SUBMARINE OUTGASSING REVEALS EXPLORATION POTENTIAL VIA REMOTE SENSING DATA: CASE STUDY FOR THE BLACK SEA

A.A. Kitchka

CASRE IGS, Nat’l Ac.Sci. Ukraine, 55-B Gonchar St., 01601 Kiev, Ukraine; kitchka@casre.kiev.ua

 

During last decade application of remotely sensed data acquired by synthetic aperture radar sensors installed onboard of ENVISAT, ERS, JERS, ALMAZ, and other satellites stimulated successful exploration testifying rather good confidence level of the technique in various shallow and deepwater petroleum-prone basins worldwide. This research is a continuation of the program developed at CASRE to apply space-born data for oil and gas prospecting in the Black Sea basin. The program includes processing and thematic interpretation of space-born imagery coupled with analysis of available geological, geophysical, hydrophysical, environmental and meteo information. The repetitive oil slicks in two areas, west of Tarkhankut Peninsula and south of Cape Opuk of Crimea (known for numerous submarine gas seeps and pockmarks, allowed delineation of hydrocarbon emission zones and selection of first-order prospects to increase success ratio in this highly promising but still immature hydrocarbon-prone basin. Implementation of this study (Kitchka and Kostyuchenko, 2004) has contributed to discovery of Subbotin oil and gas field in the South Kerch offshore that proved commercial productivity for the whole East Black Sea sub-basin.

The applied technique is based on rather simple and clear theory the prognostic power of which is proved by experiments, numerical modeling and exploration practice. It is based on immanent attribute of oily material to attenuate higher harmonics of sea waves (so-called capillary ones) due to surface tension forces of the film it produces (Marangoni damping effect) at water-air interface. That is why a microwave radar signal (of few cm wavelength, 5.67 for ERS-1,2) beamed from the orbit onto a smoothed sea surface backscatters to the sensor with a low impedance (visible dark areas) that drastically, up to 20dB, differs from surrounding wavy medium (visible light-gray background) if wind velocity ranging from 3 to 12 m/s. That is why in case if wind fronts cross a radar scene oil slicks usually can be detected on their windward sides.

It is worthy to emphasize that oil slicks are not ordinary anomalies of a radar image; first of all the slicks are objects because they can reflect just that matter what explorationists are searching for. The principle of stationarity of search features through criterion of repetition / spatial compactness of slick populations allow delineation of hydrocarbon seeps zones and improvement of prospects ranking and assessment owing to accumulation of useful signal. The archive series of ERS SAR quick-looks and some georeferenced images covering the areas of interest was visualized by using ENVI and BEAM software and analyzed with ERDAS Imagine package. Further analysis was to select temporarily repetitive slicks indicating perspective zones of higher confidence. It was also found that vast majority of large slicks is spatially coincides with the toe of the continental slope where numerous and intensive gas seeps (or submarine geysers) have been detected by sonar surveying.

As to the NW shelf the data looks much complicated due to significant pollution coming from Danube and Dniper rivers, accidental spills along main tanker routes and ship lanes to Odessa, Illichivsk and Nikolaev ports, leaks from exploration platforms and intensive algal bloom during summer months. Several slick groups of higher population density were recorded nearby Zmeiny Island on the western part of studied area, however they were deselected from the consideration for the moment due to an ambiguity caused severe pollution of the sea with oil products.

Figure 1. An example of repetitive oil slicks registered over Pribiyna prospect west of Tarkhankut Peninsula (circled), subsets of ERS-1 SAR images, ESA Catalogue.

Nevertheless, it was possible to discriminate natural oil manifestations from spills and confidently delineate several emission zones and one of them to mention is Pribiyna prospect located west of Tarkhankut Peninsula, nearby of Krymske gas field (Fig. 1). An important peculiarity of the area is a dense network of latitudinal normal and reverse faults of the Gubkin – Tarkhankut buried deformation zone subjecting the acoustic basement and the whole sedimentary cover up to the base of Pliocene. The section is characterized by presence of several lines of undulated and upright anticlines with elevated Cretaceous strata.

