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 DvC2-06
Using D3D-impedance data, this Figure shows an EI27/46 Kingdom time-structure-on-amplitude map, and a west-to-east arbitrary line, through the two EI-27 key Odeco and EPL wells. Both show D3D-impedance data but, otherwise, comprise a fairly conventional Gulf of Mexico drilling prospect "sales package": a time-structure-on-amplitude map and a key seismic line connecting the proposed location with a nearby key well.
(LEFT) - A CIB CARST sand "layer" trough-(impedance-) amplitude map, with the lowest impedance values colored yellow, and the highest impedance colored dark blue (per the yellow-red-cyan-blue color bar). The lowest impedance-amplitude point within a negative amplitude CIB CARST sand seismic trough, was "picked" on every D3D-impedance trace in the 1.5 OCS block seismic volume (55-feet X 55-feet, over 7,500 acres or 11.7 square miles). A Kingdom 3dPAK event-tracker was used, similar to one used by the Landmark system for the conventional amplitude map. The black TWT-structural contours reveal the (reflected echo two-way travel) time structure of the lowest-impedance point INSIDE the CIB CARST sand (not the top of the sand). These minimum-impedance points naturally move up and down within the real, heterogeneous sand body, as the sand quality (mainly porosity, but cementation also alters the velocity) influences the local acoustic impedance of the gas- (red and yellow?) and water- (pink and cyan) filled sandstone ... and siltstone (dark blue?). A sand with higher porosity, filled with gas, will be lower impedance than the same sand (same cementation), containing lower porosity, so when the porosity (and cementation) distribution is three-dimensionally complicated, so is the (D3D-) impedance distribution. A D3D-impedance amplitude value was then "extracted" at each of these minimum-amplitude points, on every trace, and the resulting, so-called horizon-amplitude map (or horizon slice) is displayed, using the yellow-red-cyan-blue color bar.
Note the curved, yellow (meandering channel?) portion within the overall low-D3D-impedance EI-27 gas sand anomaly. The reservoir quality logged by EPL in their #1 well supports its position in the center of such a clean sand channel ... unlike the rather randomly speckled, conventional (un-reprocessed reflectivity) amplitude map, on the left side of Figure DvC2-05. And notice, too, how the Odeco #4 well spot is quite far west of the western edge of the red-and-white, conventional-data amplitude anomaly. This Figure DvC2-05 image as the interpreted edge of the anomaly accentuate by both the chosen amplitude scaling, and the use of a discontinuous color-bar that puts a dark green rim around this conventional Bright Spot. This dark rim serves to isolate it from the nearby, water-out Odeco well. The D3D-impedance map (this Figure), on the other hand, shows the Odeco #4 well to be just barely within the up-dip, pink fringe of the meandering gas sand D3D-CIO anomaly. This relationship was confirmed by the similar slight reduction in pressure found in EPL's (re-) discovery well CIB CARST reservoir and reported in the Odeco #4 reservoir, at abandonment. The Odeco well is, in fact, in an edge position similar to that of the Norcen #2 well, with respect to its D3DSP-proposed CIB CARST gas reservoir sand anomaly. The biggest difference is that the Norcen #2 well perforated only 18-feet of the very uppermost lobe of a complex, gas- and water-filled CIB CARST sand package (note the SP curve and many thin shale breaks on the log in Figure DvC2-07), and in a structurally high position. Odeco chose to perforate a 32-foot interval spanning two apparently (SP-) separate sand stringers (shown on the "zoomed log" on the caption to Figure DvC2-08).
The D3D-impedance-amplitude map is overlain by 100 ms (approximately 400 feet, black) TWT structural contours. Note that the 2.7 second contour, wrapping around the down-dip (eastern) part of the red-and-yellow EI-27 reservoir, marks the level of the interpreted gas-water-contact, shown exquisitely as a flat base to the low-impedance (yellow) CIB CARST gas sand on the right-handD3D-impedance arbitrary line.
