Delineating Thin-Bed Tight Gas Sand Reservoir with Prestack Joint PP/PS Inversion*

The thickness of single sandstone is about 30-60m and the thickness of single gas sandstone is less than 10m. However, the extent of gas sandstones is greater than 80,000 km 2 in the Ordos Basin of Northwestern China. It poses challenges to predict the gas sand reservoir because of drastic lateral lithological variations in the fluvial sediments in Dyas. The P-velocity or impedance of tight gas bearing sand decreases, which may be the same as that of shale surrounding rock. A prestack stochastic inversion of joint PP and PS data conducted recently provided an effective technique for solving the problem and successfully delineating the characteristics of the reservoir. Converted wave (Sv-wave) velocity analysis approach is always a difficult problem in 3C seismic data processing. Conventional 3C velocity and image are generally computed in different time scales; PP wave is processed with PP time scale and PS wave with PS time scale. PP and PS wave data are basically processed separately causing errors in horizon calibration between PP and PS waves. Joint inversion of PP and PS reflection data has been hindered by the difficult task of registration or correlation of PP and PS events. It can perhaps be achieved by registering the events during inversion but the resulting algorithm is generally computationally intensive. In this paper, we report on a converted wave velocity analysis approach from 3C data that can image P and Sv-waves in the same PP or PS wave time scale. In fact, we carry out the velocity analysis in depth domain such that common conversion points are updated at each iteration of velocity analysis. Thus, mapping to PP and PS time scales is trivial. This method circumvents the horizon calibration problem in the data interpretation between PP and PS waves and image them accurately. At the same time,