Iranian Journal of Earth Sciences
Volume:14 Issue: 1, Jan 2022

Biomarker fingerprints of crude oils are useful indicators of origin of organic matter input in source rocks and depositional conditions which are useful indices for petroleum systems development within a hydrocarbon producing horizon. Twenty five (25) crude oil samples from the five depobelts of the Niger Delta basin, Nigeria were studied to describe their biomarker fingerprints, provide information on the origin of organic matter input in the source rock(s), determine depositional environmental conditions and thermal maturity of the crude oils. The study was based on biomarkers (steranes and hopanes), normal alkanes and acyclic isoprenoids (pristine and phytane) obtained from the gas chromatography–mass spectrometry (GC-MS) analyses performed on the saturated fractions of the crude oils. The results of pristine/phytane (Pr/Ph) ratios, Pr/n-C17 ratios, Ph/n-C18 ratios, C29/C27 sterane ratios, sterane/hopane ratios, %C27, %C28 and %C29 regular steranes, oleanane index, waxiness index, Ts/Tm ratios and Carbon Preference Index (CPI) values, indicated that the analyzed crude oils belong to the same family of oil and originated from terrigenous clastic source rock (s) containing land plant organic matter with minor marine organic matter input, deposited under oxic to sub-oxic paleoenvironmental conditions. The Pr/n-C17 versus Ph/n-C18 cross plot indicated marine algal type II and a mixed type II/III kerogen. The C32H:22S/(22S+22R) values together with Ts/(Ts + Tm), C29:20S/20S + 20R, oleanane index and CPI values indicated that most of the crude oils have reached thermal equilibrium with  high thermal maturity levels.

This study investigates the Sequence stratigraphy and depositional environment of Wadi Es Sir Formation (Turonian) in Irbid City. The Wadi Es Sir Formation consists of thick bedded to massive limestone, dolomitic limestone and dolomite. Based on Petrographic study Eight facies wreeidentified in the studied sections These are mudstone, gastropoda peloidal packstone, algal-lithoclast wackestone to packstone, peloidal foraminifera wackestone to packstone, bioclastic wackestone to packstone foraminifera wackestone, rudist framestone and pelletal grainstone. These facies weredeposited on the followingenvironments: restricted marine, shallow open marine and shoal environments.Sequence stratigraphystudy in Wadi Es Sir Formation identified forth-order cycles which are asymmetrical and consist of transgressive systems tract (TST) represented by shallow open marine and sometime shoal facies, followed by highstand systems tract (HST) represented by restricted marine facies. They are bounded below by a transgressive surface (Ts) and above by marine flooding surface (mfs). They represent successive episodes of relative sea level rise and stillstands. The nature of these cycles (composition and symmetry) reflects the combined effect of local tectonics and eustasy.

The Band-e-Narges magnetite deposit is located in central part of Urumieh–Dokhtar Magmatic Arc (UDMA). Wide I-type calk-alkaline and alkalin magmatic activity in the Koh-e- Latif region has been reported due to Eocene intrusive processes in UDMA. The iron ores are hosted by Cretaceous limestone intruded by granite and granodiorite units. Genetic model of this deposit was determined using petrological, stable isotope, fluid inclusion and mineralographical data. Five stages of paragenesis were observed in terms of mineralization in this area: prograde stage, retrograde stage, sulfide-quartz stage, carbonate stage and oxidation stage. According to mineralogy and geochemistry studies, formation of the skarn has resulted from a hydrothermal fluid changing carbonate units to hydrosilicate minerals. The ore minerals showed magnetite features with slight chalcopyrite and pyrite. The δ34S values ranged from +3.31 to +6.29 for the early retrograde stage pyrite and from +5.51 to +7.1 for that of late retrograde stage pyrite + anhydrite pairs. All the δ34S values of pyrite and anhydrite + pyrite were positive with a magmatic sulfur origin in these deposits. Fluid inclusions were observed according to petrographic and microthermometric inclusions within garnet, quartz, and calcite minerals at various stages. Due to high temperature (414 -448 ºC ) and middle salinity (up to 13.186 wt% NaCl) of fluid inclusions in prograde skarn-stage (garnet), the fluid inclusions showed a composition related to magmatic fluids following reaction with calcareous wall rock and fluid inclusions were trapped at pressures of 400 -500 bars, corresponding to depths of 1.5 -2 km in prograde stage. Fluid inclusions in quartz had moderate temperatures (152-303 ºC) and low salinity (7.9-11.3 wt% NaCl) indicating quartz-sulfide stage and late retrograde stage. The presence of fluid inclusions with moderate homogenization temperature (303 ºC) suggested that reboiling has occurred under hydrostatic pressure of 150-250 bars, equivalent to a depth of 1 -1.5 km in the late retrograde skarn and quartz-sulfide stages. Fluid inclusions in calcite had moderate temperatures (160 -287ºC) and low –to- high salinities (0.406-23wt% NaCl). A greater number of the fluid inclusions in the Band-e-Narges deposit had salinity (0.4-23.74 wt% NaCl) and homogenization temperatures (152-448 ºC) showing them as a moderate-high temperature and low–to-high salinity type of deposit. A decline in temperature and variation in salinity documented for the Band-e-Narges deposit would cause a notable decrease in Fe solubility and ore precipitation. Fluid compositions indicated that ore-forming fluid had a high fO2 value and rich Fe concentration in the early stage, while having relatively lower fO2 value and poor Fe concentration in the retrograde and sulfide stages. The data obtained from geology, mineralogy, geochemistry, salinities, and homogenization temperatures of the fluid inclusion populations at the Band-e-Narges iron deposit followed a model of boiling as a result of decrease in pressure, mixing, and cooling.

