|
Riphean and Vendian-Paleosoic sediments of the northern part of the Tungussian syneclise, bedded under thick Permian-Triassic tuffaceous-effusive rock mass, have high oil and gas prospects based on the regional gelogical and geochemical data. However coverage of the promising horizons by deep wells is extremely low. So the drilling of the two key wells (4.0-4.5 km depth) and four structural wells (1.0-1.5 km depth) within the Ledianskaya and Chirindinskaya areas at the end of 80th-beginning of 90th was of great importance for the more precise definition to ideas of structure and oil and gas content of the geological section of the north-eastern Tungussian syneclise. As a result of drilling of above-mentioned wells essential differences of geological section structure of the areas remote from each other by 400 km were ascertained. Besides the complex analysis of lithological, stratigraphical, geochemical, petrophysical, hydrogeological and tectonic data the statistic working of lineament net and gas-hydrogeochemical sampling of the ground sediments of present water flows and reservoirs have been carried out. Both kinds of these independent investigations was carried out for the prediction of most probable localities of large hydrocarbon (HC) accumulations
Prediction of buried platform uplifts is based on analysis of a combination of independent statistical indices of lineament network defined from interpretation of aerial photographs at a scale of 1:40000 -1:80000. One of these indices shows mechanics of fault formation due to amalgamation of initial fractures. The other important statistical index of lineament network reflects anomalies of geologic layered medium affecting realization of tectonic stress fields under surface conditions.
Spatial coincidence of first-index minima, its relative maxima being around, with second-index maxima, its relative minima being around, indicates an uplift in this area. Absence of the latter suggests high probability for occurence of positive structure at depth. Presence of hydrocarbon accumulation at depth of the predicted uplift is estimated by character of changes of heavy hydrocarbons and helium contents in gases of the surface water within the uplift. This technique has been successfully tested on the Australian and Siberian platforms. Modernized variants of proposed technique can be used for the prediction of buried ore-bearing intrusions, diamond-bearing tubes, reef massifs and zones of the improved collectors of fracture and fracture-cavern types.
The comparison of the prediction results of buried uplifts and gas-hydrogeochemical sampling shows the most contrast anomalies revealed by these independent techniques practically coincide. The composition and distribution characteristics of hard hydrocarbons within the Verkhnehugdiakitsk uplift makes it possible to consider them as passing, diffusing from a large oil deposit, probably containing gas condensate. Obviously such a coincidence is not occasional and it rather testifies to the fact of large accumulation of carbons within Verkhneugdiakitsk buried uplift. |
|
To the North of the Lediansk area a powerful, rather stratigraphically complete geological section of Paleozoic-Mesozoic formations with a relatively small number of trap intrusions was overburdened; lithostratigraphical discrepancies were fixed on the contacts of Riphean andVendian, Ordovician, Silurian, Devonian and hard coal formations and apparently within Devonian.
Water beds (by the data of the geophysical study of wells) were obtained by well L-358 in limestone of Riphean within the range of 3801-3885 m (Kn - 3.2-14.4%), in cavernous dolomites of Lower Cambrian in the depth 2724-3267 m (Kn to 5%) and in cavernous onganogenic dolomites of the upper part of wenlock in the range of 850-894 m (Kn3.2-14.4%). The water with mineralization 34 (well L-358) and 21 g/l (well L-3) was obtained during water self-running in wenlock. The water from well L-3 is sulphate-chloride, hydrogen sulphate with a high (8.4-9.5 mg/l) concentration of naphthene acids. The study of the kern material showed that the pores, caverns, fractures and stylotitic joints of the dolomites, limestones and sandstones composing the geological section from media Ordovician to upper Devonian are saturated with naphthoids of the different transformation degree. This oil of light yellow color with strong smell was found in the sediments of the lower and upper Silurian in well L-2, dark-brown oil and black maltha, dabs of black bitumen scattered in the rock of media Ordovician and Lower Silurian were found in the kern of well L-2, L-3. In the examined part of the geological section there are three oil layers with the capacity up to 20m each, and water-oil contacts of the middle deposit is situated near the contact of the Lower and Upper Silurian between the wells, closer to well L-2.
In the examined territory grey limestones, conditionally related to Riphean clayed Vendian dolomites, dark-grey secondary dolomites of the Lower Cambrian, grey-colored argillites and carbonates of the tops of'media Ordovician, separate layers among limestones of Llandoverian, dolomites ofwenlock, limestone layers in the upper Silurian, dark grey lumpy clayed limestones of the upper Devonian, argillites and clayed alleurites of the media-upper carbon can be considered as mother-of-oil potentially. The rocks of the following stratigraphic classes are related to potential collectors. Quartz red-colored sandstones of the main Vendian represent the products of oversediments of the Prevendian weathering crust. In the Lediansk area of the structural cape about 5 m of these sandstones with poriness in ground edge more than 11 % were overburdened: the poriness collector corresponds to class III. Separate layers of fracture-poriness collectors were found in clayed carbonates of Vendian. Poriness-cavern potential collectors can be considered as secondary dolomites of the Lower Cambrian; by definitions in ground edges poriness of them is up to 12-15%, the definition of poriness by laboratory data is up to 3.4%, penetration is 0.1x10-15 m2 corresponds to classes III-V. According to the Geophysical Study of Wells data one can assume the presence of three levels of collectors with the total capacity about 140 m in the geological section. Separate lenses of penetrating rocks (IV-V classes) as organogetic limestone type dolomitized limestones among deposits of upper less than medium Cambrian are supposed to be there. |
|
Sufficient clayed carbonates of the Upper Cambrian, sulphate-bearing rocks of the Upper Silurian, sulphate-clayed and sulphate-saltbearing of the Media-Devonian, as well as tuff-laval rock mass formation of the Lower Trias armouring sedimentary syneclise implementation from the surface can be fluidstops.
Taking into consideration the availability of mother-of-oil, collector and shielding qualities in the Riphean-Paleozoic sediments of the Lediansk area fixed during drilling the direct signs of oil-bearing and the location of the Verkhnekhugdiakitsk uplift on the ways of the regional hydrocarbonate migration from the Kureisk syneclise to Anabarsk anticlise in front of the zone of the secant trap intrusion thickening the total volume of hydrocarbon deposits in Vendian terrigenous, Lower Cambrian carbonate, Media Ordovician terrigenous, Silurian and Devonian organogenic carbonate and the Late Paleozoic terrigenous horizon can be estimated at the first approximation. If the coefficient of oil extracting for terrigenous rocks is 0.2-0.3, and for carbonate is-0.3-0.4, than the total volume of the oil mining could exceed 16 billions m3 at average amount of poriness and the capacity of each horizon within Verkhnekhugdiakitsk uplift having the area of 2500 m2. However, taking into consideration that real deposits will not cover all this area being located in the traps anticlined and shielded by trap intrusions the summary area of which will occupy at least one of the fourth part of the Verkhnekhugdiakitsk uplift territory, the mentioned volume should be reduced to 4 billions m2. The accumulation in such a scale in the region with a wide complex of hard natural resources (diamonds, platinum, apatites, phlogopites, hard coal, salt, etc) near the sea port Khatanga the possible bridge-head for the exploitation of the Laptev sea natural resources deserve a special attention during exploitation of the Russian mineral-raw resources in the conditions of market economy despite the severe climatic conditions and the lack of ready infrastructure. |