Basis Of Petrographic Analysis Research Articles

Can Eu Anomaly Indicate a Hydrothermal Fluid Si Source? A Case Study of Chert Nodules From the Permian Maokou and Wujiaping Formations, South China

The Middle–Late Permian witnessed an unusual chert accumulation event along the margin of the Pangea and Paleo-Tethys realms, known as the “Permian Chert Event (PCE).” The PCE is well recognized in the Permian limestone from South China, in the forms of nodular and bedded cherts. Previous studies suggested that PCE was caused by hydrothermal fluids related to the Emeishan large igneous province (ELIP). Meanwhile, another hypothesis supported the biogenic origin of PCE, i.e., the Permian chert derived from biosilicification of abundant sponges and radiolarian. Thus, sources of silica from the Permian chert remain uncertain. To understand linkages among PCE, biosilicification mechanism, and the ELIP event, this study focused on chert nodules collected from the Permian Maokou and Wujiaping formations in the Lianziya and Maoertang sections, South China. We measured germanium/silicon ratios (Ge/Si) and rare earth element (REE) compositions of chert nodules on the basis of petrographic analysis. Ge/Si ratios range from 0.14 to 0.63 μmol/mol with an average of 0.33 μmol/mol (n=18) in the Lianziya section and from 0.02 to 0.75 μmol/mol with an average of 0.18 μmol/mol (n=45) in the Maoertang section, both of which are close to the seawater value. The REE pattern is characterized by LREE depleted with a positive Eu anomaly ranging from 0.66 to 2.16 in the Lianziya section and from 1.05 to 9.57 in the Maoertang section. Our results indicate that the silica of the Permian chert predominantly originated from seawater with limited contributions from hydrothermal fluids. To further quantify the contributions of hydrothermal fluids, we applied a binary (seawater and hydrothermal fluid) mixing model based on two geochemical proxies, i.e., the Ge/Si ratio and Eu anomaly. The modeling results suggest a mixing of 0.5 vol% to 1 vol% hydrothermal fluids with contemporaneous seawater, verifying the dominant seawater source of silica in the PCE. Although it has been widely accepted that positive Eu anomaly points to the hydrothermal fluid origin of silica, our study demonstrates that positive Eu anomaly could also be present in cherts that was predominantly derived from normal seawater. Therefore, the analysis of the Ge/Si ratio or REE compositions is highly recommended when determining the Si source of cherts.

Diagenetic facies and reservoir porosity evaluation of deep high-quality clastic reservoirs: A case study of the Paleogene Shahejie Formation, Nanpu Sag, Bohai Bay Basin, China

Petrological analysis, thin-section observation and laboratory analysis data were selected to systematically study the physical and diagenetic features of the first member of the Paleogene Shahejie Formation (Es1) in the No. 3 structural belt of the Nanpu Sag, Bohai Bay Basin. The intensities of different diagenetic processes were determined, the diagenetic evolution sequence was reconstructed, the typical diagenetic facies were identified and the effects of different diageneses on the reservoir were quantitatively analyzed. The results show that the main intergranular fillings include authigenic-quartz, quartz secondary enlargement, clay minerals, carbonate cement and matrix. The pore types include intergranular porosity, dissolution porosity and microfractures. The reservoir has experienced compaction, early cementation, dissolution and late cementation, among which compaction is the most important porosity reducer. Compaction was the main diagenetic process involved in porosity reduction, accounting for about 24.4% of the loss of thin-section porosity. The dissolution process clearly improved the porosity, increasing thin-section porosity by 2.7%. Five diagenetic facies were identified on the basis of petrographic analyses, namely, (a) strongly compacted-weakly cemented-weakly dissolved facies; (b) weakly compacted-strongly cemented-weakly dissolved facies; (c) moderately compacted-moderately cemented-weakly dissolved facies; (d) strongly compacted-weakly cemented-moderately dissolved facies; and (e) strongly compacted-weakly cemented-strongly dissolved facies. According to the analysis of diagenesis intensity, the porosity evolution model of various diagenetic facies was reconstructed, and the reservoir quality of various diagenetic facies was quantitatively predicted. The reservoir quality of different diagenetic facies clearly changed with depth. The best reservoir quality was in strongly compacted–weakly cemented–strongly dissolution facies, which have good sorting, contain a large amount of feldspar and soluble debris, and are mainly developed in the main part of the river channel. Our study can provide a reference for the subsequent exploration and development of deep petroleum systems.

