Palaeodepositonal conditions and hydrocarbon source characteristics of lignites from Bikaner-Nagaur Basin (Rajasthan) western India based on organic petrographic studies

ABSTRACT: In Indian subcontinent Cenozoic lignites are found at several places along the boundary of Indian plate which was the shoreline of palaeotethys. However, the western part of the Indian subcontinent is particularly rich in lignites and shales along with hydrocarbon reserves. Thus, this location of the country is an important producer of hydrocarbon and lies at a significant position in the mineral map of the country. In the north western state of Rajasthan, lignites are reported in associated with the Cenozoic sedimentary rocks extending an area of 70,000 sq.kms in Bikaner, Barmer, Nagaur, Jalore and Jaisalmer districts. The present study focuses on the lignite bearing sequence exposed in the Matasukh and Barsingsar lignite mines of Bikaner-Nagaur Basin, Rajasthan. The lignite belongs to the Palana Formation of early Palaeocene age (~66-56 Ma). The lignite seam extends over an area of 2.50 sq.kms and is estimated to have consisted of 10.10 million tonnes of lignites.The lignites of Matasukh and Barsingar lignite mines are studied based on organic petrographic data to elucidate the palaeodepositional conditions and the hydrocarbon source potential. The lignites of Matasukh are predominantly composed of huminite macerals (av. 60 vol. %), followed by moderate liptinite content (av. 23 vol. %). Inertinite macerals (av. 9 vol. %) and mineral matter (av. 8 vol. %) are in lesser proportions. However, in Barsingsar, the lignites are predominantly composed of huminite macerals (av. 74 vol. %), followed by Inertinite macerals (av. 10 vol. %), liptinite content (av. 9 vol.%) and mineral matter (av. 6 vol. %). The dominance of detrohuminite (attrinite + densinite) concerning telohuminite (textinite + ulminite) suggests that the organic matter has undergone a higher degree of degradation; as is also indicated by the frequent occurrence of funginite. In Matasukh, Low TPI and GI values indicate limno-telmatic and mesotrophic-rherotrophic conditions of the palaeomire during the deposition of the lignite forming peat. The deposition took place in varying depositional settings. However, in Barsingsar, the lignites are formed in limnic and rheotrophic conditions. The deposition took place possibly in a marshy depositional setting. Matasukh lignite has huminite reflectance values (av. VRo= 0.26%) indicating that the studied lignites attained ‘brown coal’ as German Standard or \u27lignitic’ stage/rank (ASTM) and is of low rank B (ISO: 7404-5, 2009). Altogether, the lignites of Bikaner-Nagaur Basin show varying petrographic characteristics indicating the variation in the source floral composition, microbial degradation and depositional conditions in different parts of the basin

Three-dimensional probability tomography of self-potential anomalies of graphite and sulphide mineralization in Orissa and Rajasthan, India

International audienceCharge occurrence probability (COP) and dipole occurrence probability (DOP) tomographic imaging of graphite and sulphide orebodies, which were earlier detected by a self-potential (SP) survey, were carried out. Three typical examples, two shallower graphite lenses in Bender and Berni, Orissa State, and one relatively deeper sulphide deposit in Kayar, Rajasthan State, are presented. An exploratory test hole, drilled on the basis of the SP results in Bender and Berni, struck graphite. A mise-à-la-masse survey was also carried out on these two deposits to delineate the orebody. The graphite deposits have subsequently been mined. The shapes of the graphite bodies, especially the upper parts, are now known. The Bender deposit is a single graphite lens, whereas the graphite occurrences in Berni are complex multilens-type deposits. Both the COP and DOP techniques clearly imaged the single lens at Bender, as well as the multilens deposits at Berni. The Kayar sulphide deposit, unlike the graphite deposits, has not yet been mined, and therefore it is not delineated as precisely as the graphite orebodies. However, its outline is known from the drilling data. The imaging results of COP and DOP show good agreement with the drilling results of the sulphide deposits

Thin lithosphere–asthenosphere boundary beneath eastern Indian craton

The lithosphere–asthenosphere boundary (LAB) separates the hard and rigid outer layer of the earth (lithosphere) and the weaker, hotter, and deeper part of the upper mantle (asthenosphere) and plays a pivotal role in plate tectonics. However, its definitive detection, especially beneath the cratons, is proving elusive. One of the geophysical tools used to map the LAB beneath the cratons is through magnetotelluric (MT) observations. The resistivity at boundary falls in the range of 5–25 Ω-m and can be explained by the presence of a small amount of water in the asthenosphere, possibly inducing partial melt. Here, we report thickness of the LAB in one of the oldest dated ancient cratons of India—Eastern Indian Craton (EIC) of ~ 3.3 Gyr, from MT studies. The two prominent phase-sensitive strike directions, one each for crust and mantle, and the presence of resistive continental lower crust act as a window to mantle in resolving deeper electrical conductivity structures beneath EIC. Our results show that the LAB beneath the EIC is at 95 km. The region is interesting as the electrical properties of the crust and mantle and the Moho depth are similar to those of the Slave Craton, Canada (~ 4.0 Gyr) but the depth of the LAB beneath the EIC is half that of the Slave craton. As cratonic signatures, depicted by ultrapotassic rocks from Gondwana coal fields close to EIC, are preserved at least till early Cretaceous (117 Ma) it is likely that Himalayan orogeny could have played a major role in delamination of the lithospheric roots of the EIC in addition to attendant seismicity

Evolution of eastern segment of the Central India Tectonic Zone: an insight from a magnetotelluric study

The collision process between the South Indian Block (SIB) and North Indian Block (NIB) resulted in the development of the complex crustal nature of the Central India Tectonic Zone (CITZ). The evolutionary past of CITZ covers a long geological time (similar to 1000 Ma), which corresponds to the assembly and spreading of Columbia and Rodinia supercontinents. Despite several studies in the western and central parts of the CITZ, the location of the suture zone between the SIB and NIB is still under debate. In addition to that, the crustal structure in the eastern segment of CITZ is yet to be resolved. Therefore, for the first time, a dense station coverage magnetotelluric (MT) study is carried out along a 275 km transect in the eastern segment of CITZ from Pandaria to Rewa. The complexity of the Central Indian Shear (CIS) and Tan Shear Zone (TSZ) are reflected as anomalous phases (beyond 90 degrees) in the MT data. A deep crustal resistivity model derived from 2-D and 3-D inversion of the MT data brought out a high-to-moderate conductivity structure (10-100 omega-m) in the middle of the surface expressions of CIS and TSZ. The conductive structure could be related to a deformation zone formed by tectonic interaction of the CIS and TSZ or multiple tectonic boundaries in the middle of the CIS and TSZ. The conductive structure observed in the southern limit of the CITZ also may indicate the tectonic boundary between the SIB and NIB. The high conductivity in the deformation zone may be explained by the collision-related metallic rich sediments and/or mylonite associated with interconnected fluids. Moderately conductive vertical features delineated from the MT model correlate with the intrabasinal faults which might have acted as the pathways for Deccan volcanism. This study suggests that the CITZ could have been developed under the transition of oceanic subduction to continental collision processes at multiple geological times