Organic Matter of Maykopian Series — Possible Source of Gases in Mud Volcanoes of the Sorokin Trough

Natalia P. Fadeeva, Elena V. Kozlova, and Elena N. Poludetkina
Geological Department, Lomonosov Moscow State University, Moscow, Russian Federation

Black Sea with Sorokin Trough as its part is perspective region for oil and gas. Multiple oil shows, outlets of HC gases and gas hydrates are revealed at the sea bottom. Sorokin Trough is situated in the southern continental slope of Mountain Crimea Megaanticlinorium submerging under waters of the Black Sea. The trough is filled by Mezozoic-Cenozoic complex up to 8 km thick (Tugolesov D.A. et.al., 1985). Rocks of Maikopian Series (MS, Oligocene-Lower Miocene) more then 5 km thick are dominating complex, its foot is submerged onto the depth of >9 km. Plastic MS shales are crumpled in diapiric folds complicated by collapsed, dome-like structures of mud volcanoes and venting fissures (Ivanov M.K. et.al., 2000) with connected outlets of HC gases and gas hydrates. OM of mud volcanic breccia by palinological data (spore-pollen complex, dinoflagellates) is dated as Oligocene - possibly Miocene (Fadeeva N.P. et.al., 2001). Judging to elemental, petrographic, biomarker and Rock Eval data Oligocene OM is characterized by marine sapropelic kerogen (type II) with high petroleum generation potential (TOC=1,2-2,5%, S1+S2=2-10 kg HC/g rock, HI=265-337 mg HC/g TOC). By Rock Eval data rocks are situated in the beginning of the “oil window” (Tmax=428-434oC) with methane homologues prevailing in generated gas fluids. Younger MS deposits (Upper Oligocene-Lower Miocene?) are related to kerogen type III partially bacterially reworked (max of n-alkanes C21-C22) with high gas-generating potential (TOC=6,5%, S1+S2=13 kg HC/t rock, HI=192 mg HC/g TOC) not realized (Tmax=420-426oC). Composition and maturation of OM are reflected in composition of HC gases of mud volcanoes and gas hydrates. Judging to data of Stadnitskaia, etal. (2007) HC gases are of mixed origin. δ13C of C2-C5 is -40÷-30‰, as isotopic composition of methane points on its secondary microbial reworking (δ13C= -70÷-55‰).

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