Characterization of oil-bearing fluid inclusions in calcite cement of oil reservopirs of Kupal oil field (Asmari and Bagestan Formations), Dezful embayment, Khuzestan province

Introduction: Oil-bearing fluid inclusions within oil reservoirs are usually trapped in the course of migration and during the formation of diagenetic neomorph mineral phases and associated cements. Their size often varies from 2mµ to 50µm (England et al. , 1987). They preserve the physic-chemical data such as temperature and composition of the fluids at the time of entrapment (Roedder, 1986; Barker & Goldstein, 1990; Sisson et al. , 1993). Since the oil reservoirs are commonly filled by oils of many origins with different maturity at various stages, investigation of the oil inclusions can help researchers determine the history of accumulation of oil within the reservoirs (England et al. , 1987). Microthermometric studies have been done in order to determine the maximum burial temperature, paleogeothermal gradient and maximum depth of burial (Burruss, 1987; Lowenstein & Brown, 1998; Ceriani et al. , 2002; Ceriani et al. , 2006; Ceriani et al. , 2011) The main objective of this research is to use the petrographic and fluorescence spectroscopy along with oil and aqueous inclusions microthermometry in cements of Asmari and Bangestan sedimentary formations of the Kupal oil field. For recognition of oil inclusions flourescenc microscopy was applied. The phenomenon of fluresence in organic matter consists of the emission of photons by chemical entities. Flurophores are able to absorb part of the incident energy, rising from a fundmental energy state of higher energy. The return to the ground state may then produce a luminescence phenomenon such as fluorescence (Hercules, 1965; Mousseron et al. , 1969). Essentially the fluorescence of organic molcules is related to the energy tranistion phenomena in the c=c bands. The conjugated system reguires a lower excitation energy therefore, aromatic and polyaromatic compounds are mainly responsible for the flurescence properites of sedimentary organic matters (Wehry, 1967). Oil inclusions are Classified with respect to changes in color of fluorescence: Orange colors with maturity (API): 15-20, Yellow colors with maturity (API): 20-30, blue colors with maturity (API): 30-40, white colors with maturity (API): 40-50 and White with maturity (API) > 50 (Lang & Gelfand, 1985).

Geologic setting: Kupal oil field is one of the main oil fields which is located about 60 km northeast of Ahwaz, Khuzestan Province. The area is located in central part of northern Dezful embayment. This oil field includes Asmari and Bangestan reservoirs both of which are currently producing oil.

Methods: In this study, samples were collected from sedimentary rocks and core logs of the Asmari reservoir (No. 40 and 44) and Sarvak reservoir (No. 20 and 4). To carry out this study, wafers of doubly polished sections (150 -200 µm) were prepared for fluorescence studies and discremination of inclusions (aqueous and petroleum) was done by using Olympus microscope equipped with UV light (with 340 nm filter). The microthermometric measurements were performed using a Linkam THMS600 heating - freezing stage.

Discussion: Fluid inclusions petrograohy: Based on petrographic studies, The majority of the studied fluid inclusions are hosted by sparry calcite cements and calcites filling the solution casts and cavities. The sparry calcite cements exhibit a variety of primary and secondary fluid inclusions. Most of them are hydrocarbon bearing fluid inclusions. Fluid inclusions micro termomerty: The obtained results showed that the highest frequency of homogenization temperatures in oil inclusions are 60-70 ºC and 70-80 ºC in Asmari Formation and 65-85 ºC and 100-120 ºC in Sarvak Formation. Also the salinity of fluid inclusions in the studied samples vary from 5 to 15 wt% NaCl eq. Fluid inclusions fluorescence: The fluorescence studies on oil reservoirs of Asmari and Bangestan (Sarvak) Formations illustrate two types of fluorescent colors, yellow and blue. Combination of the results obtained from micro-thermometry and fluorescence studies indicate that Asmari and Bangestan (Sarvak) reservoirs were fed by two different sources or were filled through several stages of diagenetic processes. The blue fluorescent color indicates the higher degree of maturity (API: 50-40) while yellow fluorescent color reflects relatively immature and heavy (API: 30-20) oil. Aqueous fluids are non-fluorescent.

Conclusion: The sparry calcite cements exhibit a variety of primary and secondary fluid inclusions. Most of them are hydrocarbon bearing fluid inclusions. Highest frequency of homogenization temperatures in oil inclusions are 6070 ºC and 70-80 ºC in Asmari Formation and 65-85 ºC and 100-120 ºC in Sarvak Formation. The results of fluorescence stuies showed that the oils from Asmari and Bangestane (Sarvak) reservoirs have two types of fluorescent colors (yellow and blue) that indicating the reservoirs were fed by two different sources. The blue fluorescent color indicates the higher degree of maturity (API: 50-40) while yellow fluorescent color reflects relatively immature and heavy (API: 30-20) oil. Combination of fluorescent, geochemical, and microthermometric data in Asmari and Bangestan Formations revealed that there were various generations of hydrocarbons filling the reservoirs.

Acknowledgments: The authors would like to thank the National South Oil Company for providing samples and funds for this research project. Our appreciation also extend to University of Tabriz, Research Institute of Petroleum Industries, and Iran Mineral Processing Research Center for furnishing laoratory facililities for this research