Analysis of Sedimentary Environment and Sequence Stratigraphy of Middle-Late Permian Sediments in Coastal Fars , Iran ( Zireh Gas Field , Well ZHA )

The current paper is focused on the facies, sedimentary environment and depositional cycles study of the Middle-Late Permian sediments in the South Fars Zireh Gas Field, the subsurface section of well ZH-A. Four hundred thin-sections obtained from cores and cuttings were examined under standard petrographic microscope. For this study research, Corel Draw X6, Corel Photo-Paint, DN2 Microscopy Image Processing System, Scope Photo, Autodesk Map, Geocalc and Auto Cad 2014 were utilized. Based on microfacies analysis and significant founa and flora, fourteen major facies in four facies associations comprising tidal flat (A), lagoon (B), shoal (C) and open marine (D) identified in the well ZH-A. This formation was deposited in inner part of a homoclinal ramp. Based on depositional cycles, seven main 3 order sequences were revealed in the Well ZH-A. The ooid grainstone facies with interparticle and oomoldic porosity has high reservoir potential. The diagenetic processes like dolomitization and dissolution have significant effect in the reservoir quality. Based on research results, a major framework can be weaved and used to correlate reservoir layering.


Introduction
The Middle-Late Permian Dalan Formation is an economically significant gas and condensate reservoir in the Fars region of southwest Iran.This formation overlies the Faraghan Formation and conduces to the Kangan Formation.This formation consists of limestone and dolomite accompanied by some evaporite intervals with an overall thickness of 815 m.Edgell [1] has divided the mentioned formation into three members containing: 1) Lower Dalan Member, 2) Nar Evaporite Member and 3) the Upper Dalan Member.The geographical coordinates of the studied subsurface section are 51˚56'57.83"Eand 28˚10'23.82"Nand overlie Zireh anticline which is located about 169 km from Shiraz city (Figure 1).

Methodology
Four hundred thin-sections obtained from cores and cuttings were examined under standard petrographic microscope.Carbonate rocks were classified using textural classification proposed by Dunham [2].Diagnostic microfacies criteria of Flügel [3] were used in detecting the microfacies types in carbonate rocks.Evaluation of vertical and lateral changes accomplished according to [4] [5] [6].Sequence Stratigraphy concepts developed by previous workers (e.g., [7]- [12]) were extended to the current study.

Facies A1 (Massive to Layered Anhydrite)
The facies A1 consists of high percent anhydrite (about 80 percent) with divers forms such as nodular and bladed types (Figure 2(a)).This facies formed in supratidal environments due to dry and intense evaporation condition [13] [14] [15] and is mostly developed through the Nar Member.These brecciated dolomudstones represent evapo-karstic and karstic zones that have undergone evaporite and carbonate dissolution resulting in small vugs, dissolution seams and brecciation collapse.They can occur in both shallow-water limestone and evaporites sequences, and mark the top of the low-frequency regressive cycles [4].
The upper parts of intertidal settings in arid condition practice high salinity and intense evaporation which restrict the activity of browsers and burrowers.The upper intertidal pond is an environment where this facies is formed [5].

Facies A4 (Dolomitic Mudstone with Evaporite Casts)
The Facies A4 contains dolomitic mudstone with anhydrite nodules and crystals with fan type, bladed and needle fabrics (Figure 2(d)).This facies has formed in intertidal setting with attention to rare fossils, sparse anhydrite casts and fine-grained dolomites [3] [5].

Facies A5 (Peloid Intraclast Packstone)
It consist high percent of intraclasts (100 μm -1 mm) and few amount of peloids sized of 50 -150 μm.The matrix includes micrite and spary cement, but anhydrite cement is also found (Figure 2(e)).The generation of intraclasts is common in the supratidal and intertidal environments [3] [5].This facies seems to be developed in tidal channel sites at the adjacency of facies A4.  ooids which develop in brakish protected lagoons [16].In carbonate ramps, ooids are common in shorelines or sand shoals [3].With attention to ooid type and adjacency with tidal flat facies, theses ooids are considered to be deposited in protected lagoons near the shoreface.

Facies B3 (Bioclast Wackestone/Packstone with Algal and Benthic Foraminifera)
This facies contains calcareous green algae such as Mizzia sp. and Permocalculus sp. and benthic foraminifera (such as Hemigordius sp. and Geinitzina sp.) accompanied by gastropods, ostracods, echinoderms and bivalves.There is no spary cement and the oil show is rarely found (Figure 3(c)).The lagoonal setting is a place that this facies is formed [3] [19].

Facies B4 (Mudstone with Lagoonal Foraminifera)
This facies consist of lagoonal foraminifera such as Longa sp. and Dagmarita sp.

