PRE-NEOGENE RESERVOIRS, PLIOCENE STRAT TRAPS LIKELY TARGETS IN HUNGARY

Matthew R. Silverman, Eric W. Barton
Gustavson Associates Inc.
Boulder, Colo.

Hungary stretches from the Soviet Union in the east to Austria in the west (Fig. 1).

This range of neighbors reflects not just geography but the significance of Western European culture and trade, and Hungary's remarkable recent efforts to reform its politics and economy along increasingly Western lines. These reforms include broad changes in petroleum exploration and production.

The country has an area approximately that of the state of Indiana and consists primarily of a flat plain with low foothills of the Carpathian Mountains in the north.

Hungarian is the official language, but English is widely s ken and suitable for technical discussions. Hungary's population is 10.6 million, of whom 2.1 million live in the cosmopolitan capital city, Budapest.

OIL, GAS FEATURES

The Nation's principal geologic feature is the Pannonian basin, a large Neogene feature consisting of several small, deep subbasins separated by shallow basement blocks. Extensional and strike-slip deformation are characteristic. Rapid Miocene and Pliocene sedimentation deposited thick deltaic and lacustrine sandstone reservoirs.

Hydrocarbon source rocks and seals are thick Neogene shales that are relatively low in organic content. The geothermal gradient is unusually high, and most pre-Miocene strata are post-mature.

A recent deep drilling program with assistance from the U.S. Geological Survey had negative results, and the potential of significant deep hydrocarbon reserves is remote.

Oil and gas have been trapped in compactional anticlines associated with basement highs, primarily in Pannonian (Miocene-Pliocene) reservoirs. Significant production has also been obtained from Mesozoic carbonates and fractured metamorphic basement rocks.

Exploration for stratigraphic and other non-structural traps represents the best potential for future discoveries.

Hydrocarbon exploration and production has continued since the 1930s, with all activity since the mid-1950s coming under the control of the national oil company, Hungarian National Oil and Gas Trust (OKGT).

Cumulative production is approximately 486 million bbl of oil and 4.5 tcf of gas.

OKGT recently estimated undiscovered recoverable reserves at 274 million bbl of oil and 5.5 tcf of gas. The Pannonian basin is well explored for hydrocarbons, but state-of-the-art seismic techniques and the deliberate search of subtle stratigraphic traps may be expected to lead to substantial reserve additions.

EXPLORATION CONSIDERATIONS

On an international scale the geologic favorability of hydrocarbon exploration in Hungary is considered only fair, primarily because of the lack of any large unexplored structures, secondarily because of the high geothermal gradient.

In contrast, the investment favorability of Hungary is considered average to good because of the national desire for international oil company participation in exploration/production as seen against the current background of political normalization.

In spring 1991 the Hungarian legislature passed a concession law granting OKGT authority to license foreign exploration and production companies. The Ministry of Industry and Trade formally invited the international oil exploration industry to examine data and purchase data sales packages beginning May 1, 1991.

Purchase of one or more of the three basic data sales packages is a prerequisite for participation in the first concession round, scheduled to close in 1993.

The plays that will probably best satisfy the technical and financial criteria of the international petroleum exploration companies include exploration for pre-Neogene reservoirs (fractured "basement" rock, Paleozoic and Mesozoic carbonates) and Pliocene stratigraphic traps.

This effort will include reprocessing and reinterpretation of existing seismic data and acquisition of new data with these targets in mind. The plays will focus on the sparsely drilled deep basin and basin flank areas in the southern Great Hungarian plain and in the Transdanubian basin.

HISTORICAL SUMMARY

Oil seeps from fractured Mesozoic and Tertiary rocks in the mountainous northeastern part of Hungary have been known for many years.

As described by Sikabonyi1 oil and gas exploration efforts before World War I were made in areas beyond the borders of modern Hungary. Oil was first discovered in the old Hungarian territories north of Vienna, in what is now Czechoslovakia.

