Martin G. Baum
RWE-DEA Aktiengesellschaft
Hamburg
Manfred Schmitt
GCA-Laboratories
Lehrte, Germany
Geochemical applications in hydrocarbon exploration can be a useful tool to resolve exploration problems.
Stable carbon isotope investigations on surface and subsurface samples from the Southeast Turkish Adana/Iskenderun basin have been carried out to refine the exploration assessment of this province.
The Adana/Iskenderun basin represents a forearc basin developed during Neogene time in front of the Taurus orogenetic belt. During Pliocene time the basin became subdivided by the intrusive Misis uplift.
The sedimentary section ranges from Paleozoic to Pliocene intervals exposed at the surface along the basin rims. Tertiary thickness in the depocenter exceeds 6,000 m.
Intercalations of allochthonous ophiolite nappes within Early Mesozoic/Late Tertiary indicate significant compressional tectonic activity.
Zones of prospective interest are Lower Middle Miocene limestone buildups developed on paleo highs or ramps as well as Upper Miocene/Lower Pliocene clastics deposited as marine fan lobes.
The sole productive model is the marginal field Bulgurdag, which produces 37 gravity oil from a reefoidal carbonate body of Lower Middle Miocene age (Karaisali formation).
Generation of hydrocarbons is further indicated by a 17 gravity oil accumulation at the 1 Efe well, by active oil and gas seeps at the surface, and by numerous shows recorded during past drilling. A geological cross section shows the individual investigation and sample sites (Fig. 1).
Principal objectives of the study:
- Characterization and correlation of gases adsorbed in drill cuttings and in surface sediments.
- Identification of generating source for the liquid oil accumulations of Bulgurdag field (Adana subbasin) and the 1 Efe well (Iskenderun subbasin),
- Characterization of active surface hydrocarbon seeps.
SAMPLES, DESCRIPTION
The active gas seeps (Table 1, #1) near the village of Kurtbagi, about 20 km southwest of Iskenderun at the Amanos mountains west-flank, is the only known gas seep in the area. Burnable gas escapes from ophiolite fractures at several sites.
Samples 2 through 8 are derived from deep wells in the Iskenderun subbasin. All wells bottomed in ophiolites after penetrating thick Tertiary sections.
Washed drill cuttings from predominantly shaly intervals were selected for adsorbed gas extraction. Sample 4 represents a sediment sample from the seafloor collected near the Gulcihan-1 well location.
Samples 9 through 11 are source rock samples.
Sample 9 derives from the Feke outcrop in the central Taurus mountains, where Devonian black shales are exposed at the surface. The unit is a kerogen type I/II source with 6% total organic carbon (TOC) content and 1.2% vitrinite reflectance (VR) maturity.
Sample 10 originates from a well known outcrop site near the Sabunsuyu river valley east of the Amanos mountains. The selected Cretaceous limestones have source rock character with 1% TOC content.
Sample 11 represents a canned subsurface sample of the Iskenderun-S1 well, which penetrated a 100 m thick source rock sequence of Lower Middle Miocene age. Routine laboratory examinations proved potential source properties with TOC 3-5%, kerogen type II/III, and 0.8% VR maturity.
Samples 12 through 14 are oil samples.
Sample 12 derives from the active oil seep near the Kepirce village about 15 km southwest of Iskenderun, where outcropping Upper Miocene/Lower Pliocene sandstones are impregnated with live oil. Several wells have been drilled by early wildcatters in close vicinity without commercial success.
Sample 13 represents a sample from the producing Bulgurdag field about 30 km northwest of Adana. The field, discovered in the 1960s by Mobil Oil Corp., is a marginal accumulation about 1,500 m deep. The oil is 37 gravity.
Sample 14 is a 17 API oil sample recovered by reversed circulation in the 1 Efe well, drilled in 1980 by AGIP SpA. The oil recorded about 1,100 m deep is biodegraded due to lacking an effective seal.
ANALYTICAL PROCEDURES
GAS ANALYSIS
Adsorbed gases were extracted from surface sediments and drill cuttings using an acid vacuum treatment.
About 200-300 g of the sieved and water washed < 63 micron fractions were decomposed by concentrated phosphoric acid under vacuum conditions. Carbon dioxide developed by degradation of carbonates was trapped in a 50% potassium hydroxide solution.
When the reaction was finished, the remaining gas phase of the reaction chamber was analyzed by isotope geochemical methods.
The molecular gas composition of the adsorbed gases or gas samples was examined by gas chromatography. From selected gas compounds (methane, ethane, and propane) the 13C/12C isotope ratio was determined by high resolution mass spectrometry.
The results of the 13C/12C and D/H isotope measurements are calibrated vs. the international standards in PDB and SMOW.1 2
"Delta" = ([Rsample - Rstandard]/Rstandard) x 1,000 (%O)
"R" = 13C/12C or D/H
LIQUID ANALYSIS
The liquid fraction of fine grained rocks and cuttings was generated by soxlet extraction with dichlormethane. Live oils were analyzed without any pre-treatment.
