Western Cordillera has observed a recognizable and predictable bouguer gravity pattern or signature associated with the structural closures of the Blackburn and Grant Canyon oil fields within Pine and Railroad Valleys. We have termed this signature the Block and Arch Model and feel that it represents Tertiary extensional fault blocks and arches in structurally high positions within Cenozoic basins. These structurally high blocks and arches are formed along high-angle planar and listric normal faults, and have created traps for Tertiary generated and migrated hydrocarbons.

The proposed model is based upon the interpretation of regional bouguer gravity data which can be used to map the overall basin configuration, including basin bounding and smaller intra-basin high-angle faults. The regional bouguer gravity data indicate that the Cenozoic fill is less dense than Paleozoic lithologies, so that gravity "lows" are indicating the deeper areas of basin fill. On a regional scale, this has been used in conjunction with well data to construct the Cenozoic Basin Fill Isopach Overlay II.

In some cases, a simple model assigning a low density of 1.9 gm/cc. to the Cenozoic basin fill, and a density of 2.6 gm/cc. to Paleozoic carbonates is quite adequate. However, the presence of Tertiary volcanics with densities approaching those of Paleozoic carbonates, local shallow intrusive masses, and Chainman shale with lower densities than enclosing Paleozoic carbonates will complicate the final solution of two dimensional gravity models.

Closely spaced contours (steep gradients) in the gravity data indicate the major basin bounding fault system that controls most of the Cenozoic basins within the evaluation area. The major basin bounding fault system can be predicted and mapped with assurance from the gravity data, and can be confirmed with seismic data. In any given valley there may be an observed saddle or arch in the gravity contours that separates deeper depressions or sub-basins within the basin (Figure 1). These arches represent remnant blocks of Paleozoic rocks that have not been downdropped as deeply as the remaining basin (Figure 2). The mosaic of high-angle fault blocks shown in Figure 2 are the result of two periods of Tertiary extension which occurred at oblique angles to one another. The resulting high blocks are closures with potential hydrocarbon entrapment. The retention of hydrocarbons within these fault blocks and arches is largely dependent on the sealing capacity of the block bounding faults that place generating source rocks against fractured reservoir sections. These faults are probably fluid migration conduits at one point in time, and tight and sealing at another point in time.

Positive aspects of the resulting traps are the shallow depth to the prospective horizon, defineable limits of the closure area, and permeability and porosity enhanced by fracturing. The remaining unknown structural aspect is the uncertainty of internal structure within the fault block.

Updip truncation of Paleozoic through Tertiary sediments into high-angle normal faults beneath the sealing basal Tertiary unconformity, is the most probable and readily discernable trap-type in northeastern Nevada. Dip-slip movement of strata in the hanging wall of concave listric normal faults often results in anticlinal closure as a result of "reverse drag". These complex anticlinal closures provide additional trap possibilities.

An analogous structural setting is present in the Sirte Basin of Libya, the Gulf of Suez and Viking Graben. The Gulf of Suez and Viking Grabens were as a result of two periods of extension. Initial extension produced relatively closely spaced faults with moderate displacements. A later extension produced fewer and more widely spaced faults that formed platforms or agglomerates of small fault blocks (Harding, 1984; Hay, 1978; Robson, 1971). This structural development is essentially geologically identical to the Cenozoic evolution of northeastern Nevada.

The large hydrocarbon accumulations in the Sirte Basin of Libya and in the Gulf of Suez and Viking Grabens produce principally from rotated fault blocks in both high and low structural positions that are truncated by an unconformity. This setting is similar to the know accumulations in Pine and Railroad Valleys within the evaluation area.