Impact of sedimentary environment on mechanism of mass, fluid and energy transport in the South Caspian basin

Mud volcanism represents one of the unique natural phenomenon that reflects subsurface processes characterized by movement of large volumes of sediments and fluids in the Earth’s crust. The significant part of world mud volcanoes is concentrated within the on- and offshore Azerbaijan, South Caspian basin (SCB).

The study integrates comprehensive geological, geophysical and geochemical database and presents the results of the analysis of the sedimentary environments in SCB to understand origin and development of mud volcanism in the context of the evolution of the basin. The interpretation of new 2D and 3D seismic data shows that the roots mud volcanoes do not extend below the base of Oligocene-Lower Miocene Maikop Group. The features of sedimentation were studied in details and the sedimentation model of Maikop Group was suggested.

According to the geochemical data and thermobaric conditions Maikop Group considered as the main hydrocarbon source rock within the South Caspian basin. Sedimentary environment during Oligocene-Early Miocene within the territory of Azerbaijan and adjacent Caspian Sea characterized by the complex interaction of various sedimentary processes resulted in the formation of a unique sedimentary complex. The accumulation of Maikop deposits is associated with the beginning of the formation of the folded mountain structures of the Greater Caucasus, the Kopetdag, and the Elbors mountain systems surrounding the SCB, which were the main provenance of clastic material. According to seismic data, the thickness of Maikop sediments currently reaches 3000 m indicating high sedimentation rates over such a relatively short period of geological time. The recent paleontological and geochemical data suggests that Maikop deposits accumulated mainly in the deep marine environment. The rapid burial and subsidence of Maikop sediments occurred because of subsequent intense (avalanche) sedimentation, especially in the Pliocene-Quaternary period. Due to rapid burial, Maikop sediments remain underconsolidated, water-saturated and represent a plastic clay mass, overlapped by thick sequence of denser sediments of the Low Pliocene Productive Series. Thus, as a result of deep subsidence and subsequent catagenic transformations Maikop medium turned into closed physico-chemical system characterized by specific elision processes. The sedimentary environment providing the 20.5 km thick sedimentary filling of the SCB controls mass, fluid and energy transfer within the basin, resulted in formation of mud volcanoes and hydrocarbon accumulations.

Structural interpretation of seismic and geological data, carried out over the past 25 years, indicates that the hydrocarbon deposits discovered in SCB are confined to rootless structures, i.e. injection folds. The formation of injection folds in the sedimentary cover occurs at different stages of basin development due to the alternation of transgressive and regressive series of sediments represented by the layers of clay and sand. All the folds are complicated with mud volcanoes indicating genetic relationship between them, thus the results of paleoreconstructions suggest that mud volcanism in the study area appears to be synchronized in time and space with the growth of anticlinal folds. The mechanism of mud volcano formation based on the gravitational instability has been suggested.