The Influence of Stress Regimes on Hydrocarbon Leakage
Hydrocarbon leakage through faults and fractures commonly limits in-place hydrocarbon reserves. Faulting and fracturing are controlled by effective stress changes, and such changes may therefore alter hydrocarbon column heights. The predictive power of stress history analyses in seal evaluation depends on how accurately the stress history and relationships between effective stress changes and hydrocarbon leakage can be determined.
Stress history and hydrocarbon occurrence were examined in four different overpressured provinces of offshore Norway in the search for such relationships. These provinces have experienced different geological histories and variable amounts of hydrocarbon leakage. Because all these areas received fairly recent hydrocarbon charge, the work focused on the identification of recent geological events that may subsequently have influenced recent stress history, including the present-day stresses. Areas of recent structuring were found to be characterized by more extensive hydrocarbon leakage than areas with less such structuring. This increased frequency of hydrocarbon leakage was interpreted to be the result of shear failure at the trap crests, induced by the combined effects of elevated pore pressures, stress anisotropy, and recent stress changes.
These results suggest that identification of recent stress changes based on the geological history of the study area could aid the prediction of hydrocarbon occurrence. It is inferred that stress history analyses can also reduce the uncertainty involved in seal analyses elsewhere.
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This volume constitutes the proceedings of the AAPG Hedberg conference on seals held in Barossa Valley, South Australia, in 2002. The key driver for both the Hedberg conference and this publication was the recognition that knowledge of risk in the estimation of sealing capacity and fault-seal potential is important in making judgments at the exploration, appraisal, and development stages of the petroleum business. In addition, incorporating seal risk in the overall assessment of hydrocarbons in place can affect decisions to drill prospects and the location of appraisal and development wells, as well as reserve estimation. Improved methods to estimate seal capacity and fault integrity can lead to savings in well costs, improved recoveries through optimum placement of wells, and improved estimates of hydrocarbon in place. This volume contains 18 chapters that reflect the spectrum of presentations at the conference. The knowledge imparted by these chapters will be a window on the state of seal knowledge at this juncture of time and includes topics such as seal failure related to basin-scale processes, the role of geomechanics in seals, and the economic evaluation of prospects with a top seal risk.