Anomalous strong reflections on high resolution seismic data from the Turkish shelf of the Eastern Black Sea: possible ındicators of shallow hydrogen sulphide-rich gas hydrate layers

Doğu Karadeniz Türkiye şelfi ve üst yamacından alınmış olan derinden çekilen 5 kHz mühendislik sismiği verilerinde, 250–700 m su derinliklerinde, deniz tabanından 25–60 m derinliklerde uzanan 3–5 m kalınlıklarda çok güçlü yansıtıcılar gözlenmiştir. Bu tür güçlü yansımalar, araştırmacılar tarafından genellikle yersel ve sığ gaz birikimleri olarak yorumlanmaktadır. Bununla birlikte, bu çalışmada bu yansımaların polaritelerinin pozitif olduğu gözlenmiş olup, bu durum, bu tür yansımaların olası bir gaz cephesinin üst sınırından alınamayacağı anlamına gelmektedir. Bu çalışmada, bu yansımaların hidrojen sülfürce zengin sığ gaz hidrat tabakalarını yansıtıp yansıtmadığı incelenmiştir. Çalışma alanının sahip olduğu termobarik koşullar altında (sığ su ve yüksek sıcaklıklarda) gaz hidrat oluşumu ve durağan kalması hidrat yapısının gaz bileşimine bağlıdır ve bileşiminde metan ile birlikte belirli miktarlarda hidrojen sülfür bulunması durumunda olasıdır. Çalışma alanının güçlü yansımaların alındığı kısmında, yüzey tortullarındaki toplam sülfür üretimi 5550 ppm’e kadar yükselmekte olup, hidrojen sülfürce zengin sığ gaz hidrat birikimlerinin oluşması için uygun görünmektedir. Akustik ve jeokimyasal verileri kullanarak, bölgede bu tür hidrojen sülfürce zengin sığ gaz hidrat birikimleri için kavramsal bir oluşum modeli geliştirilmiştir. Bu modele göre, bölgede var olması olası hidrojen sülfürce zengin gaz hidrat birikimleri, metan gazının hidrojen sülfür ile temas haline geldiği sülfat indirgeme zonu ve altta uzanan karbonat indirgeme zonu sınırında oluşmalıdır. Buna ek olarak, bu tabakaların mühendislik sismiği verilerinde çok güçlü yansımalar olarak görülmesi, hidrat zonunun ince gaz hidrat lens veya tabakalarından oluştuğunu, bunların kalınlığının deniz tabanına doğru azaldığını işaret etmektedir.

Doğu Karadeniz Türkiye şelfinde yüksek ayrımlı sismik kayıtlardaki anormal güçlü yansımalar: hidrojen sülfürce zengin sığ gaz hidrat tabakalarının olası belirteçleri

Some strong reflections about 3 to 5 m thick were observed at depths of 25–60 metres below the sea floor using deeptowed, 5 kHz subbottom profiler data in the Turkish shelf and upper slope of the Eastern Black Sea at water depths of 250 to 700 m. Strong reflections of this kind are generally attributed to shallow and localized gas accumulations. We, however, observed that the reflection polarity of these strong reflections was positive, suggesting that they do not correspond to reflections from the upper boundary of a possible gas front. In this study, we evaluate these reflections to determine if they represent hydrogen sulphide-rich shallow gas hydrate layers, which would be an unusual gas hydrate occurrence in a shallow marine environment. The existence of gas hydrate formations in thermobaric conditions, as in our study area (shallower water depths and relatively higher temperatures), depends completely on the gas composition in the hydrate structure; it is possible for gas hydrates to be stable only if they are formed by a certain amount of hydrogen sulfide together with methane. We closely examined the hydrogen sulphide potential of the area and found that the maximum total hydrogen sulfide concentration in the surficial sediments in the area was, at 5550 ppm, enough to produce hydrogen sulphide-rich gas hydrates. Using geoacoustic and geochemical data, we propose a conceptual model for the formation of shallow hydrogen sulfide-rich gas hydrates. According to this model, conjectural gas hydrate layers in the area should be formed along the boundary between a sulphate-reducing zone and an underlying carbonate reducing zone, where methane comes into contact with hydrogen sulphide. We also propose that the strongly reflective appearance of these layers on the subbottom profiler data indicates that the hydrate zone consists of a number of gas hydrate sheets with a decreasing thickness towards the seabed, interbedded with non-hydrate-bearing sediments.

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