William C. Pearson

Spectral Balancing of Aeromagnetic Data: A View From the Bend In the Road

A newly developed technique for selectively enhancing portions of the magnetic field spectrm bocsts the signal of desired geologic bodies while it dampens noise. Magnetic bodies Of specific sire, shape and depth can be selectively enhanced. For instance, shallow detrital, volcanic or epigenetic signatures can be enhanced at the expesse of basement signatures and deep regional bodies. Alternatively, faulted mgneti.c basement signatures or deep seated plutonic or geothemal signatures cm be selectively enhanced.

Interactive 3-D Gravity Modeling of a Pennsylvanian Age Salt Ridge In Paradox Valley, Colorado is Integrated With Seismic And Subsurface Geology to Produce Structural Leads For Gas Exploration

Integrated 3-D and 2-D gravity modeling, 2D seismic profiling and geology of salt anticlines in the Paradox Basin, Colorado and Utah have a benefit over Gulf Coast salt studies in that they involve large vertical relief salt ridges that displace high density Paleozoicage overburden. Density contrasts between the salt and overlying Pennsylvanian and Permian age clastics and minor carbonates are larger than seen in the Gulf Coast Mesozoic and Cenozoic rocks. New interactive 3-D gravity modeling can be performed very quickly and effectively by integrating seismic and well data in multiple windows interactively.

ABSTRACT: The Aeromagnetic Definition of Wrench Faults and Their Influence on Hydrocarbon Entrapment and Production Fairways

Abstract Structurally and stratigraphically entrapped hydrocarbons, as well as the trends of reservoir facies and hydrocarbon migration pathways, appear to be strongly influenced by the wrench fault systems present in basins throughout the Rockies and elsewhere. At least three types of basement structural features influence production: 1) basement structural highs; 2) relatively short basement faults, which mostly define the margins of basement structural highs, and 3) regional crosscutting wrench faults, which define and create major structural and compositional discontinuities. These three structural feature types are most readily interpreted from patterns seen on the Second Vertical Derivative and SUNMAG displays, as well as structures defined by line profile analysis. Integration of these data with log, facies, hydrocarbon show, and production information indicate that motion along wrench faults is instrumental in controlling where entrapment of hydrocarbons takes place. Production bears a direct and obvious relationship to either the juncture of basement structural highs, with the cross-cutting wrench faults that can be interpreted from discontinuity of patterns present on the aeromagnetic displays, or to certain structural features orthogonal to these wrench faults. Essentially, all fields occur on the tops or immediate flanks of mapped basement structures, an indication that even subtle structures at basement level are important in the stratigraphic entrapment of hydrocarbons. In basins such as the Williston, Big Horn, Powder River, Piceance, Uinta, and Greater Green River, fields are located on mostly northeast trending wrench faults, or on orthogonal structures limited by these cross-cutting wrench faults. Major fields such as Little Knife, Billings Nose, Fryburg, the Dickinson-Eland Wausortian Mound Fields, Oregon Basin, Cottonwood Creek, Jonah, Highlight, and the Rulison-Parachute-Grand Valley fields bear a distinct relationship to these major, but mostly subtly defined faults. Production commonly ends abruptly or changes trend at wrench fault discontinuities. Consequently, these faults appear to be of critical importance in controlling not only structural development, but also the updip productive limit of many stratigraphic entrapments, whether being caused by diagenetic pore throat entrapment, or a change in facies. In either case, these relationships are an indication that regional wrench faults were active during deposition or influenced diagenetic fluid movement through the reservoir system at a later time.

Three-Dimensional Gravity .Modeling of Salt Diapirs In the Paradox Basin: Lisbon Valley Case History

grid (Figure 2) was created through the inversion of selected anomalies. The grid of the initial susceptibility values was calculated at a uniform basement depth of 50 m below ground surface. Figure 3 shows poor resolution, especially where the basement is deeper. Calculation of susceptibility at a deeper surface, 500 m below ground for example, would certainly improve the map in the area of the large anomaly. However, the shallow sources would appear greatly amplified and distorted. Using the grid of depth values the final susceptibility map (Figure 4) was generated using the variable depth susceptibility program. The anomalies on this map are much more well-defined, and the map’s appearance is cleaner and smoother. The most noticeable improvement is in the area of the deepest sources, The resolution does not suffer when compared to the shallow regions. As expected, the calculated values for susceptibility in the deep area are much higher than those on the previous map. Figure 5 shows the geologic interpretation, derived in part from the final susceptibility map of the test area. Such detailed interpretation could not have been obtained using conventional susceptibility mapping techniques. The technique of mapping susceptibility at a variable depth-to-source shows considerably better definition in comparison with previous methods of mapping to a level surface. The technique does not assume that the magnetic field is measured on a horizontal plane. Hence, the method may be extended to allow for the continuation of the field from any one specified surface to another. For example, the magnetic response measured on a drape-flown aeromagnetic survey could be regenerated at a constant barometric height, or vice-versa.