Seismic AVO Analysis for Shallow Hazard Assessments in Stratigraphically Complicated Areas in Onshore Alaska Locations

Abstract

Abstract We show how amplitude variation with offset (AVO) analysis can be used to distinguish among 3D seismic indications of shallow gas, coal seams, and coal seam gas (gas contained within coal seams) during shallow pre-drilling hazard assessments for both onshore and transition zone wells in Alaska. These conditions are difficult to distinguish using more conventional seismic interpretation methods, which can lead to incomplete or incorrect shallow drilling hazard assessments in the absence of AVO analysis. The study area is located within the Cook Inlet Basin with a thick Tertiary non-marine strata. Sediments of the strata consists of interbedded claystones, siltstones, sandstones, and conglomerates with a few coal beds. We simulated and analyzed AVO responses for shallow gas, coal seams, and coal seam gas using 2D seismic data collected in the area. Our results indicate that each condition presents a characteristic AVO behavior. Shallow gas accumulations are characterized by Class III AVO behavior, with low impedance gas sands underlying high impedance clays and amplitude increasing with offset. Coal seams are characterized as Class IV AVO behavior, with low impedance coal underlying high impedance clay but amplitude decreasing with offset. If a coal seam contains gas, amplitude phase reversal could occur from the middle angle stack to the far angle stack. Our well log example in existing well 1 indicates predicted gas and coal intervals from AVO analysis are consistent with the findings of the well logs. Predrilling shallow hazard assessment by using this AVO analysis indicates that potential shallow gas and coal seam are anticipated to be encountered.