Depositional Model of Middle-Late Permian Sediments in Well ZH-A
Studies on facies associations and their lateral and vertical patterns based on depositional models, facies zones and standard microfacies reveal that the depositional model of Dalan Formation is a carbonate ramp (Figure 6).Gradual changes between relatively widespread facies belts, lack of breccias, ample skeletal particles, ooids, peloids, intraclasts and calciturbidites, presence of high energy grainstones in shoreface depositional settings accompanied by dasyclads and forams designate a homoclinal ramp setting.
The cortoids, oncoids, pisoids and aggregates are rare [3].Development of supratidal facies (A1) in the studied sequence points to the regression and arid climate resembling today's climate in Persian Gulf.The Dalan Formation formed on the passive margin of the Neo-Tethys Ocean extended from Saudi Arabia to Iran [4].

Interpretation of Sedimentary Cycles
Stratigraphically, this study is focused on the Dalan Formation in the Zireh Gas Field.Cycle 1 (LDS-1): This cycle has been identified according to the electrical logs (Gamma-Ray) and Paleologs due to lack of samples (Figure 7).The Maximum Flooding Surface (MFS) of this unit is marked by argillaceous and shaly deposits comprising "condensed zone" (Picked by sudden increase in gamma-ray response).The lower boundary of this cycle coincides the Global Stratotype Section and Point (GSSP, CC 268 Ma) [12] and sea-level falling [8] [11].The MFS of this cycle is according to Wor1 [8] and sea-level rising [11].The upper boundary of this cycle would correspond to the GSSP (CC 265.8) [12] and sea-level falling [8] [11].
Cycle 2 (LDS-2): This sequence includes Middle Lower Dalan Member encompassing the middle part of K5 reservoir (Figure 7).TST of this unit includes lagoonal and shoal facies in retrogradation stacking pattern.The MFS of this unit is marked by coarse grain intraclast bioclast grainstone (C3).HST is designated by lagoonal and intertidal facies that is capped by supratidal facies (A1).The MFS of this cycle coincides to the Wor3 [8].
Cycle 3 (LDS-3): LDS-3 encompasses the upper part of K5 reservoir (Figure 7).TST comprises of intertidal facies in retrogradation stacking pattern.MFS of this unit is marked by lagoonal facies (B3).HST of this cycle which passes through the Nar member is recognized by intertidal dolomudstone with evaporite cast (A4) that is capped by supratidal evaporitic facies (A1).MFS of this cycle would correspond to the Cap1 [8] and sea-level rising [11].
Figure 7. Lower Dalan member depositional cycles and their correlations with last calibrated sea-level changes and GSSP.
Cycle 4 (Nar-S): This sequence includes Nar member (Figure 8).TST includes a succession of intertidal and lagoonal facies in retrogradation stacking pattern.The maximum accommodation of this unit is distinguished by ooid grainstone facies (C2).
The MFS of this cycle is according to the Cap3 [8].This cycle is characterized by global warming and sea-level falling evidenced by intertidal to supratidal evaporitic facies.

Conclusions
Based on microfacies analysis and significant founa and flora, fourteen major facies in four facies associations containing Tidal flat (A), Lagoon (B), Shoal (C) and Open-marine (D) were distinguished in the well ZH-A.Lagoon facies are frequent, but are well distributed in Upper Dalan Member.The Ooid Grainstone facies (C1) has a good reservoir potential due to intergranular and oomoldic porosity accompanied by mimic dolomitization.This facies is well developed in UDS-1 comprising K4 reservoir.The open-marine facies (D1) is established in UDS-1.
Gradual changes between relatively widespread facies belts, lack of breccias, ample skeletal particles, ooids, peloids, intraclasts and calciturbidites, presence of high energy grainstones in shoreface depositional settings associated with dasyclads and forams designate a homoclinal carbonate ramp.
Based on depositional sequence studies, seven main 3 rd order sedimentary cycles were revealed in the Well ZH-A.The MFS of depositional cycles matches the best with the MFS proposed by Snedden and Liu [8].Dolomitization and high evaporation are common in the late HST due to global sea-level falling.The transgressive-regressive cycles have good correlation with the last calibrated sea-level changes and GSSP.

3. 4 .
Facies Association D (Open Marine Facies) Facies D1 (Bioclast Mudstone) This facies contains low percentage of spars ostracod and brachiopod fragments in the micritic matrix (Figure 5(a) and Figure 5(b)).The abundant micrite, rare skeletal fragments and scarcity of evaporites and intraclasts denotes open-marine low energy settings.

Figure 9 .
Figure 9. Upper Dalan depositional cycles and their correlations with last calibrated sea-level changes and GSSP.