پديد آورندگان :
فتحي، نوشين نويسنده دانشجوي كارشناسي ارشد، دانشگاه آزاد علوم و تحقيقات تهران , , رحيم‌پور بناب ، حسين نويسنده استاد دانشكده زمين شناسي، دانشگاه تهران , , دارايي، مهدي نويسنده دانشجوي دكتراي رسوب‌شناسي و سنگ شناسي رسوبي، دانشگاه تهران , , اسعدي ، علي نويسنده دانشجوي كارشناسي ارشد زمين‌شناسي نفت، دانشگاه تهران ,
چكيده لاتين :
Introduction
The Upper Jurassic strata of the Arabian Plate, known as the Arab Formation, are the most prolific reservoir rocks of the world (Beydoun, 1988, 1991; Cantrell 2006; Lindsay et al. 2006). This formation encounters in many parts of the Arabian plate, mostly occurring as reservoir in the Persian Gulf, Abu Dhabi, offshore U.A.E., Saudi Arabia, Bahrain, and Qatar (See Alsharhan and Kendall 1986; Alsharhan and Nairn, 2003 for more information). The type section (127.5 m) of the formation has been selected in the well Dammam-7 of the Dammam Field, located in eastern Saudi Arabia (Steineke et al., 1958; Powers, 1968). First lithostratigraphic description of the strata by Steineke et al. (1958) led to their subdivision into 4 members which are currently applicable over the Arabia and neighboring countries. Accordingly, the formation is comprised of Arab D, Arab C, Arab B and Arab A members, from base to top, each containing a shallowing-upward cycle of carbonate to evaporite (Fig. 1a). The last evaporite unit, overlaying carbonates of the Arab A Member, has been named as Hith Formation (Powers, 1962, 1968) which makes the principal regional seal for the Jurassic reservoirs in the south and southwest of the Persian Gulf (Murris 1980, 1981).
Although the formation has been studied for many years, concerning different aspects of the formation (e.g. Cantrell, 2006; Lindsay et al. 2006; Eltom et al. 2013), but no published data is available from the Iranian part of the formation. This study focuses on this part of the formation. In this way, three exploration wells, located within the Ferdowsi heavy oil field were examined for this study (Fig. 1c).
Materials and Methods
All three exploration wells (Named F-1 to F-3) of the Ferdowsi Field were selected for this study, among which F-3 is the only one that the core samples were taken. A total of 93 m cores of the F-3 were investigated for lithofacies analysis. Based on the core studies (lithofacies analysis) some 365 plug samples were acquired for petrographic studies (microfacies analysis) and laboratory measurements. All standard thin-sections, prepared from the plugs were stained for their mineralogical composition using Dickson (1965) method, and 55 samples impregnated with blue-dyed epoxy resin. Microfacies classification was based on Dunham (1962), and Embry and Klovan (1971) methods. For porosity examination, definitions proposed by Choquette and Pray (1970), are considered. Frequency of allochems in each facies, their cement and matrix content were determined using comparison charts.
Based on the sedimentological characteristics of each facies and comparison with the standard facies from well-known environments (Wilson 1975; Pomar 2001; Flügel 2010), depositional environment of each facies was determined. Facies with comparable sedimentological characteristic (genetically related) were considered as facies association (Miall 2000).
Discussion of Results & Conclusions
In the studied interval, based on petrographic analysis and core description, 9 facies were recognized. The Arab Formation formed in a high energy carbonate ramp composed of extensive shoal complexes, lagoons and vast peritidal sub-environments.
Marine cementation, hypersaline precipitation and meteoric leaching were the main diagenetic processes impacting both reservoir quality (partially) and diagenetic evolution. Early marine cementation along with over-pressured system resulted from fluids entrapment in evaporite capped cycles, led to retention of primary poroperm values. Furthermore, meteoric leaching enhanced primary porosity. Namely, intra-framework cavities of some encrusting micro-organisms and intense meteoric leaching especially led to extraordinary high reservoir quality in the lagoon facies. Sea-level fluctuations and arid climatic conditions were the main allogenic controlling factors in development of petroleum system of the Arab Formation.
porosity-permeability cross-plot for different facies, show discrete clusters for given petrophysical attributes, and each can be regarded as a single petrophysical group. On the other hand, similar pattern can be seen among different sub-environments. This suggests the primary or depositional parameters may have forced the main petrophysical characteristics of the reservoir (the case that was referred to as “depositional reservoir” by Ahr 2008). This sedimentary environment exerted the primary control on petrophysical characteristics such that the shoal made main reservoir rocks and peritidal facies acted as seals for the underlying porous rocks.
Keywords: Arab Formation, Microfacies, Sedimentary model, Diagenesis, Reservoir Quality