This led to later discoveries in the Vienna basin and in the present territory of Hungary. In the mid-1930s oil was discovered by gravimetric exploration in broad structures near the Yugoslavian border. Reservoir rocks were Pannonian (Upper Miocene) sandstones interbedded with marls.

Modern petroleum exploration began in 1933 when a subsidiary of Standard Oil Co. (New Jersey), and the European Gas & Electric Co. (Euro-gasco), obtained concessions from the Hungarian government.

Bukkszak oil field was discovered in 1936 and produced for a brief time from Mid-Oligocene andesitic tuffs in a faulted dome. The first commercially successful drilling brought in the small Budafapuszta field in southwestern Hungary, where oil and gas were found structurally trapped in Pliocene sandstones in 1937 .2

After World War II oil exploration rights in western Hungary that had been leased to Standard were nationalized. Exploration rights in the eastern half of the country were seized from the German concern Wintershall AG and granted to a Hungarian-Soviet company, Maszolaj.

OTHER HUNGARIAN FIELDS

Nagylengyel field was discovered in 1951 in the Zala highlands. It produces from Mesozoic carbonates and shallow Pliocene sandstones, and remains one of the country's most important fields. In the mid-1950s all oil and gas exploration and production came under the control of OKGT.

Continued exploration efforts revealed the presence of additional prospective areas, and it soon became apparent that Pannonian sandstones, shales, and marls had been deposited over a large portion of the country and were the logical exploration targets. Consequently, deeper pre-Pannonian strata have remained relatively unexplored, although present efforts are extending to unconventional reservoirs, such as fractured "basement" rocks.

In the last 40 years, OKGT has intensively explored the Pannonian basin, especially in southeastern and southwestern Hungary. In the northwestern and northeastern parts of the country, seismic exploration has been hindered by deposits of surface gravels and tuffs, respectively.

The most important recent discoveries have been made in the Szeged area (Fig. 1) in southeastern Hungary, including Algyo, Ulles, Szank, and Tazlar fields.

Several important gas fields have been found in the Great Hungarian plain, among them Hadjuszobszlo, Ebes, and Endrod. As of January 1988, OKGT had reserves of approximately 206 million bbl of oil and 4.2 tcf of gas.

GEOLOGIC SETTING

The key geologic feature in reviewing the petroleum potential of Hungary is, of course, the Pannonian basin.

This large basin extends over portions of Austria, Yugoslavia, Czechoslovakia, Romania, Poland, and the U.S.S.R., in addition to Hungary. It can be subdivided into numerous subbasins (Fig. 1); most are productive of hydrocarbons and share similar geologic histories as portions of the Greater Pannonian basin.

Considerable confusion has been generated by the use of the term "Pannonian" for both a late Miocene biostratigraphic stage and, in Hungary, for a stratigraphic sequence that includes strata of this stage through latest Pliocene.

In numerous publications it is difficult or impossible to determine which use of the term the author intended. The lower portion of the Pannonian stratigraphic sequence corresponds closely to the Pannonian biostratigraphic stage. The upper portion includes strata of youngest Miocene and Pliocene ages.

MULTIPLE BASINS

As described by Royden and Horvath 3 the Pannonian basin (Fig. 1) is actually a series of small, deep basins separated by relatively shallow basement blocks.

The Neogene-Quaternary section exceeds 7,000 m in places, and the entire basin is about 400 km from north to south and 800 km from east to west. Horvath 4 and many others consider it a Mediterranean back-arc extensional basin of mid-Miocene age.

The basin overlies most of the internal Mesozoic thrust sheets of the Carpathian and Dinarides Mountains, and Rumpler and Horvath5 have demonstrated some largescale overthrusts in the Pannonian basement as well.

However, Cenozoic tectonic events have strongly overprinted most of the Mesozoic structure of the Mesozoic, Paleozoic, and crystalline basement rocks. Cenozoic faulting may have been localized along major tectonic lines by older zones of weakness. Royden and Baldi6 note that Paleogene sediments occur only locally beneath the Neogene basin.

