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Over the years, several observers have reported on temperature observations which show a particular oil or gas field to be "hotter" at the pay depth than the surrounding rock at the same depth. Our study of 22 oil and gas fields from six states in the Rocky Mountain region demonstrates that at least 15 of these fields have positive temperature gradient anomalies at the pay level. Nine of these "hot" fields are contained in structural traps and six are primarily stratigraphic accumulations. Three of them are gas and 12 are oil fields.
All of our temperature measurements were recorded during drill-stem tests except for a few values from temperature logs. Drill-stem test temperatures usually are recorded a longer time after mud circulation has ceased in the well bore than are wireline log temperatures. Therefore, the former generally are a truer measure of the formation equilibrium temperature than are the latter.
Speculating on the causes of these temperature anomalies over oil and gas fields, we conclude that upward convective fluid movement at depth is the most important factor. The upward moving fluids carry heat along with them and both heat and fluids are trapped whenever suitable trapping conditions are encountered in the reservoir rocks through which the fluids pass. The main evidence for this conclusion is that observed
temperature anomalies occur over fields which are contained in stratigraphic traps.
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