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By Louis J. Mazzullo1
Search and Discovery Article #40045 (2002)
*Adapted for online presentation from an article by the same author in AAPG Explorer (September, 1998), entitled “Past Data Goes Back to the Future.” Appreciation is expressed to the author and to M. Ray Thomasson, former Chairman of the AAPG Geophysical Integration Committee, and Larry Nation, AAPG Communications Director, for their support of this online version.
1Petroleum geological consultant and general partner with Ameristate Exploration of Albuquerque, N.M., and Midland, Texas ([email protected]).
Figure 1. The location of Cisco and Canyon fields in Eddy County, a southeastern New Mexico portion of the Permian Basin. Dashed red lines represent some of the time lines associated with various productive zones.
Figure 2. Schematic cross-section across a composite Cisco-Canyon algal mound, with the time units (t1 through t6) shifted in response to changes in relative sea level, often bringing multiple units close to each other.
Figure 3. Geologic map showing algal mound and related facies defined by sample evaluation and 2-D seismic data. Dashed blue line is approximate trend of the seismic line shown in Figure 4.
Figure 4. Seismic expression of Cisco-Canyon algal mounds (yellow) in a sparsely drilled area, confirmed with sample analyses from surrounding wells. Note wavelet anomalies and offlap beds associated with algal facies. Pink arrow points basinward.
Figure 5. Seismic expression of Cisco-Canyon algal mounds (yellow) in another area of southeast New Mexico, showing pronounced mounding in the upper (Cisco) portion of the section. Cisco Algal mounds here are developed upon a deeper, structurally high block formed from faulting that appears to penetrate through the canyon. Again, the pink arrow points basinward.
The introduction of 3-D seismic several years ago sparked a new way of exploring for complex reservoirs and for developing mature fields. It has enjoyed widespread application in all types of prospecting, often at the expense of other methods of study.
In mature basins such as the Permian Basin of southeastern New Mexico, there is a wealth of geologic information available in the form of well logs, 2-D seismic data and samples that allow exploration geologists to focus on new or remote plays prior to the design of a 3-D seismic survey. We have all learned to use convergent evidence to analyze a scientific problem. However, we often weigh one method, such as 3-D seismic, more heavily than others or, at worst, to the exclusion of other evidence. With all the information available in mature basins, the use of multiple lines of reasoning is not only good science, it can save considerable amounts of exploration and lease acquisition dollars.
Upper Pennsylvanian Carbonates in Southeastern New Mexico
Figure 1 shows that in southeastern New Mexico there are a number of sub-parallel trends of established gas and oil production from carbonate algal mound facies in the Cisco and Canyon Formations. Production in the region occurs where several algal mound complexes coalesce or are closely laterally offset to one another. To date, only a handful of multi-well Cisco-Canyon fields have been discovered in this area, the most significant being the Indian Basin and Dagger Draw fields. Because of the way in which the Cisco-Canyon mound trend developed in this region, there should be more opportunities for production from these rocks, albeit from smaller features.
This Cisco-Canyon sequence was deposited as individual algal mound and related carbonate units over a wide area of the present-day Permian Basin. Detailed paleontological age dating of the upper Pennsylvanian in Texas has defined over 20 separate time units that were deposited in response to changes in relative sea level. Since the entire Permian Basin area was affected by these changes, there should be as many discrete depositional units present in the play area.
During the Late Pennsylvanian, the sea floor across most of southeastern New Mexico had a gentle slope. Slight changes in sea level and hence optimal environmental deposition caused algal mound deposition to shift laterally by several miles in an updip or downdip direction. Such shifts in depositional focus occurred over a width of more than 20 miles across parts of the area, resulting in the field pattern of Figure 1.
Figure 2 is a schematic cross-section that shows how several different depositional units may develop within a relatively narrow fairway--in this case, perhaps no more than two miles wide. The shifts in depositional sites of the various algal mound-related facies occurred as a result of changes in relative sea level. The algal mounds are commonly found at the shelf margin and are composed of phylloid and red algae, and associated bryozoans and encrusting forams. They grade basinward to foreshelf carbonates and shales, and shelfward to shallow marine and supratidal carbonates and clastics.
Indian Basin Field is constructed by many of these individual depositional units stacking vertically because of recurrent movement on faults. This type of buildup is not expected in other, more stable areas of southeastern New Mexico. Instead, one might expect to find:
Single unit buildups produce economically from depths less than 7,000 feet, and searching for these smaller reservoirs can make economic sense. This is where a thorough understanding of subsurface geology integrated with 2-D seismic comes into play.
Potentially Prospective Areas
Outside of the existing fields in southeastern New Mexico, the Cisco-Canyon has received less attention than zones such as the Morrow sandstones and Bone Spring sandstones and carbonates. Cuttings from existing wells in some of the less densely drilled areas of the basin can be used to identify depositional environments. The algal mound facies of the Cisco-Canyon are diagnostic, and so are the facies that are found immediately behind (shelfward) and in front of (basinward) the mounds. In any particular time unit, the shelf-margin algal mound trend will be fairly narrow (often less than a mile wide).
If enough sample control is available, it is often possible to bracket individual mound fairways by identifying the foreslope and back-mound facies. To identify these fairways, an isopach map of the portion of the Cisco-Canyon section of interest was constructed (Figure 3). This map shows a significant increase in thickness in proximity to the algal mound buildups. Samples were evaluated to determine gross depositional facies. Figure 3 also shows three distinct Cisco-Canyon time units identified in samples, leading to a tentative identification of their respective algal mound fairways. To further narrow down the fairways, several 2-D seismic lines were purchased.
Figures 4 and 5 show high quality 2-D lines across part of this trend. They both clearly identify the shelf margin and offlap beds associated with the fairways as determined from samples. Figure 3 shows the type of prospect that can be generated from coordinated subsurface mapping and commercially available 2-D seismic data. By using good subsurface geology and relatively inexpensive 2-D data, general sweet spots and individual prospects can be identified.
In this case, a lease acquisition program was recommended that encompassed only that acreage identified by the initial geologic mapping. The map shows that each individual algal mound trend will be discontinuous, and comprised of “pearls on a string.” A regional 3-D seismic program shot prior to this geologic work would have evaluated far more “goat pasture” acreage than necessary.
Today 3-D seismic seems to be necessary to sell prospects. This is true regardless of whether the prospect is a field extension or a new, remote wildcat idea. When it comes to exploring for smaller targets or unconventional plays in maturely developed areas, however, we should remember the old adage “don’t put the cart before the horse.” Shooting 3-D seismic data should only follow a thorough geologic study of an area, not the other way around