Neogene sedimentation and extension is of different ages in various portions of the basin system. Rapid sedimentation in the northernmost portion of the basin occurred primarily in the early to middle Miocene.

In middle Miocene time rapid subsidence began in Hungarian basins (the Danube, Zala, and Drava basins and the Derecke, Mako, and Bekes depressions) farther south. The Little Hungarian and Great Hungarian plains and the southeastern Drava area experienced maximum subsidence in the late Miocene.

Large syndepositional normal faults of Miocene age are associated with rapid Miocene basin subsidence in most Pannonian sub-basins. However, in the Little Hungarian and Great Hungarian plains few large-displacement normal faults are present in the thick Miocene-Quaternary sedimentary sequence. 5

Instead, mid-Miocene subsidence in these areas produced deep basins that were rapidly filled by large prograding delta systems .7 8 The widespread, complex system of faults is probably related to late Miocene strike-slip and extensional deformations 9 10 11

Wrench-fault structures are prevalent, 12 and local compressional deformation is present along many strikeslip faults.

STRATIGRAPHY

Royden and Horvath 3 note that excellent subsurface data coverage exists for almost the entire Pannonian basin.

In Hungary more than 11 million m of wildcat and exploratory footage have been drilled and more than 30,000 line km of multifold common depth point seismic data have been acquired and interpreted. 13 The stratigraphic information provided by the wealth of data makes the Pannonian basin one of the best-known hydrocarbon provinces in the world.

Miocene and early Pliocene strata (Fig. 2) comprise the key section in terms of Hungary's petroleum geology. Berzci et al.14 have reviewed the Neogene sedimentation of the area in detail.

They reveal that the last major cycle of sedimentation in the Pannonian basin began in the early Miocene and has continued into the Holocene. In northern and southern Hungary the Miocene section is mainly continuous and more complete than in the nation's central plains and low mountains.

Miocene to Quaternary sediments locally attain total thickness of 7,000-8,000 m, of which pre-Pannonian Miocene strata comprise 1,0003,000 m, including thick volcaniclastics. These sediments were deposited in deeply subsiding subbasins separated by horsts of the Paleozoic-Mesozoic complex.

In northern Hungary, continuous Oligocene to Miocene sedimentation is present; whereas elsewhere the Precambrian-Mesozoic basement complex is overlain directly by Middle Miocene and younger sediments.

In the deep basin areas, sedimentation began with deposition of variegated terrigenous clastics, overlain by clastic and calcareous marine sediments.

In early Badenian (Middle Miocene) time and later, slight topographic relief in the form Of small islands created areas of non-deposition surrounded by areas receiving clastic sediments. The islands were all buried by transgressive sediments by early Pannonian (latest Miocene) time.

Basinal sedimentation was continuous from Badenian to Quaternary, but local gaps are present in the sedimentary record due to redeposition by Sarmatian (Middle to Upper Miocene) turbidity currents.

SOURCE ROCKS

In a study of the hydrocarbon-bearing subbasins of the Great Hungarian plain (southeastern Pannonian basin), Szalay15 found that two potential Neogene source rock zones occur.

These shales and shaly sandstones may be as thick as 1,000 m, but the average total organic carbon (TOC) content is fairly low at 0.51.5%.

The upper zone is located in the upper part of the lower Pannonian sequence and has just entered the oil generation window. Hydrocarbons from this zone migrated laterally and upwards, and were sealed by and trapped in Pannonian strata.

The lower zone is contained in Middle Miocene to lower Pannonian strata, and passed into the oil 'window about 8 million years ago, at which time it became significantly overpressured. It is already overmature in the deepest parts of the basin.

One of the critical exploration problems in Hungary is an unusually high geothermal gradient, thought to be to a thin crust. The average gradient is about 3.60 C./100 m, and in places this figure exceeds 5.40 C./100 m.

Downward and lateral migration probably occurred, sourcing relatively deep traps. Analysis of other subbasins suggests similar migration paths, although upward migration is also documented, of course.

