Geophysical Study of the Gas Show Area in the South Western Region of the Proterozoic Cuddapah Basin

摘要

In the western Cuddapah basin several bore wells were identified to be associated with methane gas gushing out along with the ground water in the Tadipatri area. In order to gain insights in to the subsurface structure and lithology in the gas show area, deep electrical soundings together with resistivity scanning along a 4.5 Km. long traverse was carried out. Magnetic field measurements were also carried out along the resistivity scanning traverse. The subsurface model obtained from the inversion of electrical data brought out in general a three layer structure with the top and the bottom layers being conductive (50-100 ohm.m) representing the Tadipatri shales and a middle intermediate moderately resistive layer (a few hundread Ohm.m) interpreted to be a hidden sill. The intermediate layer exhibited conspicuous discontinuities/breaks, which are also observed to be associated with well defined sharp magnetic anomalies. Interestingly all the bore wells located along the scanning traverse and whichexhibitd gas shows fall very close to these segments of breaks in the intermediate layer suggesting that these features represent fault/fracture zones. These structures are inferred to facilitate the outward movement of the gas that remained presumably trapped underneath with the sill itself acting as a cap rock. The area of gas shows falls over the unique "gravity high" in the south western region of the basin which is interpreted to be a zone of magmatic intrusion. It is inferred that the source for the observed gas shows has a generic link to the hidden magmatic intrusive feature which might have created conditions favourable for thermogenic origin of the CH_4 through low-temperature fluid-rock interactions in the crustal layers related to active magmatism and metamorphism.