L GEOLOGY & GEOLOGICAL EXPLORATION
UDK 553.98(439) © Group of authors, 2014
The Algy Turbidite Gas Play in the Mak Trough, Pannonian Basin, Hungary1
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Углеводородный потенциал турбидитных отложений в формации Альджио по блоку Мако Трог в венгерской части Паннонского бассейна
Г Бада, Э. Домбради, А. Хораньи, Г Молнар (ООО «Фалкон Ойл энд Газ Разведка»), О. Стано
(ООО «Фалкон Ойл энд Газ Разведка», Университет им. Лоранда Этвёша), М.Б. Шевелев
(ООО «Паннон Нафтагаз» (дочернее общество НИС - Газпромнефть в Венгрии))
Адреса для связи: shevelev.mb@nis.eu, gabor.bada@txm.hu
Ключевые слова: газонасыщенный песчаник, сейсмические атрибуты, турбидитные отложения, Миоцен, Паннонский бассейн.
Принимая во внимание геологические условия и площадное распределение открытых на сегодня месторождений в Венгрии и в регионе Паннонского бассейна, следует отметить, что большинство классических структурных ловушек углеводородов уже открыто и находится в разработке. Значительный потенциал Паннонского бассейна остается в стратиграфических ловушках, расположенных глубже 2000 м. Данные геологические объекты в настоящее время являются одной из основных целей геолого-разведочных работ в Венгрии и Сербии.
Одним из примеров таких объектов является блок Мако Трог в Венгрии, представляющий собой одну из самых глубоких депрессий в регионе с мощностью осадочных отложений до 6000 м и расположенный вблизи крупных открытых нефтяных и газовых месторождений.
Газовые залежи в формации Альджио (нижний Паннон) по блоку Мако Трог Паннонского бассейна, расположенного на юго-востоке Венгрии, представляют углеводородную систему Миоценового возраста, где в турбидитные песчаники газ мигрировал из темных глин, нижезалегающих неф-тегазоматеринских пород, и покрышкой служат вышележащие выдержанные толщи глин. Выявление стратиграфических ловушек среди различных типов турбидитов основано на материалах высокоразрешающей 3D сейсморазведки, отображении сейсмических атрибутов и AVO анализа. Присутствие серии газонасыщенных песчаников толщиной 10-50 м и турбидитных каналов подтверждено скважиной КЛуо!ду-1, пробуренной летом 2013 г.
Основные характеристики по блоку Мако Трог можно использовать для идентификации и выявления дополнительных нетрадиционных систем углеводородов по региону Пан-нонского бассейна в Центральной Европе.
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G. Bada, E. Dombradi, A. Horanyi, G. Molnar
(Falcon-TXM Oil and Gas Exploration Ltd., Hungary, Budapest), O. Sztano
(Falcon-TXM Oil and Gas Exploration Ltd,
Hungary, Budapest, Eötvös Loränd
University, Hungary, Budapest),
M. Shevelev
(Pannon Naftagas Kft.
(NIS - Gazpromneft Neft subsidiary
company in Hungary), Hungary, Budapest)
E-mail: shevelev.mb@nis.eu, gabor.bada@txm.hu
Key words: gas sand, seismic attributes, turbidite, Miocene, Pannonian basin.
Introduction
The Pannonian Basin is a Neogene sedimentary depression in Central Europe formed during the last ca. 20 million years. As such, it represents a juvenile yet productive petroleum system with several well established and some new, emerging play types. Hungary has seen thousands of wells drilled producing approximately 350 million tons of oil and 400 billion cubic meter of gas over the last 75 years. Most production is concentrated in and around deep sub-basins that contain mature source rocks. Most large fields are associated with structural highs or with the combination of structural-stratigraphic traps around the margins of the deeper sub-basins. The creaming curve shows a characteristic temporal pattern with the majority of fields discovered in the middle of the last century and only a few new fields identified in recent years. Successful exploration efforts have been made to target upper Miocene deep-water turbiditic sands where gas accumulations occur in structurally controlled stratigraphic traps (e.g., [4, 6]). The global economic value of similar turbiditic gas plays is proven by worldwide examples of giant gas fields discovered mainly in offshore continental margin settings. With the advent of high-resolution 3D surveys and related technologies, the identification of subtle gas bearing stratigraphic traps has become feasible in offshore as well as onshore environments.
печатается в авторской редакции.