Diagenetic controls on the reservoir quality of the tight gas Collyhurst Sandstone Formation, Lower Permian, East Irish Sea Basin, United Kingdom

The Lower Permian Collyhurst Sandstone Formation is an important gas reservoir both offshore and onshore in the East Irish Sea Basin of north-western UK, and is considered to be a tight gas reservoir. Diagenetic processes have significantly influenced the reservoir quality of the sandstone. This study employs petrographic, mineralogical, and stable isotopic analyses on two different onshore core successions to investigate the composition and texture, the diagenetic history, and the impact of lithology and diagenesis on reservoir quality. The Collyhurst Sandstone Formation consists of fine- to very coarse-grained, poorly to well-sorted, sub-arkosic to lithic arenite sands. The reservoir properties of the sandstone are relatively poor, with average porosity and permeability of 6.6% and 0.387 mD respectively. On the basis of petrographic analyses, five sandstone lithofacies were identified: fine-grained, conglomeratic, carbonate-cemented, quartz-cemented and matrix-free. Contrasting characteristics between the reservoir units suggest the development of divergent diagenetic pathways. Features observed during early diagenesis include compaction, growth of pore-filling early carbonate cements (calcite I, dolomite and siderite I) and vermicular kaolinite. Late diagenetic features include: further carbonate cementations (ferroan dolomite/ankerite, calcite II and siderite II-III), clay mineral cementation (illite and chlorite) with both grain-coating and pore-filling textures. Also common in late diagenesis are syntaxial quartz overgrowths. Albite, barite and anhydrite are minor late diagenetic cement phases. Compaction processes rather than cementation accounts for the major destruction of the initial sandstone porosity. But cementation still accounts for up to 32% of porosity loss. Carbonate cements mostly fill intergranular pores, thus block pore throat and prevent migration of fluid. Clay minerals bridge and occlude pores, resulting in further destruction of the reservoir quality. The study provides insights into how lithofacies (primary composition, texture and sorting) and diagenesis can influence reservoir qualities of tight gas sandstones.

Diagenesis and reservoir quality in tight gas sandstones: The fourth member of the Upper Triassic Xujiahe Formation, Central Sichuan Basin, Southwest China

Mineralogical, petrographic, and geochemical analyses were performed to investigate the sandstone composition and texture, pore system, and diagenetic minerals of Upper Triassic sandstones in the Central Sichuan Basin, Southwest China. The results show that authigenic quartz, clay minerals, and carbonates are the main pore‐filling constituents. The pore systems consist of intergranular porosity, secondary porosity, microporosity, and microfractures. Loss of depositional porosity is greater due to compaction than to cementation. Eodiagenesis includes mechanical compaction, precipitation of early calcite cements, kaolinite, and smectite, and leaching of feldspars by meteoric water. Mesodiagenesis consists of compaction, framework grain dissolution, and subsequent precipitation of illite, quartz, and late carbonate cements. Six lithofacies were identified on the basis of petrographic analyses, namely, (a) argillaceous sandstones; (b) poorly sorted sandstones; (c) quartz‐cemented sandstones; (d) carbonate‐cemented sandstones; (e) clay‐mineral‐cemented sandstones, and (f) clean sandstones with abundant secondary porosity. The best reservoir‐quality rocks have high percentages of detrital quartz but low percentages of matrix, quartz, and carbonate cement. Differences in textural and compositional attributes of various lithofacies significantly affect porosity‐depth trends. The diagenetic evolution pathways and reservoir‐quality prediction models of various lithofacies are reconstructed by integrated petrographic data. This work provides insights into describing the different lithofacies by petrographic analysis and helps to investigate how detrital composition and texture influence the diagenesis and reservoir quality evolution in tight gas sandstones.

Predicting the yield of coke-plant products on the basis of petrographic analysis

Mathematical models are developed for predicting the yield of coke and the primary coking byproducts from mixed batch on the basis of petrographic data. Coefficients are calculated for conversion from laboratory yields to industrial yields.

Ceramic regionalism in Prepalatial Central Crete: the Mesara imports at EM I to EM II A Knossos

This article presents results obtained through detailed stylistic analysis of a body of EM I–EM II A pottery at Knossos in association with a programme of petrographic analysis and scanning electron microscopy. From the Knossian ceramic assemblage, four specific ware groups were chosen for this study on the basis of shape, decoration, and fabric: fine painted, fine grey, painted semi-fine to coarse, and slipped and burnished. It is argued here, on stylistic grounds and on the basis of petrographic analysis, that these groups were imported to Knossos from south central Crete. In addition, scanning electron microscopy characterizes the technology of production of the Mesara imports, and demonstrates the use of oxidation–reduction–oxidation techniques to produce black in this early period. The Mesara imports at EM II A Knossos mark the first sizeable exchange of pottery at this site in Minoan times. They suggest that in central Crete by EM II A there may have been an inter-regional distribution of a broad range of pottery types from various specialized production centres.

Minimize Formation Damage by Rapid, Inexpensive Method of Completion- and Stimulation-Fluid Selection

Summary Minimizing or removing formation damage is a major objective in completion and stimulation operations. Formation damage is minimized by selection of "nondamaging" fluid systems and operations on the basis of petrographic analysis and field experience. A technique known as the capillary suction time (CST) test, adopted from the drilling-fluid industry, provides a quick and inexpensive method for qualitative selection or screening of the least damaging fluid system for a particular formation. The CST test is a fundamental filtration method for determining the electrolyte concentration that will produce the maximum inhibiting effect on a formation. The method uses an instrument to measure the time required for a liquid to travel a calibrated distance on a standard porous filter paper. The CST procedure currently is used very successfully to select completion fluids and to evaluate stimulation-fluid additives. This paper explains the procedure, discusses limitations of the test, and presents laboratory and field data to show the value of the technique.