فصلنامه پژوهش های چینه نگاری و رسوب شناسی
سال سی و نهم شماره 2 (پیاپی 91، تابستان 1402)

In this paper, a method is proposed for the prediction of thermal maturity in the source rock using indirect methods. The applied data are well logs (neutron, density, resistance, and acoustic) in 13 wells and seismic data in six oil and gas fields in the central part of the Persian Gulf. Well-logs and seismic data are much more abundant than geochemical data and cover an extensive area in the oil and gas fields. These properties compensate for the lack of geochemical data that are scattered and limited to a few wells. This study is carried out in two steps. First, the amount of thermal maturity in the Kazhdumi Formation is calculated from well logs and is presented as an index in each well. Data obtained from organic thermal evaluation analyses are used to validate the results of thermal maturity prediction. These data include Rock-Eval pyrolysis in two wells. Then, seismic data are processed and studied in two-dimensional sections at the location of the target fields. In this step, seismic attributes are extracted from the seismic data using the multi-attribute regression analysis method, and thermal maturity is calculated using these attributes. Prediction is performed by probabilistic neural network analysis, and a seismic section is extracted indicating variations in thermal maturity in the Kazhdumi Formation.

The Sarvak Formation is one of the most important oil reservoirs in southwestern Iran. The Hendijan–Bahregansar–Nowrooz strike-slip Fault (HBNF) is known as a major fault system in the northwestern of the Persian Gulf, extending for approximately 700 kilometers in a north-northeast to south-southwest direction. This fault line passes through the northwestern region of the Persian Gulf, exerting a significant influence on the geological evolution of the area. This study emphasizes on importance of the microfacies, sedimentary environments, and sequence stratigraphy of the Sarvak Formation in the western part of the Hendijan–Bahregansar–Nowrooz palaeohigh, specifically those of the Hendijan, Bahregansar, and Mahshahr oilfields in southwestern Iran. A total of 186 thin sections of rock samples was examined in terms of their petrographic, sedimentological, and stratigraphic aspects. This led to the identification of seven microfacies distributed in four facies belts of tidal flat, lagoon, shoal, and open sea. The lack of turbidites and continuous reefs indicates that carbonates of the Sarvak Formation in the studied area formed on a homoclinal ramp. Additionally, five third-order depositional sequences were identified in the strata studied northwest of the HBNF. It can be concluded that the studied area was structurally stable during the Early Cenomanian. However, starting from the Late Cenomanian, significant tectonic phases resulted in the uplift of the area along an old ridge. Furthermore, the data indicate that the uplift of the Arabian Plate during the Early Turonian had significant effects on sedimentary processes in the region. This resulted in the retreat of the sea and the occurrence of a subsequent notable erosion phase at the Cenomanian–Turonian boundary in many areas including the Bahregansar and Hendijan oilfields. The interpretation of sedimentary characteristics and depositional environments in the upper part of the Sarvak Formation in the Mahshahr Oilfield relies on seismic sections, petrophysical logs, and microfacies analysis. Based on the available information the sediments apparently accumulated in a north-northwest to south-southeast trend, forming an onlap over both sides of a palaeohigh.

Nowadays, the use of artificial intelligence is common to increase the accuracy of the study and, close to reality, is used in the oil industry to increase the accuracy of studying and understanding the relationship between various parameters. The main purpose of this study is to compare the performance of the two methods of Extreme Learning Machine (ELM) and Radial Basis Function (RBF) in porosity estimation, which is static oil modeling. The data from seven wells in the offshore field (Hendijan Oilfield) of the northwestern Persian Gulf were examined. In this regard, post-stack seismic attributes which have a significant relationship with porosity and porosity log for each well were used to compare the performance of the ELM and RBF networks under the same conditions. Eventually, it reveals that ELM is quite sensitive to the data set and needs more data points to prepare a map (quantitatively), but is better than RBF in terms of classification (qualitative). On the other hand, RBF is one of the most powerful algorithms in mapping, especially in low numbers of data points, which can be challenging for others.

The Oligocene–Miocene Asmari reservoir is dominated by heterogeneity in various aspects, especially porosity and permeability caused mainly by dolomitization. The Asmari Formation has been deposited along a homoclinal ramp-type platform with a gentle slope divisible into an inner ramp, mid ramp, outer ramp and basinal settings. The distribution of dolomite through the Asmari carbonate platform in the Shadegan Oil Field is not uniform nor is it random. All 12 facies associations are variably affected by dolomitization and have influenced reservoir quality. Inner ramp facies associations are the most dolomitized  while, the mid- and outer ramp facies associations are moderate to least dolomitized intervals. Six third-order depositional sequences were recognized within the Oligo–Miocene succession. They are bounded by sequence boundaries (SB) with significant evidence of subaerial exposure, diagenetic alteration, oxidizing conditions, and an abrupt change in facies or facies bathymetry. The stratigraphic distribution of facies associations proves more dolomite percent is formed near the sequence boundary as well as in the high stand system tract (HST) deposits. The main dolomitization model of the Asmari carbonate platform is seepage-reflux. Thus, the highest dolomite percentages occur near the sequence boundary when the sea level was low in the shoreface facies (inner ramp), the lowest percentage and dominant fabric destructive dolomite in the offshore facies, and fabric selective dolomite near the maximum flooding surface (MFS) due to the slow rate of dolomitization and low volume of dolomitizing fluids.

The carbonate deposits of the Taleh Zang Formation are exposed extensively in the southeastern to northern Lorestan zone, SW Iran. The thickness of the shallow water carbonate Taleh Zang Formation in Ritt Anticline is 84.5 meters. It overlies on top of the turbidity Amiran Formation and it is overlain by the clastic-dominated Kashkan Formation. Petrographic examinations revealed seven facies deposited in tidal flat, lagoon, shoal and open marine. Detailed analysis of sedimentary facies indicates that in the Late Paleocene interval, the depositional system of the Taleh Zangi Formation was a ramp carbonate platform. Changes in the depositional facies and cycle stacking patterns indicate one transgressive-regressive sea-level cycle from the bottom to the top of the section, which is equivalent to the last eustatic sea level rise in the Late Paleocene (Thanetian).  This depositional sequence is separated by type 2 and 1 sequence boundaries at its lower and upper boundaries, respectively. Elemental geochemical evidence indicates a closed to weakly open digenetic system, with low water-rock interaction for carbonates of the Taleh Zang Formation. The relatively open digenesis system in this formation could be due to the influence of meteoric fluids in the erosional discontinuity at the border between the two formations, the Taleh Zang and Kashkan formations.

Abstract The Dalichai Formation in the Ahvanu section, north of Damghan city, with a thickness of 78 meters, comprises alternations of bluish-gray marls and limestones. Microfacies and palynofacies studies, along with fossil evidence, including foraminifera diversity and abundance, the ratio of epifaunal to infaunal benthic foraminifera, and the ratio of surface to deep infaunal, were utilized to identify organic-rich layers and reconstruct trophic levels. These factors confirm a mesotrophic environment with average oxygen and nutrient levels for the Dalichai Formation (Bajocian to Lower Callovian) in the studied section. The primary reasons for low organic matter in the Dalichai Formation are attributed to low production and poor preservation of organic matter. However, dark-colored deposits with high organic matter at the Bajocian–Bathonian boundary and the lower and upper Callovian boundaries suggest increased production rates in anaerobic and eutrophic conditions. Macrofossil fragments (ammonites and echinoids) within limestone deposits at the uppermost part of the Dalichai Formation, exhibiting low organic matter, and the presence of Zeophycus trace fossils, suggest an oligotrophic environment with dysoxic conditions in the Upper Callovian.