Characteristics of unconformity structure in the south slope of Dongying sag, Bohai Bay Basin, China

Abstract:

After the exploration of structural and lithologic reservoirs, stratigraphic reservoirs have gradually become the main direction of exploration. The stratigraphic reservoirs are closely related to the unconformity surface. The concept that a complete unconformity should include upper, middle and lower layers has been widely accepted. However, it is difficult to identify unconformity structure layer because of its thin thickness and complex distribution law. It is one of the difficult problems in petroleum geological exploration to accurately divide the internal structure of unconformity. Distinguishing the development of unconformity structure is helpful to reveal the main controlling factors of the effectiveness of stratigraphic traps.

There are many regional or local unconformities in the south slope of Dongying Sag in Bohai Bay basin. It has good conditions for the development of stratigraphic traps. However, the effectiveness of the traps in this area is lack of systematic study. On the basis of multiple regional sections and seismic data, the unconformity characteristics of the Es₄-Ek are mainly studied. The results show that the unconformity types are diverse and inherited, and the distribution becomes complex from shallow to deep. On the logging curve, Comprehensive identification parameters U, φ_c and V_sh are constructed by dividing unconformity structure. The U shows that there are obvious abrupt changes in the upper, lower and middle layers of the unconformity structure. The values in middle layer is low and stable and its φ_c and V_sh are different from the normal mudstone far away from the unconformity structure. This study provides technical support for accurate evaluation of stratigraphic reservoirs and can improve the effectiveness of oil and gas exploration and prediction.