INTEGRATION OF SEISMIS DATA, IODINE GEOCHEMISTRY YIELDS LODGEPOLE EXPLORATION MODEL

Sept. 18, 1995
Steven A. Tedesco Atoka Geochemical Services Corp. Englewood, Colo. John A. Andrew StratSeis Geophysical Inc. Englewood, Colo.
Steven A. Tedesco
Atoka Geochemical Services Corp.
Englewood, Colo.
John A. Andrew
StratSeis Geophysical Inc.
Englewood, Colo.
The recent discovery of prolific Mississippian Lodgepole mounds near Dickinson, N.D., has resulted in one of the most exciting U.S. exploration plays in several decades. The presence of the mounds in the Williston basin1 around Dickinson in North Dakota; in outcrop in central Montana; and in the subsurface of eastern Montana, northern Alberta, southwestern Manitoba, and southeastern Saskatchewan suggests a potential target that could be found over the entire basin and especially on the eastern margin. However, as prolific as the Lodgepole wells near Dickinson are, they are not easily found by simple subsurface mapping, interpreting existing 2D seismic, or newly acquired 3D seismic data (as proved by dry holes located on 3D seismic data). Presented here are a section and map examples of two different exploration methods used together over two producing areas near Dickinson, N.D. (Fig. 1)(30959 bytes). When the two methods are used together, in synergistic fashion, they provide a higher degree of success than when used apart. This approach can be used as a model for future exploration. The empirical signature of a Lodgepole mound on conventionally acquired and processed, high-resolution 2D and swath 2D seismic data is clearly expressed in the nature and character of the seismic event representing the contrast between the Lodgepole and underlying rocks. Furthermore, and more importantly, there is direct correlation between the occurrence of surface iodine geochemical anomalies and the location of Lodgepole mounds on map and profile views of the seismic data.

Geochemical data

Iodine geochemical data, obtained and measured from surface samples, are very useful in collaborating the presence of hydrocarbons in the subsurface.2 A strong increase in inorganic iodine compounds is an indirect indicator of a petroleum accumulation somewhere at depth. A reaction occurs between leaking hydrocarbons and iodine in the presence of sunlight in surface soil. The source of iodine is either minerals in the soil and/or the atmosphere with UV light as the initiator of the reaction. The inorganic iodine molecules formed are large, relatively stable, and immobile (in the presence of hydrocarbons). The accumulation of iodine is a more reliable and repeatable tool than such transitory methods as soil gas. Iodine background typically ranges from 0.1 ppm to 2.0 ppm but can be higher. Anomalous areas worth noting are typically greater than 50% over background or more than one standard deviation above the mean. Strongly anomalous areas are usually 2-20 times above background. A surface iodine anomaly only indicates hydrocarbons in the subsurface. Other evidence (such as seismic, well, or production data) is needed to identify the target horizon for exploration. A section display of seismic data collected in 1977 (Fig. 2)(18488 bytes) and a map view of seismic data collected in 1995 (Fig. 3)(26220 bytes), are shown with the seismic mounds noted and geochemical data added. The direct correlation of anomalously high iodine values and the geographical occurrence of seismically identified Lodgepole mounds is striking.

Seismic section

Fig. 2 (18488 bytes) displays 6-fold seismic data, now owned and marketed by Seitel, that were acquired in August 1977 in southern Eland field, 23-24-139n97w, with 48 channel instruments recording 330 ft groups using 40 lb dynamite shots in 200 ft holes spaced at 1,320 ft. Permission to use these data was obtained by Atoka Geochemical, and the data were reprocessed by H.T. Geophysical. Stratigraphic and lithostratigraphic information can be interpreted from visual inspection of reflection seismic data, especially when the data are displayed with an aspect ratio matching common geologic scales.3 The aspect ratio of these displayed seismic data is nearly 1:1 (horizontal, 1:12,000; vertical, about 1:11,200). Seismically identified mounds are indicated by variations in the nature and character of seismic event amplitudes where the Lodgepole reflector dims and broadens. The amplitude of a seismic event shows the strength of the contrast between two acoustic (velocity times density) units. On these seismic data, a black event represents the contact (here called L/B) and strength of acoustic contrast between the higher velocity and density main Lodgepole carbonate unit and the underlying lower velocity and density complex unit. This low velocity unit consists of the Cromer shale (false Bakken), Scallion limestone (base Lodgepole), Bakken shale, and Three Forks clastics. These units (Lodgepole to Three Forks) are, in general, regionally continuous and are represented on seismic data as a persistent reflector. Lateral variations in reflector strength indicate lateral changes where units become acoustically similar. Many factors can cause a change in seismic character, but the most likely in the vicinity of known mounds are variations caused by mound lithology and porosity. Varying seismic response simply pinpoints where such changes occur. The L/B seismic event dims where regional Lodgepole litho-facies change near mounds. On higher frequency seismic data the L/B event splits into a double event where the Lodgepole mound facies are sufficiently thick (100+ ft) to provide an upper and lower acoustic contrast. Thicker mound facies (250+ ft) are clearly marked by two events, and the thickest seem to contain internal accretionary surfaces. Steeply dipping lateral faults, marked by event terminations and classical flower structures, are clearly associated with many mounds. The drape in shallow reflectors used to locate some mounds1 is not universally present. Some inferred drape can be interpreted as fault offset on low frequency seismic data. The two producing wells projected to the seismic line were drilled in 1995 for the Lodgepole and were located on modern seismic data. The iodine surface geochemical data presented in profile form on Fig. 2 (18488 bytes) were collected as part of a much larger survey. The profile of iodine data shows high values directly over seismic mounds. Anomalous iodine values begin at 1.8 ppm and exceed 5.0 ppm. Iodine background values are generally less than 1.0 ppm. Samples for iodine analysis were acquired prior to the drilling of the two producing wells.

