Sagnik Dasgupta

Characterizing Thin Sand Reservoirs In Onshore Northern China

The study area in onshore north China contains significant oil reserves in numerous narrow river channels, so far developed using vertical wells resulting in a high risk of bypassed pay. There exists 36 sand units identified using well log data covering a potentially huge area. One key challenge to develop the area is being able to accurately map sand geo-bodies versus shale. A second, more difficult challenge is in determining the porosity variation, clay and fluid content of the sand facies. A target is therefore set to be able to map 3-5 meter thick sands which are part of the multi-level 3D fluvial channel system; and be able to identify residual oil distribution within these sands. Only a couple of available wells had measured shear log which was a big challenge for reservoir characterization. A robust rock physics model was built in order to predict reliable pseudo shear logs for the field. As seismic data is band limited most of the thin sand reservoirs could not be individually resolved and were seen as packages of thin sand and shales in the seismic inversion volumes. A detailed rock physics study was able to integrate the AVO inversion attributes to predict high porosity clean sands and also predict averaged porosities and water saturation distributions from the seismic inversion volumes. These predictions were limited to the resolution of the seismic data. As a final step geostatistical inversion is carried out using the well data and the deterministic seismic inversion as a constraint. The multiple realizations of the geostatistical seismic inversion is used to understand the uncertainty associated with fine sub seismic resolution predictions and at the same time better characterize the thin sand distribution which can be used to identify bypassed oil for future wells planning.