INTRODUCTION

The Cretaceous La Luna Formation has long been ascribed to as the source rock for much of the oil found in the Maracaibo basin of Venezuela (e.g., Hedberg, 1931; Young, Monaghan, and Schweisberger, 1977). Crude oils in the Middle Magdalena Valley, Colombia, which have accumulated in Tertiary fluvial-sand reservoirs, also have been generated, in part, from the Upper Cretaceous La Luna Formation, based on geochemical-correlation techniques such as stable-carbon-isotope ratios and geochemical fossil distributions (Zumberge, 1980). The objective of this present study is to describe both the gross organic geochemical properties (e.g., percentage of total organic carbon and quantities of extractable hydrocarbons) and the detailed geochemical character on a molecular level (e.g., sterane and terpane biomarker distributions) of this widespread and prolific calcareous source rock.

Outcrop samples of the La Luna Formation were collected along the eastern flank of the Nuevo Mundo syncline in the Middle Magdalena basin approximately 20 km (12.4 mi) west of Bucaramanga, Colombia (Fig. 1). Although the samples are from outcrops, care was taken to collect unweathered specimens; however, some of the observed variability of the data may be due to differential-weathering effects. All three members of the La Luna Formation (Galembo, Pujamana, and Salada) were sampled; the older Salada Member samples were collected ~5 km (3.1 mi) south of the Galembo and Pujamana Members sample-collection area (Quebrada La Sorda). In outcrop, the thickness of the formation ranges from 150 to 600 m (492-1,968 ft). In general, the La Luna Formation in this region can be described lithologic lly as consisting of dark-gray to black calcareous shales with varying amounts of interbedded limestones and some thin chert beds. In thin section, La Luna samples from the Pujamana and Salada Members contain abundant calcareous planktonic Foraminifera and other pelagic organisms, suggesting deposition in moderately deep water with restricted bottom circulation (M. W. Longman, personal communication). Photomicrographs of a Salada thin section (LL16) are shown in Figure 2; the textural relationship between organic matter (brown material in Fig. 2a) and inorganic minerals (especially the calcite-filled globigerinid Foraminifera) can be observed. Except for eolian-transported clays, little evidence of land-derived material, such as detrital quartz, was found. Foraminiferal tests that are no filled with calcite are commonly filled with oil, as was determined by fluorescence microscopy in reflected light of polished whole-rock specimens (S. E. Palmer, personal communication). These Colombia La Luna samples are similar with respect to lithology and organic-carbon content to La Luna rocks from northwestern Venezuela as described by Hedberg (1931) more than 50 years ago.

EXPERIMENTAL PROCEDURES

Rock samples were ground to <100 mesh with subsequent carbonate dissolution with HCl and organic-carbon combustion using a Leco Carbon Analyzer in order to determine the approximate percentages of carbonate and total organic carbon (TOC). Powdered samples were Soxhlet extracted with chloroform/methanol (90:10, V:V) to remove extractable organic material followed by deasphalting in pentane after solvent removal. The pentane-soluble material was then separated by liquid chromatography into aliphatic hydrocarbon, aromatic hydrocarbon, and NSO (nitrogen-sulfur-oxygen-containing organic compounds) fractions on alumina and silica columns, and the quantity of each fraction was determined gravimetrically. The stable-carbon-isotopic compositions of both the aliphatic and aromatic hydrocarbo fractions were determined using the combustion method of Sofer (1980) with subsequent analysis on a V.G. Micromass 602 mass spectrometer. Capillary-gas-chromatographic separation of the aliphatic fraction was accomplished on a Hewlett-Packard 5880 gas chromatograph fitted with an SP-2100 fused silica column (12.5 m 0.02 mm i.d.). Combined gas chromatography/mass spectrometry was performed on a Finnigin 4000 GC/MS/DS system by Global Geochemical Corp., Canoga Park, California, to determine sterane and terpane distributions. Microscopic and elemental analyses of kerogen concentrates (HCl and HF dissolution of inorganic

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matter followed by flotation in ZnBr₂) were made using a Zeiss Universal Microscope equipped for vitrinite reflectance and UV fluorescence measurements and Carlo Erba CHNO analyzers (Williams Brothers Co., Tulsa, Oklahoma).

RESULTS AND DISCUSSION

Total-organic-carbon values and approximate carbonate percentages for the three members of the La Luna Formation are presented in Table 1. The Galembo (youngest) Member samples (LL1-LL6) contain only small quantities of carbonate minerals (average = 2.4%) and average less than 0.9% TOC. The Pujamana and Salada Member marlstones, however, contain an average of 43.2 and 40.4% carbonate, respectively (excluding samples LL7 and LL8, which are from thin interbedded chert layers). These two relatively carbonate-rich lower members also contain almost five times more TOC (Pujamana average = 3.51%, and Salada average = 4.51%) than the carbonate-poor Galembo member. Sample LL11 (Pujamana Member) is a marlstone with fractures in-filled with asphaltite, a solid bitumen that is soluble in organic olvents (Hunt, 1979). An asphaltite vein that typically is up to 1 m thick also was sampled (LL20). This fracture-filling organic material is similar to the gilsonite common in the Uinta basin, Utah, and may represent an early, preserved stage of migration in which oil generated from La Luna kerogen has migrated short distances into associated fractures within the La Luna marlstones.

