Mudrock Resistivity Reversal in the Riserless Section of Deep-Water Well, Western Black Sea: A Geological Phenomenon Unrelated to Shallow Hazards

Pratap V. Nair, Dr Biswajit Thander

Abstract: Uncertainty related to Shallow Hazards associated with overpressure just beneath the seabed in the deep water Western Black Sea prompted drilling of a riserless pilot hole in the top hole section of an exploration well as a mitigation measure. The riserless section indicated mudrocks of low resistivity to the extent of as low as 0.2 ohm-m. The pilot and the main holes revealed that the pore pressures estimated from these low mudrock resistivities were much higher than actual pore pressures. However, the benign well behavior invalidated the pore pressure estimates, signifying it as a false alarm. Therefore, it is imperative to investigate what causes the significant lowering of mudrock resistivity. Understanding the geological history of the region holds the key to the underlying process that shaped the electrolog response. Analogues were consulted, and the geological history of the Black Sea was studied to explain the low resistive mudrocks just beneath the seabed. With periodic saline water incursions through the Bosporus Strait in the Pliocene to recent sediments of the Black Sea, as documented in various studies, it is not unusual for the unconsolidated mud rocks to be influenced by the salinity of the incursions from the Mediterranean Sea. The sediments, characterized by their softness and porosity, along with the percolation of the weight of seawater above them, contributed to the retention of the denser saline water within the interstices. Hence, the resistivity responses are subdued. The outflow from the Black Sea is characterized by lower temperatures and reduced salinity, allowing it to float above the warmer, saltier inflow from the Mediterranean. This phenomenon arises from variations in density attributed to differences in salinity, resulting in the formation of a substantial anoxic layer situated well beneath the surface waters. Therefore, the lower mudrock resistivities are not the reflection of higher pore pressures but only the imprint of the preservation of environmental conditions of that geological period. These shallow formations just beneath the seabed are inferred to be connected to the seabed and therefore of hydrostatic pressures, with the overburden pressure being borne by the rock framework. This indicates that the environment of deposition plays a key part in the pressure generation in the Black Sea Basin, and understanding it is important for predicting the pore pressures. This attempt will serve as a platform for future ventures in the Black Sea.

Keywords: Low resistivity, pore pressure, shallow hazards, salinity, pilot hole, main hole, Bosporus, Mediterranean