LACUSTRINE SANDSTONE RESERVOIRS OF THE LAKE PANNON AND ALLUVIAL SEQUENCES, CENTRAL PARATETHYS - CASE STUDIES AT ALGYÖ AND HOSSZÚPÁLYI SOUTH

I. Magyar, A. Fogarisi, V. Soreg, L. Buko, V. Lemberkovics, K. Kiss, G. Vakarcs, M. Vincze and P. Zahucski
MOL Hungarian Oil and Gas Co.

Lake Pannon deposits filled up significant volumes in the Pannonian Basin that extends to northern Croatia and Serbia, eastern Romania, southwestern Slovakia and to substantial areas of Hungary. The maximum sediment thickness reaches more than four kms in the deepest subbasins. An additional 2-3 kms of subsequent alluvial sequence accumulated in the depocenters.

More than 31 million tons of oil and 70 billion m³ of gas have been produced from the lacustrine sandstones of Algyö, the largest petroleum field in Hungary, since its discovery in 1965. Remarkable reserves were ascertained in a series of other major petroleum fields in the same geological succession: Budafa (located by the first successful well in Hungary in 1936), Lovászi, Hajdúszoboszló, Pusztaföldvár, Ferencszállás, Kiszombor and Cherestur, Pordeanu, Mokrin, Kikinda etc. Despite the mature exploration activity new finds has been made in recent years (e. g. Hosszúpályi South). Any improvement in the understanding of the reservoir habitat has considerable economic significance.

The huge sedimentary input from the surrounding mountain ranges (from NW and NE) through alluvial systems accounts for the rapid delta progradation across the highly subsiding subbasins. Thick meandering river type deposits and lacustrine clastics filled the accommodation space generated on the delta and alluvial plain, while turbidite sandstones and silty marls were generated in the semi-starving distal lake.

Lobe switching is not characteristic or hardly recognizable even on 3D seismics, balanced progradation along the coastline reflects noticeable linear clastic input. Depositional packages bounded by "sequence boundaries" and generated by probable relative lake level changes were delineated. Debates about the comparative importance of global sea-level changes, climatically induced lake level changes and tectonism arose, but any model of the sedimentary pattern should explain the apparent absence of seismic-scaled sequence backsteps.

Sandy layers of all the depositional environments from sheet turbidite sandstones and sublake fan distributary channels to point bars and crevasse splays may contain petroleum. The reservoir performance, compartmentalization and boundaries are based on the internal characteristics defined by the depositional setting: levees along channel sandstones, crevasse splays and point bars in floodplain fines, etc. Stratigraphic traps are relatively rare, however. Most pools can be classified into compactional anticline or structural inversion, tectonic or mixed tectonic-stratigraphic trap types. Hydrodynamics and the permeability pattern (>100 mD for reservoirs and <0,1 mD for seals) played an important role in the hydrocarbon accumulation and preservation potential of the prospects.

At Algyö, the delta system gradually covered the basinal elevation. Early laminated, silty turbiditic sheet sandstones pinch out at the thin marlstone caprock over the highs. Full sandy cover appears at the level of a dominant turbiditic layer, pool AP13, the internal sedimentary structure of which probably resulted from sheet amalgamation during a long-lived system. Distinctive fan distributary channels were shown on 3D seismics near this level, at the adjacent subbasin margins. Delta slope turbidites interfinger with mouth bar sandstones. The main exploration targets were outlined in the delta plain sequence where sandy sediments of a number of meandering river depositional environments are stacked to 20-30 m thick beds dissected by some meters of fines.

An erosional valley generated on a local strike-slip push up was transgressed by Lake Pannon at Hosszúpályi South. The relative effects of the structural movements and the “absolute” lake level changes on the “low-stand” sedimentation may be debated, however, the typical succession of stacked braided river sandstones deposited on the delta plain and meandering type point bar-crevasse splay-floodplain system above were shown.