Abstract

Shale gas is considered as an unconventional source as the gas is derived from adsorbed gas in organic matter. Methane in shales is generated from the transformation of organic material by bacterial and geo-chemical processes. The gas so generated gets stored by multiple mechanisms as free gas in micropores/ microfactures and as adsorbed gas on the internal surfaces.

Determination of potential organic content of shale section is one of the method to determine the shale gas potential of the given area. As Passey (AAPG, 1990) has shown, there are several methods that can be used to determine the organic content by interpretation of various electric logs. One such method, referred to as the ? log R technique, used for identifying and calculating total organic carbon in organic rich rocks can be quickly estimated by cross-plotting sonic logs (DT) and log (natural logarithm) of resistivity data and determining the shale line that can then be used to calculate a pseudo-sonic log that is then displayed over the existing sonic logs to determine the organic shale potential for a zone in an individual well. Sediments can be regarded as consisting of heavy and light fractions. The heavy fraction is the mineral matter, and the light fraction is the formation fluids. In source rocks, the contained organic matter is also part of the light fraction. During compaction, water is expelled; consequently, the density increases and the sonic transit time decreases. Because of the presence of organic material, source rock retain a greater amount of the light fraction than organic-lean sediments, and thus appear on sonic and density logs to be somewhat less compacted. Organic matter is normally not electrically conductive. Organic-rich sediments, therefore, have a higher resistivity than organic-lean sediments.

A statistical approach has been adopted to find a meaningful parameter which is a function of resistivity and sonic logs to find out shale gas potential of cambay shale in India. Cambay Shale can be cited as good shale gas reservoir having a considerable thickness (500–1900m), TOC range (1.5–4%), Ro range (0.75–0.85) and kerogen type is mainly II & III.

Petrophysical interpretation and existing seismic data shows study area is having considerable thickness of Cambay shale. The ? log R crossplot has shown cambay shale in the study area is organic rich. Organic rich cambay shale section has shown increase in sonic transit time and an increase in resistivity. As the gas market in India is rapidly growing and various innovative hydrofracturing and multi-lateral horizontal drilling techniques are available Cambay Shale section can be regarded as unconventional energy sources.

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