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Bouguer Gravity and Geothermal Gradient as Exploration Tools for Gas in the Denver Basin

Moore, W. Richard
Independent Geologist, Crawford, CO

Bouguer gravity and geothermal gradient anomalies occur over areas of shallow and deep basin gas in the Denver Basin. Both the Wattenberg Field, in the deep portion of the Denver Basin, and the Niobrara gas trend on the shallow, eastern flank of the basin, are defined by areas of bouguer gravity minima and high geothermal gradient. It is believed that this combination of data is related to basement fracture trends. The bouguer gravity minima are probably related to intrusion of less dense granitic materials into metamorphic rocks through deep- seated basement fracture zones. These same fracture zones conduct heat through the basement and sedimentary column more readily than non-fractured areas resulting in areas of high geothermal gradient. Furthermore, the fracture zones are the major determining factor in the location of the gas fields. At Wattenberg the fractures have provided most of the reservoir volume for the field. In the shallow Niobrara trend the fracture zones have caused salt dissolution. This dissolution has resulted in anticlinal and fault closures which are the primary mechanisms for gas entrapment in the shallow Niobrara. This combination of exploration tools, geothermal gradient and gravity, is very easy and inexpensive to obtain and can be used as an efficient and quick exploration tool.