CATEGORY DESCRIPTION AND FLUID DETECTION FOR LOW PERMEABILITY RESERVOIRS IN HY GAS FIELD OF XIHU SAG

The HY Gas Field is located in the south of the central inversed structural belt of the Xihu Sag. The upper part of Huagang Formation is dominated by shallow water deltaic deposits, and the reservoir is characterized by medium-low porosity and low permeability, of which the seismic response characteristics are of class II AVO. Affected by compaction, the reservoir porosity gradually decreases from top to bottom, as AVO type changes from Class IIb to Class IIa. In this paper, a detailed description method for the low permeability reservoir with different AVO characteristics is proposed from petrophysical analysis, forward simulation, phase conversion and AVO projection angle rotation. It is considered that the medium-porosity and low-permeability reservoirs in the area are characterized by class IIb AVO response characteristics, and the post-stack seismic sand top is calibrated in the trough. Reservoir description should adopt post-stack 90 degree phase shift waveform interpretation, post-stack envelope attribute fluid detection, etc. Low porosity and low permeability reservoirs are characterized by class IIa AVO response characteristics. Reservoir description should use post-stack 0 degree phase shift waveform interpretation, pre-stack extended elastic impedance fluid detection and other methods. Based on the fine reservoir description method of "one method for one layer", multiple wells were successfully drilled in the study area, and the success rate of reservoir prediction and fluid detection was 100%, which has greatly improved the development of the gas field.