P 221 Detection of Thin Sand by Using Seismic Inversion in Gandhar field of Cambay Basin , India-A Case Study

Deltaic sands of Hazad formation of Mid-Eocene age are hydrocarbon bearing in Gandhar field of Cambay basin. Greater depth of burial, lesser thickness of reservoir sands and encompassing shale as compared to wavelet duration, their discrete nature of occurrence make precise mapping of individual pay sands a challenging task despite sands having higher impedance than shale. The challenge has been met in delineating the GS-1 sand; the bottommost of 12 sands in Hazad unit by utilizing the fact that the thin sand overlies the shale and its base is easily correlatable on inverted seismic volume. Other observation utilised for the mapping has been that the lower part of GS-1 sand is tight and fine grained resulting in higher average impedance despite the producing upper part of the sand which is highly porous and clean having low impedance comparable with shale. Impedance log of all the wells filtered at seismic band shows the complete sand unit as high impedance confirming the fact that high impedance characterizes the reservoir facies, whereas the low impedance corresponds to poor/shale-dominated reservoir. This led to the inversion of 3D seismic data for mapping acoustic impedance over GS1 interval. GS-1 top has been constructed over the area mathematically using the correlated base of the sand and two way time thickness of GS-1 sand unit observed at well locations. The sand distribution mapped by calculating mean impedance over the GS-1 interval is validated by all the wells in the area. The facies map explains sand distribution encountered in the wells and also shows additional area available for delineation.