Seismic detection of overpressuring and fracturing: An example from the Qaidam Basin, People's Republic of China

Journal Article · Fri Oct 01 00:00:00 EDT 1993 · Geophysics; (United States)

DOI:https://doi.org/10.1190/1.1443369· OSTI ID:5719706

Paul, R J ^[1]

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1. Halliburton Geophysical Services, Beijing (China)

Shallow hydrocarbon reserves were discovered in 1959 in the Nan Yi Shan structure located near the western corner of the Qaidam Basin. The first successful deep well encountered an overpressured zone at 3,000 m that resulted in a well blowout. To improve the structural definition of the field and delineate the overpressured layer a 3-D 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. The distribution of stacking (NMO) velocities in this region was regarded as abnormal; stacking velocities derived on the steeply dipping flanks adjacent to the sag anomaly were found to be slower than those on the shallower crest. Ray-trace modeling of a buried low-velocity anomaly provided a possible geometric solution to explain both the time variant nature of the sag and the unusual stacking velocity signature associated with it. A significant difference in seismic and sonic travel times was also observed for wells that penetrated the sag region and was 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 a thin-bed reservoir. Wells drilled through high-amplitude anomalies encountered overpressuring, those drilled in a region of moderate seismic amplitude tested significant quantities of gas, and wells located outside the region of good coherent signal encountered poor or no hydrocarbon shows. These results demonstrate that with good quality seismic data and sufficient lateral and vertical resolution, thin fractured hydrocarbon-bearing reservoirs can be delineated and overpressure zones identified.

• OSTI ID: 5719706
• Journal Information: Geophysics; (United States), Vol. 58:10; ISSN 0016-8033
• Country of Publication: United States
• Language: English

Related Subjects

02 PETROLEUM
03 NATURAL GAS
58 GEOSCIENCES
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SEISMIC SURVEYS
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FRACTURED RESERVOIRS
GEOLOGIC FRACTURES
GEOPRESSURED SYSTEMS
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