Depositional environment and diagenesis of Asmari Formation in the well-A (oilfield at western Lorestan basin) and Syah-koh Anticline (Zagros fold–thrust belt, west Iran): microfacies, sequence stratigraphy and geochemistry

The Asmari Formation (Oligocene-Miocene), as one of the main carbonate petroleum reservoirs of the Middle East, represents strongly variable reservoir quality on regional and local scale. One outcrop analogue (Syah-koh Anticline) and a well-A (oilfield at western Lorestan basin), both situated in western Iran, were investigated to address the reservoir characteristics. The 12 microfacies identified reflect deposition on a broad carbonate ramp reflecting an outer, mid and inner ramp setting. Three third-order depositional sequences are deduced based on interpreted Gamma-ray, sonic logs and stratal stacking patterns. The identified sequences and existence of a thicker evaporite succession in well-A points to more restricted depositional conditions with reduced connection to the open sea from the Syah-koh Anticline towards well-A. The diagenetic modifications revealed micritization, dissolution, cementation, fracturing, dolomitization and compaction. Petrographic and δ13C and δ18O analysis of cements points towards marine, meteoric and burial diagenetic alteration, whereby the cements with δ13C and δ18O values similar to those of enclosing micrite (0 to -2‰ V-PDB) reflect a marine diagenetic environment. The micrite phases enriched in aforementioned isotopes resulted from evaporation which is supported by widespread associated dolomitization. The J trend present in the δ13C and δ18O cross plot reflects the meteoric diagenetic overprint. The cements with highly depleted δ18O signatures (<-10‰ V-PDB) are linked to burial diagenetic processes. The isotopic values are in agreement with the luminescence characteristics of the cements including dull for marine, alternating bright and dull zones for meteoric and bright for burial cements.

Rejuvenating The Concept of The Salodik Group Reservoir of The Banggai Basin Through Surface Geological Mapping And Multi-Method Analyses

Carbonate formations of the Banggai Basin have been proven to be hydrocarbon producers. This research examines Salodik Group properties and provides an analogue to the subsurface reservoir for further development. The methods used in this study are the outcrop samplings at some traverses through fieldwork and laboratory analyses, including petrography, biostratigraphy and SEM. Based on the analyses results and lineament imaging, formation distributions, traverse profiles and cross-sections were generated. Furthermore, facies and reef systems were determined in every formation based on petrographic and biostratigraphic results, by considering organisms, composition, and texture. Based on facies, reef system, and diagenetic environment distribution, a paleogeographic model were interpreted in every age from Middle Eocene to Early Pliocene to represent a better understanding of Salodik Group depositional environment and tectonic events. Through this fieldwork, Salodik Group on the surface was characterized into several equivalent formations in the subsurface, including Lower Tomori Formation, Upper Tomori Formation, Minahaki Formation and Mentawa Member. The formations distribution was greatly influenced by southwest-northeast thrust faults, determined based on lineaments and biostratigraphic analyses, resulting in repetition of the age on some traverses. Formations thickness varies, from approximately 180 to 300 meters. Each formation contains specific facies developed on back to off reef, and depends on organism and texture found through petrographic and biostratigraphic analyses. SEM analysis shows a series of tectonic events that affected the diagenetic process that developed in every formation and age. Banggai Microcontinent collision and further carbonate exposure that produced intense vuggy porosity were indicated by the meteoric vadose diagenetic process since Upper Tomori developed in Late Oligocene. The diagenetic process has a significant role. It generated significant porosity, including in dolomitic and planktonic facies, and possibly influenced further development in carbonate reservoirs, especially in Salodik Group.

Genesis of dolomite in the upper assemblage of the Ordovician Majiagou Formation in the southeastern Sulige gas field, Ordos Basin, China: Evidence from C, O, and Sr isotopes and major and trace elements

Typical dolomite reservoirs exist in the upper assemblage of the Ordovician Majiagou Formation in the southeastern Sulige gas field, Ordos Basin, however, the current understanding of dolomite genesis needs to be clarified. This study confirms the dolomitization characteristics, diagenetic environment, and genesis of dolomite through the use of core observations, thin section identification, and geochemical data (carbon and oxygen stable isotopes, strontium isotopes, and major and trace element contents). The results showed that dolomite samples from the study area includes micritic dolomite, powder crystal dolomite, and micrite to powder crystal dolomite, in which the main reservoir space consisted of intergranular pores, intercrystalline pores and various dissolved pores. The δ13C value of the dolomite samples ranged from −7.26‰ to1.28‰ with an average of −0.45‰, which is equivalent to that of seawater during the Ordovician. The δ18O value ranged from −9.94‰ to −6.32‰ with an average of −7.86‰, which is clearly more negative than that of the Ordovician seawater. The 87Sr/86Sr ratio varied from 0.70867 to 0.71033 with an average of 0.70946, which is significantly more positive than that of Ordovician seawater. The Mg/Ca ratio was lower than that of the seawater and close to 1:1. Overall, all of the samples were characterized by high Fe and Mn contents, while low Sr and Na contents. These results clearly indicate that the dolomitized fluid was closely related to seawater or a sea-source fluid. Furthermore, the restored paleosalinity, paleotemperature, and paleodepth also indicate that the dolomite was formed in a marine reducing environment. Combined with the petrological characteristics and geochemical indicators, two dolomitization models (mixed water dolomitization and burial dolomitization) were established for the upper assemblage of the Ordovician Majiagou Formation in southeastern Sulige gas field. We infer that mixed water dolomitization was dominant before the Carboniferous, whereas burial dolomitization was dominant after the Early Permian.