First, the asphaltenic fraction of the extract/oil was precipitated by addition of petroleum ether.
The volume of the remaining extract was reduced, followed by a medium pressure chromatographical separation using an Al203/Si02 column and different organic solvents to separate the extracts into saturated (SAT), aromatic (ARO) and heterogenic (HET) fractions.
The proportion of each fraction was then determined. Finally all individual fractions were oxidized in a vacuum line with CuO to CO2 for the determination of their 13C/12C isotope rations.
RESULTS, INTERPRETATION
The records of the gas investigations (Fig. 2) allow a reliable "finger print" correlation. Obvious is the different spike pattern of the Gulcihan record, which shows more unsaturated compounds than the other measurements.
Another anomalous feature is evidenced at the seafloor and the Iskenderun-S1 (1,750 m) sample, where a pronounced iso-heptane peak indicates a different bulk gas composition. These two investigation sites are located above a geopressured interval encountered during drilling.
The surface gas seep Kurtbagi, however, shows almost identical signal characteristics as the peak pattern observed below the overpressure zone with the exception of the anomalous Gulcihan registration.
The gas chromatography of the liquid oil/source rock fractions (Fig. 3) leads to following diagnostic evaluation.
A normal pattern is evident for the Bulgurdag oil.
The Efe and Kepirce oils are biodegraded due to lacking hydrocarbon compounds of lower order. Devonian and Miocene source rock extracts show significant hydrocarbon response of different compositions.
The analytical results are displayed by various cross plots (Tables 2, 3).
Gulcihan's abnormal character is evident by gases abnormally enriched in 13C-methane isotopes associated with extremely low D-methane isotope content (Fig. 4). This phenomenon is interpreted as an artificial gas mixture generated through bit metamorphism during drilling . 3
Differentiation of biogenic and thermogenic gases is possible with the so-called "Bernard-plot."
Subject plot (Fig. 5) illustrates the absence of biogenically generated gases and, with the exception of the Gulcihan and Kurtbagi values, demonstrates the thermocatalytic origin of the desorbed gases, which plot very well in the established "oil window."
Source maturation for the gases can be estimated with isotope cross plots. 4
Gases are being produced from mature source units with a vitrinite reflectance ranging from 0.8 to 1.2% (Fig. 6). Obvious is an apparent separation between deep (below overpressure) and shallow (above overpressure) readings, which might be an indicator for two different oil kitchens.
Unusual is the higher maturation of the shallow value pair.
Isotope determinations on source rock and live oil samples were performed to investigate oil/source relations. Analytical values are shown, and the interpretation is presented using the "Galimov" method (Table 3).
An "isotopic-type-plot"' illustrates three families (Fig. 7). Interesting are the increasing isotopic values from older to younger sediments.
This phenomenon has apparently no relation to the source maturity but more likely to the kerogen type classification. The lowest values are measured for the Devonian source/Bulgurdag oil pair, the highest for the Efe/Kepirce/Miocene source troika.
Devonian and Cretaceous source units are not responsible for the oils in Efe and at the Kepirce seepage because the generating formation must carry higher isotope values than the corresponding oil occurrence.
Based on this condition the Bulgurdag oil must have been generated by a mixture of predominantly Devonian and to a smaller extent by younger source units. The Efe and Kepirce oils originate consequently from younger source beds.
DISCUSSION
The principal analytical results summarize the 13C/12C isotope ratios for the gaseous and liquid hydrocarbon investigations (Fig. 8).
Obvious is the sensitivity of the methane values to the deviations of the "artificial" gas mixture in the 1 Gulcihan well (2, 3) and the abnormal character of the Kurtbagi gas seep (1).
This gas seep carries a significant amount of molecular hydrogen (from separate bulk gas analysis) and is extraordinarily enriched in 13C isotopes of the methane.
This phenomenon has been observed as well on other ophiolite gas seeps as in the Philippines 6 and in the Oman mountains (verbal communication), which leads to the assumption that the serpentinized ophiolite body in connection with ground and surface water influence may be responsible for the abnormal gas composition.
The presence of higher homologues, however, indicates a thermogenic origin from an organic source.
The 13C/12C isotopic ratios of propane approach the band width of the established oil window suggesting a cogenetic origin of the hydrocarbon occurrences of the basin.
Source maturation determinations for the desorbed gases suggest the existence of two separate kitchens. The "deep" gases are generated from units with approximately 0.87% VR-maturity, and the shallow desorbed gases derive from organic rocks with about 1.15% VR.
The higher maturity of the shallow measurements is unusual, but a possible explanation is the nearby Misis intrusion causing the thermal alteration of younger source beds.
