Upthrown Side Of Fault Research Articles

Geologic and Geothermal Fluid Flow Models of Cerro Prieto Field: ABSTRACT

A detailed hydrogeologic model of the liquid-dominated Cerro Prieto geothermal field in Mexico (located in Baja California about 35 km [22 mi] south of the U.S. border) has been developed to identify subsurface geothermal fluid flow paths. This was accomplished by integrating downhole temperature profiles and depth of production intervals with a geologic model derived from the interpretation of geophysical and lithologic well logs. Two different methods were used to define the geologic model of the Cerro Prieto field. In the first, the lithology was divided into seven different formations based on correlatable groups of beds having distinct lithologic, structural, and fluid production characteristics. In the second method, the lithology was classified into three basic types of lithofacies: shales, sandy shales, and sands. Faults identified by the first method were incorporated into lithofacies sections. Figure Geothermal fluid flow paths were identified by integrating well temperature profiles and the depths of geothermal well production intervals with the lithofacies cross sections. Hot fluids are believed to enter the field at depths greater than 3,000 m (10,000 ft) from the east through a thick, porous sand unit, called Sand Unit Z, which dips gently to the east. This unit underlies a 450 to 600 m (1,500 to 2,000 ft) thick low porosity shale unit, named Shale Unit O, which acts as a local caprock. The fluids flow westward until they encounter a normal fault, Fault H, with downthrow to the southeast. The fluids then move up along this fault until they again encounter Sand Unit Z along the upthrown side of Fault H. Whereas most of the fluids continue to flow westward through Sand Unit Z, a small portion rise along Fault H, as evidenced by high temperatures found at shallow depths in the faulted region. The westward flow continues until it reaches a gap in the overlying Shale Unit O. There, some of the flow rises up and into the western Shale Unit O, which is sandier in this region, while the rest continues to flow westward through Sand Unit Z. Eventually, some of the geothermal fluids leak to the surface, whereas the rest mix with colder waters that surround the geothermal system. A number of cross sections detailing the subsurface movement of the geothermal fluids are shown. This hydrogeologic model is consistent with other geologic, geophysical, geochemical, and reservoir engineering studies carried out at this field. End_of_Article - Last_Page 477------------

LIMESTONE WALLS OF OKINAWA

Wall-like ridges of limestone that stand well above the surrounding terrain are an interesting phenomenon on the island of Okinawa. These ridges rim a variety of topographic features, but all are believed to represent the same formative processes. Rimming ridges or walls occur along the banks of streams crossing areas of limestone, along the upthrown sides of faults, around a sink hole, and along terrace edges. The limestone walls of Okinawa are believed to have formed by the cementation, or casehardening, of steep exposures of poorly consolidated limestone, followed by differential erosion which brings the cemented zones into relief. Cementation occurs wherever poorly consolidated limestone is exposed to alternate wetting and drying. It is best developed, however, on steeply sloping exposures where no soil accumulates to hinder drying. Differential erosion results in part from slower solution of cemented limestone, due to its greater density, and in part from accelerated solution in the surrounding soil-covered areas. Rampart walls along the seaward edges of terraces have been described from many Pacific islands and are present on many others. They are believed to be comparable to the walls of Okinawa and all are believed to have been formed under similar conditions.

Geology of Green and Yampa River Canyons and Vicinity, Dinosaur National Monument, Utah and Colorado

The Green and Yampa River Canyon country within the boundaries of Dinosaur National Monument represents the deeply dissected eastern end of an east-west anticlinal fold, the Uinta Mountains, and its accompanying smaller flank folds. Oldest formation exposed in this area is the Uinta Mountain group (Algonkian) and the youngest, Browns Park formation (Pliocene). Along the crest of the main arch, in the deeper canyons, and at some places along the upthrown sides of faults are exposed massive dark red blocky-weathering quartzitic sandstone, shale, and conglomerate beds of the Uinta Mountain group, and the shale and quartzitic sandstone beds of the Lodore formation (Cambrian). In much of the area, Mississippian, Pennsylvanian, and Permian rocks form long dip-slopes toward the outh. Typical of Uinta Mountain structure and representative of the smaller folds that flank the main arch of the Uinta Mountains is the Split Mountain-Yampa Plateau area. Formations along the crest of this anticlinorium have low attitudes in contrast to the sharply flexed, steeply dipping, younger flanking formations. These strata with their well developed flat-iron and race-track patterns of exposure exhibit a striking, colorful geological cross section ranging in age from Lower Pennsylvanian (Morgan) to Upper Cretaceous (Mancos). Lower Pennsylvanian and Lower Mississippian are exposed in the deeper part of Split Mountain Canyon. High-angle normal faults occur in the Monument area. The Split Mountain-Yampa Plateau section is traversed by the Yampa fault which seems to die out in the western part of the fold. Split Mountain Canyon has been carved by the Green River along the western end of this zone. Stream piracy and deep entrenching of the meandering stream courses during Pleistocene have produced a spectacular scenic area.