Mudrock overpressure, fracturing, and mud volcanism in the Lower Kura Depression, Azerbaijan

Abstract

The Lower Kura Depression (LKD) in Azerbaijan is a unique place on Earth where enormous oil and oil–gas-condensate deposits coexist with mud volcanoes. Large mud volcanoes developed in this area as a result of favorable tectonic processes, depositional settings, and subsurface pressure conditions. Disequilibrium compaction leading to overpressurization of mudrocks, as well as gas generation, have been previously proposed as the main factors that cause overpressure and trigger mud volcanism. To assess the mechanisms contributing to mudrock overpressure, we conducted a 2D basin modeling work to simulate the burial, temperature, maturation, and pressure histories of the sedimentary pile along a 120-km-long geological cross-section in the ENE‒WSW direction perpendicular to major structures in the LKD. The results of the calibrated model suggest that the main petroleum source rocks of the LKD, namely the Oligocene–Middle Miocene Maykop and the Eocene Middle Koun mudrock formations, are still in the oil generation zone. Therefore, previously speculated gas generation effect on overpressurization is insignificant in the LKD. Modeling also predicts overpressure of varying magnitude in the potential hydrocarbon source rocks of dominantly mudstone lithology. We have verified that disequilibrium compaction caused by rapid sedimentation in the last 3 million years has led to mudrock overpressurization that exceeded rock strength. We take model-predicted fracturing as a proxy indicator of mud ascent and suggest that fracturing of the mudrocks enabled ascend of the mud via fault-associated weakness zones.

Graphical abstract