INTERPRETING SHALE-GAS RESERVOIR USING SEISMIC AND WELL-LOG DATA

Abstract

India is poised to enter the exploration of natural gas from unconventional reservoirs which may hold great potential for future energy supply. Unconventional Shale—gas plays are sedimentary rocks which are fine grained and have organic richness (Total Organic Carbon or TOC) in them. They have very tight porosity and ultra-low permeability but their commercial production is possible through advanced drilling and stimulation techniques. Due to continuous research (17 years), shale gas plays have come to reality in United States of America. In India, First gas well was struck in the Barren Measure shale recently. Traditionally, logs are used to identify the organic rich zones but characterising shale-gas reservoirs using seismic methods (thickness, fractures etc.) is still in the development stage. In my work, I have used the logs like sonic, resistivity, density, neutron porosity, gamma ray etc. to identify the possible TOC zone. I used DT-resistivity overlay method (Passey e/ al., 1990) to make cross plots to clearly distinguish between reservoir shale and non- reservoir shale. Also, a Pseudo sonic log was made from the cross plot and using this, TOC zone was highlighted. Shale gas sequences were mapped through integrated seismic interpretation. Possible signatures could be seen by simultaneous display of the logs but subtle features could not be seen in these seismic sections. So, I used impedance inversion volume to identify the low impedance areas which could possibly be TOC zone. AVO modelling was done to see the type of anomaly (Class Ill type identified). AVO cross plotting was also done between the intercept and gradient to further confirm the class identified. Also, fractures along the prominent shale markers were mapped using geometric attributes like Event similarity, Curvature and discontinuity.