Nevada
This short article suggests a causative relationship between lineaments and fractures in oil fields of the Great Basin. Because the lineaments are definable with regional gravity and magnetic data, they may be used as a predictive tool to delineate analogs to the existing production.
Spring 1997 Update: Gibson Consulting is actively involved in play and prospect generation in east-central Nevada in association with Thomasson Partner Associates. For more information on a Grant Canyon look-alike in White River Valley, contact Terry Mather or Jay Leaver at TPA, 303-436-1930.
Gravity anomalies have been shown to have a direct relationship to the general locations of oil fields in Railroad Valley, Nevada (Guion and Pearson, 1979, RMAG Guidebook). Gravity gradients mark the density contrast between Tertiary valley fill and pre-Tertiary rocks, thus defining normal faults. In many cases, such faults do not reach the surface and do not coincide with range-front faults. While the basin-range normal faults are important contributors to the hydrocarbon traps in Railroad Valley, the oil accumulations are more complex and are related to other structural and stratigraphic features.Guion and Pearson (1979) pointed out that Eagle Springs Field is in a gravity "embayment" or low, bounded on the south by a small ridge in Paleozoic rocks and on the east by two normal faults. The southern margin of the embayment is a subtle east-west linear feature visible in magnetic data and possibly in gravity data as well (Nev. BuMines, 1983).
Lineaments have long been related to structures and regional features in Nevada. Rowan and Wetlaufer (1981, AAPG Bull., Aug.) described lineaments as parts of three broad structural zones. In my study, five linear features are correlated with the locations and geometries of all the hydrocarbon production to date in Railroad Valley.
Linear disruptions in east-central Nevada were identified initially on maps of magnetic data (USGS MF-902); many had been defined earlier by Ekren and others (1976, USGS Prof. Pap. 986). Most of the grabens in this part of Nevada are segmented along their lengths, and are expressed as chains of sub-circular to oval gravity lows separated by high saddles. When the linear features defined from magnetics were compared to gravity maps, it was found that many of them corresponded with the gravity high saddles that segment the grabens.
The linear geophysical features have abundant surficial geological expression and are also evident in Landsat images. Rowan and Wetlaufer have summarized this evidence for the lineaments they described. In many cases, these features also coincide with marked excursions in the boundaries of playa deposits and the dog-leg geometry of range-front faults as mapped by Stewart and Carson (1978, Geol. Map of Nevada).
Portions of five regional lineaments are shown on Figure 1. The three east-west features, from south to north, are 1) the Warm Springs Lineament, somewhat north of the feature defined by Ekren and others (1976); 2) an extension of the Pancake Range Lineament; and 3) a lineament that passes near the town of Currant.
The NE-SW linear is one of several that are well expressed in magnetic data, especially northeast of Ely; it is marked by a subtle but clear gravity anomaly (Guion & Pearson, 1979, Fig. 10). The NW-SE lineament is the extension of the Northern Nevada Rift of Zoback and Thompson (1978; Geology Vol. 6, p. 111) (Oregon Nevada Lineament of Stewart and others, 1975, Geology). The magnetic expression of the latter lineament is much weaker in the area of Railroad Valley than in areas to the northwest, where the magnetic anomalies are related to a linear mafic dike swarm (Zoback and Thompson, 1978). Rocks similar to the surficial rocks corresponding to the strongest magnetic anomalies are mapped as basalt and andesite flows. These rocks crop out along the lineament from the Idaho border to Kobeh Valley, and in isolated small outcrops in the Grant Range and White River Valley, just southeast of Railroad Valley.