Coupling between Diagenetic Environment and Porosity Evolution—A Quantitative Study of the Zhuhai Formation in the Huizhou Sag, Pearl River Mouth Basin, South China Sea

The diagenetic environment is vital in controlling reservoir quality by influencing diagenetic processes, especially porosity evolution. The Zhuhai Formation is a critical exploration target in the Huizhou Sag. However, there are considerable differences between the reservoirs in the west (Huixi) and east (Huidong) Huizhou Sag with only a 400 m difference in burial depth. To investigate causes of these differences, scanning electron microscopy (SEM), cathodoluminescence analysis, fluorescence analysis, and fluid inclusion analysis are employed for the petrological and diagenetic characterization of the target formation. Moreover, image analysis software is used to qualitatively evaluate the impacts of diagenesis on porosity. The results show that the digenetic environments in Huixi and Huidong are different. Huixi is characterized by a high content of feldspar with a stronger dissolution phenomenon than that observed in Huidong. The main cement types in Huixi are siliceous cement and kaolinite. Moreover, Huixi had a low palaeo-temperature, low palaeo-salinity, and low palaeo-pH diagenetic environment. In contrast, Huidong is accessible via deep faults and rich in lithic fragments which show visible deformation. Moreover, cements in this area include carbonate, gypsum, quartz overgrowth, kaolinite, and chlorite. This evidence confirms that the diagenetic environment in Huidong was a high palaeo-temperature and high palaeo-salinity environment with high palaeo-pH. Additionally, the palaeo-pH evolved following the sequence of weakly alkaline, acidic, alkaline and acidic. Microscopy evidence indicates that the Dongsha Uplift was a likely source of Huixi sediments during the deposition of the Zhuhai Formation. The determination of the porosity evolution shows that compaction and lithic fragment content are the main controlling factors on the reservoir quality of the Zhuhai Formation.

MICROFACIES AND GEOCHEMICAL STUDY OF THE DALICHAI FORMATION: A CASE STUDY OF CENTRAL ALBORZ IN SEMNAN PROVINCE, NORTHERN IRAN

Top Bajocian progressive deposits are marly accumulation and limestone filled with ammonites and other marine fossils that are called ‘Dalichai formation’ with 1420m in thickness. There are erosion and inconsistent boundary that distinct Dalichai formation from Shemshak formation. The boundary of Dalichai and Lar formations is isocline and graded. The presence of Benthic foraminifera, sponge sparkles, and ammonites in the Sharaf section alludes the existence of shallow to deep open marine. Such diagenetic processes as cementation, micritization, porosity, accumulation, iron oxide and bioturbation are observed in Dalichai formation. The carbonates in this formation could have primary aragonitic composition according to the studies on main and secondary elements, carbon and oxygen isotopes, and drawing these values against each other. In such condition, non-marine diagenesis influenced the carbonates mostly. By comparing Sr/Ca and Mn, a semi-closed to open diagenetic environment can be suggested to have impacts on carbonates. Dalichai formation and Lar formation can identify a boundary in 1200m thickness since there are changes in Sr/Ca, Sr/Na, Sr, and carbon and oxygen isotopes to study. Dalichai formation limestone temperature was calculated 32.9 °C. Regarding lithology and analysis of Sharaf section microfacies, three microfacies are specified that appear as deposits in outer ramp. In this study, the model of Homoclinal ramp is suggested to utilize for such formation. Sharaf section Dalichai Formation entails aragonitic mineralogy composition on which burial diagenesis has substantially influenced.

High-fidelity preservation of the Scarabaeoidea (Insecta) exoskeletons from the Miocene of Shanwang

The Shanwang Fauna has been one of the well-known Miocene Konservat-Lagerstätten for a long time, but the high-fidelity preservation of the fossils has been often ignored. This paper illustrates the exceptional preservation displayed by some fossil insects from the Shanwang Formation. The microstructures of the preserved exoskeletons were analyzed using a scanning electron microscope and a transmission electron microscope. The epicuticle, procuticle, and some fine details such as cuticular scales could be recognized from the structural remains. These details could enable the identification of valued taxonomic and phylogenetic features and contribute to understanding the diagenetic environment.