.07 1.120.57 0.910.45

8.225.59 14.551.17 10.175.92 11.204.93

2.130.58 1.400.66 0.950.33 1.360.65

0.05 0.040.03 0.130.11 0.080.09

n.d.e 0.100.04 0.330.37 0.250.31

0.150.07 0.340.08 0.290.09 0.280.10

Pujamana B B Average Organic facies B

pcSM2 mlW2 pcSM2, mlW2

23 0.160.05 0.450.44 8 0.190.02 1.200.60 31 0.170.05 0.520.49

1.100.18 2.331.06 44.576.12 23.465.71 0.290.08 1.090.56 1.320.34 2.601.23 39.122.67 26.204.50 0.350.11 1.080.50 1.130.22 2.361.05 43.946.06 23.775.58 0.300.08 1.090.54

11.865.01 11.005.47 11.764.96

1.270.49 0.940.65 1.230.51

0.080.11 0.030.02 0.080.11

0.320.28 0.300.28 0.310.27

0.200.09 0.280.16 0.85 0.100.04 0.220.02 n.d. 0.190.09 0.280.16 0.85

Salada

plW mlW3 pcSM3 mlW3, pcSM3 cL

6 8 6 14

0.460.28 0.310.19 0.640.49 0.460.38

1.030.09 1.100.09 1.240.38 1.170.28

3.010.78 2.730.93 1.780.72 2.320.95

38.994.16 41.745.29 38.914.98 40.535.17

27.364.89 26.122.88 24.235.81 25.314.29

0.310.04 0.210.05 0.760.66 0.460.51

0.980.36 0.930.24 2.972.31 1.801.78

9.547.68 12.325.90 5.706.60 9.496.86

1.380.25 1.430.32 0.780.43 1.150.49

0.190.15 0.050.06 0.030.01 0.040.05

0.260.15 0.230.23 0.150.06 0.200.19

0.240.05 0.280.11 0.210.12 0.260.11

0.18

0.92

1.90

41.22

26.63

0.17

0.65

17.00

1.31

0.02

0.09

0.270.00 0.21

Average

Average

a b c d e

BC B B Organic facies B C

2 3 4 7

0.28 0.25 0.260.01 0.260.02

0.310.17 0.190.04 0.330.22 0.250.16

1 0.16

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

n.d. 1.130.16 1.350.32 1.280.28

C20/C23 tricyclic

Pr/Ph

0.610.45 0.200.02 0.520.51 0.420.37

1.01 n.d. 0.60 0.81

0.790.71 0.380.23 0.250.05 0.340.21

0.52 0.98 0.49 0.73 n.d.

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Member

1273

1274

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

Fig. 5. Histograms showing the average values and standard deviation of some geochemical bulk parameters and representative biomarker ratios for each lithofacies in the La Luna Formation.

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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 d13C saturates

