فصلنامه پژوهش های چینه نگاری و رسوب شناسی
سال سی‌ام شماره 4 (پیاپی 57، زمستان 1393)

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. The goal of this study is to interpret depositional environment and sequence stratigraphy of lower zones of the Asmari Formation in Well No. 281، 342 and 312in Marun Oilfield based on changes in the shape of γ- Ray، isolith maps and microfacies properties. Accordingly، identification of 9 carbonate microfacies and 2 siliciclastic petrofacies were identified 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 γ- Ray log، There sediment were deposited in marine environment. In open marine and barrier environments، The shape of γ- Ray log is serrated bell-shaped، serrated funnel-shaped، left bow-shaped، serrated shape and right boxcar shape، Whole 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 of lower zones of the Asmari Formation in Marun Oilfield expanded by deltaic system along the southwestern margin of the basin and influenced by changes in sea level constantly. Sequence stratigraphic analysis led to identification of three third- order (DS1، DS2 and DS3) depositional sequences.

Palaeonvironment of the Dalichai Formation، northeastern Semnan، is envisaged based on 69 samples studied palynologically. Diverse and well-preserved palynofloras، viz.، miospores، dinoflagellate cysts، foraminiferal test linings، acritarchs، wood debris (plant tissue) and Amorphous Organic Matters (AOM) occur in the material examined. The presence of relatively abundant proximate dinoflagellates and ±laevigate acritarchs in the lower part of the section studied implies sedimentation in a turbulent shallow environment. However، occurrence in the upper part of the section of abundant chorate dinoflagellates as well as notable proportion of blade-shaped to equal dimensional opaque palynomacerals seem to signify marine incursion and accumulation in an open marine setting. In most of the samples examined، prevalence of transparent AOM denotes a low oxygenated environment and slow rate of accumulation. Moreover، incidence of three palynofacies types (II، IV، VI) in the pertinent material also corroborates a shallow low oxygenated environment with low sedimentation rate which gradually deepened upward in the section.

The present study was performed on 59 drillhole cuttings from Pabdeh Formation in Nosrat oil field using Rock- Eval VI pryrolysis. Geochemical analysis indicated that Pabdeh Formation possesses poor to good hydrocarbon potential. Plotting S1 against TOC suggests that samples were not affected by polluting substances such as crude oil and lubricants while drilling operation. Jones organic fancies diagram shows B-BC area indicating that Pabdeh Formation was deposited in marine anoxic to oxic environments. HI vesus Tmax shows that most samples initially have had type II kerogen and now reflecting a mixture of type II to III kerogen (capable of generating oil) that have already entered oil generation window. In addition، S1+S2 versus TOC plot also suggests that Pabdeh Formation can be considered as a capable hydrocarbon generating source rock in the study area.

Takht mine، SE Minoodasht contains well-preserved plant macrofossils belonging to 27 species allocated to 22 genera of various orders viz.، Equisetales، Osmundales، Filicales، Peltaspermales، Bennettitales، Ginkgoales and Coniferales. The plant macrofossils in this area are studied for the first time. Based on the occurrence of Equisetites muensteri، Clathropteris meniscoides، Dictyophyllum exile، Anthrophyopsis crassinervis، Scytophyllum persicum، Pterophyllum bavieri، and Baiera muensteriana a Rhaetian age is suggested for this assemblage. Since، there was no differentiation between formations in geological map these flora emphasized spreading the Kalariz Formation in this area. The Minoodasht flora is correlated to the plant macrofossil assemblages of Zirab، Tazareh، Narges-Chal، Hiv، Jajarm (Alborz)، Parvadeh mines (Tabas)، and Darbid-Khun (Kerman Basin). Therefore، there were close floristic relationships between North and Central-East Iran (i. e. Kerman Basin and Tabas Block) and two areas were palaeogeographically closely related، probably forming a uniform paleoenvironment. On the basis of relative abundances of taxa، Filicales، Bennettitales، Ginkgoales and Pinales were 42. 02%، 38. 11%، 7. 17% and 6. 51%، respectively. It is noteworthy that variety and relative abundance of the species of Filicophyta and Bennetittales were high within Iran during the Late Triassic epoch. Therefore، a warm wet climatic regime dominated. In addition، a more uniform climate in continental scale is suggested. On the basis of similarity indices and relative abundance of taxa، Iran located within Eurasia climatic belt، Euro-Sinian Region، and the Middle Asia Province of Vakhrameev’s subdivisions and south-western region of Dubroskina’s subdivisions during this epoch.