The deep gases are undoubtedly produced from basinal Lower Middle Miocene beds proven by drilling and analyzed with 0.8% VR, which matches the value derived from isotopic investigations.
Oil/source correlations are possible with the "isotopic type-curve" (Fig. 7). The generating source, however, must carry higher isotopic values than the corresponding oil. Based on this condition and keeping in mind the increasing isotopic amounts from older to younger rocks, the following oil source relations are concluded:
- The Bulgurdag oil is pre dominantly generated from Devonian source unit mixed with minor quantities from younger source beds.
- Efe and Kepirce oils ar not sourced from Devonian, because their 13C/12C isotopic ratios are much higher than the Devonian values. Consequently these oils originate from younger beds, from the proven Lower Middle Miocene interval "blended" by minor contributions of Upper Miocene/ Lower Pliocene units.
Gas/source correlations are not possible with the isotopic method. However, the recorded desorbed gases below the proven geopressure interval must have been generated from the nearby Lower Miocene source and the gas indications above the overpressure zone have most likely been produced from Upper Miocene/Lower Pliocene source material.
Although a vertical gas diffusion through a 2,000 m thick geopressure body appears to be unlikely, it cannot be entirely excluded that certain quantities migrated through micro-fractures and faults to the surface especially in basinal portions with significant tectonic activity like the Iskenderun basin.
The interrelation between the oil seep Kepirce and the nearby gas seep Kurtbagi cannot be determined by direct analytical comparison. The gas, however, is not biogenic and contains thermocatalytic compounds.
The gas source is most likely the Middle Miocene kitchen, the same source that generates the liquid oil at the Kepirce seep. The active seeps of hydrocarbons are a sign for continuing generation of hydrocarbons in the basin.
CONCLUSION
This study has established a case history for:
- Characterization of gases entrapped in deep subsurface drill cuttings and in surface sediments.
- Determination of source maturity generating the gases.
- Identification of abnormal gases.
- Source/oil correlation.
The encouraging results obtained by the stable carbon isotope investigation contributed a beneficial value to the assessment of this geological province and the resulting geological interpretation (Fig. 9).
- The Iskenderun subbasin contains liquid hydrocarbons generated predominantly from Middle Miocene source units. Structural traps in sufficient depth are potential prospects. A secondary source interval is most likely present within the Upper Miocene/Lower Pliocene sequence making this formation prospective.
- The Adana subbasin contains hydrocarbons produced mainly from Devonian source rock. Productive accumulations appear to be depending on favorable access to the source.
Bulgurdag oil field, the only productive model of the basin, contains Devonian sourced oil mixed with hydrocarbons from a younger source. Consequently, similar to the Iskenderun basin, Tertiary source intervals must be in this basin as well.
This successful application of carbon isotope geochemistry was performed at very low cost. The results may direct the future exploration strategy to other targets like Upper Miocene/Lower Pliocene fan lobes, which have not yet been tested.
With geochemical investigations the subsurface presence of hydrocarbons can be directly detected, which cannot be accomplished with the conventional seismic tool. Seismic provides a quantitative evaluation, whereas geochemistry contributes a qualitative assessment.
More research and routine applications are needed to increase the inventory of successful case histories. Successful applications of high sensitive isotope geochemistry as integral part to pre-drilling seismic activities as well as post-drilling subsurface examinations fill a gap in prospect maturation. The reward will be high when the exploratory risk can be minimized.
ACKNOWLEDGMENTS
The authors thank Turkish Petroleum Corp. (TPAO), INA-Naftaplin, and RWE-DEA for permission to publish this study. Special gratitude is expressed to TPAO for assisting in the field sample acquisition. Dr. E. Faber and Dr. P. Gerling, Federal Institute of Geosciences and Natural Resources-BGR-of Germany, are thanked for stimulating discussions and beneficial suggestions.
REFERENCES
- Craig H., Isotopic standards for carbon and oxygen and correction factors for mass spectrometric analysis of carbon dioxide, Geochem. Cosmochem. Acta, 12, 1957, pp. 132-149.
- Craig H., Standard for reporting concentrations of deuterium and oxygen-18 in natural waters, Science, Vol. 133, No. 3,467, 1961, pp. 1,833-34.
- Faber, Gerling, Dumke, Gaseous hydrocarbons of unknown origin found while drilling, Organic Geochemistry, Vol. 13, Nos. 4-6, 1988, pp. 875-879.
- Faber E., Zur Isotopen-Geochemie gasformiger Kohlenwasserstoffe, Erdol und Erdgas Z., Vol. 103, 1987, pp. 210-218.
- Stahl W.J., Source rock-crude oil correlation by isotopic type curves," Geochem. Cosmochem. Acta, 42, 1977, pp. 1,573-77.
- Abrajano T.A. et al., Methane-hydrogen gas seeps, Zambales ophiolite, Philippines: Deep or shallow origin ?, Chemical Geology, Vol. 71, 1988, pp. 211-222.
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