Abstract
AVO (Amplitude Versus Offset) is one of the powerful methods in identifying the fluid type and lithology in subsurface exploration. In this paper, a comprehensive method has been designed which comprises of three parts: 1- Direct modeling 2-Inverse modeling 3- Cross plot interpretation. Each step has special roles. Direct Modeling can indicate the identity of attributes (lithology or fluid dependent attributes). The analysis has been done using both P-P and P-Sv attributes. Inverse modeling deals with real data and is fed from the results of direct modeling for characterization of the light hydrocarbon zones. The main role of cross plot interpretation is satisfaction of the results of the Inverse modeling and consequently increases the validity of them. This cross plot is also able to identify the lithology. Because of using Hodogram – Cross plot in this methodology, We able to manage the distribution of all points (anomalous and background points), So the small hydrocarbon zones is identifiable too. Distribution of points in wet zone is dense and in gaseous zone is expanded at Gradient – Normal Incidence Reflectivity crossplot. Using this methodology in Gorgan Plain (located in Iran) showed that fluid factor, SIGN, and possion reflectivity are fluid-dependent attributes. It also showed the normal incidence reflectivity and P-wave impedance reflectivity are lithology-dependent. Making the inverted sections of fluid-dependent attributes indicated two light hydrocarbon zones under the Cretaceous-Tertiary unconformity. It is expected that there is a wet zone under the one of this zones. Wet and gaseous zones are also satisfied by cross plot. Points distribution in cross plot of lithology-dependent attributes or fluid-dependent attributes is regular. Whereas considering simultaneous effect of both lithology and fluid attributes in cross plot results dispersive points distribution.
