Abstract The mobilized sediments expelled by the mud volcanoes in Trinidad correspond to liquefied argillaceous and sandy material in which the solid fraction is systematically polygenic and originating from several formations (Cretaceous to Pliocene). The mud is notably rich in thin-grained quartz that is angular and frequently mechanically damaged related to shearing at great depth, during the sedimentary burial, and/or hydraulic fracturing processes. The exotic clasts are mostly fractured fragments from various formations of the tectonic wedge (mostly Palaeocene to Miocene). The origin of the solid particles of the mud is polygenic, including deep Cretaceous-Palaeogene horizons close to the décollement, and various materials from the stratigraphic pile pierced by the mud conduits. Moreover, the fluids expelled by the mud volcanoes have a deep origin and notably the gas phase is thermogenic methane generated probably below a depth of 5000 m. The effusions occur either during cycles of moderate effusion of mud and fluids (quiescence regime), or during catastrophic events responsible for the expulsion of huge volumes of mud, clasts and fluids (transient regime). Available subsurface data suggest that the deep structure of the mud volcanoes includes: (1) a focused deep conduit at depth in the zone of overpressure; (2) a mud chamber intruding the surrounding formations around and above the top of the abnormal pressure zone; and (3) a superficial outlet leading to the surface vents.

Abstract Deep-seated mud volcanoes are observed in a variety of geological settings, which has led to considerable debate on their origin. This paper summarizes the geological features common to mud volcanoes around the world and possible mechanisms of their extrusion. Field and laboratory data from Trinidad and Taiwan are discussed to assess the possible sources and causes of the volcanoes. A close association between mud volcanoes and compressional tectonics leads to the conclusion that tectonic activity plays an important part in mud volcano development. Experimental data are presented to explain the role of tectonic activity and the association between shear stresses and mud volcanoes. It is demonstrated that shear stresses applied to low permeability sediments can produce a dramatic increase in pore pressure and can cause sediment flow. This is proposed as one possible contributing cause of mud volcanoes.

Abstract Bubbling gases from the mud volcanoes of Trinidad and gases associated with oil in deeper reservoirs were sampled and analyzed to understand their possible relationships. Numerous geochemical analyses were performed on the gas samples. The chemical concentrations of organic compounds, CO 2 , and noble gases (from He to Xe) were measured, and isotopic signatures (δ 13 C from C 1 to C 5 ,CO 2 , and noble gases) were also determined. In the southern part of the island, our data show a typical thermogenic origin for the gases from oil reservoirs. However, the gases from the mud volcanoes, all located in the Southern Range of Trinidad, exhibit intermediate values between thermogenic and bacterial signatures. The hypothesis of simple mixing between these two end members can be discarded because of an incompatible δ 13 C for the bacterial end member (ranging from −52 to −33‰). We thus conclude that most hydrocarbon gases found in the mud volcanoes are purely thermogenic gases. The isotopic δ 13 C of CO 2 versus CO 2 /C 1 elemental ratios from the oil reservoir gases and the mud volcano gases follow a global trend. We therefore conclude that all gases come from the same source. This is corroborated by the isotopic composition of the hydrocarbons and of CO 2 and by the radiogenic fraction of the associated noble gases. However, mud volcano gases show an extreme dryness, which cannot be directly related to gas generation but instead to postgenetic processes. A process of migration involving local process of dissolution and diffusion in water and preferential adsorption of the larger organic molecules onto the solid mud particles should be involved. Finally, mud volcano gases have never been trapped but are permanently expelled to the atmosphere and thus have a lower time of residence than oil reservoir gases as indicated by noble gas data.

Abstract This paper illustrates the diversity of subsurface sediment mobilization features in tectonically mobile regions in a shale-rich environment. In the studied area of the southeastern Caribbean, new geophysical acquisition in Trinidad and Barbados spectacularly show the widespread development of mud volcanoes and massive sedimentary extrusions in the interference area between the southern part of the Barbados prism and the active turbidite system of the Orinoco. Mud volcanoes are well developed along ramp anticlines, or on top of sigmoid rises, which are oblique with respect to the trends of the main folds of the accretionary wedge. The area also exhibits trends of structures corresponding to massive extrusions of well preserved turbidite and hemipelagic sediments that cut the surrounding sediments. Some of these extrusion structures are complicated by the development of collapse structures, calderas, and superimposed mud volcanoes. On some active mud volcanoes, heat flow measurements show high positive anomalies and bottom simulating reflectors that are shallower compared to the surrounding areas and show high fluxes of fluid expulsion. The mobilized sediments expelled by the mud volcanoes are liquefied argillaceous and sandy material from deep horizons, and various shallower formations pierced by the mud conduits. Both in the Barbados prism and in Trinidad, the mud expelled is rich in thin, angular, and mechanically damaged quartz grains related to shearing and/or hydraulic fracturing processes. The exotic clasts and breccia result mostly from hydraulic fracturing. In Trinidad, the gas phase is mainly deep thermogenic methane associated with hydrocarbon generation at depth. This paper emphasizes that subsurface, clay-rich formation mobilization notably differs from salt mobilization by the role taken by the fluid dynamics that control overpressured shale mobilization, and induce sediment liquefaction. Mud volcanism corresponds to fluid displacement, whereas massive sedimentary extrusion corresponds to large movements of stratified solid levels for which the deep cause could be the intrusion of mud plugs. Both are dynamic phenomena controlled by the development of overpressure at depth, contributing to sediment mobilization by reducing the strength within the overpressured layer. The regime of the expulsion of the fluids varies according to cyclic phases. Low-frequency cycles, notably the catastrophic events, are controlled by the dynamic development of overpressure. They could be related to the fact that when high excess pore pressure occurs at depth, hydraulic fracturing is responsible for opening the fracture network favoring successive fluid release and cyclic pressure decrease. Such processes could be enhanced by a threshold effect when fluids are oversaturated in gas. In that case, massive degassing of a large volume of dissolved gas at depth is possible, resulting in a sudden rise in fluid pressure and damage to the sealing properties of the sediments above gas-charged mud chambers.