Quantitative reconstruction method of source-reservoir paleo-pressure in tight oil

The Triassic Yanchang Formation in the QingChengBei (QCB) area of the Ordos Basin, China, hosts an “upper-generation and lower-reservoir” type of tight oil reservoir, indicating that the source rocks must have experienced abnormally high paleo-pressure during the reservoir-forming period, resulting in a strong source-reservoir pressure difference. However, the amount of erosion during paleo-pressure restoration has rarely been considered, and previous studies have typically relied on a single compaction coefficient for restoration. The determination of paleo-porosity is usually based on the porosity-depth trend fitted within the study area, without accounting for burial time. Changes in paleo-porosity caused by undercompaction of source rocks and secondary dissolution of reservoirs are rarely incorporated. Paleo-pressure reconstruction typically utilizes the trendline method or compressional wave velocity, without incorporating shear wave velocity, which may lead to biased pressure predictions. In this study, the mudstone compaction curve extrapolation method is employed to determine the thickness of strata erosion. Considering the burial time and depth of the strata, a bivariate function for calculating paleo-porosity is constructed, and the paleo-burial depth is inversely obtained. By utilizing the relationship between porosity increment and depth during the maximum burial depth period, the porosity increment during the reservoir-forming period is inversely deduced to determine the paleo-porosity of the source rocks. Considering diagenetic effects, a model for reducing and increasing sandstone reservoir porosity is constructed to restore paleo-porosity. Additionally, a quantitative reconstruction method for tight oil source-reservoir paleo-pressure incorporating shear wave velocity is proposed. The resulting pressure difference between the source and reservoir provides a basis for predicting favorable exploration directions.