پديد آورندگان :
ميرمرقابي، احمد نويسنده دانشجوي كارشناسي ارشد گروه زمين‌شناسي دانشگاه فردوسي مشهد , , موسوي حرمي ، سيد رضا نويسنده استاد گروه زمين‌شناسي دانشگاه فردوسي مشهد , , محبوبي، اسد اله نويسنده استاد گروه زمين‌شناسي دانشگاه فردوسي مشهد , , خانه باد، محمد نويسنده استاديار گروه زمين‌شناسي دانشگاه فردوسي مشهد , , جعفري، جليل نويسنده كارشناس شركت ملي مناطق نفت‌خيز جنوب ,
چكيده لاتين :
Introduction
More than 65 percent of the oil reservoirs and nearly 34 percent of the gas reservoirs of the world have located in giant hydrocarbon ?elds of the Middle East, that Zagros basin is one of them (Alavi 2007). The closure of the Neo-Tethyan Ocean during the Late Cretaceous was due to the convergence and northeast subductionof the Arabian Plate beneath the Iranian sub-plate thatled to the emplacement of pieces of the Neo-Tethyan oceanic lithosphere (ophiolites) onto the northeastern margin of the Afro-Arabian plate (Babaie et al. 2006).The Oligo- Miocene Asmari Formation is one of the most important reservoir units of the Marun Oilfield in Dezful Embayment SW Iran, deposited in Zagros foreland basin.Lithologically, this reservoir is one of the Iranian carbonate- sandstone reservoirs (limestone, shale, sandstone and dolomite) which are,in this field, located about 40 km east of Ahwaz city.The goal of this study is to interpret deposition environment and sequence stratigraphy of lower zones of the Asmari Formation in Well No.281, 342 and 312 in Marun Oilfield based on petrography, facies variations (microfacies analysis), the shape of ?- Ray log and isolith maps.
Material & Methods
For the purpose of interpretation of depositional environment and sequence stratigraphy,petrographic studies of more than 130 thin sections obtained from cores samples and ?- Ray log from the lower zones of the Asmari Formation in three subsurface borehole profile (Well No.281, 342 and 312) in the Marun Oilfield has been used in this study. Petrographic facies were described based on Dunham(1962) and Folk(1980) classifications. Carbonate facies analysis are done according to Schlager (2002) and Flügel (2010) models. Also the shape of the ?- Raylog is used to identify lithofacies variations for interpretation of depositional environments and sequence stratigraphy.Constructed isolith mapsof sandstone zones for 215 drilled well also used for interpretation of depositional environment and sequence stratigraphy.
Discussion, Results and Conclusion
Detailed microscopy observations of thin sections inWell No.281, 342 and 312 Marun Oilfield led to identification of 7 carbonate microfacies including Planktonic foraminifera wackestone- packstone (A1), lepidocyclinides numulitides wackestone- packstone (A2), bioclastic red algea wackestone- packstone (A3), coral- red algea floatstone- boundstone (A4), imperforate and perforate foraminifera packstone- grainstone (A5), perforate foraminifera wackestone- packstone (A6) and fenestrate mudstone- dolostone (A7) and 2 siliciclastic petrofacies (shale and quartzarenite),that are deposited in four depositional environment including open marine, barrier, lagoon and tidal flat in a homoclinal ramp (consisting of outer, middle and inner ramp).
Also, based on the shape of ?- Ray log, these sediments were deposited in marine environment. In open marine and barrier environments, The shape of ?- Ray log is serrated bell-shaped, serrated funnel-shaped, left bow-shaped, serrated shape and right boxcar shape, while in the beach environment, it is cylinder and funnel shape and in lagoon and tidal flat environment can be seen on right bow to cylinder-shaped.
Based on the isolith maps ,sandstone deposits of lower zones of the Asmari Formation in Marun Oilfield distributedalong the coastal area by deltaic system in the southwestern margin of the basin which is influenced by changes in sea level constantly.
In this case study, facies variations, strata patterns, ?- Ray log and isolith map has led to recognition of three third- order depositional sequences.
Sequence1 (DS1): The lower boundary of this sequencein the studied wells(281, 342 and 312) is not clear and must be presentin the lower zones and Pabdeh Formation. TST of this sequence iscomposed of limestone, shally limestone and shale (Well No. 281 and 312), pyritic shale and lepidocyclinidea wackestone microfacies (Well No. 342). Planktonic foraminifera wackestone- packstone (Well No. 312), lepidocyclinides numulitides wackestone- packstone(Well No. 281 )and shale with high ?- Raylog (100API)( Well No. 342) are identified as the maximum flooding surface (mfs).HST of this sequence is composed ofShally limestone with bioclasticred algea wackestone- packstone (Well No. 281), deltaic sandstone (Well No. 281), limestone, and deep sea shale and lepidocyclinides numulitides wackestone- packstone (Well No. 281).The upper boundary of this sequence(SB2) may be associated with the Ch10/ Ru30 sequence boundaries recognized by Ehrenberg et al (2007).
Sequence2 (DS2): The TST package of this sequence consists of deep sea shale and showing a gradual increasing in ?- Raylog toward the top. Maximum flooding surface is determined by the largest amount of the?- Ray log.The HST is characterized by deltaic- beach sandstones (Well No. 281 and 342 ), lepidocyclinides numulitides wackestone- packstone (Well No. 342), bioclastic red algea wackestone- packstone, coral- red algea floatstone- boundstone, imperforate and perforate foraminifera packstone- grainstone and beach sandstones (Well No. 342). The upper boundary of thissequence (SB2)may be associated with the Ch30 sequence boundary recognized by Ehrenberg et al (2007).
Sequence3 (DS3): This sequence in Well No. 281 is composed of transgressive systems tract (TST) with about 30 m thickness and consists of fenestrate mudstone- dolostone (tidal flat) and perforate foraminifera packstone- grainstone (open marine). Also the sequence in Well No. 342 is composed of transgressive systems tract (TST) with about 70m thickness and consists of perforate foraminifera packstone- grainstone, bioclastic red algea wackestone- packstone and lepidocyclinides numulitides wackestone- packstone, which reflect increasing in water depth.This sequence in Well No. 312 is composed of transgressive systems tract (TST) and highstand systems tract (HST). The lower part of this sequence is characterized by limestone with perforate foraminifera wackestone- packstone, that interpreted as TST. Floatstone- boundstone with coral- red algea is identified as the mfs. The upper part of this sequence is characterized by limestone with imperforate foraminifera packstone- grainstone, that interpreted as HST.
Keywords:Wire line Log, Asmari Formation, Depositional Environment, Marun Oilfield, Sedimentary Sequence.
