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OIL AND GAS PERSPECTIVES OF THE TRIASSIC SEDIMENTS IN CENTRAL SOUTH MOESIAN PLATFORM MARGIN

Nikola Botoucharov

Abstract

The oil and gas perspectives of the Triassic rocks in the Central South Moesian Platform Margin are evaluated on the basis of a complex geological analysis. The geodynamic evolution of the region during Triassic was dominated by failed rifting phases, followed by Early Cimmerian orogeny. These events have predestined tectonic processes and sedimentation settings, leading to deposition of source rocks, reservoirs, permeable migration pathways and seals. The initial extensional stages and then compression at the end of the epoch have favored also formation of the structural, stratigraphic and mixed type traps. Nevertheless, the source rocks spreading in appropriate maturity stage and different scale hydrocarbon occurrences are of the primary significance for the successful exploration. Considerable part of the discovered oil and gas fields during the last decades are concentrated in the Central South Moesian Platform margin (Devetaki and Uglen) or in the close proximity (Dolni and Gorni Dubnik, Pisarovo). They are located mostly in Triassic (Anisian) limestones and dolomites. The presence also of some other small hydrocarbon accumulations and shows in different Triassic series determine their reservoir quality and relatively high level of the exploration perspective. The promising source rocks among Triassic sediments are established only in Mitrovo Fm (mainly Ladinian in age). The Formation contains shales, marls and silty-shaly carbonates with an organic matter content exceeding 0.80 \% TOC. The dominated kerogen types II/III and III determine mostly gas potential. The maturity parameters (\%Ro and Tmax) vary from mature to postmature stage, depending on the depth and location of the sediments. The Triassic reservoirs are diverse and heterogeneous. These permeable rocks are concentrated in the Lower and Upper Triassic coarse-grained clastics (Red colour sandstone unit and Moesian Group), Middle and Upper Triassic carbonates (Doiren Fm with Dolni-Dubnik member and Russinovdel Fm). They are most often porous and mixed types. Porous rocks are encountered in Lower and Upper Triassic; porousfractured in Lower Triassic; fractured, porous-fractured and fractured-porous-cavernous type in Middle and Upper Triassic. The open and effective porosity varies, but rarely exceeds 8 \% and hundreds mD permeability. Nevertheless, in porous, porous-fractured and cavernous carbonates are reported higher porosity values. The intensive diagenesis and erosional events improve the filtration parameters of the Triassic succession. When the reservoirs are close to the Triassic/Jurassic erosional surface the porosity and permeability increase. Seals comprise of shales, marls and clayey carbonates in Middle Triassic and dense carbonates, terigenous and shaley carbonates in Upper Triassic sediments. The positive oil and gas perspectives with relatively good reservoir intervals, effective seals in some locations, permeable migration pathways and diverse traps outline an optimistic scenario for future exploration in the Central South Moesian Platform Margin.

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