13

ÿ27.40 ÿ27.43 ÿ27.24 ÿ27.33

ÿ28.45 ÿ27.42 ÿ27.35 ÿ27.32

ÿ27.38 ÿ27.37 ÿ27.21 ÿ27.36

25.9 34.0 34.3 58.2 74.4 102.5 112.0 130.0

ÿ27.76 ÿ27.27 ÿ27.33 ÿ27.90 ÿ26.91 ÿ26.08 ÿ26.61 ÿ26.47

ÿ27.91 ÿ27.57 ÿ27.27 n.d. ÿ27.52 ÿ28.18 ÿ27.78 ÿ27.41

ÿ27.40 ÿ27.28 ÿ27.31 ÿ27.94 ÿ26.99 ÿ26.78 ÿ26.34 ÿ26.49

142.0 179.5 192.5 204.0 221.0 228.0 233.0 235.0

ÿ26.13 ÿ27.51 ÿ27.62 ÿ27.61 ÿ27.46 ÿ27.41 ÿ27.39 ÿ27.96

ÿ27.98 ÿ27.99 ÿ28.43 ÿ27.55 ÿ28.31 ÿ28.31 ÿ27.56 ÿ28.33

ÿ26.56 ÿ27.47 ÿ27.84 ÿ27.67 ÿ27.60 ÿ27.43 ÿ27.79 ÿ27.86

ÿ27.24

ÿ27.82

ÿ27.30

Member

Organic facies

Lithofacies

Sample ID

Galembo

B B B B

pcSM1 phPW pPh pcSM1

CD07 CD15 CD22 CD41

3.0 9.0 14.0 18.2

Pujamana

B B B B B B B B

pcSM2 pcSM2 pcSM2 pcSM2 pcSM2 pcSM2 mlW2 pcSM2

CD50 CD59 CD60 CD82 CD98 CD104 CD110 CD114

Salada

B BC BC B BC B BC B

pcSM3 plW plW mlW3 plW pcSM3 plW pcSM3

CD119 CD129 CD135 CD143 CD154 CD157 CD158 CD160

Average

Cummulative thickness

13

C Bitumen

C Aromatics

1276

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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 is high, varying between 0.04±0.13 (Tables 2 and 3; Fig. 5). The phosphatic packstone lithofacies of the Galembo member shows the highest values of HI, the highest ratios of sterane/hopane (0.34) and C23 tricyclic/C24 tetracyclic (14.55) and the lowest ratios of C20/C23 tricyclic (0.20) and C24 tetracyclic/C26 tricyclic (0.59) of the La Luna Formation. This parameter could correlate with increased algal material within this lithofacies. These characteristics are consistent with the observed abundance of phosphates indicating an upwelling event. The HI versus OI plot, the modi®ed Van Krevelen diagram (Fig. 7), shows that organic matter is predominantly 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. Talukdar et al. (1986) also noted, based on microscopic analyses and molecular geochemistry, that the bulk of the organic matter is algal and bacterial in origin. Summarizing, Organic Facies B is the most representative organic facies of the La Sorda Creek section with a total percentage of 80.5% in volume. This facies is 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 characteristics, a facies substitution diagrams (Fig. 3) for the La

Luna formation in La Sorda Creek was interpreted. This diagram provides information about the natural succession and substitution of lithofacies under increasing or decreasing accommodation conditions. The area of each lithofacies is proportional to its abundance within the section. This diagram helps to identify stratigraphic cycles 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 by Reyes et al. (1998), using well logs and cores. From base to top, the transition of the argillaceous shales of the SimitõÂ Formation to calcareous shales, and the appearance of thin layers of foraminiferal wackestones (Salada Member), indicates a generalized low frequency hemicycle of base level fall. This hemicycle reaches a maximum fall (minimum in A/S) where the limestone 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 of shales 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 organic matter and low percentages of carbonates. After the maximum deepening within the Pujamana Member, the amount of phosphates progressively increases. Packstone phosphorites (lithofacies pPh) and phosphatic packstones (lithofacies phPW) appear. This demonstrates a base level fall in the basin (Fig. 5). The shallowing is not completely recorded in the column of the La Sorda Creek, because of a fault at the top of the La Sorda Creek section. Stratigraphic events in the basin are shown in detail by the intermediate frequency cycles (third order cycles, sensu Vail et al., 1977) (Fig. 3). 4.3. Paleoceanographic events and environmental considerations

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

The ®ne lamination and the calcareous character of Salada Member and the presence of very ®ne planktonic foraminifera arranged in laminae, demonstrate a low energy marine environment of deposition. The shale sequence and the minor calcareous character, which characterize the Pujamana Member, evidence 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 consistent with the occurrence of upwelling currents. The high contents of organic matter, as well as the presence of pyrite are evidence of high productivity and suboxic to anoxic condition, which favored accumulation and preservation of organic matter. The bentonites represent

