3D Seismic for Development of the Nebraska Niobrara Chalk

From ESER oil and gas

Overview

• The Niobrara is characterized by extensive structural faulting attributed to basement features as well as salt-edge dissolution and collapse in the underlying Permian units.
• The faulting sets up low relief, fractured, structurally high features that dominate commercial production.
• The complex faulting and discontinuity of the Beecher Island interval make 3-D seismic data fundamental to identifying the subtle structural features that control success in the Niobrara play.

Role of 3D seismic

• 3-D is used to identify the following:
  1. Increased gas saturation pay zones identifed through amplitude anomalies at the top of the Beecher Island Zone.
  2. Structural elevation
  3. Faults
• 3-D seismic data is not only useful for identifying areas of high structure, and therefore good potential trap areas, but also for identifying amplitude anomalies.
• Increased gas saturation in the Beecher Island reservoir section produces a 10-12% decrease in seismic velocities that results in increased acoustic impedance contrast relative to the surrounding rocks.
  • This increased impedance contrast manifests itself in the seismic data as an increased trough amplitude at the top of the Niobrara Formation.
    • This ampltiude increase can be as much as 140%
  • The highest observed amplitudes in the niobrara Chalk are located ins tructurally high and trapped positions.

Using 3D Seismic to identify faults

• The structural faults in the nio are characterized by 20-50 feet of throw.
• As a result, the reservoir is highly compartmentalized.
• Using 3D to map the faults turns the Niobrara into a structural play and allows the wells to be placed within the compartments and away from the faults.
• Drilling a well too close to a fault will cause screen-out during hydraulic fracture stimulation.

Eser Corporation Design of 3D seismic surveys in the Niobrara Chalk

• energy source: vibroseis
• source/receiver spacing: 220 feet
• source line spacing: 880 feet
• receiver line spacing: 440 feet
• large active patch: 16 lines by 60 stations
• source sweep: 10-120hz
• sweeps at each source point: 2, 4, or 6 (depending on field testing)
• Primary target formation top: beecher island niobrara chalk: 1970
• Secondary target formation top: Lansing Kansas-City: 5,500ft
• halo: 0.5 miles (based on 5,500ft formation top for Lansing Kansas-City formation)
• total 3D seismic survey size, including halo: 4 square miles
• location of seismic acquisition project: Chase County, Nebraska
• record length (listen time): 3 seconds
• sweep length: 10 seconds
• No. of Active Vibrators: 2 sets 2
• Sample Rate: 2ms
• Geophone array: Inline over 30ft
• Geophones per station: 6
• Fold at 1970ft: best effort

Seismic Response of the Beecher Island Niobrara Chalk

* Amplitude response from the Niobrara Chalk in Yuma County, Colorado.
* Dry holes D1 and D2 were drilled before 3D seismic
* The best performing well is G1. G1 was drilled in a structural high with good amplitudes and logged 9-12 ohm resistivities and has an EUR of over 900MMCF.
* The doublet peak below the Niobrara in G1 is a better indicator of well performance than the amplitude alone.

Seismic amplitude anomaly over Bonny Field, Yuma County, Colorado.

Links

1. Seismic surveys: what property owners and seismic companies need to know
2. Eser Corporation's use of 3D seismic in drilling site selection for Project Nebraska