Temperature method can help locate oil, gas deposits

Lloyd Fons
Lloyd Fons Exploration Inc.
Houston

One can expect hydrocarbon deposits to be accompanied by overlying negative temperature anomalies. The method entails the construction and interpretation of temperature anomaly maps to locate subsurface hydrocarbons (quantitatively) without the need for previous exploration data. It can locate stratigraphic trap reservoirs, predict reservoir shape and boundaries, explore below the bottom of drilled holes, and faulting or the presence of intervening salt do not adversely affect it.

The method is simple, rapid, safe, environmentally perfect, and very inexpensive. Generally, no permitting is required, and answers are obtained within several hours of survey completion.

Reservoirs as thermal barriers

Heat flows from within the earth to its surface, according to the Second Law of Thermodynamics.

Hydrocarbons are considerably more insulative to heat flow than are common earth materials. Hydrocarbon-connate water combinations can be much more insulative to upward heat flow than are hydrocarbons alone.

Hydrocarbon-rich reservoirs form relatively efficient, though imperfect, thermal barriers, resulting in a dynamic equilibrium condition of negative temperature anomalies above and positive temperature anomalies below hydrocarbon reservoirs.

The greater the negative anomaly, the greater the hydrocarbon deposit below the point of measurement. The greater the positive anomaly, the greater the hydrocarbon deposit above the point of measurement. A wintertime, cold climate analogy would be to equate the amount of attic insulation in occupied homes to the amount of snow on the roof.

Thermal conductivity is a widely misused term. Published values for earth materials such as quartz are more than 200 times that of air, yet the observed gradient for near-surface sediments (about 1° F./100 ft) is three to four times that of the near-surface atmosphere (0.24-0.39° F./100 ft). This shows that air can be a very much better heat transfer agent than the common sediments.

The author believes this method to be a major breakthrough in exploration geophysics. It might well lead to the finding of a considerable part of the world's remaining undiscovered hydrocarbon deposits.

Temperature indicators

Temperature-versus-depth plots taken from well log bottomhole temperature measurements in the Province of Alberta, Canada, demonstrate a 15-34° F. increase in temperature below hydrocarbon deposits as compared to average temperatures for the depth in question in areas where hydrocarbon reservoirs are absent (Fig. 1). [98,314 bytes] A positive temperature anomaly below a single pay zone is accompanied by an equal and opposite direction (negative) anomaly above the hydrocarbon deposit.

Data used to construct the graph of Fig. 1B included field wells. If one were to remove the field well data, the differences between average temperature versus depth and that below oil fields at the same depth would be even greater. When one considers the probability that additional hydrocarbon zones lie below the drillstem test depth, the temperature difference projection is greater yet.

Average air-temperature maps can locate oil and gas deposits. A 1941 Department of Agriculture map of average annual temperature for the U.S., with temperatures recorded 41/2 ft above earth surface at an estimated 10,000 stations, shows an essentially "fail-safe" relationship between negative anomalies and known fields (Fig. 2) [46,482 bytes]. Particularly well defined are the hydrocarbon accumulations in the Arkoma, San Juan, Anadarko, Delaware, Fort Worth, and Midland basins and the Sweetgrass arch, where many known oil and gas fields were discovered after 1941.

This map represents a broad brush approach resulting from sparse control points. Forty-two years later, in 1983, there were still only 13,000 official control points in the U.S. Consider the time and money that could have been saved had such maps been properly utilized at their inception.

Texas temperature indicators

A Texas Department of Agriculture Weather Bureau map, "Mean Isotherms and Prevailing Winds in Texas for the Year 1916," indicates the most favorable counties for oil and gas exploration to be Panola, Rusk, Stephens, Eastland, Fort Bend, and Brazoria (Fig. 3) [92,603 bytes].

The Texas map betrayed the location of the East Texas oil and gas field long before its conventional discovery in 1930. The simple mapping of infrared measured surface temperatures under acceptable conditions appears to be faster, simpler, more inclusive, and more accurate (Fig. 4) [159,041 bytes].

Existing subsurface temperature gradient maps can be of considerable value in identifying "new" reserves. The simple observation of a 1947 map shows the most favorable area for deeper drilling to be the prolific Delaware basin that would wait until the early 1960s to be "discovered" by drilling existing wells deeper (Fig. 5) [70,211 bytes].

Nonvugular reservoirs contain untold millions of pore spaces within which oil is found in contact with connate water that wets the internal pore surfaces (Fig. 6) [23,522 bytes]. In addition to oil being much more thermally insulative than water, oil and water are immiscible, with every oil-water interface providing a perfect barrier against buoyancy type (natural) convective heat transfer.

