The La Luna formation: chemostratigraphy and organic
facies in the Middle Magdalena Basin
A. Rangel
a,*, P. Parra
b, C. NinÄo
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 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 geo-chemical 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 bio-marker parameters display a relatively narrow range of variation. However, the geochemical variations are sucient to dierentiate 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/C34hopane 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/C23tricyclic, Ts/Tm) increase during base level falls. Regardingd13C/12C isotope composition, the
aro-matic fraction and whole bitumen display an isotopic shift associated to the main deepening event in the section.
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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 processes controlling the Upper Cretaceous-La Luna Formation
deposit in the MMB is minimal and there are few inte-grated stratigraphic and geochemical studies on this formation.
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 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
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Organic Geochemistry 31 (2000) 1267±1284
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* Corresponding author.
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 evalu-ate geochemical changes relevalu-ated to existing lithofacies and organic 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 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 uni-directional 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 chromato-graphy±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, aro-matic 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 tem-perature programmed from 60 to 320C at 4C/min and
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 Pre-cambrian 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 con-tinental and volcanic rocks in the backarc setting. Dur-ing the Early Cretaceous, a marine transgression led to the backarc basin to be ®lled with a prograding sequence. Maximum transgression during the Tur-onian±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)
of the La Luna Formation in La Sorda Creek display a relatively narrow range (Table 1). However, the sedi-mentologic and geochemical variations are sucient to dierentiate 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 cal-careous shales. The lithologies observed in this member are 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 dark gray, thin bedded foraminiferal wackestones and calcareous 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 crystalline limestone with laminae of organic matter.
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 S2between 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.
Table 1
Average values (meanstandard deviation) of bulk geochemical parameters for each lithofacies of the La Luna Formation
Member Organic
Pujamana B pcSM2b 49.3 63 3.2
characterized by the high ratios of C30/C29sterane and
relatively low ratios of C29/C30hopane and C23tricyclic/
C24tetracyclic (Fig. 5; Tables 2 and 3).
The organic facies BC is related to the plW litho-facies. 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).
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.
An organic Facies C associated with the cL litho-facies, 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/C30sterane (0.09), diasterane/sterane
(0.18) and C35/C34hopane (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 phospha-tic shales with abundant foraminifera, phosphaphospha-tic cal-careous mudstones, cherts and bentonites. Calcal-careous 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 mud-stones (pcSM):This lithofacies consists of shales and calcareous mudstones that are ®nely lami-nated and slightly phosphatic, with abundant for-aminifera 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, respec-tively, and S2 varies between 14,41 and 15,37. This organic facies shows very little variation in the average values ofTs/Tm(0.16±0.19), C35/C34hopane (1.10±1.32),
C24tetracyclic/C26tricyclic (1.08±1.09) (Tables 1, 2 and
3; Fig. 5).
Organic facies C, associated with the bentonite litho-facies, has HI values of about 14, and TOC of about 0.3%. 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 Member
The 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 phosphos-phatic shale. Layers are tabular, with a thickness range between 5 and 30 cm and concretions up to 2 m in dia-meter are present toward the base of this member. Three lithofacies 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 consists of packstone phosphorites with abundant for-aminifera, pellets, ®sh bone fragments, and oolites. 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 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
Fig. 6. Schematic evolution of the carbonate ramp.
