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The Barmer Hill Formation is a Palaeocene lacustrine deposit occurring in the Barmer Basin of northwestern India. The lake developed entirely within the rift basin and its sediments comprise a complex of shallow to deep water deposits. In much of the northern part of the basin, the upper part of the Barmer Hill Formation is characterised by the presence of a finely laminated siliceous unit. Millimeter scale laminations in this unit consist of highly siliceous lithofacies, interbedded with organic material and subordinate clay. This syn-rift unit varies from 50 to 250 meters in thickness and is known to be hydrocarbon-bearing through much of the basin from numerous well penetrations. It is characterised by high porosities (25-35%) but low permeabilities (0.1-4mD). Hydraulic fracturing well operations in this reservoir had produced stabilized oil flows in the range of 30-250 BOPD demonstrating commerciality.Detailed reservoir characterisation of this siliceous Barmer Hill Formation unit has been carried out in advance of planned field development and further hydraulic fracturing activities. Outcrop samples and subsurface core data have been integrated to understand the unusual properties of the reservoir. Scanning Electron Microscope (SEM) examinations of subsurface core samples reveal microcrystalline quartz (chalcedony) as the main mineral present, whereas equivalent outcrop samples are dominated by both Opal-CT and chalcedony and contain opaline diatom tests. This integration of core and outcrop data indicates that the siliceous unit was likely formed through diagenetic alteration of biogenic silica from opal-A through opal-CT to microcrystalline quartz. Early oil migration may have helped to preserve the porosity in the subsurface resulting in high porosity-low permeability reservoir properties unusual for microcrystalline quartz.Geomechanical properties were assessed from bulk rock X-ray diffraction (XRD) information and core data to identify the most suitable intervals for hydraulic fracture initiation. This XRD data has been used semi-quantitatively to predict reservoir brittleness using a ternary plot of quartz-carbonate-clay abundance whereas geomechanical properties viz. Poisson's Ratio (PR), Young's Modulus (YM) and Unconfined Compressive Strength (UCS) of each units are estimated from wireline densitysonic log data and calibrated with core plugs to quantify the fracability of the units. The in-situ stress orientation is estimated from image log interpretation to predict fracture propagation orientation and aid optimal well placement.This integrated workflow combining geological, petrophysical and geomechanical properties for reservoir characterisation is being applied to support commercial development of the hydrocarbon resources of this low permeability siliceous unit.

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D31 cluster is located in the prolific Mumbai Offshore Basin, Western part of India. B-192Afield is part of this cluster, where Bassein (Middle Eocene),Mukta and Panvel (Early Oligocene)Formations are the main reservoirs. The reservoirs are complex in terms of reservoir heterogeneity. They were deposited in a shallow marine carbonate platform.Sea level fluctuations andchange in depositional environment impacted the porosity development. The main objective of this study was to integrate spectral gamma ray signatures with seismic interpretation for demarcating significant stratigraphic surfaces and differentiating depositional environments for robust reservoir characterization. Regionally, Bassein Formation (Middle Eocene) is characterized by thick foraminiferal and algal wackestone, packstone and occasional grainstone facies.The Mukta Formation (Early Oligocene),which unconformably overlies the Bassein Formation, is characterized by presence of fossiliferous limestone with shale intercalations. In the present study, data from four exploration wells data have been analyzed, where spectral gamma ray log patterns in carbonate reservoirs appear to have a distinctive relationship to depositional facies and stratigraphic surfaces in the Bassein and Mukta Formations. Different cross plots have also been utilized for analyzing the depositional conditions (i.e. oxic or anoxic).Later, the spectral log interpretations have been integrated with seismic interpretation. This study is part of a larger effort for reservoir characterization, as a basis for seismic interpretation and integrated reservoir modelling. The spectral gamma ray signatures demarcated significant stratigraphic surfaces. In BasseinFormation, three different units have been marked as Upper, Middle and Lower Bassein. The major lithological boundary between the Bassein and Mukta Formation is also well demarcated with spectral GR signature. The carbonate strata of Bassein & Mukta Formation have also been subdivided with U-Th-K abundance.The "Low Th-Low U" units indicative of pure carbonate and deposition in oxidizing environment whereas "Low Th-High U"is indicative ofreducing environment, which gave a relative sea level fluctuation in the area.The major stratigraphic boundaries identified from these spectral GR logs has been incorporated in the seismicinterpretation and used for regional seismic mapping.As porosity development is governed by thesea level fluctuations,this study also gave an indication of the possibility of porous zonein the reservoir section. These results can be useful as a basis for applying spectral GR signature as a tool for stratigraphic interpretation in un-cored heterogenous carbonate sections. Along with the petrophysical interpretation, integration of core analysis, biostratigraphy and seismic attribute are critical for detailed carbonate reservoir characterization incorporating depositional environment.This approach can be applied to support commercial development of the complex carbonate reservoirs.