Structural and sedimentological analysis of the Pliocene–Late Miocene deposits from the Romanian Black Sea offshore based on borehole electrical imaging

Abstract

This study presents the results of the structural and sedimentological interpretation of borehole electrical imaging data recorded in a gas exploration well from the Romanian offshore area of western Black Sea. The well intercepted on the investigated interval Pliocene to Late Miocene deposits, including a gas reservoir hosted in Early Pliocene shallow marine sands and silts. A total of 824 features were picked on the electrical image, such as bedding, soft sediment deformations, open (conductive) fractures and closed (resistive) fractures. An analysis of the dip angles and azimuths data identified four structural domains within the study interval: two are quasi-horizontal, one shows slightly inclined bedding dips and one is dominantly deformed (slump features and possibly sediment creep). The open fractures show a preferential WNW–ESE strike, whereas the closed fractures show, besides a WNW–ESE strike, mostly ENE–WSW to N–S strikes. The opposite directions of the two fracture sets suggest a relationship to the regional tectonic stress, with open fractures being nearly parallel to maximum horizontal stress SHmax direction and closed fractures being nearly normal to SHmax direction. The variable bioturbation intensity observed on the electrical image may reflect salinity fluctuations in the Black Sea basin.

Keywords

• bioturbation
• black sea offshore
• borehole imaging
• fractures
• soft-sediment deformations
• tectonic stress