Table 4
Carbon isotopes ratios in whole bitumen, saturates and aromatic fractions for dierent 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 ÿ27.38 B phPW CD15 9.0 ÿ27.43 ÿ27.42 ÿ27.37 B pPh CD22 14.0 ÿ27.24 ÿ27.35 ÿ27.21 B pcSM1 CD41 18.2 ÿ27.33 ÿ27.32 ÿ27.36
Pujamana B pcSM2 CD50 25.9 ÿ27.76 ÿ27.91 ÿ27.40 B pcSM2 CD59 34.0 ÿ27.27 ÿ27.57 ÿ27.28 B pcSM2 CD60 34.3 ÿ27.33 ÿ27.27 ÿ27.31 B pcSM2 CD82 58.2 ÿ27.90 n.d. ÿ27.94 B pcSM2 CD98 74.4 ÿ26.91 ÿ27.52 ÿ26.99 B pcSM2 CD104 102.5 ÿ26.08 ÿ28.18 ÿ26.78 B mlW2 CD110 112.0 ÿ26.61 ÿ27.78 ÿ26.34 B pcSM2 CD114 130.0 ÿ26.47 ÿ27.41 ÿ26.49
Salada B pcSM3 CD119 142.0 ÿ26.13 ÿ27.98 ÿ26.56 BC plW CD129 179.5 ÿ27.51 ÿ27.99 ÿ27.47 BC plW CD135 192.5 ÿ27.62 ÿ28.43 ÿ27.84 B mlW3 CD143 204.0 ÿ27.61 ÿ27.55 ÿ27.67 BC plW CD154 221.0 ÿ27.46 ÿ28.31 ÿ27.60 B pcSM3 CD157 228.0 ÿ27.41 ÿ28.31 ÿ27.43 BC plW CD158 233.0 ÿ27.39 ÿ27.56 ÿ27.79 B pcSM3 CD160 235.0 ÿ27.96 ÿ28.33 ÿ27.86
ratios of sterane/hopane (0.34) and C23 tricyclic/C24
tetracyclic (14.55) and the lowest ratios of C20/C23
tri-cyclic (0.20) and C24tetracyclic/C26tricyclic (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 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. 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 repre-sentative 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 character-istics, a facies substitution diagrams (Fig. 3) for the La
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 wack-estones (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
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 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. 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
sedimentary input from volcanic activity during this period.
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 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 For-mation was deposited on a large carbonate platform. On this platform, sedimentation took place in a through, limited to the West by the submerged Central Cordil-lera, 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 Her-naÂndez (1992), the deepest parts of the ramp were loca-ted 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 Rock-Eval pyrolysis.Tmaxvalues (average 436C) listed in Table
1. This suggests the section is early mature to mature. AverageTmaxandS1values suggest that the section is
not greatly aected 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 and sequence stratigraphy
Several authors have described the relationship between the characteristics of organic matter and sea
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 regres-sive 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 con-tinental 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 pro-ductivity 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 strati-graphy is a useful tool to follow oil prone strata.
4.6. Sedimentological controls on geochemical composition
Biomarker fragmentograms appear similar for the dierent 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 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 by Zumberge (1984), the most abundant cyclic compounds throughout the La Luna formation are the tricyclic ter-panes and hoter-panes 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 character-istic (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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The extended hopanes have relative abundance of C35
hopanes (C35/C34hopane ratios higher than 0.92). High
C35hopane 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 a highly reducing marine condition during deposition.
The high relative abundance of C35over C34hopane in
the La Luna Formation con®rms its association to car-bonate environment.
The C35/C34hopane 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. 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)d13C saturates versusd13C aromatic hydrocarbons. Sofer (1984) cross-plot.
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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 char-acterized by high relative abundance of tricyclic ter-panes. Fig. 10c shows a relatively good correlation between carbonate content and tricyclic terpanes/ hopanes ratio.
TheTs/Tmratios are less than 0.33 (Table 3), and the
relative abundance ofTsincreases 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 Pujamana-Salada 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/C30hopane 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/C30hopane,%C29steranes,
and C20/C23tricyclic versus TOC show inverse
correla-tion (Figs. 10d±f). This inverse correlacorrela-tion is consistent with the opposite trends displayed for this parameter in Fig. 8. Higher oleanane/C30 hopane,%C29sterane 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 markedd 13C C
org isotopic excursion in the middle part of a
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 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 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 phos-phatic 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/C30hopane,
C20/C23tricyclic,Ts/Tmshow 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 sequence stratigraphy cycles.
Sedimentation of the La Luna Formation occurred on a large carbonate ramp, with restricted water circu-lation 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 deposi-tional environment of the La Luna Formation (e.g. diasterane/sterane ratios < 1, Ts/Tm average < 0.33,
C35/C34hopane >0.92, and oleanane/C30hopane ranging
from 0.02 to 0.19). The C35/C34hopane 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 dierentiate 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. Thed13C isotope composition of
section. Based on these results, organic geochemistry could be considered as an important tool to support the sequence stratigraphy architecture of a sedimentary succession.
Acknowledgements
We are grateful to Drs. R. Sassen and S. Talukdar for helpful suggestions and constructive reviews and Drs. M.N. YalcËyÂn and S. YÂnan for major linguistic and edi-torial corrections.
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