Other methods compared

Important comparisons between temperature and seismic methods for exploration can be made.

Aside from surface solar heating, earth heat flow occurs in only an upward direction. Seismic survey energy essentially originates at the surface and is reflected back to the surface to include both downward and upward (two-way) travel.

Also, differences in thermal conductivity and acoustic travel time properties of oil, gas, and water are significant (Table 1) [6,901 bytes].

Under the standard temperature and pressure conditions in Table 1 and with other factors being equal, the presence of oil affects the thermal conductivity of a porous material to a far greater degree than it does acoustic travel time (4.63/1.28 = 3.62/1). The same is true for gas (19.04/3.37 = 5.56/1). Although comparative lithology effect between thermal and seismic methods is beyond the scope of this article, the temperature method appears to be the less complex.

Airborne surveys made under ideal atmospheric conditions have resulted in a fail-safe efficiency rate of more than 75% in locating and delineating known fields. One 640 mile survey clearly identified 11 of 14 known fields within a 3½ hr period and provided for interpretation and correct answers before breakfast on the day of the survey. One should compare the surveying of 650 miles in 3½ hr with the time and costs involved in surveying a similar distance using other exploration methods.

A series of shallow (5 ft depth in this case) temperature measurements across giant Webster oil and gas field, Harris County, Tex., (Fig. 7) [95,583 bytes] shows this method to be a reliable hydrocarbon prospecting tool. Such surveys can present fewer variables to interpretation but are more time consuming than noncontact infrared measurements and commonly require landowners permission.

A wealth of "public record" information on bottomhole temperatures is available from well log headings. A statistical study of nearly 150,000 temperature-versus-depth data points has shown that some industry logic related to subsurface heat flow is incorrect.

The effects of hydrocarbons on temperatures above and below a pay zone in Live Oak County, Tex., are shown on Fig. 8 [103,410 bytes]. The first-run negative anomaly at a total depth of 7,704 ft indicates that one can expect to encounter at least 38 ft of pay by deepening the well by less than 3,000 ft. The positive anomaly at the total depth of 8,103 ft indicates the likelihood of no additional hydrocarbon potential below the present total depth. A temperature gradient chart for Live Oak County is shown on Fig. 9. [105,734 bytes]

Offshore temperature surveys in Galveston Bay show near-perfect relationships between negative water-temperature anomalies and underlying hydrocarbon deposits (Fig. 10) [138,437 bytes]. Offshore surveys eliminate the consideration of possible variations in surface material emissivity and pore content.

Salient findings

It is hoped that the following findings will prove helpful and eliminate some common misconceptions.

1. Considering sedimentary basins, essentially all subsurface temperature gradient curves are exponential. Present service company log analyses imply linear gradient "curves." In Bee County, Tex., at 1,000 ft the gradient is about 0.8° F./100 ft, whereas at 14,000 ft the gradient is about 3.5° F./100 ft.
2. Most industry definitions of "thermal conductivity" (including the Encyclopedia Britannica's) are incorrect as they fail to limit such heat transfer ability to conductive flow.
3. Subsurface water has the highest upward transfer ability of any common sedimentary material. Models demonstrate that water transmits heat in an upward direction at more than 20 times that of downward flow.
4. The author has yet to encounter a positive temperature anomaly over a salt dome.
5. One cannot construct accurate subsurface isotherm maps by observing values at one depth and projecting temperature values to a predetermined depth datum.
6. Most published values for thermal conductivity are of little practical use because sedimentary materials have differing degrees of porosity and pore fluid content and are at different temperatures.
7. Almost certainly, more than 20% of all service company BHT values that appear on log headings are not related to the actual reading of a properly used maximum reading BHT thermometer.
8. Industry literature states that thermal conductivity increases with depth because of decreasing porosity. However, the thermal conductivity of sediments is quite temperature sensitive, decreasing with increasing temperature. In fact, upward heat transfer may actually decrease with depth because of a decrease in porosity.
9. Temperature gradients through pay zones can be as high as 100° F./100 ft.
10. Subsurface heat flow cannot be determined by the arithmetic averaging of published values of thermal conductivity. Thermal conductivity parallel to the bedding planes is generally more than 30% greater than that across the bedding planes.
11. Convective heat flow may, in many instances, be the dominant means of upward energy transfer.
12. Variations in time since circulation in using BHT methods are of no major consequence in the interpretation process because of the magnitude of most anomalies and uniformity of error.
13. Remote sensing of earth surface temperature by satellite is greatly affected by the temperature of atmospheric moisture between the sensor and target whereas fixed distance, very near surface collected temperature anomaly mapping can eliminate this problem.

    The author holds five patents related to the above described methods.

The Author