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

AUTHOR/S: N. BOTOUCHAROV
Sunday 1 August 2010 by Libadmin2008

8th International Scientific Conference - SGEM2008, www.sgem.org, SGEM2008 Conference Proceedings/ ISBN: 954-918181-2, June 16-20, 2008, Vol. 1, 581-588 pp

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.

Keywords: Central South Moesian Platform Margin, Triassic sediments, source rocks, permeable migration pathways, reservoirs, seals, traps.