Porosity estimating of Parfenovo horizon of the Angara-Lensk step central part: complex interpretation of seismic, well-log, and petrophysics

Skuzovatov M. Y.

A.A. Trofimuk Institute of Petroleum Geology and Geophysics SB RAS, Novosibirsk, Russia

SkuzovatovMJ@ipgg.nsc.ru

Introduction

At the present time big gas-production centre is being created at the southern part of Siberian platform. The reason for its developing is the discovery of Kovykta gas field in 1987.

Formation of such unique object is due to complex interaction of many geologic factors such as reservoir lithology, tectonics, high degree of katagenesis of main sourсe rocks and others.

The foundation of Kovykta gas field effected on activization of hydrocarbon exploration in the central part of Angara-Lensk step, that, in turn, resulted in new discoveries. The Levoberezhnoe and Angara-Lensk fields was discovered to the east of Kovykta field in 2004 and 2006 correspondingly. Later, in 2007 Chikan and Khanda fields was discovered. It is obviously that at the initial stages of gas exploration the main criteria of Kovykta field hydrocarbon prospects was used for prognosing reservoir properties of the other perspective territories. Results of such prognosation were often not satisfactory. For example, one of Pravoberezhnaya site wells that was defined on the base of seismic reflection data as situated in high-amplitude zone did not show any indication of hydrocarbon presence.

Multidisciplinary research created by specialists of A.A. Trofimuk Institute of Petroleum Geology and Geophysics SB RAS including seismic reflection, well-log, and core data interpretation allowed them to distinguish the differences between Angara-Lensk, Levoberezhnoe and Kovykta field geological structures. It was shown that the main perspective terrigenous horizons was formed in subsequently diverse facial conditions [1,2]. Angara-Lensk and Levoberezhnoe fields are characterized by more complicated petrophysical properties distribution than Kovykta field. This facts allow us to conclude that using prognosation criteria of Kovykta field for other perspective territories is not rightful enough.

To define prognosation criteria for Angara-Lensk and Levoberezhnoe fields we made this research on the base of the same approach that was earlier used for Kovykta field [3]. This approach includes seismic reflection, well-log and core (petrophysics) data analysis.

Geological setting

Parfenovo horizon is the main perspective terrigenous horizon at the studying area to which the giant gas pool of Kovykta field is related. The sandstones of Parfenovo horizon are widespread within central part of Angara-Lensk step but their lithological and petrophysical characteristics differ greatly either in lateral or vertical direction. The main perspective zone (Kovykta field) is interpreted to be formed in deltaic depositional system. Within Angara-Lensk and Levoberezhnoe fields Parfenovo reservoirs are interpreted as the deposits of large-scale alluvial plain where the highest porosity of sandstones is determined in bars and channels [1].

The Parfenovo sandstones of Kovykta field are divided into two units P1 and P2, which may be distinguished either by core or well-log analysis (generally by gamma-ray, neutron, and sonic logs). The total thickness of sandstones is up to 80 m. The lower sandstones (unit P2) are characterized by bigger wave-traveling time on sonic log (200-250 µsec/m) than higher sandstones of P1 (180-220 µsec/m) and the mudstone unit lying under sandstones of P2 is of the same wave-traveling time as higher sandstones of P1.

Within the territory of Angara-Lensk field the thickness of Parfenovo horizon sandstones is 50-60 m. It decreases to 15-20 m at Levoberezhnoe field. The sandstone top and bottom can be distinguished in the section by gamma-ray and neutron log values. The sonic characteristics of Parfenovo sandstones, and also of overlying and underlying mudstones are of slite difference or almost equal.

Seismic reflection synthetics and petrophysical properties.

Effective mapping of Parfenovo horizon top through seismic reflection data interpretation is due to dramatic difference between seismic P-wave velocities in terrigenous Parfenovo horizon and overlying carbonate Katanga formation. At this geological boundary P-wave velocity may increase from 4600 to 6500-7000 m/sec. The entire upper part of Vendian terrigenous complex is reflected on seismic time sections as a single composite wave (wavetrain M), mainly contributed by reflections generated on the Parfenovo horizon top and bottom as it is observed at Kovykta field. Earlier it was shown that at this area the averaged porosity values of Parfenovo horizon sandstones (either only P2 or both P1 and P2 units) have strong linear relationship with averaged wave-traveling time [3].

Within the area of Angara-Lensk and Levoberezhnoe fields the velocity differences between sandstones and underlying mudstones almost disappear. As a result, the main contribution in summary amplitudes of wavetrain M is made by the reflector at the top of Parfenovo sandstones. It was also concluded that some increase of amplitudes and time thicknesses of wavetrain M in Angara-Lensk field wells is due to thin layers (7-10 m) of sandstone characterized by relatively lower P-wave velocities than overlying and underlying deposits.

Fig. Crossplots of averaged porosity coefficient vs. averaged P-wave traveling time from wells of Angara-Lensk field (A) and from all wells of the studying area (B).

During this study the petrophysical parameter database was created using core data of A.A. Trofimuk Institute of Petroleum Geology and Geophysics SB RAS core collection. This database was used to make thick-layered and thin-layered models accordingly to the velocity models of wells. Figure displays crossplots of averaged porosity coefficient vs. averaged P-wave traveling time from wells of Angara-Lensk field (A) and from all wells of the studying area (B). Crossplotss analysis provided the conclusion that in Angara-Lensk field wells where lower-velocity layers are presented in the section there is stronger correlation of sandstone porosity values and its wave-traveling time.

Conclusion

Results of this study prove the conclusion that using prognosation criteria of Kovykta field for other perspective territories is not rightful enough. The strongest correlation of porosity and wave-traveling time is observed within the area nearest to the Kovykta field where the total thickness of Parfenovo horizon sandstones is up to 50-60 m. It is likely to be concerned with facies distribution. The thickness of sandstones of less than 30 m is a serious problem for 2D seismic survey that requires for futher investigations.

References

1. Khabarov Е.М., Varaksina I.V., Krotova М.М. et al. Lithologic-petrophysical characteristics of the Vendian Parfenovskiy horizon of the central part of Angara-Lena step // Collection of materials of V International scientific congress «GEO-Siberia-2009». – 2009. – P. 103-107. (in Russian)

2. Sovetov Y.K. The Upper Precembrian sandstones of south-western Siberian Platform. Novosibirsk: Nauka. - 1977. - 228 p. (in Russian)

3. Skuzovatov M.Y. Integrating of well-log, seismic and petrophysical data for prognosing reservoir properties of Parfenovo horizon of Kovykta field // Materials of all-Russian scientific conference with foreign scientists participation «Trofimuk readings of young scientists-2011». - 2011. – P. 285-286. (in Russian)