Understanding the synergistic impact of stress release and cementation on sandstone using sound waves — Implications for exhumation estimation

Abstract

Exhumation is the process that encompasses uplift and erosion, leading to the removal of overburden and the release of effective stress exerted on rocks. When estimating exhumation magnitude using the compaction trend method, it is commonly assumed that the physical properties of rocks are insensitive to stress reduction. However, recent laboratory evidence has indicated that porosity exhibits weaker sensitivity to stress release compared with velocity that can be significantly affected by stress release. This raises questions regarding the validity of irreversible compaction assumed by compaction trend method. It remains unclear whether the impact of stress release can be observed in real rocks in exhumed areas because there is a lack of methods to directly measure the impact of stress release on field data. In addition, studying real rocks is further complicated by the presence of rock diagenesis and its interaction with stress release. To address these knowledge gaps, this study uses stress-dependent burial and uplift modeling and interprets an extensive well-log data set using the modeling-derived evaluation metrics. Conceptual modeling suggests that methods that neglect the combined effect of cementation and stress release tend to underestimate the exhumation magnitude. Furthermore, we discover that the disparity between porosity sensitivity and velocity sensitivity to stress release can be leveraged to derive a metric we call “porosity inconsistency” that can serve as a qualitative and quantitative measure for identifying and evaluating stress release in sandstone using geophysical field measurements. We gather a significant amount of sonic velocity and porosity data from normally compacted and uplifted clean sandstones in the Norwegian Sea and the Barents Sea. Notably, we observe significant porosity inconsistency in the exhumed well 6510/2-1 in the Norwegian Sea. In the Barents Sea, which has experienced extensive Cenozoic exhumation, the well data reveal a varying pattern of porosity inconsistency increasing toward the north and decreasing toward the west. This distribution of porosity inconsistencies in the Barents Sea wells not only aligns with the spatial variation of exhumation reported in various studies but also exhibits a positive correlation with the magnitude of exhumation. Furthermore, the exhumation magnitude derived from velocity-depth trends is considerably lower than the magnitude obtained from porosity/density-depth trends for wells displaying significant porosity inconsistency. These observations provide support for the predictions made by the conceptual modeling. The results of this study enhance our understanding of the synergistic impact of stress release and cementation on sandstone. Moreover, these findings have implications for pore pressure prediction and core evaluation in exhumed areas. They also provide insights into the feasibility and interpretation of time-lapse data of reservoir injection, for which the effective stress is likely to decrease due to pore pressure buildup.