Uttam K. Bhui

Chemical substitutions, paragenetic relations, and physical conditions of formation of högbomite in the Sittampundi layered anorthosite complex, South India

Abstract Layered anorthosite, chromiferous pyroxenite, and spatially associated mafic and felsic rocks of the 2.9 Ga Sittampundi layered complex (SLC), South India, underwent high-grade metamorphism at ca. 2.5 Ga and were subjected to amphibolite-facies metamorphism accompanied by intrusion of granitoid plutons during late-Neoproterozoic tectonothermal activity (0.72-0.45 Ga). During the latter event, anorthosite developed millimeter- to centimeter-thick compositional layers rich in clinopyroxene+amphibole+clinozoisite+chlorite±chromite and corundum+spinel+chlorite. Tiny grains of högbomite replaced only the grains of corundum and spinel and are in textural equilibrium with chlorite. The studied högbomite contains appreciable TiO2 (>4.4 wt%) but insignificant NiO and ZnO. Cr2O3 content reaches up to 0.35 wt% only in chromite-bearing samples. Systematic partitioning of Fe and Mg between högbomite and associated Fe-Mg minerals demonstrate attainment of chemical equilibrium among these phases. Integrating textural relations and algebraic analyses of the phase compositions, several reactions were constructed involving the associated oxide phases (spinel, corundum, högbomite), calcite, and silicates (amphibole, chlorite, anorthite, clinozoisite). Interpretation of the reaction reveals that (1) Mg2+ and Ti4+ were mobile for more than 2 cm during the formation of högbomite and chlorite, and (2) chloritization of amphibole in the clinopyroxene+amphibole-bearing layers released Ti that was transported to the spinel+corundum-bearing layers to develop högbomite. Stability fields of some critical mineral assemblages in P-XCO₂ and T-XCO₂ space combined with geothermobarometry in the associated rocks tightly constrain the growth of högbomite in the presence of aqueous fluids (XCO₂ < 0.15) to the P-T range of 7 ± 1 kbar, 650 ± 50 °C. These aqueous fluids, presumably derived from the Pan-African granitoid batholiths, chloritized amphibole grains, and transported the released Ti4+ to the spinel+corundum-bearing layers to develop högbomite. Topological relations in isothermal-isobaric fugacity diagrams (log fO₂-log fS₂ and log fO₂-log fH₂O) in the system FeO-Al2O3- TiO2-O2-S2-H2O-CO2 (+MgO, Cr2O3) indicate that the stability and compositional characteristics of natural högbomite are strongly influenced by fO₂, fS₂, fH₂O, and concentrations of other soluble species (Ti, Mg, Cr, etc.) in the metamorphic fluids

Magmatic evolution of mafic granulites from Anakapalle, Eastern Ghats, India: implications for tectonic setting of a precambrian high-grade terrain

Abstract Mafic granulites showing intrusive relationships with enclosing pelitic, calcareous and quartzofeldspathic gneisses at Anakapalle, Eastern Ghats belt, share a common retrograde metamorphic history (decompression followed by near-isobaric cooling) and are, therefore, considered to be syn-metamorphic. Detail textural, phase chemical and bulk chemical analyses of the mafic granulites show that (a) these are melts derived through fractionation of a primary tholeiitic magma and (b) they crystallized at temperatures

Characterisation of crude oil for enhanced oil recovery: study with anionic surfactant

ABSTRACT A chemically enhanced recovery technique is used where thermal recovery is not feasible. It reduces the interfacial tension by forming a microemulsion. In this paper, characterisation of crude oil in terms of chemical bonds present is investigated by Fourier transform infrared spectroscopy (FTIR). The peaks at different wave number show saturated groups such as n-alkane –CH, –CH2, –CH3 and short chain of n-alkane C–C bond present in the oil samples. Since the crudes are collected from an anionic reservoir, sodium dodecyl sulphate (SDS) of 0.4% critical micelle concentration is suitable. It has been seen that the particle size increases in the case of a surfactant solution with oil compared to one without oil indicating entrapment of oil components inside the surfactant micelle. The larger absorptions of SDS from UV spectroscopy are caused by electrons moving between π and π* orbitals due to the presence of more unsaturated groups in the solution.

Brittleness modeling of Cambay shale formation for shale gas exploration: a study from Ankleshwar area, Cambay Basin, India

Unconventional gas shales are described as organic-rich, fine-grained reservoirs and are typically dominated by clays. The shale gas reservoirs have received great attention in the past decade, because of their large reserves as well as recent technical advances in developing these resources. Accordingly, there are increasing demands to understand the petrophysical and mechanical properties of these gas shale rocks. The mineral composition and the presence of organic matter can influence not only the distribution of pores and fluid saturation, but also the effectiveness of stimulation. The geomechanical study of a shale gas reservoir is useful in identifying the intervals which can be fractured effectively. The estimation of geomechanical properties from well logs and their calibration with laboratory-derived properties on cores has been attempted in the present paper for Cambay shale of Cambay Basin, India, which is very much prospective for shale gas exploration. Powder X-ray diffraction (XRD) analysis was carried out on drill cutting samples in the study area, and it was seen that the major mineralogy is quartz, kaolinite, pyrite, calcite and mixed clays. Petrographic observation and Fourier transform infrared spectroscopy (FTIR) results also conform to the same minerals which are identified from XRD. Geomechanical properties (Young’s modulus, Poisson’s ratio, brittleness) of Cambay shale derived from sonic logs and density logs and are validated with the available predicted brittleness index (BI) from mineralogy through petrographic observation, XRD and FTIR interpretation results. Modeling using petrel software with log data and P-impedance was carried out and a relation between log results and P-impedance volume was established. The study concluded that (BI) varies from 0.44 (less brittle) to 0.75 (highly brittle) using both mineralogy and sonic logs. This study successfully identified the areas of high BI in the study area which can be an input for effective stimulation for shale gas exploration and exploitation.