Microscopic examination of La Luna kerogen revealed predominantly fine-grained amorphous material with few well-defined vitrinite particles and pollen, suggesting little input of terrestrial organic matter. The amorphous kerogen fluoresces under UV light, indicating that it is prone to oil generation. Although few vitrinite-reflectance measurements were available for maturity estimates, an average value of 1.0 ± 0.2% R₀ (N = 73) for all samples was obtained, which is well within the main phase of petroleum generation (Tissot and Welte, 1978). A spore-coloration index of 6.5 ± 0.8 (N = 216) and a T_max value of 446°C or 835°F (from Rock-Eval pyrolysis of a Salada marlstone) are consistent with this level of thermal maturity.

Table 2 lists the results of elemental analyses of five La Luna samples from the Pujamana and Salada Members; the atomic ratios of hydrogen to carbon and oxygen to carbon also are given. The H/C and O/C averages for the five kerogen samples are 0.85 and 0.05, respectively, which are characteristic of type II kerogen, placing them within the principal zone of oil generation on the van Krevelen diagram as defined by Tissot and Welte (1978). These results are consistent with the visual analysis of the La Luna kerogen.

Quantities of the various C₁₅₊ extractable fractions are presented in Table 1. As in the case of TOC, the marlstones of the Pujamana and Salada Members average more extractable indigenous organic matter than the shaly Galembo Member. Typically, the aromatic hydrocarbons are more abundant than the aliphatic (saturate) hydrocarbon fraction (averaging 1,638 ppm and 860 ppm, respectively) in the relatively carbonate-rich samples (excluding the asphaltite-containing LL11 sample). It is interesting to note, however, that the aliphatic hydrocarbons are more abundant than the aromatic hydrocarbons

Fig. 1. Location map showing the Middle Magdalena Valley, Colombia.

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Click to view image in JPEG format. Fig. 2. [Grey Scale] Photomicrographs of a thin section from carbonate-rich La Luna sample LL16 (Salada Member); a, plane light; b, crossed nicols. The textural relationship between the brown organic matter and the calcite-filled globigerinid Foraminifera can be observed.

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in the carbonate-poor Galembo samples. The nonhydrocarbon extractable fractions are yet more abundant than the hydrocarbon fractions. The asphaltite sample (LL20) contains primarily pentane-insoluble asphaltenes and NSO organic compounds; aromatic hydrocarbons are an order of magnitude greater than aliphatic hydrocarbons. Almost 6% of the TOC consists of extractable C₁₅₊ hydrocarbons in the relatively carbonate-rich La Luna samples, which is compatible with maturities estimated for La Luna kerogen and is indicative of good possible oil source rocks.

In Figure 3 a representative capillary gas chromatogram (sample LL15) of a La Luna C₁₅₊ aliphatic fraction is displayed. The isoprenoid phytane is typically more abundant than pristane (Pr/Ph = 0.70), and the n-alkane distribution indicates only a slight preference in odd- over even-numbered alkanes in the C₂₈₊ region (average OEP = 1.09). An even carbon preference is noticeable in the n-C₂₀ to C₂₈ range (average OEP = 0.956). Most of the aliphatic fraction consists of branched and cyclic alkanes, as evidenced by the large, unresolved "hump" under the n-alkane peaks. The appearance of this chromatogram is suggestive of organic matter derived from marine plankton and microorganisms deposited in a reducing environment.

The stable-carbon-isotopic compositions of the C₁₅₊ aliphatic- and aromatic-hydrocarbon extractable fractions

Table 1. Organic geochemical data for La Luna source rocks.

Table 2. Elemental composition of La Luna kerogen.

Table 3. Identified sterane and triterpane components.

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average -27.5 ± 0.4^pmil and -27.1 ± 0.4^pmil, respectively (relative to the PDB standard), for the Pujamana and Salada samples. These values are typical for oils of marine origin as reported by Sofer (1984). Oils produced from the Payoa, La Salina, and Corazon fields in the Middle Magdalena Valley (~30 km [18.6 mi] northwest of La Luna outcrop locations; Fig. 1) average -27.6 ± 0.2^pmil and -27.2 ± 0.1^pmil for the aliphatic- and aromatic-hydrocarbon fractions, respectively (Zumberge, 1980). The ratios of the oils in the reservoirs are essentially identical to the La Luna extractable hydrocarbons--i.e., the proposed source rock.