Geochemical anomaly map

StratSeis Geophysical Inc. conducted a source test and data demonstration across known Lodgepole mounds in 28-30-139n96w, southwest of Dickinson, in July.

Three individual seismic data sets consisting of 3-mile-long, three-line swaths were acquired by employing three different seismic sources (dynamite and two nonlinear Vibroseis sweeps) and rolling through the middle receiver line with 400 ft vibrator/shot points into the same geophone array consisting of 80 ft groups. Data were acquired by Northern Geophysical of America as directed by StratSeis Geophysical and were processed by ExCel Geophysical Services. The subsurface seismic lines cross all of the quarter-quarter well locations. The conventionally processed, varying fold data obtained with the three sources are generally quite similar and are equally useful for identifying and locating Lodgepole mounds. Surface samples for geochemical analysis were collected by the seismic crew at 400 ft intervals along the receiver lines for analysis by Atoka Geochemical Services. The map view of iodine distribution (Fig. 3)(26220 bytes) contains the location of Lodgepole mounds noted on the seismic lines according to the criteria described above to identify mounds. The mapped iodine geochemical data indicate that anomalies begin at 1.8 ppm and are as high as 6.5 ppm. Background values are generally less than 1.0 ppm iodine. A direct correlation is indicated between the location of observed seismic mounds and anomalously high geochemical values. There are areas on the east side of the survey that indicate anomalous amounts of iodine but where there are no Lodgepole mounds. The source of hydrocarbons related to this anomalous iodine concentration is believed to be the Tyler (Pennsylvanian) sand reservoir (open boxes on the map). The four wells penetrating the lower Lodgepole indicated on the map are located directly on the swath seismic lines. The Shell Kostelecky 31-30 (NW NE Section 30) was drilled in 1979 for a Red River objective. The well missed an L/B seismic mound by 240 ft. The well is located on the northwest edge of an iodine anomaly. If the well had been drilled in the NE NE instead of the NW NE, the Lodgepole play would have been completed in the 1980s. The Summit/Armstrong Haller 29-1 (NW NW Section 29) was announced as a Lodgepole discovery, but the well is now said to have production problems. The seismic evidence indicates that the well was drilled in the heart of a 350 ft thick seismic mound but in a unique seismic facies that extends laterally along the line for 720 ft. The iodine anomaly shows no evidence of an anomaly at the well location. The Columbia Krank 28-1 (SW NE Section 28) is a dry hole drilled for the Lodgepole. There is no evidence on the seismic data to suggest a mound at the well location. The well is located on the edge of a strong iodine anomaly and south of a seismic mound noted on the adjacent seismic line. The latest well drilled for Lodgepole mounds in the area is the Summit/Armstrong Hondl 30-1 (SE NW Section 30). The well reached total depth during July. The vertical well is said to be dry with no mound facies, and the well is presently being sidetracked. There is no evidence on the seismic data or iodine anomaly map data to suggest a mound at the vertical well location,

Data integration

The empirical exploration model presented here utilizes the integration of iodine surface geochemistry and seismic data to define Lodgepole mounds in the Williston basin. Different data types and interpretation techniques identified the same prospective geographical areas and mutually supported the interpretation.

If the synergistic correlation of data types were not done, the importance of the interpretation of each data type may not have been recognized as such. The failure to properly utilize this type exploration model, when exploring for a target such as that described, will lead to dry holes. However, the objective and synergistic use of multiple methods will minimize the number of dry holes and maximize the number of successful wells.

References

1. Johnson., M.S., Dickinson Field Lodgepole Reservoir: Significance of This Waulsortian-Type Mound to Exploration in the Williston Basin, The Mountain Geologist, Vol. 32, No. 3, July, 1995, pp. 55-79. 2. Tedesco, S.A., Surface Geochemistry in Petroleum Exploration, Chapman & Hall, New York, 1994. 3. Andrew, J.A., The Art and Science of Interpreting Stratigraphy from Seismic Data, in Seismic Exploration of the Rocky Mountain Region, Robbie Rice Gries and Robert C. Dyer, editors, Rocky Mountain Assoc. of Geol. and Denver Geophy. Soc., 1985, pp. 95-103.

The Authors

Steven A. Tedesco is owner of Atoka Geochemical Services Corp., formerly Atoka Exploration Corp. He worked for Mobil Oil Corp. and Shell Oil Co. as well as several independent oil and mining companies and has more than 20 years of experience with surface geochemistry in exploration for natural resources. He is author of Surface Geochemistry in Petroleum Exploration, Chapman & Hall, 1994. He has a BS from Northeastern University and an MS from Southern Illinois University.

John A. Andrew is chief executive officer of StratSeis Geophysical Inc., responsible for business development, research, seismo-stratigraphy, marketing, and sales. He has 32 years of postgraduate geology and geophysics experience. He worked for Shell Oil Co. in 1968-76 and Shell Development in 1976-80 as a geologist, geophysicist, and seismo-stratigraphic interpretation specialist in exploration, offshore lease sales, and research. He worked for Marathon Oil Co. in 1980-82 as an in-house exploration and seismo-stratigraphic consultant.
Andrew cofounded and was chief executive officer of StratSeis Inc., a seismic data acquisition and marketing firm during 1982-86. He has a PhD and MS (geology) from the University of Wisconsin (Madison) and a BS (geology) from the University of Tulsa. Copyright 1995 Oil & Gas Journal. All Rights Reserved.