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Mössbauer spectroscopy is widely used for the study of geological samples including all types of sediments for mineralogical identification, investigation of the oxidation states and site population of the iron and identification of iron-bearing minerals, and the semi-quantitative analysis of iron distribution in each mineral and lattice site [1]. It is well known that the oxidation state of iron metal in sediments is a measurement of the oxidation-reduction condition of sedimentation. It is the only technique that provides crucial information about the ferrous/ferric ratio in sediments. To get better insight into the application of ⁵⁷Fe Mössbauer spectroscopy for geological samples [2].⁵⁷Fe Mössbauer spectroscopy is particularly useful for the characterization of iron-bearing species because it probes the local environment of iron nuclei sensitivity. This method offers certain advantages over other conventional techniques such as chemical, optical, electron microscopic analysis, etc. Indeed each technique has its own strength, but where Mössbauer spectroscopy can give results, it offers a quick reliable and simple method. Being a non-destructive technique in the sense that the sample either in powder form or thin slice is not altered during the experiment also in a single run, one can get information about all the iron phases present in the sample by proper de-convolution of the Mössbauer spectrum. Mössbauer spectroscopy is also used widely to study organic-rich sediments (source rocks) from the different petroliferous basins. In fact source rocks are tiny generators of oil/gas or both. Source rock characterization is one of the important aspects of the exploration of oil/gas. This distribution of minerals suggests that North Sea offshore sediments were deposited in a highly reducing environment. It is worthwhile to note that the offshore region is a major oil field off the North Sea. The detailed study of the chemical state of iron in subsurface sediments for four differ tiny of minerals including iron-bearing minerals, e.g. if the rate of sedimentation is fast it quickly cuts off sediment from the environment and this may favor the formation of minerals like pyrite which are digenetically stable in reducing environment. Simultaneously organic matter also escapes oxidation and becomes more favorable for the generation of oil. But if the sedimentation rate is slow sediments will remain in contact with the atmosphere for more time. This may result in oxidation of organic matter making it unfavorable for the generation of oil and favors minerals like siderite, iron oxide etc. depending upon the degree of redox condition. This correlation can also be viewed that the physicochemical transformation of organic matter during the geological history of the sedimentary basin cannot be regarded as an isolated process. It is controlled by the same major factors that determine the variation of the composition of the inorganic solid phase of sediments; that is to say, biological activities in the early stage, and temperature and pressure afterward decide the evolution of both the organic matter and the inorganic solids[3-7].

Mössbauer spectroscopy is also used to study various shale deposits from different basins (Cluff, 1987; Cole et al., 1978; Karl and Zukermann, 1981; Leventhal and Hosterman, 1982; Lineback, 1970; Ross and Bustin, 2007 and Pisuotto et al., 1992). On the basis of these studies it has been hypothesized that the organically rich oil/gas shales were formed in an anoxic environment while the kerogen poor shales were formed in an environment high in oxygen cf. Maynard, (1982). On the basis of these studies it is expected that iron in minerals will be in more reduced state in those shale deposits which are favorable to produce shale oil/shale gas relatively to those shale deposits which are not favorable for shale oil/shale gas. Mørup et al., (1985) have reported relative distribution of iron bearing minerals as a function of depth in inorganic rich Cenozoic and Mesozoic subsurface sedimentary samples (source rocks) from North Sea Danish oil field. It is worth noting that in Danish North Sea though offshore area show large reserve of oil/gas but on-shore wells were found to be dry. In their study they found that relative distribution of iron bearing minerals was markedly different in these two regions. While offshore wells show presence of iron in Fe2+ state in pyrite and clay minerals in onshore wells there is appreciable amount of iron was present in Fe3+ state and siderite. This is consistent with the hypothesis of (Maynard, 1982). In follow up this chemical state of iron was also studied by our Mössbauer group, at Jodhpur in sub surface sediments of different petroliferous basins of India cf. (Tripathi et al., 2008; Ram et al., 1997; Ram et al., 1998; Nigam et al., 1989; Kulshrestha et al., 2000 and Bhatia et al., 2012 etc). These studies also suggest that Mössbauer spectroscopy can be used as one more additional tool for characterization of organic rich sediments.

The Bikaner-Nagaur basin could probably be an extension of the infra-Cambrian tectonic-depositional system of the Arabian platform. The eastern flank of the Indus Shelf including Bikaner-Nagaur, Jaisalmer and Barmer-Sanchor remained uplifted from Middle Eocene to Pliocene and supplied classics to the fluvial Basin to the southwest. However, during Quaternary, thin veneer of the fluvial sediments covered middle Eocene sediments prominent east-west basement of high trend acts as a structural trap for oil accumulation. Broadly two sedimentary systems clastic-carbonate system and the clastic dominant system had been observed in this basin[8-15].In the present investigation, we have further extended Mössbauer spectroscopic studies of deep subsurface sediments collected from two representative wells Lunkha-1, Pinodha-1 and Phulasar-1 wells of Bikaner–Nagaur basin.

Mössbauer absorber was prepared by sandwiching finely ground sediment samples between two paper discs, in a sample holder (25 mm in diameter). Thickness of the absorber was always kept nearly constant (about 100 mg/cm2 ). The isomer shift (IS) values are reported with respect to the centroid of a 25 lm thick an iron foil obtained from M/S FAST COM Tech., Germany. The 57Co embedded in Rhodium matrix was used as a Mössbauer source (the initial strength of source was 10 m Ci). During the work the stability of the system was checked periodically by taking standard iron spectrum. All the Mössbauer spectra were recorded at the room temperature (300 K). Computer programme written by Meerwal [16] was used for data fitting after suitable modifications. The programme assumes the spectrum to be the sum of Lorentzians. In most of cases, the width and intensity of the two halves of a quadruple doublet were constrained to be equal. In the case of a sextet, the width of all the peaks and the intensities of 1st and 6th, 2nd and 5th, and 3rd and 4th peaks were constrained to be equal. The solid lines in the spectra represent computer-fitted curves and dots represent the experimental points. The value of Chi-square (χ2 ) reflects the quality of fitness. But many times the larger value of chi-square was also accepted if iteration does not improve the fitting further and also when fitting provides the physically acceptable parameters[16].

Mӧssbauer spectra at room temperature:

Mössbauer spectra of the samples of the wells Pinodah-01, Lunka-01 and Phulasar-01 recorded at room temperature as shown below

Figure1.1 Mӧssbauer spectra at room temperature of the Pinodah-01 well at different depths (retrace)

Figure1.2 Mӧssbauer spectra at room temperature of the Lunka-01 well at different depths(retrace)

Figure1.3 Mӧssbauer spectra at room temperature of the Phulasar-01 well at different depths

Pinodah-01		Lunka-01		Phulasar-01
Depths(m)	Fe2+/Fe3+	Depths(m)	Fe2+/Fe3+	Depths(m)	Fe2+/Fe3+
540	0.703	520	1.309	1127	0.235
570	0.740	570	3.545	1191	3.177
585	0.000	595	1.638	1213	0.000
618	0.000	620	12.736	1247	1.056
1244	0.937	695	0.000	1297	0.481
1364	0.985	1740	0.000	1315	0.000
1404	1.074	1910	0.616	1335	0.145
1446	2.186			1385	0.000
1460	1.888			1387	0.330
1582	5.157			1427	0.689
1676	0.732			1451	0.189
				1505	0.894

Table-01: Ferric-ferrous ratio of the sediments of the different test wells of the Bikaner-Nagaur basin

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