Abstract

There are many push-down seismic anomalies (gas chimneys or pipes with a diameter of 12–15 km) in natural gas fields in the northern part of the West Siberian basin (Urengoy, Yamburg, Zapolyarnoye, Kharasavey, Yurkharovskoye, etc.). This effect is common in gas-saturated sands. The amplitude of push-downs in West Siberian fields is approximately 300–400 ms from the horizontal surface. Using well data, we have measured how much the velocity of seismic waves decreases and study the hydrocarbon presence, fluid pressure, and other information to gain a better understanding of the cause of this effect. Several wells were drilled in three different gas chimneys. All of the wells in the gas chimneys discovered vertical columnar gas deposits in Jurassic and Cretaceous terrigenous formations. Well data from the Yurkharovskoye gas field show that seismic wave velocity in the gas chimney is 40% less than that outside this object. The wells in the gas chimneys show that formation pressure in the Neocomian and Jurassic formations is 80–90 MPa, which is approximately lithostatic pressure, so high-pressured gas in the pores begins to form cracks. There are gravity anomalies above many gas chimneys identified, indicating a density deficit. Transient electromagnetic sounding data from the Yurkharov gas chimney show a columnar zone of increased electrical resistance, as in gas-saturated formations. To explain all of these observations, we suppose that the West Siberian gas chimneys (pipes) are currently active channels for the vertical migration of high-pressured hydrocarbon gases (mainly methane), forming columnar gas-saturated zones. We think that the crucial cause of push-down seismic velocity anomalies is overpressure (approximately lithostatic). We predict that some new gas fields can be related to the gas chimneys at Gydan Peninsula and in Kara Sea. Many new deep gas deposits can be found in old gas fields with gas chimneys such as Urengoy and Yamburg.

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