In this study، 40 drilling cutting of the Pabdeh، Gurpi، kazhdumi and Gadvan Formations from the Agha-Jari Oilfield were analyzed by using Rock-Eval pyrolysis. In order to recognizing sedimentary environmental conditions of studied Formations، they are divided to 4 zones which A (Kazhdumi Formation#187) and C (Kazhdumi Formation#140) zones show reduction conditions by presence of sea organic materials and B (Gadvan Formation #140) and D (Gadvan Formation#187) zones show oxidation conditions by presence of continental organic materials to basin. Based on the Rock-Eval pyrolysis data، the Pabeh، Gurpi، Kazhdumi and Gadvan Formations have variable hydrocarbon generative potential. HI vs. OI plot revealed that the kerogen type in this Formations is a mixed of types II & III. The intensity of matrix effect in the Pabdeh، Gurpi، Kazhdumi and Gadvan Formations was compared by using S2 vs. TOC plot and calculating its regression equation. The results show that the significant amount of S2 adsorption by matrix was happened in the Pabdeh (4. 98-6. 96 mg HC/gr rock in wells 113 and 121) and Gurpi Formations (4. 33 mg HC/gr rock in well 113) which is due to their low thermal maturity (Tmax<430) and low Gas-Oil Ratio. In the well number 187 the amount of S2 adsorption by matrix effect in the Gadvan Formation is about 0. 13 mg HC/gr rock showing relatively high thermal maturity. Due to the fact that most of samples are mature، they have reached the oil generation window and show high GORP. Accordingly، the least amount of mineral matrix effect is related to the Kazhdumi and Gadvan Formations. The amount of TOClive in the Pabdeh، Gurpi، Kazhdumi، and Gadvan Formations lies in the ranges of 0. 27-1. 64 (lean to good source rocks)، 0. 14-0. 64 (lean to fair source rocks)، 1. 85-2. 27 (good to excellent source rocks) and0. 30- 0. 51 (lean to fair source rocks). GORP factor for studied formations were (0-1) which reveals variable range of oil and gas generation potential.

The siliciclastic Dahu Strata (Early Cambrian)، in the Central Iran، 280 metres thick، in the Gazouieyeh area، rests with an erosional surface on Protrozoic-Early Cambrian sedimentary rocks (Dezu Series). This strata disconformably overlain by Middle Cambrian-Late Cambrian marine carbonate rockse (Kouh-Banan Formation). Based on field and Laboratory studies، 3 association facies، shale-sandstone and conglomerate have been identified. Mainly، sandstones are rich in quartz، feldspars، and rarely contain rock fragments (metamorphic and sedimentary). The sandstones have a wide compositional range from quartzarenite to arkose، feldspathic litharenite and rarely litharenite (chertarenite). According to plots of feldspar garins، total quartzose grains، and total unstable lithic fragments، they were derived from craton interior، transitional continental، and recycled orogen sources. The Dahu sandstones experienced diagenetic events that included compaction and pressure solution، cementation (mostly by silica، carbonate، Fe-oxide، clay and rarely by barite)، grain fracturing، alteration of unstable grains، dissolution and replacement. Based on petrological and geochemical studies، we interpreted the diagenetic history for the Dahu sandstones، which consists of early، deep burial and late stages. The above results are based on surface studies، but it might be changed during increasing the depth.