Figure 2. Natural oil slicks detected west of Tarkhankut Peninsula according to interpretation of ERS SAR data for 1992-2004. Faults are shown in black, anticlinal crests in yellow, oil slicks are indicated as red objects, and probable emission zone is circled. Land is featured by JERS OPS image.

The traps in Cretaceous and Paleocene reservoirs are main targets of exploration in the area under consideration. The water depth is around 50 m here, thus it is quite possible for JSC Chornomornaftogaz to use jack-ups to drill these prospects. The radar images of this area have demonstrated a compact group of slicks (5 repetitions, Fig. 2) shown on ERS scenes over structures Pribiyna, Albatros, and Martivska. Besides, ASAR sensor of ENVISAT satellite and OPS (very near-infrared range) of JERS-1 one have also detected slicks related to this emission zone. These radar-derived prospects is in good correspondence with interpretation maps featuring negative thermal anomalies of sea surface caused by seepage-driven rising of cold bottom waters (detected by near-infrared sensors of NOAA and Landsat satellites) according to the technique developed at CASRE.

It in necessary to mention that Pribiyna prospect is located not far from and in the same structural zone as onshore West Oktyabrske oilfield, one of the few oil fields discovered in this part of Crimea to the date. Another exploration advantage of this area is very spectacular ring structure that has recently been delineated by potential fields surveying (Shnyukov Ye.F. et al., 2007). The Pribiyna, East Arkhangelsky, Albatros prospects roughly coincide with rim zone around this probable impact paleo-crater.

As to the restrictions affecting effectiveness of the technique it is necessary to mention that depending on the season local sea currents vary from ~ 5 to few tens of cm/s that could significantly offset slicks from original place. Other problems are related to the lack of direct slick observations at sea corresponded to the available images and few available compositional analyses of submarine seeps in the Black Sea, and rather high cost of georeferenced radar images of full resolution. The study has demonstrated that radar imagery usually bear lots of useful information. For example, it can be clearly distinguished some features of seawater dynamics like currents, eddies, internal waves, as well as production platforms or ships, and even jet contrails could be recognized on radar scenes under some specific conditions. There are indications that some phenomena like earthquakes, tides (of solid Earth, because usual ones are insignificant in the Black Sea) and strong baric fronts affect submarine seepage activity and produce higher slick population density, however, these assumptions need to be proved with proper statistic retrieval of the data.

Finally, it is worth to mention that remote sensing data has greatly improved the global assessment natural oil discharge into oceans. Although only a few new seeps were identified and estimates of known crude-oil deposits throughout the world have not changed greatly, new technologies, particularly remote-sensing techniques, have improved seep detection and assessment. The 'best estimate' of the global crude-oil seepage rate was revised to 600000 mt/a, with a range of 200000 and 2000000 mt/a (Kvenvolden and Cooper, 2003). In conclusion: Simple calculations based upon conservative estimates of the average present rates of hydrocarbon seepages from the sea bed establish that the world’s proven reserves of conventional oil should disappear in less than 1 Ma.  This fact fundamentally contradicts the conventional time period required by the bio-organic notion of the origin of petroleum.

References

·               Kitchka A.A. and Kostyuchenko Yu.V., 2004. Radar Imaging Data Applications to Hydrocarbon Prospecting in the South Kerch Offshore, Black Sea Basin. 66th EAGE Conference, Paper E031. Ext. Abs. CD-ROM, , 7-10 June 2004, Paris.

·               Шнюков Е.Ф., Коболев В.П., Богданов Ю.А., Захаров И.Г., Климчук А.Б. Западно-Тарханкутская кольцевая структура в Черном море // Геология и полезные ископаемые Мирового океана, 2007, №2. С. 137-139.

·               Kvenvolden, K.A., and Cooper, C.K., 2003, Estimates of the rates at which crude oil seeps naturally into the oceans: American Association of Petroleum Geologists, Annual Convention, Official Program, v. 12, p. A97.