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

sedimentary input from volcanic activity during this period. The packstone phosphorites and phosphatic packstones that characterize the Galembo Member indicate a ¯ow regime greater than the one during the sedimentation of the Salada Member and the Pujamana Member. The presence of wavy and lenticular lamination, ripples, ¯aser laminations, reworking of ®sh fragments and presence of quartz demonstrates a shallow water environment within wave base. These lithofacies correspond to the shallowest level within the carbonate platform in which 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 interaction of important regional and global paleogeographic events, such as Ekman water transport (closely related to upwelling regimens), and oceanic anoxic events described by Macellari (1998), Martinez and HernaÂndez (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 Formation was deposited on a large carbonate platform. On this platform, sedimentation took place in a through, limited to the West by the submerged Central Cordillera, which restricted water circulation and contributed to an anoxic environment of deposition. During some less restricted periods, upwelling currents favored high primary productivity, and organic matter accumulation and preservation that characterize the sediments of the La Luna Formation. According to Martinez and HernaÂndez (1992), the deepest parts of the ramp were located near the present area of Maracaibo, where a maximum depth of approximately 600 m was attained during the Campanian. 4.4. Maturity level Because of the predominance of amorphous organic matter, the samples contain little vitrinite, and therefore the vitrinite re¯ectance measures are unreliable. The maturation level in this study was determined using RockEval 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 geochemical variations can largely be considered as indicative of depositional conditions and associated variations in the type of organic matter. 4.5. Relationship between geochemical parameters and sequence stratigraphy Several authors have described the relationship between the characteristics of organic matter and sea

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level changes (e.g. Middleburg et al., 1991; Pasley et al., 1993). The relationship between some geochemical parameters commonly used to characterize petroleum source rocks and interpreted sequence stratigraphy cycles in the La Sorda Creek section are graphically shown in Fig. 8. The trends in variations of geochemical parameters in this ®gure are generalized. In the regressive carbonate shelf (Salada Member and Galembo members) an increase in the relative values of HI and a decrease in TOC contents is observed, while in the regressive siliciclastic shelf (Pujamana-Salada Member), both TOC and HI values decrease continuously during the shallowing stage. Some biomarker parameters such as oleanane/C30hopane and C20/C23 tricyclic ratios usually used to re¯ect relative contribution of continental organic matter, increase during regression cycles. This occurs on both carbonate and siliciclastic shelves. Other parameters suggested by some authors (e.g. Waples and Machihara, 1990) as sensitive to lithology, such as Ts/Tm ratios, increase in carbonate levels. In the carbonate shelves, HI values increase despite the higher oleanane/C30 hopane and C20/C23 tricyclic ratios, probably suggesting that algal productivity and preservation are predominant processes in this depositional environment. Based on these results, organic geochemistry could be considered as an important tool to support the sequence stratigraphy architecture of a sedimentary sequence. Conversely, sequence stratigraphy 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 and Fig. 5), show a narrow range. Therefore, these values can be considered typical for the La Luna Formation and its depositional conditions. This is useful for oilsource rock correlation in the MMB. The pristane/phytane (Pr/Ph) ratio is lower than 1.01 con®rming an anoxic/reducing depositional environment for the La Luna Formation. As was noted by Zumberge (1984), the most abundant cyclic compounds throughout the La Luna formation are the tricyclic terpanes and hopanes while steranes abundance are relatively low. The sterane distributions display a predominance of C27 steranes. Low concentration of rearranged C27 steranes relative to regular steranes are also characteristic (Fig. 9). Most extracts display diasterane/sterane ratios less than 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 should be related to a carbonate/anoxic environment, typical of the La Luna Formation.

Fig. 8. Variation of geochemical bulk parameters and some typical biomarkers correlated with stratigraphic sequence in the La Luna Formation, La Sorda creek.

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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). High C35 hopane are commonly associated with marine carbonate environment (Mello et al., 1988; Clark and Philp, 1989). Additionally, Peters and Moldowan (1993) interpret this phenomenon as a general indicator of a highly reducing marine condition during deposition.