Horvath et al .16 compared the known oil and gas fields with the thermal and maturation history of the Great Hungarian plain. They found that 49 of 53 fields are located at some distance from the areas of hydrocarbon generation. Promising future prospects include deeper sediments and the fractured basement, in which hydrocarbons may have accumulated in stratigraphic and unconformity traps.

Dank concluded that the main source rocks are Lower Miocene to lower Pannonian shales, which may reach thicknesses of a few thousand meters in the deepest troughs of the Pannonian basin.

Small but commercial volumes of oil in northeastern Hungary may have been generated by thick Paleogene sediments below thinner Neogene cover.

During abrupt Miocene subsidence, previously immature Mesozoic strata may have become important source rocks.

Hydrocarbons were sourced by Miocene shales in deep troughs, experienced significant secondary migration, and were trapped in fractured Mesozoic and Paleozoic basement rocks.

RESERVOIRS, SEALS, TRAPS

Reservoirs range in age from Paleozoic to Pliocene and are of several lithologies.

Neogene reservoirs are primarily basal conglomerates and fluvio-deltaic and lacustrine sandstones; whereas, Mesozoic limestones and dolomites are the principal pre-Pannonian reservoirs.

Approximately 62% of Hungary's oil production is from Miocene and Pliocene strata. More than 72% of the gas production is from reservoirs of these ages, as well.

Jurassic and Triassic rocks account for almost 24% of the oil production, and Paleozoic reservoirs contain about 20 percent of the nation's gas .3 Production has also been established from strata of Cretaceous to Oligocene age.

Most of Hungary's hydrocarbon traps are extensional folds and compactional anticlines associated with basement highs. Many researchers have concluded that the best of these features have long since been discovered.

Positive structures along strike-slip fault zones are also productive. Seals are primarily Pannonian marls and shales; fault and evaporite seals are rare or nonexistent.

Future prospects will include stratigraphic and unconformity traps in both Pannonian and pre-Pannonian strata and in fractured basement. The deeper parts of the basin and the flanks of the deep troughs are natural targets.

FIELD EXAMPLES

Dank 13 reviewed six key oil and gas fields in the Hungarian portion of the Pannonian basin. Three of these are described briefly below.

Algyo is by far the most important field in Hungary. Nagylengyel and Sarkadkersztur fields are examples of different reservoirs, trapping mechanisms, basinal settings, and portions of the country.

Algyo field was discovered in 1965 and is the largest oil and gas field in the country. Algyo apparently accounts for more than 50% of the total hydrocarbons currently produced in Hungary.

Ultimate recovery has been estimated" at 200 million bbl of oil and 3.69 tcf of gas. The field covers about 80 sq km and is located near the city of Szeged on the west flank of the Mako trough in southeastern Hungary.

Oil and gas are trapped at Algyo in a Pannonian compaction anticline above a large basement high (Fig. 3). Source rocks are probably Miocene shales that passed through the oil window in the adjacent troughs. Reservoir beds vary in thickness from 5-70 m.

Production occurs in the Badenian-Sarmatian conglomerate that overlies the Paleozoic basement. Other reservoirs include the upper part of the lower Pannonian, and the lower part of the upper Pannonian sections.

These units are slightly deformed sandstone bodies that are associated with prograding delta lobes and delta plains. The basement core of the anticline is also fractured locally and contains some oil.

OTHER LARGE FIELDS

Nagylengyel field 13 was discovered in 1951 and is the largest field producing from Mesozoic strata. It covers approximately 75 sq km and is located in a small deep depression adjacent to the larger Zala basin in southwestern Hungary. The field is now essentially depleted.

Most of the reservoirs at Nagylengyel are Upper Cretaceous limestones, and individual pay zones are 7-90 m thick. The rest of the reservoirs occur in Upper Triassic (Norian) dolomite and vary in thickness from 20-160 m.

Oil is also contained in a Lower Miocene (Karpatian) sandstone. The productive units are associated with tilted fault blocks separated by Lower Miocene normal faults. Mesozoic source rocks reached oil maturity during Neogene burial and heating, and are the probable hydrocarbon sources.

