نشریه رسوب شناسی کاربردی
پیاپی 12 (پاییز و زمستان 1397)

Petrography and geochemistry (major and trace elements) of siliciclastic rocks from the Jeirud Formation (Upper Devonian) in the Central Alborz, have been investigated to understand their provenance. Petrographical and geochemical analyses of the white to gray sandstones of the this Formation display three categories which consist of quartz arenite, sub-litharenite and sub-arkose.These sandstones, characterized by medium to very well roundness, high sorting and high compositional and textural maturity of monocrystalline quartz, chert and k-feldespar, were derived from a distal Arabia-Africa plates recycled orogen and stable cratonic source. Most major and trace element contents of studied samples are generally depleted (except Si) to upper continental crust (UCC) values that is mainly due to the high percentage of stable minerals such as quartz and chert and low content of un-stable Al-bearing minerals like feldspar and clay minerals. Modal composition (e.g. quartz, feldspar, lithic fragments) and geochemical indices of sandstones, indicate that they were derived from felsic source rocks in a humid climate and were deposited in a passive continental margin.

Sarvak Formation, as part of Bangestan group, is important for the production and exploration of hydrocarbons in the Zagros and Persian Gulf basins. In this study, for identifying flow units and interpreting reservoir quality changes in Sarvak Formation in the sequence stratigraphic framework, combining many data like core description, petrographic and porosity-permeability studies on core samples in one of the large field of the Abadan plain have been used. These data are including of 388m core samples, 420 microscopic thin sections and 800 porosity-permeability plugs from two key wells. Based on sequence stratigraphic side of view, the upper part of Sarvak Formation in the studied field can be divided in to tow 3rd order sequences. Based on the stratigraphic modified Lorenz plot (SMLP), flow units were identified and interpreted. Sequence stratigraphy and reservoir quality studies show that the best flow units are related to the middle and the upper part of high stand system tract and the middle of transgressive system tract. Packstone and rudstone related to the proximal open marine and also talus are the facieses associated with this system tract.

The Upper Red Formation (URF) was chosen to review due to tectonic and stratigraphic complication of the Avaj area and the extent of the URF in this area. The URF was investigated through sedimentological and tectonic aspects. The URF deposits in the Avaj area were divided into two members (M1 and M2). Based on sedimentological investigations (mineralogy, lithology and sedimentary structures), the URF two members are different in lithology (e.g., quartz, feldspars, volcanic rock fragments and carbonate cementation differences in sandstones and clay mineral composition difference in mudrocks). The paleocurrent direction changed from SE to NW in the lower member to NNW to SSE in the upper member. Field observations revealed an unconformity between these two members. The Shurab salt dome deformed the lower member but deformation did not reach the upper member. According to the mentioned evidence, two previously known members of the URF (M1 or the Avaj Red Beds and M2 or the Bi-Ab conglomerate) did not belong to the same formation, and the upper member (Bi-Ab conglomerate) should be considered as a new lithostratigraphic unit. Based on tectonic evidence, the late Alpine movements (<20 Ma.) triggered the inversion of Avaj Fault and deposition of the URF. The Atikan orogeny (5 Ma.) caused the deformation of the URF and the unconformity at the base of the Bi-Ab conglomerate. The Atikan movements also reactivated the Hassan Abad Fault, causing the creation of the Khar Rud shortcut. The uplift of the Khar Rud block provided the Bi-Ab conglomerate sediment supply. The Bi-Ab conglomerate might be deposited during Pliocene and deformed by the Pasadanian movements (2-1.8 Ma.).

In present research, depositional environment of the Qelli Formation (Late Ordovician) was investigated in NE Alborz Basin (Robat-e-Qarabil Area; Kuh-e-Kurkhud). This study was performed based on field observations, thin sections petrography, sedimentary structures and palynological studies. Utilizing this multidisciplinary approach, four litofacies assemblages were recognized. The Qelli Formation in studied area comprises alternation of thick to thin shale beds, medium to thin bedded sandstones, sandy limestones, laminated siltstones and diabasic sill. These lithofacies assemblages consist of arkose-subarkose, sandy allochem limestone, siltstone and shale. According to the properties of lithofacies, sedimentary structures and vertical stacking of these lithofacies suggest a storm and wave dominated shelf, wherein the depositional setting evolved from lower shoreface with lower- middle lithofacies (with hummocky cross stratification, truncation surface, parallel lamination and cross lamination) that gradually changed into upper shoreface with upper lithofacies (with swaley cross stratification, wave ripple-mark, parallel lamination, normal grading, wedge-shaped and tabular cross-lamination) which storm and waves induced currents had important roles in sediment reworking and redeposition. Using the Tyson diagram, four palynological facies (III, IVa, V and VII) were identified. Meanwhile, statistical studies on the palynological parameters (percentage of phytoclast, amorphous organic matter and lability parameter) indicate a shallowing upward trend in Ghelli Formation. Comparison of the relative abundance of marine palynomorphs with terrestrial palynological elements indicate less abundance of acritarchs and chitinozoans in comparison with cryptospores and plant fragments which indicate that Qelli Formation is deposited in shallow marine environment.

The first essential and key step for optimum development of hydrocarbon fields is the determination of reservoir properties. More precise determination of reservoir properties provides stronger infrastructure for development management of the reservoir. Therefore we are observing significant attempts in order to develop more effective methods for determining and to present a more accurate view of the reservoir petrophysical properties, including reservoir rock types. Of the most effective recent methods is the RQI/FZI flow zone indicator method, which is very appropriate for the identification of hydraulic flow units (HFUs). In this study, a discrete rock type (DRT) was determined on the basis of HFU and with using the Sequential Indicator Simulation (SIS), its spatial structure and probabilistic distribution in the three-dimensional network of the reservoir has studied. As a result, five discrete rock types were identified while determined DRT3, DRT4 have the best reservoir quality; and the DRT1, DRT2 and DRT5 have the lowest reservoir quality. In the longitudinal section, DRT3 is more extensively developed in the middle part of the reservoir, indicating better reservoir quality in this part of the reservoir than in other parts. In the transverse section, the DRT3 and DRT4 are high reservoir quality in the southern and western part of the reservoir, but in the central and eastern part of the reservoir, the lower reservoir quality are observed; also the developed model with high accuracy shows rock type dispersion in the reservoir.

In this research, the Mishrif Formation was studied in Sirri Esfand field in order to investigate the pore system and reservoir zonation using velocity deviation log. The results show that there is a significant and consistent correlation between the variations of this log with the facies characteristics and core porosity and permeability values of reservoir rocks, which based on three reservoir zones were differentiated in the field. In these zones, variations in the quantitative and qualitative values of velocity deviation log can be related with pore system properties of the reservoir facies with respect to their primary texture and the effect of the diagenetic processes. Comparison of differentiated zones shows that zone-1 in upper part of the reservoir under the effectof sedimentary texture as well as the effect of diagenetic processes such as dissolution has high reservoir quality. Zone-3 in lower part of the formation, due to the mud-dominated texture of the facies has a lower reservoir quality. Zone-2 has an intermediate state in terms of reservoir quality.