Porosity Prediction in Tight Reservoirs via Elastic Decoupling Factor-Constrained Prestack Density Inversion: A Case Study from the Bohai Bay Basin

The Paleogene deep tight glutenite reservoirs in the Bohai Sea area feature strong heterogeneity and complex pore structures, posing significant challenges to the seismic prediction of sweet-spot reservoirs. Uncertain reservoir-sensitive factors and insufficient large-angle information from seismic gathers, together with the low accuracy of conventional prestack density inversion, lead to considerable porosity prediction errors and make it difficult to identify high-quality reservoirs. To address these issues, this study proposes a novel prestack density inversion method constrained by an elastic decoupling factor (EDF) for porosity prediction in tight reservoirs. First, rock physics modeling is performed using a variable aspect ratio Xu–White model, based on which a reservoir-type indicator and a porosity-sensitive factor are constructed to establish differentiated solution equations among density, Poisson’s ratio, and porosity. Second, to overcome the strong dependence of density inversion on large-angle information, an EDF is formulated to decouple density from S-wave impedance, thereby reducing inversion non-uniqueness. Simultaneously, compressed sensing technology based on L1-norm regularization and basis pursuit is introduced to obtain high-resolution P-wave impedance, S-wave impedance, and Poisson’s ratio, providing high-quality inputs for density decoupling. Based on the above methods, the solution equations between density and porosity are integrated to achieve classified and quantitative porosity prediction for tight reservoirs. Application to the deep Kongdian Formation glutenite reservoirs in the Bozhong 28 area, Bohai Sea, shows that the predicted porosity matches measured porosity with an accuracy of 82% (average relative error of 18.5%), and the correlation coefficient of the inverted density is improved from 62% (conventional simultaneous inversion) to 85%. Furthermore, an approximately 10 m thick mudstone interlayer is clearly resolved. Blind well BZ28-L validation confirms good agreement between predictions and actual drilling data. This work provides a valuable reference for sweet-spot prediction in similar deep fan-delta tight reservoirs.