The promise of significant hydrocarbon production from fractured metamorphic basement rocks is illustrated by Sarkadkeresztur field, located in the Bekes depression on Hungary's eastern border with Romania. This oil and gas field was discovered in 1976 and accounts for about 5% of the country's total production.

Sarkadkeresztur is related to a very narrow horst that is unconformably overlain by thin Middle Miocene conglomerates and Pannonian clastics. About 99% of the hydrocarbons are trapped in the fractured Paleozoic basement in the overlying conglomerates. The thickness of the pay section exceeds 370 M.

The hydrocarbons were sourced by Miocene shales and migrated downward and laterally into this structure.

Upward hydrocarbon migration from deeper shales into fractured basement has been demonstrated in other Hungarian fields. This discovery established that the metamorphic basement is locally permeable and can be a good hydrocarbon reservoir.

EXPLORATION FAVORABILITY

The declining trend in discoveries reflects the fact that about 60% of the conservatively estimated oil and gas reserves in place have already been found.

Most or all of the large conventional structures have been discovered; therefore future discoveries of such structures will be of an increasingly small size. OKGT's recent exploration of deeper horizons has proven difficult and has not reversed the decline in reserve replacement.

The likelihood of future large structural discoveries in pre-Pannonian strata is not great.

However, it is likely that considerable reserves remain in stratigraphic and other subtle, nonstructural traps. Discovery of these reserves will require modern, integrated exploration methodologies.

Hungary's exploration efforts would benefit from a greater diversity of technical approaches than can be found in one organization, such as OKGT. The participation of foreign oil companies would serve to advance this diversity.

Continued successful exploration efforts will require more sophisticated exploration management, integrated exploration methods, and the increasing use of the most modern seismic techniques, such as those now employed by the best American and European operators.

SELECTED PLAYS

Three exploratory plays (Fig. 4) stand out as the prime investment opportunities for foreign oil companies.

Each of the plays is limited geographically and geologically, and all are described briefly below. The three plays represent distinct types of exploration opportunities.

Two areas offer significant opportunities in the deliberate exploration for traps in Mesozoic strata. The first of these is in the interfluve of the Danube River and Tizsa River, south of the town of Kecskemet. This area also offers opportunities in the exploration for subtle traps in Neogene strata as well.

The area of interest is located in southern Bacs-Kiskun County and western Csongrad County, between Budapest and the Yugoslavian border in south-central Hungary. This play offers the greatest opportunity for wildcat prospecting for Mesozoic reservoirs. Seismic and subsurface data are made available as part of Package B currently being offered by OKGT to the industry.

In addition to structural traps, several other plays are evident on Line As-27-8224 (Fig. 4). The top of crystalline basement, the deepest reflector, is overlain by the Mesozoic section that thins or pinches out near the crest of the anticline.

Mesozoic reservoirs may be juxtaposed by normal faults against the upthrown basement. Post-Miocene events, which lie above the third-deepest reflector, indicate shallower pinchouts.

Seismic data quality allows only structural interpretation of a few good reflectors; predictably, most exploration has been limited to these plays. The sections do not exhibit much character, probably due to poorly applied amplitude leveling and paucity of higher frequencies.

Stratigraphic interpretation will require reprocessing (or reshooting) to ensure that the field tapes contain the broadest possible frequency spectrum.

Additional stratigraphic plays are evident on lines (Fig. 5) that contain a broader frequency content. Miocene and Pliocene plays that appear to be prograding delta wedges and channels are among the features indicated. Anticlinal and fault-related plays may be present in the Mesozoic.

TRANSDANUBIAN BASIN

The second area prospective for hydrocarbons trapped in Mesozoic strata is in the Transdanubian basin of southwestern Hungary (Fig. 1). This play is centered around Nagylengyel field near the town of Zalaegerszeg in Somorgy, Zala, and Vas counties on the Yugoslavian and Austrian borders with Hungary.

