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Examples of high-resolution temperature logs measured in oil and gas fields in the United States are presented and the pertinent features useful in basin analysis are discussed. We point out that wells suitable for equilibrium or near equilibrium temperature logs usually are available, and we describe by examples criteria for the evaluation of the quality of a high- resolution temperature log. Examples of temperature gradient logs from two fields in the Paleozoic-age Anadarko Basin in Oklahoma, one field in the Cenozoic Gulf Coast Basin, and one field in the Mesozoic/Cenozoic Sacramento Basin in northern California are described and their application to the analysis of basin thermal structure discussed. The major criteria that can be used to evaluate the quality of the log are level of (temperature) noise, presence/absence of negative/zero gradient sections, degree of correlation with other geophysical logs, and well to well comparisons. Even logs that are not in complete equilibrium contain significant information compared to a typical set of BHT points. The development of memory P/T tools and their deployment in the field for production logging indicates that the potential now exists for routine collection of high-resolution temperature data in hydrocarbon settings worldwide. Because the thermal regime of many boreholes in producing fields may be closer to equilibrium than has been thought in the past, the new temperature capability can be used in a practical way. High-resolution logs can furnish detailed information on the gradient, the ratios of the thermal conductivity values in hard-to-sample lithologies to those in lithologies easier to characterize, and ultimately more precise understanding of the thermal regime (whether conductive, convective, etc.) in individual wells and sedimentary basins.

Gabbro, gabbroic anorthosite, basaltic breccia, diabase and basalt samples were dredged from a fracture zone of the Galapagos Rise, a fossil spreading center on the Nazca plate. Serpentinite was dredged from the Ecuador fracture zone of the Galapagos Spreading Center. Although the rocks are variably altered, minerals and chemistry of gabbroic rocks indicate that extensive fractionation occurred throughout the suite. Basaltic breccias have textures indicating prefracture zone brecciation.

Magnetic anomalies with unusually large amplitudes are found along the Galapagos spreading center near the Galapagos Islands from long. 85° to 95° W. On either end of the Galapagos spreading center, however, normal amplitudes occur. Modeling of these magnetic anomalies, which were measured by both surface and deep-tow methods, indicates that the amplitude differences are probably caused by variations in the magnetic intensity of the rocks in the area. Tholeiitic basalt samples that were recovered from the ridge crests involved showed the expected variation in magnetic intensity. Basalt samples from the ridge crests with normal-amplitude anomalies contained 9 percent total Fe (calculated as FeO) and 1 percent TiO 2 , whereas basalt samples from the ridge crests with large-magnetic amplitude contained 14 percent total Fe and 2 percent TiO 2 . Variations in MgO, CaO, P 2 O 5 , K 2 O, and Al 2 O 3 , as well as in FeO and TiO 2 , are consistent with the theory of Fe enrichment by fractional crystallization as a mechanism for the formation along ridge crests of tholeiite with high magnetic intensity. This phenomenon may be related to the proximity of those ridge segments with large magnetic amplitudes to the Galapagos melting anomaly. Furthermore, the transition from large magnetic amplitudes at the ridge crest near the Galapagos Islands to normal magnetic amplitudes on the flanks suggests that the Galapagos melting anomaly may have formed 3 to 4 m.y. ago.