Paleogene Canyons of Tethyan Margin and Their Hydrocarbon Potential, Czechoslovakia

PICHA, FRANK J., Chevron Overseas Petroleum Inc., San Ramon, CA

Two Paleogene canyons buried below the Neogene foredeep and the Carpathian thrust belt in Southern Moravia have been outlined by drilling and seismic profiling. The features, as much as 12 km wide and over 1000 m deep, have been traced for 40 km. They are cut into Mesozoic and Paleozoic carbonate and clastic deposits and underlying Precambrian crystalline rocks. The sedimentary fill is made of late Eocene and early Oligocene marine deposits, predominantly silty mudstones and siltstones. Sandstones and conglomerates are distributed mainly in the lower axial part of the valleys. Proximal and distal turbidites, grain-flow and debris-flow deposits have been identified in the fill. The common occurrence of slump folds, pebbly mudstones, and chaotic slump deposits indicate that mass movemen played a significant role in sediment transport inside the canyons.

The canyons are interpreted as being cut by rivers, then submerged and further developed by submarine processes. Depositional filling coincided with a major marine transgression over submerged margins of the European platform during late Eocene to early Oligocene time.

The canyon fill bears some similarities to the time equivalent deposits of the Alpine Molasse and the Carpathian Menilitic Formation. All these sequences represent marginal marine facies of the Tethyan seaway which were supplied from the European foreland rather than from the advancing Alpine-Carpathian orogenic belt.

The organic rich mudstones of the canyon fill are significant source rocks (1-10% TOC). They reached the generative stage only after being tectonically buried below the Carpathian thrust belt in middle Miocene time. Channelized sandstones and proximal turbidites provide reservoirs of limited extent, although more substantial accumulations of sands are possible further downslope at the mouth of these canyons. Additional attractive reservoirs could be found in adjacent Devonian and Jurassic reefal carbonates and weathered granites. So far, several

oil fields have been discovered both within the canyon fill and the surrounding rocks.

Similar Paleogene valleys may be present elsewhere along the ancient Tethyan margins buried below the Neogene foredeeps and frontal zones of the Alps and Carpathians. Their recognition could prove fruitful in the search for hydrocarbons.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)