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

Abstract: Paleogene Valleys of Northern Tethyan Margins and Their Hydrocarbon Potential

Two large Paleogene valleys buried below the Neogene foredeep and the Carpathian thrust belt have been discovered in southern Moravia, Czech Republic. The features, called Nesvacilka and Vranovice depressions or canyons, are as much as 12 km wide and more than 1500 m deep. By drilling and seismic surveys, these paleovalleys have been traced for more than 40 km. Comparable in size to the Grand Canyon of Arizona or the Hudson submarine canyon of the East Coast of North America, these features are cut into Mesozoic and Paleozoic carbonate and clastic sequences and into the Precambrian crystalline basement rocks, and are filled with Paleogene clastic deposits.

The tectonically controlled valleys are interpreted as being excavated by rivers during the Late Cretaceous to early Paleogene-Laramide uplift of the Carpathian foreland and then submerged and converted into submarine canyons and further developed by submarine processes.

The sedimentary fill of the canyons is made predominantly of organic-rich (1-9% TOC) mudstones and siltstones with subordinate channelized sands, proximal and distal turbidites, and debris flows. The common presence of slump folds, pebbly mudstones, and chaotic slump bodies indicates that the mass movement played a significant role in sediment transport inside the canyons.

The organic-rich canyon fill represents a significant type II-III source rock that reached the generative stage only after being buried below the Neogene foredeep and the Carpathian thrust belt in the early Miocene. The channelized sands and proximal turbidites form reservoirs that are well sealed within the muddy canyon fill. Additional reservoirs are found in porous rocks adjacent to the paleovalleys, including the Jurassic clastics and carbonates and the weathered and fractured Precambrian crystalline basement rocks of nearby topographic highs. So far, several oil and gas fields have been found within both the canyon fill and the surrounding rocks. An existence of large subsea fans may be anticipated farther downslope at the mouth of these valleys/submarine canyons. If accessible by drilling, these potential fans may become yet another target for exploration. The tectonically controlled valleys also may have played a role in migration of hydrocarbons from deeper zones of the Carpathian system into the more distal foreland.

The valley fill represents the most distal facies of the Paleogene foreland basin. It is partly coeval with the Fish shales of the Alpine Molasse and the Menilitic shales in the Carpathians, both of which are considered to be the main sources of hydrocarbons in their respective areas. Unlike the deep-water Menilitic shales, which were detached and incorporated into the Carpathian thrust belt, the marginal valley fill remained mostly in its original autochthonous position. Similar Paleogene valleys might be present elsewhere along the northern margins of Tethys. One such feature, named Tulln canyon, may exist in the eastern Alps, and also the Istria and Kamchia depressions in the western Black Sea shelf are at least partly erosional in origin. It is possible that similar autochthonous organic-rich marginal deposits may be preserved underneath the entire length of the Carpathian thrust belt. If so, these Paleogene rocks, after being buried below the thrust belt in Miocene times, may have generated significant quantities of hydrocarbons that would migrate into traps both in the Carpathian belt and the underlying European foreland. The assumed presence of autochthonous Paleogene deposits beneath the Carpathian thrust belt would thus greatly increase the prospectivity of the entire region. 


 

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AAPG Search and Discovery Article #90922©1998-1999 AAPG International Distinguished Lectures