TNGG: Structure and Stratification of Triassic-Jurassic Formations in the Northern Part of Western Siberia: Part 2
A.A. Nezhdanov, V.V. Ogibenin, M.V. Melnikova, A.S. Smirnov, LLC “TyumenNIIgiprogaz”
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The transition from the well data to the seismic data requires obtaining the information about the seismic wave’s distribution speed in the deep horizons of the mantle. Due to the anomalously high formation pressure that increases down the section of the sedimentary mantle and the pre-Jurassic foundation of the northern and arctic regions of Western Siberia, the interval velocities for the reflected seismic waves in the lower part of this section are anomalously low. Thus, based on the vertical seismic profiling (VSP) conducted for well SG-6, the lower-Jurassic part of the section (depth interval 3950-6010 m), the interval velocity (Vint.) is 3850 m, in terrigenous Triassic rock (6010-6560 m) – below 3300 m/s, in the Triassic basalts (int. 6560-7430 m) these are somewhat higher – 4462 m/s [7]. Based on the VSP data from the Enyakhinskaya ultradeep well SG-7, Vint. are similar. Based on the one-dimensional seismic geological modeling for the Tota-Yakhinskaya well 25, Vint. for the mid-Jurassic – Toarcian interval is below 3700 m/s. Considering this data, the “time thickness” of the Pliensbach-Hettangium near the Tota-Yakhinskaya well 25 is about 760 m/s (Figure 4).
We index the Triassic-Jurassic boundary as MH Iв, unlike L.S. Girshgorn et all [1], V.S. Bochkarev et all [2], who marked this boundary at least 500 m higher and confined MH Iа to that boundary. Unfortunately, the available time section for Tota-Yakhinskaya area where the thicknesses were calculated for well 25 show the reflection recording time of 4 s, whereas even on MH Iв in Jurassic bottom is fixed at about 4 s time (see figure 4). Therefore to illustrate the structure of deeper horizons, the composite time section is shown in Figure 4 along with the time section for the regional seismic profile 41 intersecting the Gydan peninsula (figure 5). Based on the estimated Triassic thickness near well 25 which exceeds 4 km and the seismic waves’ interval velocities in the Triassic interval for the ultradeep wells ranging from 4-4,5 km/s, it may be concluded that the “time thickness” of the Triassic may exceed 2 s. In figure 4, Triassic fills the entire volume between MH Iв and the acoustic foundation (MH A).
The correlation of the Triassic bottom was conducted using a system of regional CMP profiles and the data from the ultradeep areal works. This data substantiated the conclusion about the Triassic (Permian-Triassic) age of the formations that fill the deep troughs in the north-east of WS (figure 5). The Two second time interval starting at MH Iв also covers the entire sedimentary mantle through to MH A, which is shown in Figure 5 in south-west and north-east parts of the section along the regional profile (RP) 41. The troughs located closer towards the center of the section feature a drastic increase in the thickness of the sedimentary mantle, but it is also impossible to trace the MH A unambiguously in these troughs. Down the section dipping angles and rock dislocations increase in the troughs, and the disjunctive dislocations are manifested. At the same time, it may be concluded that the entire section of sedimentary mantle for the section in question below MH Iв pertains to the Triassic, the thickness of which drastically increases in the troughs. Why does this occur?
The West-Siberian sedimentary basin was formed as a result of the Triassic rifting processes that were widely distributed both in the Northern and Southern hemispheres. The resulting heating of the Earth’s mantle and the formation of the mantle plumes during the Triassic, the Earth’s crust cloved along rift cracks and basalt lava overflew the Earth’s surface. This emission of abyssal matter to the Earth’s surface resulted in cooling of the mantle and the increase in the crust’s mass served as an impact to intensive the sinking of crustal blocks related to these processes (basins of North, Norwegian, Kara, Barents seas, Persian and West-Siberian basins, North America, West Africa and others). The quick subsidence phase of the post-rifting sedimentary basins is a typical phase of their development [8]. Based on the historical analysis on the development for a large number of sedimentary basins, Y. V. Artyushkov [8] distinguished two types of subsidence – the first one being slow subsidence compensated with the precipitation and speeds of less than 100 m/mln years and second one with subsidence speeds of up to thousands of meters per million years. The first type of basins normally features an absence of large and gigantic hydrocarbon deposits and a second type which normally features high productivity (Persian gulf, North Sea, Western Siberia etc). In many cases, the quick subsidence phase is related to spreading of lithospheric plates and the formation of oceanic crusts. In such cases, the subsidence phase lasted about 80 million years, fading out with time. Apparently this is the occurrence observed in this territory: the fast subsidence of the basin in the late Permian-Triassic (about 80 million years) faded out gradually but lasted during the Mesozoic-Cenozoic under the weight of the accumulating precipitation and due to the periodic tectonic activation of the rifts during their lifetime (late Jurassic, Neocomian, Palaeogene etc). This downwarping of the rifts during the Permian and Triassic was compensated by the extrusion of the primarily basic magma and later with the terrigenous sedimentation.