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The high relative abundance of C35 over C34 hopane in the La Luna Formation con®rms its association to carbonate 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 conditions 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.

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The lower abundance of the C35 hopanes in the bitumen of the cL lithofacies might indicate more oxic conditions during 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 more siliciclastic 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.

Fig. 11. Isotope correlations: (a) relative sea level changes according to stratigraphic and sedimentologic analyses; (b) pro®le of carbon isotopic composition of whole bitumen and aromatic hydrocarbon fraction; (c) d 13C saturates versus d 13C aromatic hydrocarbons. Sofer (1984) cross-plot.

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Philp and Gilbert (1986), the abundance of the C24 tetracyclic may indicate higher plant marker. In extracts of the La Luna Formation (calcareous platform), variations in the abundance of this compound could not be clearly related to higher plant input. Instead, it seems to increase 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 environments and marine carbonate related environments are characterized by high relative abundance of tricyclic terpanes. Fig. 10c shows a relatively good correlation between 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 during the regressive cycles (Fig. 8). According to Peters and Moldowan (1993), many authors have associated anomalously low (Ts/Ts+Tm) ratios to carbonate source rock, which is also observed in this study. Oleanane (triterpane of higher plant origin; Ekweozor et 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 deposited in deeper environments such as the pcSM PujamanaSalada lithofacies, have lower oleanane/hopane ratios than those derived from carbonate shallow facies. Based on the sequence stratigraphy cycles and geochemical logs (Fig. 8), oleanane/C30 hopane can be proposed as a good 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 correlation (Figs. 10d±f). This inverse correlation is consistent with the opposite trends displayed for this parameter in Fig. 8. Higher oleanane/C30 hopane,%C29 sterane and C20/C23 tricyclic are considered to re¯ect relatively greater contribution of higher plant material, in this case 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, always presents an anomalous trend. 4.7. Isotope composition The average values of the saturate fractions is ÿ27.82% PDB and the range of variation is 1.18% in the saturate fractions (Table 4). In the aromatic fraction and whole bitumen, the range of variation is 1.6 and 1.88%, respectively, greater than observed in saturate fractions. The isotope log of the aromatic fraction and whole 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 13 C Corg isotopic excursion in the middle part of a

Maraca Creek section of the La Luna Formation, which was interpreted as a global depletion in 12C around the Cenomanian /Turonian and the Coniacian/Santonian boundary. Using the plot of Sofer (1984) (Fig. 11c), isotopically heavier samples correspond to extracts from pcSM lithofacies (more siliciclastic) and isotopically lighter extracts are related to more calcareous lithofacies. All extracts plot in the marine area of Sofer (1984). 5. Conclusions The La Luna Formation is a petroleum source rock with good to excellent potential for oil. About 63% of the volume of this formation is composed of phosphatic calcareous shales and laminated mudstones with abundant 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 or a deepening of the basin during the sedimentation of the Pujamana Member, and a second base level fall that permitted the deposition of the calcareous and phosphatic lithologies of the Galembo member. Regarding the relationship between organic matter characteristics and sea level changes, during shallowing stages in carbonate shelves (Salada Member and Galembo Member), HI tends to increase and TOC to decrease. In the siliciclastic shelf, 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 indicator of rise and fall of sea level in third order sequence stratigraphy cycles. Sedimentation of the La Luna Formation occurred on a large carbonate ramp, with restricted water circulation and anoxity. During certain periods, upwelling favored high primary productivity and accumulation and preservation of organic matter. Some biomarker ratios can be considered typical values for the depositional 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 ranging from 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 to redox 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 and D in the Pujamana Member and organic facies type B in the Galembo Member. The d 13C isotope composition of aromatic fractions and whole bitumens display an isotopic shift associated with the main deepening event in the

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section. Based on these results, organic geochemistry could be considered as an important tool to support the sequence stratigraphy architecture of a sedimentary succession.

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