نشریه رخساره های رسوبی
سال هفتم شماره 2 (پیاپی 13، زمستان 1393)

Carbonate successions of the Sarvak Formation (Late Albian? To Mid-Turonian) together with the carbonate of the Ilam Formation formed the second important hydrocarbon reservoirs in SW Iran and Persian Gulf. Facies analyses were carried out on the upper parts of the Sarvak Formation in western part of the Ahwaz Oilfield (AZ-63 well) to interpret depositional environment. Lithologically، the upper part of Sarvak Formation is composed of clean to argillaceous limestones، which have been partly dolomitized in some intervals. Based on this study، 15 microfacies were identified that are grouped into 6 facies associations، deposited in a homoclinal ramp-like carbonate platform in the studied well. Facies distribution is also investigated in the suggested depositional model using the frequency pie-diagram. Biotratigraphic studies were carried out on microfossils of the Sarvak Formation and resulted in presentation of three biozones including Nezzazata-Alveolinid assemblage zone (Cenomanian)، Rudist debris zone (Cenomanian-Turonian) and Moncharmontia apenninica-Nezzazatinella-Dicyclina assemblage zone (Turonian). These are closely correlated with the biozonation scheme of Wynd (1965).

Electro-facies is a unit of the vertical sediments sequence on the wire-line logs that can be differentiated from the upper and lower units. For detection of the electro-facies interval in Kangan and upper Dalan formations in south Pars Gas Field، wire-line log and core data analysis (wells A، B، C and D) were used. Note، only core data analysis in well A were available. First، thin-Sections study core plugs from this well resulted in the identification of 12 micro-facies and 7 sedimentary environment. For recognition of electro-fasies based on these data، clustering method and core data were used to categories their homogenization of data. In this study، Rock-Types (R-T) that have similar reservoir characteristics were identified using porosity/permeability cross plot from core data; therefore based on these data، 6 Rock-Type were identified. After classification of Rock-Types and micro-facies from the core data، MRGC models of these facies were identified from the wire-line logs in well A. Optimized model in well A were applied in the wells B، C and D to recognized electro-fasies in these wells with no-core data. Average petro-physical properties of each facies in different wells were compared for quality controls of predicted facies. Also facies of 4 wells were correlated to see if they are comparable. As a result، 6 electro-facies were identified with different petro-physical characteristics and the poorest reservoir quality belong to the facies (1) with dominant anhydrite lithology and the best quality reservoir belong to the facies (6) with the dominant dolomite and limetone with grainstone textue. Due to the high accuracy of the results، the suggested model can be used in other wells in this field.

Dalichai Formation at the Rey-Abad section (northeast Shahroud) along the principal road of the Mayamey-Jajarm has a thickness of 166 meters. In this section، Middle to Upper Jurassic carbonate rocks are well exposed and is typically composed of massive to medium-bedded limestones with thin shale interbeds. In the studied area and most places of this region، the lower boundary of Dalichai Formation with Shemshak Formation is covered، but in some locations، this boundary is exposed. The upper boundary with Lar Formation is not seen. Detailed fieldwork and study of 42 thin sections، using microscope، are done for identification of facies and sedimentary environment of the Dalichai Formation in the Rey-Abad section. The results of study lead to identification of 11microfacies، groups into 3 carbonate facies (A، B and C)، deposited in tidal flat، lagoon and shoal environments. The microfacies studies pointed to a shallow marine environment. On the bases of facies analysis، the carbonate sediments of the Dalichai Formation are formed in a carbonate platform of the ramp type.

For biostratigraphic studies، based on calcareous nannofossils، a thickness of 480 meters of Cretaceous deposits in Shushud section (North Birjand) has been measured and sampled. The studied section is composed of shale، marl، marine sandstone and sandy limestone. The shale and marl units were more suitable for sampling. According to taxonomical studies، 10 genera and 25 species in Shushud section were identified and photographed. Based on identified calcareous nannofossils in the studied section، CC20-CC26 biozones from Sissingh (1977) biozonation were suggested، and based on determined biozones، the age of the studied sequence in this section is Late early Campanian- Late late Maastrichtian. Based on low diversity and abundance of species، it can be concluded that environmental conditions in this basin were not good for the life of the calcareous nannoplanktons، as well as digenesis caused low to medium nannofossils preservation.