Middle-Upper Eocene volcanic and volcano-sedimentary rocks of the Torud region have been formed by the sequences of basic-intermediate lavas, pyroclastic rocks, and sedimentary layers (e.g., siltstone, sandstone, and nummulite-bearing limestone) within a shallow marine basin. According to microscopic studies, the volcanic rocks of the region include basalt, basaltic andesite, trachyandesite, andesite, and dacite. These rocks have originated from the differential crystallization processes and occasionally calc-alkaline contamination geochemical properties. Generally, they contain olivine, clinopyroxene, and plagioclase ± amphibole minerals. Porphyritic to megaporphyritic textures with microlithic and flow matrixes are observed in these rocks. Studying the main and rare elements of these rocks indicates that reducing MgO content is accompanied by an increase in Al2O3, K2O, Na2O, and SiO2 and a decrease in Fe2O3 and CaO concentrations. These rocks are mainly enriched in LIL and LREE elements but depleted of HFS elements. The prominent features of these rocks are the presence of positive anomalies in the K, Sr, Rb, and Ba elements, the depletion of some samples of Nb and Ta, and their depletion of Ti and P. This result reveals the crustal contamination of the mantle mafic magma constructing these rocks. According to the geochemical data, magmatic pollution has not been an effective process in the formation of these rocks. In addition, the relatively higher levels of Cr, Ni, and MgO in the alkali basalts of the region indicate that these rocks are originated from partial melting (5 to 10%) of a spinel-garnet peridotite. Overall, they have no subduction-dependent rock characteristics and mainly represent characteristics of alkali basaltic magmas of the preliminary back-arc basin (BAB). These features, attributed to their calc-alkaline nature, represent the formation of these rocks in a tectonic back-arc setting in the Middle-Upper Eocene.

The Permian Ruteh Formation is known as one of the most significant successions in northwestern Iran. In the studied area it exposed a thick succession (201 m) of medium to thick-bedded carbonate sedimentary rocks in the west of Kuseh-Kahrizeh village in the north of Mahabad city. This formation unconformably overlain the Cambrian Mila Formation and it is unconformably underlain by the Oligo-Miocene Qom Formation The laboratory studies on the thin sections led to the identification of 15 microfacies that are arranged in three facies associations: inner ramp, mid-ramp and outer ramp. The petrographic results and facies analysis demonstrate that the depositional environment of Ruteh Formation in the studied area exhibits the characteristics of a homoclinal carbonate ramp platform of a gentle slope. This platform is mainly composed of supratidal, intertidal, lagoon, shoal, open marine, mid-ramp, and outer ramp environments. According to facies frequency analysis, the lagoon environment accounts for the highest abundance of facies (33%), whearas the outer ramp environment shows the least abundance (2%).Vertical distribution analysis of sedimentary facies led to the identification of transgressive and regressive depositional patterns. Accordingly, a total of 4 depositional sequences of third-order, 5 sequence boundaries and 4 maximum flooding surfaces were identified. The boundaries between all sequences are identified as SB1.

Mineralogical studies play a key role in deciphering mineral’s formation and associated geochemical processes in geosciences. This paper presents the application of Raman spectroscopy to the characterization of actinolite in rock samples. The studied actinolite was formed as the pseudomorph of primary pyroxene in gabbrodiorite intrusive rock sample collected from Varan area, Urumieh-Dokhtar Magmatic Arc of Iran. The Raman spectra of micrometer-sized actinolite grains embedded in a crude rock sample are compared with the corresponding literature data for actinolite and tremolite in range of 200-1200 cm-1 and 3600-3700 cm-1 region. The results show that the quantitative estimation of Mg# = Mg/(Mg+Fe2+) can be obtained from the fractional intensities of the OH-stretching bands by applying Raman spectroscopy to micrometer-sized actinolite grains in a crude rough rock sample, which is in good agreement with the results from EMPA. The Raman spectroscopy is a fast method and low-cost for quantitative estimation of Mg# in actinolite.