The earth is a gigantic heat engine. A tremendous amount of heat is con- stantly transported from its center to the surface by thermal convection and conduction. The geothermal heat is ultimately the driving force of most large-scale geologic processes that take place on the surface of the earth (e.g., move- ment of tectonic plates, volcanic erup- tions, etc.). A portion of the heat con- ducted through the earth’s crust is used to drive the chemical reactions which transform organic matter contained in sedimentary rocks into petroleum. Without the geothermal heat, there would be no naturally occurring petro- leum on this planet.Therefore, measur- ing this heat and understanding its transport mechanisms through the crustal rocks are essential to the science of petroleum exploration. In this article, we describe how ge- othermal heat flow is measured on the seafloor and how such data are utilized in offshore oil and gas exploration, es- pecially in the Gulf of Mexico. We focus on three types of application: hydrocar- bon maturation analysis, subsalt explo- ration, and assessing the stability of marine gas hydrates. The data used here are from the heat flow programs conducted by TDI- Brooks in the last several years. Measurement techniques Geothermal heat flow through the seafloor is determined as a product of two separate measurements of the ther- mal gradient in, and the thermal con- ductivity of, the sediment in a depth interval. A single instrument can per- form both measurements. A typical marine heat flow instru- ment (Figs. 1 and 2) is equipped with a thin (1-cm diameter) metal tube of 3- to 7-m length, which contains a dozen or so thermistors spaced along its length.The temperature data ob- tained at individual thermistors are stored in the digital data recorder in a pressure-proof housing attached at the top of the metal tube. The instrument is lowered to the sea bottom by a winch cable from a ship. When the instrument reaches the seafloor, the thermal sensor tube pene- trates vertically into the sediment and records the temperature continuously at each thermistor location (Fig 2). The sediment temperatures obtained at different sub-bottom depths (Fig. 2) define the geothermal gradient. How- ever, what the instrument measures while it is on bottom is not the ‘real’ or equilibrium temperature of the sedi- ment, because the temperature of the sediment around the probe rises quick- ly upon penetration of the probe due to frictional heating (Fig. 2). As the frictional heat starts dissipating into surrounding sediment, the temper- ature measured at each thermistor grad- ually falls toward its original value. The equilibrium temperature can be extrapo- lated from this record of temperature decay. 1 In determining the geothermal gradient, we use the theoretically extrap- olated equilibrium temperatures. Five to 10 minutes after the penetra- tion, the probe applies a calibrated, in- tense heat pulse to the surrounding sediment for about 10 sec.The temper- ature of the probe rises again quickly but falls after the termination of the heat pulse (Fig. 2).The temperature de- cay is controlled by the thermal con- ductivity of the sediments. The heat dissipates relatively quickly through sediment of high thermal con- ductivity but slowly through low-con- ductivity sediment. Da- ta from the thermal de- cay after the heat pulse allows the thermal con- ductivity to be calculat- ed. A heat flow instru- ment such as the one used by TDI-Brooks can determine the heat flow with an accuracy of 1% to 3%. 2 3 The res- olution of the tempera- ture measurement made by individual thermistors is better than 1/ 5,000 of 1° C. Application of marine heat flow data important in oil, gas exploration E XPLORATION & D EVELOPMENT Seiichi Nagihara Texas Tech University Lubbock James M. Brooks Bernie B. Bernard Neil Summer TDI- Brooks International Inc. College Station, Tex. Gary Cole BHP Billiton Ltd. Houston Trevor Lew is Sidney Geophysical Consultants Ltd. Sidney, BC The marine heat flow instru- ment designed and built by Geo- logical Survey of Canada and operated by TDI- Brooks Inter- national Inc. ( Fig. 1 ) . Reprinted from the July 8, 2002 edition of OIL & GAS JOURNAL Copyright 2002 by PennWell Corporation