Mud volcanism of the northwestern Caucasus and the oil and gas prospects

Discussion of Results and Conclusions

[33] The material given in this paper supporting the concept of secondary tectonic origin or mud volcanism and clayey diapirism often causing it at the same time testifies to the diversity of concrete deformation mechanisms of diapir-forming complexes. Nevertheless the conditions that determine mud volcanism development against the background of diapir-form dislocations going on in one way of another can be definitely defined. For the most part they are as follows:

Thick diapir-forming rock complexes weakly competent (of low viscosity) in stress state (Maikopian or Jurassic-Lower Cretaceous beds);
Dislocations with a break in continuity active in the recent time underlying diapir-forming sediments not always corresponding to diapir ridge extension and to a large extent providing decreased strength characteristics of overlying sedimentary bodies (for example in the influence band of deep Timashevskiy fault-gravitational step-in sedimentary cover. Increased dislocation is manifested as systems of echelon dislocations with a break in continuity in Jurassic and Middle Miocenic complexes;
Layer fluids softening clayey rocks of which mud cone breccia is formed.

[34] The latter cause sources of considerable size with anomalously high pore (rock) pressure (AHPP) in deformed clay beds. There are a lot of hypothesis accounting for the formation of this kind of areas. Without going into consideration of them we can only note that in all cases stress concentration in AHPP sources is doubtless and stress tensor has a distinct deviation constituent in the lateral plane. Taking into consideration that the zones of AHPP development generally coincide with diapir formation areas it is reasonable to associate the formation of such zones with the development of clayey diapirism processes compensating for stress tensor ellipsoidal character. This indicates the formation of high potential energy in the interior, which is capable of causing mud volcano activity under certain conditions. In this case AHPP values (P_p/P_hydr ) as a rule reach 1.8-2.0.

[35] Lateral stress development in clays results in the fact that fluids contained in them are under geostatic pressure (or higher pressure) and flow to areas of lower pressure. Approximately since the Meotian, such areas unloading plastic clayey bed from excess fluids have been diapir cores and mud volcanoes associated with them genetically. In doing so hydrocarbons breaking through mud volcanoes or open diapirs up to the surface with water or in free phase disperse in the atmosphere or in water. If diapir core is overlapped (sealed) and is in contact with reservoirs of good permeability with relatively low rock pressure it is reasonable to expect that water and hydrocarbon excess in clays will drain to the reservoirs through the core. In this case potential energy in the diapir core area discharges ("goes down") into permeable layers and mud volcano eruption is not formed. If permeable layers are local and are only found in the zone close to diapir or are strongly argillized, then in the geological time, the rock energy pulse discharge will result in the formation of anomalous high rock pressure in the layers, which will form prerequisite conditions of mud volcano eruptions.

[36] The role should be noted especially of mud volcano formations in the process of oil and gas deposits formation. Besides evident concepts of conducting role of mud volcano vent zones and the formation of perivolcanic hydrocarbon deposits through it, the generation function of diapir crumpling zones and especially of mud volcano foci seems to be significant. The more so, that in our case diapir forming complexes including the Maikopian in all the depressions described above and the Jurassic-Lower Cretaceous undoubtedly fall with generating complexes, being saturated with organic matter. In this context, the concept of mineragenetic paragenesis "hydrocarbon-cinnabar (mercury)" proposed in the aforesaid is of great interest. Considering the problem from this viewpoint allows us to assume a certain general mechanism of formation of useful mineral deposits seemingly differing in their origin.

[37] In this case, it seem evident that echelon movement of the sediment litification front and facial zones consolidation in linearly extended blocks makes natural physical and geochemical barriers on which combined solutions seep with the formation of stratiform and a peculiar kind of telethermal deposits and of ore show of mercury, copper, molybdenum, gold, platinum, and other elements as well as of hydrocarbon deposits screened tectonically or by injections. Immediately in the areas of mud volcano development and adjacent oil and gas deposits, iodine, bromine, mercury, vanadium and other chemical elements are revealed. Apparently in all these cases, "barrier effect" appears where geodynamic activity conditions the borders of clayey diapirism and mud volcanism manifestations; in the clayey mass, the matter becomes differentiated into more fluid and less fluid components; the formed areas and zones of concentrations of one or other component in their turn make filtering or absorbing barriers. Thus it appears that very important regularities of chemical elements redistribution and the formation of a peculiar kind of mineragenetic zonality may be associated with clayey diapirism and mud volcanism manifestations including their manifestations in the previous epochs. The conclusive solution of this problem will certainly be found in the future. At the same time setting up the problem is undoubtedly timely especially in the light of certain vagueness in concepts on conditions of oil and gas formation and in a broader sense of minerageny.