One should probably agree with the opinion of S.V. Aplonov [3] about existence of the Ob paleo-ocean with oceanic crust in this territory. Moving south towards thicker continental crusts, narrow cracks-rifts could not cause such high rate of downwarping as we see in TGB (Taz-Gydan basin). This is also why the transregional tectonic structure of the Urengoy-Koltogor graben-rift does not appear as large in the seismic sections as do the TGB rifts filled with many kilometers of thick deposits (figure 6).
The Yenisei-Khatanga regional trough, which S.V. Aplonov regards as an apophyse of the Ob paleo-ocean [3], is studied with ultradeep wells to a greater extent than TGB.
YKRT in its foundation represents a wide (up to 300 km) north-east striking depression with warping amplitudes of up to 7 km. Maximum depth of the foundation in Yenisei-Khatanga regional trough’s axial region reaches 17 km, 5-6 km along the edges. The Triassic is uncapped with wells in the Malokhetskaya, Semenovskaya, Volochanskaya, Tundrovaya, Golchikhinskaya, Khabeyskaya, Balakhninskaya, Vladimirskaya and Logatskaya areas. Maximum thickness of the Triassic effusive and tuffaceous rock is uncapped in well 1 of the Golchikhinskaya area: 1576 m. The mid-upper Triassic deposits are represented by siltstones with interlayers of sandstones and argillites. The thickness of the terrigenous Triassic varies from 544 m (Balakhninskaya area) to 1380 m (Vladimirskaya area). Triassic deposits are absent in Sukhodudinskaya, Dolganskaya, Nizhnekhetskaya and the Tochinskaya areas, where the sedimentary Paleozoic deposits (P, D, S, O) are uncapped with wells. The morphology of the troughs (their amplitudes increasing down the section) suggests an increasing speed of downwarping and filling the troughs with precipitation during the Triassic. Triassic time thickness in the troughs exceeds 3 s (figure 7) on the seismic sections, although the lower boundary of Triassic may lie in rifting troughs arbitrarily deep, including as deep down as the Moho surface.
One of the peculiar attributes of YKRT is the high degree of rock dislocation along the trough edges not only for the Paleozoic, but also the Triassic. In our opinion, this is related to the fact that to the north of the area [3], the late Jurassic lithosphere plate collision zone is located and related to unfolding of the Canadian basin; the development of this zone lead to tectonic movement in YKRT (Cimmerian phase of folding in South-Taymyr zone), which is confirmed also by the scouring of the Jurassic in domes of anticlinal structures at the Malokhet arch. Tracing the Jurassic bottom and the Triassic deposits in YKRT, this also confirms the formation of this trough during the Permian-Triassic age. Moreover, it is possible that the Paleozoic deposits which are anticipated in the arches of many of the anticlinal structures on the basement highs of the YKRT and TGB may pertain to the Triassic time (West and East Messoyakh, Gydan and others).
Thus, the analysis of data for deep and ultradeep drilling and regional seismic profiling by the CMP method indicates the Triassic (Permian-Triassic) age of the deposition and the formation of the Taz-Gydan basin and the adjacent territories of Western Siberia and the YKRT. The commonality of the tectonic patterns and the development of these regions with the major part of Western Siberia (Triassic rifting) makes it possible to expect the discovery of typical, for this petroleum basin, HC deposits with unique HC reserves in traditional oil and gas bearing complexes. Moreover, the presence of the basin downwarping centered in this territory with any position on HC genesis increases the petroleum bearing prospects and allows an expectation for the discovery of new gigantic HC deposits in this area that would be similar to Urengoyskoye, Yamburgskoye and etc.
Literature
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