(RIGHT) - The NW-SE arbitrary line that connects the Odeco #4 abandoned CIB CARST producer to the EPL #1 CIB CARST proposed location (subsequently a [re-?] discovery well). The CIB CARST reservoir sand package lies within the thick, red and yellow dipping, low-impedance (trough) event. This event is not the top or base of anything. The top of the sandstone interval is interpreted to be near the white (blue-to-red) "zero-crossing" of each wiggle-trace (none shown here, but see Figure DvC2-09 for examples of how the red and blue colors match up to the troughs and peaks on a wiggle-traces display). The base of the CIB CARST reservoir sand is near the white (red-to-blue) zero-crossing. Please excuse the high-D3D-impedance color, which was supposed to be cyan, but looks like it turned out to plot as a white color, also. The white areas inside an otherwise blue event are not "zero-crossings".
Notice that the tops and bases are stated to be "near" the upper and lower zero-crossings, respectively. A very, very important aspect of the D3DSP, and one that cannot be understated, is that the object-oriented D3D-processing sequence allows an D3D-interpreter (whether geophysicist, geologist, engineer, manager, investor, "geophysician", etc.) to select, pick, and map (if so desired) both upper and lower surfaces on the three-dimensional object (CIO) target, WITHOUT the precise two-way-travel-times to these upper and lower surfaces, being limited to the minima, maxima, and zero-crossings, of any seismic trace curves. In point of fact, a single, broad (dominated by low frequencies), low-D3D-impedance trough could conceivably contain numerous, separate, Common-Impedance Objects ... with no positive-valued relative-impedance samples between them; just less-negative-amplitude samples.
Hence, the "flat base" seen on this Figure, and pointed out by the black arrow, is actually the base of the YELLOW samples, above the pink low-D3D-impedance samples, whose zero-crossing base is not particularly flat. They pink (interpreted wet sands) continue dipping on down to the SE. The wonderful SMT Kingdom interpretation system, used to capture these Figure DvC2-06 D3D-impedance images, was created for interpreting conventional seismic reflectivity data ... in layered-earth model projects. This points out another insidious effect of the wide acceptance of (and demand for) images of the subsurface that look like "layers": it is not easy to find software tools for object-oriented exploration projects.
A couple of final notes on this Figure:
(1) The Odeco #4 well was drilled by Texaco for much deeper targets than the 10,000-foot CIB CARST sands. Notice that the higher resistivity, non-pay portions of the ILD log curve correlate well with the blue and cyan areas on the D3D-impedance. This is probably because higher density rocks usually exhibit higher electrical resistivity, and sonic and density logs are often unnecessary in order to correlate a well log to a D3D-impedance section. SP, gamma ray, and resistivity curves that indicate the presence of sands and shales (in the Gulf of Mexico basin) can be used to great advantage when no other geophysical logs are available. Like many other rules-of-thumb, this "sands are low-impedance, shales are higher" becomes problematic when deeper, over-pressured rocks are investigated. Deep shales can be very low density (and velocity) and, therefore, form very low-D3D-impedance objects (CIOs). One should watch for other signs of hydrocarbons when working below the top of pressure: flat based CIOs, shows in adjacent wells, sand-channel geometries (although some spectacular examples of shale-filled channels certainly exist).
(2) Not everything yellow is hydrocarbons. In fact, there may not be any other gas sands anywhere on this particular D3D-impedance seismic cross-section. But then again many other, un-drilled low-D3D-impedance CIOs (some also with flat bases) have been seen and mapped in this area, too, so one should not be too quick to condemn an area just because there are no un-drilled anticlines, fault traps, or conventional amplitude anomalies. Open-minded, non-layered-earth model calibration to existing well control (noting that it IS unusual to come up with a good excuse to drill immediately down-dip of an 8 bcfge watered-out well), combined with an obvious flat-based CIO (on the D3D-impedance data ... not on the conventional reflectivity data) were required in this financially successful (technically accurate) case.
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