Christopher L. Liner1
Search and Discovery Article # 40040 (2002)
*Adapted for online presentation from a presentation to the Tulsa Geological Society, January 8, 2002
1Department of Geosciences, University of Tulsa, Tulsa, OK ([email protected]). Acknowledgment is made to D. Kerr and M. Kelkar, DOE project managers for geology engineering, respectively, for well data, especially those from Self #82, and to Producers Oil, Opseis, and Mercury International Technology in relation to the 3-D seismic data.
In 1996 a small 3-D seismic survey was acquired on the west edge of the Glenn Pool oil field, near Tulsa, Oklahoma, to map a producing 120-acre Ordovician Wilcox structure. The goal was to establish a template for the detection of such structures elsewhere. Among other results, the project revealed the added value of 3-D imaging even in areas of dense well control and the misalignment of time and depth structures.
The objectives of the project are:
· Leverage DOE project well information.
· Provide an analog for Ordovician Wilcox exploration.
· Get a view of the Pennsylvanian Glenn interval adjacent to Glenn Pool oil field.
· Test small-scale 3-D seismic survey in Northeastern Oklahoma.
There are a total of 17 wells in Sections 19/24, N17N, R12E (Figure 1). Production was discovered in June, 1985, in the Wilcox Sandstone (Ordovician Simpson Group) at a depth of approximately 2500 ft. Thickness is as much as 34 ft, and porosity is up to 18%. Cumulative production through 1996 was 950,000 barrels of oil. The productive feature is a nose, covering approximately 120 acres, on the southwest flank of Glenn Pool oil field.
Self unit #82, in Section 21, T17N, R12E, is located approximately 2miles from the area of the 3-D survey. The suite of logs includes DIL, LDL, BHC, CNL, GR, and SP.
– Sonic+Density => velocity X density = Impedance
– Sonic => velocity => time/depth => event ID
Using the sonic and density logs from the Self #82, an acoustic impedance log was prepared (Figure 2).
The sonic log can be ignored if the sonic values are predictable from the more common density log. In the case of the Ordovician Wilcox Sandstone (Figure 3), the relation of sonic to density values suggests that density values of themselves may be satisfactory. On the other hand, plots of the sonic vs. density values for the Pennsylvanian Glenn Sandstone (Figure 4) and for the entire stratigraphic interval (Figure 5) are such that the sonic values cannot be ignored in calculating acoustic impedance for synthetic seismograms. Figure 6 shows the velocity (from sonic values) in Self #82, from 300 ft to total depth, along with formation tops, plotted according to time so that the depth ticks are non-linear.
Features of 3-D seismic data used in this study are:
– Vibroseis, with bin size of 55 x 55 ft
– 141 E-W lines x 145 N-S lines
– 1420 acres, 2.2 square miles
– 1 sec, 2 ms
– Frequency band--15-120 Hz
Based on the calculation in equation (1), the 55-ft bin size is a little too large because fault imaging is degraded and dips greater than 34o are also degraded.
Bin <Vint / (4 fmax) = 15000 / (4 X 120) = 31 ft (1)
The vertical resolution is shown by equation (2) to be 62 ft. Correspondingly, the Wilcox, with thickness of 34 ft or less, is a “thin bed.” Lateral resolution, given in equation (3), is 155 ft, or approximately 2 bins. Structural resolution is 11 ft, as derived in equation (4) and illustrated in Figure 7.
VR = Vint / (4fdom) = 15000 / (4 X 60) = 62 ft (2)
LR = 2 X VR = 144 ft (~ 2 bins) (3)
DZ = (VavgdT)/2 = 1000 X .002 / 2 = 11 ft (4)
Data cube is illustrated in Figure 9, with a time (or horizontal) slice and two vertical slices. The data in the study area are noisy, reflecting a rough terrain and near-surface issues, but there is good frequency. The challenge is how to improve the data.
Improved quality of the data is illustrated in Figure 10, with the original time-slice map and the resulting enhanced map after smoothing, and in Figure 11, which shows a vertical profile, also with enhancement by smoothing.
Figure 14. Ordovician Wilcox structure map from well depths compared to two time-slice maps from times comparable to the top of the Wilcox; the last two are substantially different from the well-depth map.
Figure 19. Seismic tracking, illustrated by seismic profile with tracked events and overlay of time-plot of sonic log (left); Ordovician Wilcox amplitude and time structure maps on right (from Liner, 1999).
With the data described above, a well-depth structure map was made of Ordovician Wilcox sandstone (Figures 12, 13, 14, and 16) and comparable time-slice maps were prepared from seismic data (Figures 14, 15, and 16). Tracking geologic events seismically, specifically the Wilcox, was achieved by utilizing the time-plot of the sonic log and overlaying it on seismic data (Figures 17, 18, and 19). Because of a strong lateral velocity gradient (Figure 20), the maps from well data and from seismic data show significant differences (Figures 14 and 16). When both types of data are used, the result is an enhanced structure map of the Wilcox (Figures 21 and 22) and a 3-D representation (Figure 23).
The following are conclusions from this 3-D study, some 1420 acres in size, of an oil-productive area, near Tulsa in northeastern Oklahoma:
· Data improvement through smoothing by time slice (or FXY deconvolution).
· Time structure is not depth structure, reflecting a strong lateral velocity gradient.
· Postage-stamp sized 3-D seismic surveys can add detail (and detail adds value).
Liner, C.L., 1999, Elements of 3-D seismology: Tulsa, PennWell, 438 p.