Numerous small structures have been found in this area, suggesting the moderate risk and potential of this play. Seismic and subsurface data for this play are available as part of Package A.

Previous exploration in this area has been related to deeper drilling on and around structures producing from Neogene clastics; and exploration for Mesozoic structures, especially tilted fault blocks separated by Early Miocene normal faults. New exploration will focus on reprocessing and reinterpretation of existing seismic data.

Acquisition of new data shot and processed specifically to delineate Mesozoic stratigraphic and paleogeomorphic traps will represent the first substantial search for subtly trapped hydrocarbons in the area.

MAKO TROUGH-BEKES BASIN

The third exploratory play that represents a considerable economic opportunity for foreign oil and gas companies is the deliberate search for Pliocene stratigraphic traps in the Mako trough and Bekes basin in southeastern Hungary (Fig. 1).

Stratigraphic and unconformity traps in Pannonian sandstones in the deep basin and basin flank areas offer the best exploration opportunities.

This area is located east of the city of Szeged in eastern Csongrad County and Bekes County near the border with Romania.

It is adjacent to the first area of exploratory interest mentioned above, and the two areas could be evaluated in depth in a unified effort. Seismic and subsurface data for this play are also part of the Region B data package.

Unpublished seismic data (Fig. 6) in the Fabiansebestyen area of the northern Mako trough reveal a very large anomalous feature above basement. This body lacks coherent layering and may represent a stratigraphic play.

Higher in the section, analysis indicates stratigraphic pinchouts, channels and a prograding delta wedge. Further seismic and petrophysical evaluation is required, but a single well may be located to test multiple targets. There is currently no production in this portion of the Mako trough.

FIRST CONCESSION ROUND

The Ministry of Industry and Trade has recently announced that it is offering data packages for sale in anticipation of the first concession round for acreage in 1993.

The packages consist of seismic and well data, with geochemical data available at an additional cost. Purchase of one or more data packages is a prerequisite for participation in the first round. Inquiries should be addressed to: Dr. Janos Kokai, Ministry of Industry and Trade Representative, 1117 Budapest, Schonherz Z.u. 18, Hungary, Tel: 36 1 186 8754, Fax: 36 1 186 9110, Telex: 022-4762 OKGT.

The Hungarian parliament has passed a concession law that gives the government authority to assign exploration and production licenses to foreign companies. The parliament will also draft and debate legislation defining the procedure and terms for granting permits.

CONCLUSIONS

Hungary has a 60 year history of successful hydrocarbon exploration and production but faces a growing failure to replace its reserves.

Production is from a number of small, deep subbasins of the hugh Pannonian basin of Neogene age. Most reservoirs are Miocene and Pliocene sandstones in which oil and/or gas have been trapped in compactional anticlines.

Deliberate exploration for Pliocene non-structural traps represents the greatest potential for future discoveries. Significant opportunities are also present in the southern Great Hungarian plain in exploration for pre-Neogene (fractured "basement" rocks, Paleozoic and Mesozoic carbonates) reservoirs.

Most of Hungary's fields are small, and the declining rates of new discoveries are discouraging. However, Hungary's exploration efforts are likely to benefit from the diversity of approaches provided by Western technology and a competitive business arena.

Definitive legislative and executive reforms to encourage foreign participation in exploration and production are expected in 1991.

Although the area is well-explored for hydrocarbons, there is considerable geological impetus for increasing this effort.

A first concession round has been announced, and the climate for investment is progressive due to rapidlyevolving political and economic conditions. This presents a historic opportunity for international petroleum companies to be involved in the initial phase of exploration for stratigraphic and other subtle traps in a mature oil and gas province.

ACKNOWLEDGMENTS

This article is adapted from a nonexclusive report available from Gustavson Associates, Inc. The authors gratefully acknowledge the cooperation of Dr. Janos Kokai of OKGT and Dr. Gyorgy Szili of Hungary's Central Office of Geology.

REFERENCES

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