The study area of this paper, the Mako Trough in SE Hungary, represents one of the deepest depocentres in the Pan-nonian Basin system and is located in the most prolific part of the basin in terms of cumulative oil and gas production, adjacent to Hungary's largest oil and gas field, the Algyo' field. Modern 3D seismics allowed the identification of a number of leads and prospects in the turbiditic sequences within the upper Miocene Algyo Formation. These prospects have now become the targets of an exploratory drilling programme under the partnership between Hungary-based TXM Oil and Gas Ltd. (wholly-owned subsidiary of Falcon Oil and Gas Ltd.) and Serbia-based NIS (56% owned subsidiary of Gazpromneft).
The objectives of this paper are 1) to present the essential G&G elements of the Algyo Play concept; 2) to introduce the exploration methods applied for the identification of drill-able prospects and 3) after successfully drilling the first well, to provide up-to-date technical information. Accordingly, the paper first presents a general geological set-up and stratigraphic framework, then describes the procedure of prospect definition and mapping, and concludes with a summary of the G&G results of the first well.
Geological setting
The Mako Trough is situated within the Pannonian Basin comprising an up to 7,000 m thick Miocene to Quaternary sedimentary sequence. The back-arc type Pannonian Basin System, located in the Alpine-Carpathian orogenic belt, was formed due to considerable lithospheric stretching and subsequent basement subsidence [2]. Formation of the Mako Trough in Mid- to Late-Miocene times occurred as a consequence of the uplift and collapse of metamorphic core complexes that are now forming basement highs on the flanks of the trough hosting large conventional hydrocarbon accumulations [10]. During its slow-rate thermal subsidence, the basin became covered by the brackish Lake Pannon during Early Pannonian times [5]. This long-lived, large lacustrine system hosted an endemic biota and a variety of deposition-al environments, from shallow to deep water. Several hundred meters of water depth was maintained from between 125 Ma followed by high clastic sediment flux from the uplifting Alps and Carpathians surrounding the lake, resulted in long-term normal regression and gradual filling up of the basin system, the overlay shows large fluvial feeding systems and extent of Lake Pannon at selected time slices [7]. During Pliocene to Quaternary vast amount of alluvium accumulated in areas of ongoing subsidence, while other parts of the basin have been inverted [3].
The sedimentary succession in the Mako Trough is made up of five major units. First under-filled, deep basin conditions far away from significant clastic input prevailed. Regardless of water depth, a variety of plankton prospered in clear waters of the photic zone. This was a main factor in the accumulation of the Endro'd Formation up to 1,500 m thickness with high organic content in the lower calcareous part (source rocks). Mass gravity flows, initiated from local basement highs, regularly interrupted the deposition of marls. Later on, major
rivers entered the lake and through their delta systems a wide morphological shelf was gradually building up. This shelf was bordered by a slope, continuously advancing from north to south, transmitting sand into the depressions. The deepest sub-basins of the lake became the loci of accumulation of complex turbidite systems (Szolnok Formation) locally up to 1,000 m sand thickness with thick, stacked, sand-prone lobes in confined settings. As the deepest basin interiors gradually filled up and, hence, water depth decreased, the style of turbidite deposition changed to unconfined in the vicinity of the prograding slope. These turbidite bodies alternate with shales and attain a thickness of 500 m as lower part of the Algyo Formation. The slope-related Algyo turbidite system (clinoforms) comprises simple lobes, channel-levees and thick, extended slope-detached lobes controlled by the variations of lacustrine base-level and sediment input on a time-scale of 100,000 years [9]. These turbidites differ from those of the underlying Szolnok Formation as the Algyo sand bodies are more limited in size and are encased and sealed by slope shales. In addition, porosities and permeabilities are much higher in the Algyo turbidites suggesting considerably better reservoir parameters with respect to the Szolnok sands. The turbidite sand bodies are overlain by a thick sequence of slope shales in the upper part of the Algyo Formation followed by the deposition of stacked deltaic sand-lobes (Ujfalu Formation) and then by vast alluvial deposits (Zagy-va Formation; [1]).
Intense subsidence, high rate of bioproduction, lack of significant terrigeneous influx and intermittent dysoxic bottom conditions in the deep Lake Pannon contributed to the formation of dark shales representing fair to good source rocks within the Endro'd Fm. (Fig. 1). These shales of increased total organic content (TOC up to 2-3%) are making up a self-charging system where the source, seal and reservoir are forming pervasive hydrocarbon cells in the deepest parts of the basin. Accordingly, the shale gas and shale oil potential of the Mako Trough is significant concerning the bulk volume of high TOC shales. Basin modelling suggests that hydrocarbon generation has started 5-10 Ma ago and, as a result of related build-up of overpressure, source rocks in the deep basin centre are currently expelling hydrocarbons charging the overlying turbiditic sand sequences (tight gas and oil). Thus, the Mako Trough represents an active petroleum system with thick source rocks in the Endrod marls generating
Fig. 1.
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