The distributions of the geochemical fossils (biomarkers) known as steranes (tetracyclic alkanes) and terpanes (tricyclic and pentacyclic alkanes) are important in geochemical correlations. The tricyclic terpanes are especially useful in correlating nondegraded to biodegraded crude oils (such as are commonly found in the Middle Magdalena Valley) because they appear to be relatively unaffected by even severe microbial attack (e.g., Reed, 1977). Also, sterane and terpane biomarkers are often useful in determining source-rock depositional environments (e.g., Tissot and Welte, 1978). The sterane, diterpane, and triterpane distributions of a representative La Luna extract (LL15) are illustrated in Figure 4, and identified components are listed in Table 3. The C₂₇ sterane stereoi omers (peaks 6-9, Fig. 4A) are more abundant than the C₂₉ steranes (peaks 14-17), indicating a predominance of aquatic microorganisms (relative to higher land-derived plants; e.g., Tissot and Welte, 1978) that contributed to the La Luna sediments. Low concentrations of rearranged C₂₇ steranes (peaks 1-5, Fig. 4A) are present relative to regular steranes, perhaps indicating a nonacidic, carbonate depositional environment (e.g., Sieskind, Joly, and Albrecht, 1979). The roughly equal abundance of the 20S and 20R component pairs of the 5(H), 14(H), 17(H) stereoisomers ( eak pairs 6 and 9, 10 and 13, 14 and 17, Fig. 4A) is characteristic of maturities sufficient to generate significant quantities of hydrocarbons (Seifert and Moldowan, 1981; Mackenzie et al, 1980).

The most abundant tricyclic diterpane compound (i.e., C₂₃, MW 318, Fig. 4B) in the La Luna samples is also the most abundant in the oils from the Payoa, La Salina, and Corazon fields (Zumberge, 1980). The relative lack of C₁₉ and C₂₀ diterpanes (compounds generally believed to be derived from terrestrial plants; e.g., Simoneit, 1977, Palmer, 1981) in both La Luna rocks and Middle Magdalena Valley oils again suggests a marine source. The range of tricyclic terpanes extends out to the C₃₀ molecular-weight range (MW 416, Fig. 4B and 4C).

The pentacyclic triterpane components in the La Luna source rocks (Fig. 4C) consist primarily of the ubiquitous hopane series, with the C₂₉ norhopane (peak D) and C₃₀ hopane (peak F) predominating. The hopanes are believed to be derived from bacteria (e.g., Ensminger et al, 1974; van Dorsselaer, Albrecht, and Ourisson, 1977). The thermodynamically

Fig. 3. Gas chromatogram of the aliphatic-hydrocarbon fraction of a Salada Member sample (LL15) from the La Luna Formation. Numbered peaks represent n-alkanes; Pr and Ph represent pristane and phytane, respectively.

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Fig. 4. Geochemical fossil composition of a La Luna hydrocarbon extract (LL15). A. Sterane distribution as determined by the m/z = 217 mass fragmentogram; numbered peaks are identified in Table 3; RA = rearranged steranes. B. Tricyclic diterpane distribution (m/z = 191); molecular weights were determined by monitoring m/z = 262 (C₁₉), 276 (C₂₀), 290 (C₂₁), 304 (C₂₂), 318 (C₂₃), etc. C. Pentacyclic triterpane distribution (m/z = 191). Lettered peaks are identified in Table 3.

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less stable moretane series (i.e., the 17ß(H),21(H) hopane stereoisomers, peaks E, G, and K) are present in minor amounts, which is consistent with the other maturity indicators. Not shown in Figure 4C are traces of C₃₂₊ hopane 22S and 22R pairs.

CONCLUSIONS

The following listing is a summary of the significant results and inferences based on the organic-geochemistry analyses of the Upper Cretaceous La Luna Formation from the Middle Magdalena Valley, Colombia.

1. La Luna marlstones (~42% carbonate minerals) of the Pujamana and Salada Members average 4.3% total organic carbon. The carbonate-poor Galembo samples (~2% carbonate minerals) average less than 0.9% total organic carbon.

2. Microscopic and elemental analyses of La Luna kerogen suggest a marine origin, with a present maturity well within the main phase of oil generation.

3. C₁₅₊ extractable hydrocarbons of relatively carbonate-rich La Luna samples average almost 2,500 ppm, which indicate good possible oil source rocks.

4. Gas-chromatographic results of the aliphatic fractions, including pristane/phytane values less than 1, abundant C₁₅ to C₁₉ n-alkanes with respect to C₂₀₊ alkanes, and a predominance of cycloalkanes, indicate that La Luna organic matter is derived from marine plankton and microorganisms. These results agree with the kerogen analyses.

5. Stable-carbon-isotope ratios of La Luna hydrocarbon extracts are identical to those of crude oils produced from Tertiary reservoirs within the Middle Magdalena Valley, which suggests a genetic relationship.

6. Geochemical fossil distributions also indicate a marine depositional environment (evidenced by abundant C₂₇ steranes relative to C₂₉ steranes, relatively low quantities of C₁₉ and C₂₀ diterpanes, and the presence of bacterially derived hopanes) with molecular maturation parameters consistent with the generation of oil.

Fig. 4. See caption on page 132.