پديد آورندگان :
محسني، حسن نويسنده , , حبيبي، الهام نويسنده وزارت بهداشت، درمان و آموزش پزشكي,معاونت تحقيقات و فناوري , , قنواتي، كيارش نويسنده كارشناس ارشد، شركت ملي مناطق نفتخيز جنوب ايران، اهواز ,
چكيده لاتين :
Introduction
Thick carbonate intervals of the middle Cretaceous (upper Albian-Cenomanian- lower Torunian) were introduced as the Sarvak Fm. (James and Wynd, 1965). Their equivalent in the Arabian plate are the Mishrif Fm. in the UAE (Alsharhan and Nairn, 1990); the Natih Fm. in Oman (Van Buchem et al., 2001 and 2002). The Sarvak Fm. was focus of several researches (e. g. Daneshian et al., 2011; Honarmand and Maddahi, 2011; Tahavi et al., 2006; Hajikazemi et al., 2010; Rahimpour-Bonab et al., 2010; Razin et al., 2010, Ghabeishavi et al., 2010). This research focuses on the microfacies analysis, depositional environment and sequence stratigraphy of the Sarvak Fm. in the Marun oil field (Fig. 1).
Geologic setting
The Marun oil field is located 40 km east of the Ahvaz city in the simply folded belt of the Zagros basin in SW Iran as northern part of the Arabian platform. This part of the basin hosted considerable amount of hydrocarbon trapped in the Cretaceous interval (Ghabeishavi et al., 2010; Razin et al., 2010). The basin was a shallow carbonate ramp during early Cretaceous which evolved into rimmed carbonate shelf during middle Cretaceous (Granier et al., 2008) due to tectonic activities (Aqhravi et al., 2007). The Sarvak Fm. was deposited in this type of the basin since late Albian onward. Some rudistid reef facies rimmed this shallow platform (Setudehnia, 1978; Motiei, 1993). During late Albian-early Torunian four maximum flooding surface (MFS) were reported from the Arabian Plate (Sharland et al., 2001). Published data from the Zagros basin show good correlation between these MFS and 3rd order sequences ((e.g. Bibi Seyedan and Padena sections (Razin et al., 2010); Ab Teymor oil field (Rahimpor et al., 2010); the Natih Fm. in Oman (Alsharhan and Nairn, 1993)).
Method and materials
The present study was based on petrographic examinations on 300 thin sections prepared from cutting and core samples of two wells (Well# 1 and 2) drilled in the Sarvak Fm. Additional data Gamma Ray and Sonic logs as well as detail examinations of some selected samples from core lab of the NIOC, Ahvaz with SEM model JSM- 840A (Geol) in the Faculty of Engineering, Bu-Ali Sina University. Petrographic description was after Dunham, 1962; microfacies analysis after Wilson, 1975; Flugel, 2010).
Results and discussion
According to microfacies analysys on thin sections, 8 microfacies were recognized in the Sarvak Fm. in the Marun oil field which from shoreline to the offshore are as bellow:
a- Intertidal facies association: barren mudstone, indicating saline water unfavorable for biota (Shinn, 1983; Tucker and Wright, 1990).
b- Lagoon facies association: wackestone/ packstone/ grainstone with pellet, miliolid, green algae, and intraclast which suggest a restricted lagoon setting as the fauna are not abundant.
c- Open lagoon facies association: large benthonic forams, ostracods and rudistic fragments in lime mud matrix characterize this facies which were deposited in open lagoon with free circulation.
d- Shallow platform margin facies association: ooid grainstone/ coral boundstone to framestone with echinoderm and rudist fragments suggest relatively high energy condition which promoted winnowed grainstone and biogenic buildup growth in the margin of the shelf (Carozzi, 1989).
e- Open marine facies association: sponge spicules planktonic microfossils wackestone to packstone and bioclast- intraclast oligosteginid packstone with rotalipora in fine grain mtrix and poorly sorted fabric point toward open marine (Amiri Bakhtiyar and Taheri, 2010, Luciani and Kobianchi, 1999).
Presence of platform margin facies (d) supports a rimmed shelf rather than carbonate ramp. In the shelf environment facies a and b were deposited in inner shelf, ooid grainstone represents middle shelf which was protected by biogenic buildups consist of rudist and corals as well. Oligosteginid suggest shelf margin with moderate depth (Daneshian et al., 2011). Basinward the facies include sponge spicules, calcisphere, and some planktonic foraminifers (e. g. Hedbergella Favuzella) scattered in dark matrix are interpreted as basinal facies (Fig. 2). Similar facies were reported from the Sarvak Fm. (Ghabeishavi et al., 2010; Razin et al., 2010; Rahimpour-Bonab et al., 2010) from Zagros basin.
Sequence stratigraphy of the Sarvak Fm. was interpreted based on the vertical changes of the facies associations and comparison with well-known 3rd order sequences reported for the Albian- Torunian of the Arabian platform. Hence four 3rd order sequences were recognized in the Sarvak Fm. in the Marun oil field (Fi. 3). MSF of these sequences could be corelatable to those which were reported for the Arabian platform (K110 to K140 in the first and last sequences respectively). All sequences are bounded by type ?? sequence boundary excluding the upper contact with the Ilam Fm. which is a type ? sequence boundary as indicated by subaerial exposure and meteoric diagenesis. This unconformity was reported by several researches (James and Wynd, 1965; Taghavi et al., 2006; Hajikazemi et al., 2010; Razin et al., 2010; Rahimpour-Bonab et al., 2010).
Various diagenetic processes were observed in the Sarvak Fm. in the Marun oil field including cementation, dissolution, neomorphism, dolomitization, compaction and fracture related porosity. Among these processes, meteoric controlled dissolution, particularly in the upper part of the 3rd sequence and near the top of the Sarvak Fm. during exposure, had considerable impact on the development of the reservoir quality through leaving various pore spaces.
Conclusion
1- The Sarvak Fm. comprises 8 microfacies which were deposited in 5 facies associations on a rimmed shelf.
2- The Sarvak Fm. includes four 3rd order sequences bounded by type ?? sequence boundary excluding upper contact with the Ilam Fm. which is an unconformity. These sequences were accumulated during late Albian to early Torunian.
3- Meteoric diagenetic related processes affected on the the Sarvak Fm. enhanced porosity of the 4th sequence due to unconformity recorded on top of the The Sarvak Fm.
Acknowledgement
Part of this research was funded by a grant to the first author from the vice president research office of the Bu-Ali Sina University. The authors also wish to thanks to the research office of the NIOSC, Ahvaz for their financial support, providing the materials and permission for the publication of the results. Our thanks also due to the enormous reviewers of the journal who’s critical review and constructional comments improved the quality of the manuscript.