Binaloud Mountains formed during collision of Iran and Turan plate in the Late Triassic time and caused shortening of the sedimentary cover because of Cimmerian and Alpine orogeny. The thrust sheets with different generations formed as a results of the Binalood and Alborz fold- thrust belts. Northeast of Iran that was already south coast of the divergent margin of the Paleothetys Ocean at this time، has become a marine foreland basin as a result of this colliding and bending of lithosphere. Stratigraphical column of foreland basin usually is made up of a few upward coarsening and shallowing sequences that each of them represents a step loading and removal times of the lithosphere beneath the basin. The first generation of thrust faults began with thrusting of Mashhad sheet (Paleothetys remnants) over the Shemshak Formation that is syn-orogenic with Cimmerian orogeny. As a result of erosion of mountains، large amounts of siliciclastic deposits transported to foreland basin in southern parts and the Middle Jurassic conglomerate and sandstone layers of Aghonj formation and siliciclastic beds of Ferizee are deposited. These sequences show upward coarsening and thickening with flow direction to the south. Alpine movements began during the Mid-Cretaceous time، and caused the formation of the second generation thrusts of Binalud fold-thrust belt، uplift، erosion and transport of siliciclastic material with upward coarsening and thickening (Foreland basin) of Paleogene. These sequences represent depositions in different parts of the alluvial fans with movement towards the south. Third generation thrusting related to Late Alpine movements caused regression of sea water to the south and creation of lagoon in foreland basin. Evaporate sediments at the base of Neogene clastic sequence is the result of such a process. Uplift of the foreland basin deposits due to new thrusting activity and erosion of them caused transportation of large amounts of siliciclastic sediments into the foreland basin and creating upward coarsening and thickening sequence of Neogene to the present.

Two polygonal biogenic sedimentary rare structures of Koilosoma (Hitchcock 1858) have been found due to neoichnology of the Abhar-rud river fluvial sediments، northwest Iran. One of them is polygon holes، with about one square meter area. Polygons are same in size and depth، and are hexagonal in shape. They have distinct boundaries like the honeycomb. This sample is like the reported specimens from United States of America and may be referred to tadpole nests or holes. Other specimen is similar to later in shape، arrangement and size، but has been found with its trace-makers، Nemacheilus sp. Cobitidae fishes. All fish had same arrangement in holes، heading upstream current and each hole was occupied by one fish. It seems that holes are feeding structures، and produced during axial undulatory movements by fish school.

The Aitamir Formation (Albian- Cenomanian) in the Kopet- Dagh Basin consists of fine to medium grained sandstone، siltstone، shale and limestone. Its lower contact with the Sanganeh Formation is gradual and its upper contact with The Abderaz Formation is unconformable. Field and petrographic studies of the Aitamir Formation in Artenj and Ghorghoreh sections، located in Bazangan area- east of Kopet- Dagh Basin، led to identification of 3 facies association consisting of sandstone، mudrocks and limestone. Based on this study، they were deposited in marine، shoreface، barrier and Lagoon environments. Five depositional sequences in Artenj section and four depositional sequences in Ghorghoreh section were identified based on field observations، facies and sequence stratigraphic analysis. In studied sections، the sequence boundary of the first depositional sequence is in the Sanganeh Formation and upper boundary of the last sequence with the Abderaz Formation is type 1 but the rests are type 2 (SB2). The transgressive parasequences (TST) including a predominance of deep facies and regressive parasequences (HST) contains shallow facies parasequens. Interpretive sea level curve of studied sections are relatively comparable with the world wide curve. However، some differences could be occurred due to regional events.

Primary sedimentary and secondary structural and diagenetic processes are responsible for heterogeneity in carbonate reservoirs and recognition of these factors is very essential subject in modeling these reservoirs. Image logs are very useful for identification of structures and small scale sedimentary features. In this research، using image analysis of Formation Microimager log (FMI) in a drilled well in Dalan Formation، 7 patterns including: 1- band pattern، 2- spot pattern، 3- fracture pattern، 4- drilling induced fracture، 5- cross bedding pattern، 6- block pattern and 7- stylolite pattern are proposed. These patterns were compared with thin sections and core samples. Finally، with combining the FMI results، analyzed by JmicroVision (v. 1. 27) software، with thin sections and core samples of the different facies used to separate the grain supported from mud supported facies. Also among the various diagenetic processes، pressure solution، nodular anhydrite، bioturbation and dissolution were identified in image logs. This study shows that in most cases there is a good synchrony between the results of image logs، thin sections، core samples and well logs.