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.Keywords Shale oil/gas reservoir Á Brittleness index (BI) Á X-ray diffraction (XRD) Á Fourier transform infrared spectroscopy (FTIR) Á Cambay shale Á Cambay Basin

A high-amplitude anomaly was identified in the Upper Oligocene Daman Formation clastic sequence during initial interpretation of newly acquired 3D seismic data in the Tapti-Daman sub-basin offshore western India. This anomaly exhibited the depositional signature of a channel and was interpreted as bright spots associated with gas sands, consistent with the occurrence of thin gas pays in the sandstone reservoirs of Daman Formation in the area. Considering thickness and areal extent, thick gas sands were predicted, upon drilling; however, thick water-bearing sands were found. The predrill interpretation was mainly based on windowed amplitudes without validation through character of reflection events and other supporting evidence such as frequency, velocity, and AVO.

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The 3-D seismic data of NE-Panna and adjoining area within Central Graben in Heera-Panna-Bassein Tectonic block of Bombay Offshore Basin, India, have been evaluated for identification of prospects within Middle to Upper Eocene Sequence (Bassein Formation) which is hydrocarbon bearing in near by areas. In the study area, massive to laminated carbonate sections without hydrocarbons were encountered in three wells, which were drilled on the basis of 2-D seismic data interpretation. Subsequently 3-D data was acquired in 2004 to assess the reservoir and trap potential of the area.The 3D seismic along with other geo-scientific data of the area have been integrated and interpreted by applying concepts of seismic stratigraphy, seismic attribute analysis and 3D visualization techniques. On the basis of seismic reflection configuration and interpretation of electrolog facies, the Bassein Formation has been divided, seismically, into four units which were deposited under transgressive to highstand regime including late highstand sea level fall. Within units, coeval deposition in different sub-environment has been inferred from the lateral seismic facies variation. Seismic impedance volume generated through model based post-stack stratigraphic inversion, seismic attributes and attributes derived from logs, e.g., %limestone and porosity, were analysed with geostatistical cross-plots method to understand the interrelationship between lithofacies and seismic facies. Moderate to high amplitude and moderate impedances have been found to be associated with carbonates having good porosity. The lateral discontinuities and boundaries between different assemblages have been mapped with 3D visualization of volume and surfaces coupled with coherency slices. Various facies assemblages representing carbonate buildups, off-buildups at slope of big buildups and wedges of debris flows (talus) were identified with consideration of structural configurations, attributes and impedance, and depositional geometries. The inferences of this study are validated at drilled wells.

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