Abstract:

Tight sandstone of the Carboniferous Taiyuan formation in the Xunyi area represents a favorable exploration target in the southern Ordos Basin. However, it exhibits rapid variation in lithofacies and is sheltered by both the overlying continental coal rocks and the underlying marine carbonate rocks with strong reflection events, which challenge reservoir prediction. In the eastern Xunyi area covered by 2D seismic survey, seismic attribute analysis and inversion on 2D lines were often used in previous prediction of reservoir distribution, These techniques suffered from problems such as multiple solutions and low accuracy. This paper proposes a probabilistic deduction method with step-by-step constraints to predict tight sandstone reservoirs in 2D seismic areas. First, a layer-flattening procedure was applied to identify layer-thickening zones and favorable sedimentary facies zones for tight sandstones, thus setting the first-level constraint. Then, under the regional sedimentary framework, structure models of different lithologic associations were established using actual drilling data, and the seismic reflection probability type for the sandstone development area was simulated by forward modeling. Accordingly, the favorable area for tight sandstone development was determined according to the seismic response characteristics of sandstone in the favorable sedimentary facies zones. In this way, the second-level constraint was created. Finally, the wave impedance inversion was performed based on well log constraint to define sweetspots in the sandstone, achieving the third-level constraint. This three-level sandbody control approach for predicting tight sandstone reservoirs by probabilistic deduction with step-by-step constraints achieves improvements in precision (from 15 m to 10 m) and prediction coincidence (from 40% to 75%). The newly deployed exploration well encountered a tight sandstone layer of 24 m thick, recording effective support to the exploration in marine-continental transitional tight sandstone gas reservoir.

Key words: Ordos Basin, Xunyi area, Taiyuan formation, 2D seismic, probabilistic deduction, tight sandstone, reservoir prediction