Endodrainage systems and oil and gas occurrence in the Sea of Okhotsk region

Endodrainage systems containing deep flowing fluids are the “creators” of synergetic oil and gas formation in sedimentary basins of the Sea of Okhotsk region; they are the ultra-deep fluid-fault structures believed to be of riftogenic lithosphere “opening” origin. Riftogenic generation of endodrainage systems owes its origin to the Mesozoic-Cenozoic plume activation of ultra-deep mantle-crust faults (deep-seated strike-slip faults). The latter maximally extend their active zone in the Cenozoic sedimentary sequence and, along with the main shear opening channels, create a network of pull-apart fluid conductors up to 20-70 km wide. The most intensive lithosphere opening is accompanied by high-energy degassing in subsurface. It is typical of the sustained (often rich in Mesozoic ophiolite associations) suture zones separating large geoblocks. The maximum hydrocarbon potential in the Sea of Okhotsk region at the present level of its drilling exploration maturity is now typical of the East Sakhalin super-zone, which corresponds to the same-name suture zone. The pull-apart kinematics of the roughly NS trending Hokkaido-Sakhalinsky deep-seated strike-slip fault (the axial element of the zone) defines the behaviour and volume of oil and gas saturation in the pull-apart periphery of the strike-slip system. The western (right-hand) periphery is a network of numerous (up to 60 of them) small hydrocarbon accumulations in inversion and block transpressional traps. The eastern (left-hand) flank of predominant evolution in the transtensional environment contains three super-giant, one large, and several medium-sized mainly gas accumulations in the segment of the zone with the largest opening. A number of oil and gas fields were discovered in the inversion uplifts of considerable size; although they are less faulted and folded compared to the structures in the right periphery of the Hokkaido-Sakhalinsky pull-apart system