Diagenesis and sequence stratigraphy of the Fahliyan Formation in the Yadavaran oil field (Koshk and Hosseiniyeh) in the north Dezful Embayment

IntroductionInvestigation on mutual relation between depositional facies and diagenesis in a sequence stratigraphic framework is essential for systematic recognition of a reservoir (Lucia, 2004; Ahr, 2008). Spatial distribution of porosities of carbonate rocks may drastically change due to diagenetic processes (Mehrabi and Rahimpour- bonab, 2013). Facies analysis and diagenetic studies in sequence stratigraphic framework could facilitate for reservoir simulation (Masse and Masse, 2011). Hence, combination between diagenesis and sequence stratigraphy is useful for better understanding of diagenetic processes (Morad et al., 2000). This research will discuss distribution and variations of diagenetic processes of the Fahliyan Formation in a sequence stratigraphic framework in the northern part of the Dezful Embayment.
Results and discussionThe Fahliyan Formation is composed of 24 microfacies that formed four facies associations and were deposited in tidal flat, lagoon, bar and open marine (Esfandyari, 2013). The tidal flat facies association includes quartzarenite, shale, intraclast grainstone, mudstone and dolomudstone. Absence of bioclasts, frequent detrital quartz grains and intraclast, evidently suggests a tidal flat setting (Flugel, 2010). Lagoonal facies association includes peloids wackestone/ packstone, algal boundstone and bioclastic mudstone. Abundant algal fragments (e. g. Salpingoporella, Dasycladacea, Actinoporella, Clypeina, Acicularia) are distributed in this facies association. Frequent lime mud, peloid and milliolid foraminifera suggest a lagoon environment located on mid- shelf (Flugel, 2010). Peloid- intraclast grainstone, ooid- intraclast grainstone and lithocodium boundstone are representatives of a bar/ barrier. Lack of lime mud and present of skeletal grains are evidence of dynamic high- energy setting. These are similar to bar microfacies of Flugel (2010) and facies belt 5 and 6 of Willson (1975). The open marine facies association is composed of resedimented/brecciated carbonate beds. Extraclast, bioclasts and quartz grains present in lime mud matrix. Mixed open marine and lagoonal biota may represent talus deposit formed in front of carbonate platform slope. Mudstone with sponge spicules and radiolarian are being interpreted as open shelf (Flugel, 2010) and facies belt 1 and 2 of Wilson (1975). Here, these facies association are interpreted as being deposited on a shelf type carbonate platform. Various diagenetic processes affected the Fahliyan Formation in the Yadavaran oil field such as micritization and neomorphism that are more abundant in the lagoonal facies. Bladed isopachus HMC cement was precipitated in grain- supported facies (e.g. grainstone of bar/ barrier). Moldic, vuggy and intraparticle porosities are developed within mud- supported and brecciated facies. Evidences of burial diagenesis were also observed as stylolite, along late stage dolomitization and poikilotopic cement as well.
Sequence stratigraphyThe Fahliyan Formation is composed of three 3rd order sequences that can be correlated with Lower Zuni I and Upper Zuni II megasequences (Klonga and Kesling, (2002) in a global scene. They can also be correlated to the Mehrdad supersequence of the Early Cretaceous of the Zagros basin (Heydari, 2008). These sequence are composed of TST and HST facies which are separated by MFS. The first MFS is equal to MFS K20 of the Arabian plate (Sharland, et al. 2001), which was reported from the lower Fahliyan Formation (Shakib, 1994). The second FMS was also recognized in the mid Fahliyan Formation, correlatable to MFS K30 of the Arabian plate (Sharland, et al. 2001). Dolomitized lime mudstones on top of the second sequence, suggest a type I sequence boundary. MFS of the last sequence was recognized within the upper Fahliyan Formation that can also be correlated to K40 of the Arabian plate (Sharland, et al. 2001; Shakib, 1994). A global fall of sea- level (Haq et al., 1988) caused influx of siliciclastic sediments into the basin during final stage of HST of the last sequence designating type I sequence boundary. These sequences are correlated to those previously reported from the Zagros basin and Arabian plate as well (e.g.: Shakib, 1994; Sharland, et al. 2001; 2013; Haq and Al-Qahtani, 2005). Moldic porosity in shallowing- upward, grain- supported sediments are abundant just below and above the sequence boundaries in both wells. They were probably developed after influx of meteoric water as a consequence of sea- level fall. Sea- level rise might have restricted influx of coarse siliciclastics. Dolomitization of mud- supported facies is evident as dispersed dolomite crystals. Although these fine crystalline dolomites may reduce the porosity of carbonate rocks, but they could effectively increase the permeability of such rocks (Lucia, 2004). HST sediments of the sequences 2 and 3 are grain- supported and are expected to be more porous and permeable. These sediments were deposited on shelf margin facies with considerable intergranular porosity which was occluded by cements.
ConclusionThe Fahliyan Formation is composed of shallow marine to deep shelf sediments. Shallow platform facies experienced various diagenetic processes which improved their porosity. The Fahliyan Formation consists of 22 carbonate microfacies and 2 siliciclastic lithofacies which were deposited in tidal flat, lagoon, bar and open marine environments on a carbonate shelf. These sediments comprise three 3rd depositional sequences. Diagenetic processes are effectively related to sequence stratigraphy. Early marine cements are closely confined to shelf- edge sediments of TST. Maximum dolomitization was also observed in TSTs, and tidal flat facies of HSTs. Evidence of meteoric diagenesis after decline in sea- level rise are evident in sediments of late HSTs in both wells, particularly in two upper sequences.