Time-lapse multicomponent seismic data were used to detect seals within a basin-centered gas accumulation at Rulison Field, Colorado. Seals are related to structural, stratigraphic, and diagenetic mechanisms involving the formation of diagenetic clays that form the top seal to the gas accumulation.
Rulison Field produces out of Cretaceous Mesaverde tight gas sandstones (Figures 1 and 2). The pay section spans a 1200-foot thick interval that is overpressured. The reservoir consists of multiple-stacked lenticular sandstone bodies with matrix permeabilities ranging from 5 to 80 microdarcies. The reservoir interval consists of 60% shale and 40% sandstone. Sealing mechanisms for the gas have been a subject of debate for years. Our time-lapse multicomponent seismic data in conjunction with clay diagenesis studies supports the concept that feldspar mobilization prior to or during the onset of gas generation sealed the petroleum system and that the mobilization occurred along fault and fracture systems.
The Colorado School of Mines Reservoir Characterization Project (RCP) acquired three dedicated time-lapse 3D-9C surveys during 2003, 2004 and 2006 in Rulison Field, western Colorado (Figure 3). At Rulison, the reservoir is not undergoing any significant fluid saturation change because it is being produced from an interval where there is little or no free water associated with this basin-centered gas accumulation. Fluid pressure change is the only variable controlling time-lapse seismic changes.
Seismic data were acquired in a tight gas development area of 5.6 square km by Solid State Geophysical using the Input/Output Vectorseis telemetry system. P- and S-wave vibrators were used to acquire a full wave field (9-C) survey. Subsurface bin size is 16m x 16m, with 1,500 single Vectorseis sensor locations and 770 source locations. Excellent repeatability was achieved. Station repositioning for both the shots and receivers fell within +/- 1m repositioning tolerance.
Rulison Field is located on a northwest plunging anticline termed the Rulison Nose. The presence of wrench faulting within the reservoir interval causes shear deformation, resulting in a complex system of branching faults, termed “flower structures.” These faults have been confirmed by image logs and drilling information. Many of the faults are below seismic resolution due to small vertical displacements, but some of the major faults are evident in the shear wave data because they represent elastic discontinuities dependent on stress state in the subsurface. Figure 4 shows a map from the shear wave data of the seismically resolvable faults at the base of the reservoir. The S-wave seismic confirms that these faults splay and branch upwards through the reservoir. Many of the high EUR wells are located near this fault zone, but so are some average wells. The location of faults relative to a well bore may affect a well’s EUR, but faults are not the only factor.
Open natural shear fractures are predicted to occur near fault zones. The open fractures exhibit a rigidity contrast with the surrounding rock, making the larger shear zones detectable with S-wave seismic data, whereas they are indistinguishable with P-waves because of small vertical offset.
