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Seismic Detection of Overpressuring and Fracturing: An Example from the Qaidam Basin, People’s Republic of China
Shallow hydrocarbon reserves were discovered in 1959 in the Nan Yi Shan structure located in the Qaidam Basin about 2500 kms west of Beijing, P.R.C. The first successful deep well encountered an overpressured zone at 3000 meters which resulted in a well blowout. To improve the structural definition of the field and delineate the overpressured layer a 3D seismic survey was conducted.
A region of anomalous seismic time sag associated with fracturing and small quantities of oil and gas was identified on the northwest plunging nose of the Nan Yi Shan anticline. Velocity dispersion effects were encountered and attributed to localized fracturing.
In a deeper interval, seismic amplitudes were used to identify gas-saturated fracture porosity and to describe the spatial limits of overpressuring within this layer.
This paper will discuss the seismic distortion and unusual velocity signature associated with a localized fracture zone. It will demonstrate that with good quality data and sufficient lateral and vertical resolution, thin fractured hydrocarbon bearing reservoirs can be delineated and overpressure zones predicted.
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