Effects of Earth Tides on Vertical Migration
Gerry G Calhoun, James L Hawkins
New Paradigm Exploration, Inc., Midland, Texas
During the winter of 1999-2000, two planetary conjunctions affected ocean and earth tides. The first was a once-a-century close pass between the sun, earth, and moon with strong alignment on December 21-22, 1999; the second, a lunar eclipse on January 20-21, 2000.
New Paradigm personnel collected BTEX (benzene, toluene, ethyl benzene, and Xylene) gas migration data at a single station during both of these events. Subsequently, a number of daylong observations of BTEX data were made during “normal” tides.
The premise of the experiment was that vertical fractures would open more widely when the tensional component of the earth tides was at a maximum. It was assumed that this opening would occur during the two planetary alignments. Results indicate that the tensional component does, in fact, correlate with a 50-150% increase in BTEX concentrations during some extraordinary earth tides. Such evidence strengthens the theory that vertical fractures play a role in vertical hydrocarbon migration.
Figure 1. A north-south cross-section through the earth’s center in the plane of the moon’s hour angle; the dashed ellipse represents a profile through the spheroid composing the tidal force envelope.
The working hypothesis for this study is that earth tides, differentially through time, open and contract the vertical fractures through which hydrocarbon gases migrate to the surface. It is further assumed that both high vertical and low horizontal earth tides act in tandem to create tension in the crust which facilitates gas flow. The purpose of this paper is to examine such phenomena.
Describing the mechanisms which cause earth tides is not necessary to pursue our present
purpose and would be unnecessarily time-consuming. Suffice it to say that the moon affects earth tides 2.5 times more than does the sun. Studies have created a computer program to derive daily earth tides at any latitude-longitude for any time period. This program was used to define and locate these times of high tide forces.
Earth tides do not conform to ocean tides, which are influenced by traction and by inertia in the water. Continental barriers also disrupt ocean tides. Rather, earth tides reflect the location and relative position of the earth, moon, and sun with no lag. Tensile forces are present in the crust at high and low tidal conditions; however, the maximum tensile stress is located at point E (Figure 1). Such a positioning may seem counterintuitive at first glance, yet the referenced authorities indicate that the current location is, in fact, the case. The concentrations of gravity forces occur at the four points where the tide-caused ellipse (dashed line) crosses the circle representing an unstressed earth. This meridian of maximum horizontal stress is called in this paper the “45-degree window.”
Unless specifically stated, all experiments were conducted at a single site in Midland, County, Texas. Turning now to the evidence collected during a once-per-century close approach of sun, moon, and earth at a full moon in December 1999, the so-called “Millennium Tide” is shown on Figure 2. The earth was at its closest position to the sun at the same time when the moon was at its closest position in its orbit around the earth.
First, observe the extreme range of tidal swings (Figure 3) from high vertical tides at the top and horizontal tides at the bottom. The scale on the left is in meters of crustal movement in response to tidal forces. The Millennial tide movement ranges from 35 cm upwards to 8 cm downward from a neutral position. Also note that low vertical tides are inverted back above the zero values.
This mechanism is to help visualize when maximum low tide force occurs during a standard low tide. High vertical tides are labeled “H” and low vertical tides are labeled “L.” The thick dashed line represents the time when the maximum tensile stress, the so-called 45-degree window, occurs. This 45-degree window is located equi-distant between the high and low vertical tides.
On Figure 4 the benzene curve (represented by the X symbols) has a background level of about 90 ppb (parts per billion). The pulse at 9pm reaches 146 ppb at the midpoint between the low horizontal tide at 5pm and the high vertical tide at 12:30am. This is the 45-degree window mentioned earlier. No data was taken from 2am to 9pm. The window from a high vertical tide at 12:30pm until a low horizontal tide at 6pm is nearly filled by a three-hour pulse up to 147 ppb at 3pm, the center of the 45-degree window.
In Figure 5 the toluene (represented by triangle symbols) shows a background of 15 ppb with a pulse at 9pm up to 29 ppb matching the benzene data. A second pulse at 9am of 39 ppb is near the 45-degree window between a low horizontal tide at 7am and a high vertical tide at 1pm.
Figure 6 shows that the ethyl benzene curve (represented by box symbols) has a background of about 10 ppb and is uneventful until the 9am pulse, matching the toluene, and a 3pm pulse, matching the benzene peak at that time. The empirical evidence tends to support the 45-degree window timing of the BTEX pulses.
To understand better normal tidal conditions, two examples--a spring or full/new moon tide and a neap or one-quarter moon--were studied. Sun, moon, and earth are aligned in a spring tide, thus maximizing all tides. In neap tides the sun-earth line is at right angles to the earth-moon line, thus minimizing tidal effects.
The spring tide at full moon on July 6 and 7, 2001, (Figure 7) shows the large swings of tidal forces with the sun, earth, and moon aligned. Note the similar, yet weaker, response compared to the Millennium tide curves. The benzene curve (Figure 8) shows background levels of 20 ppb, and a 112 ppb pulse at 9:30am in the 45-degree window. The pulses at 7pm (July 6) and at noon (July 7) are off the 45-degree window and seem to anticipate tidal events. No samples were taken from 8pm until 7am. The toluene (Figure 9) has an average level of 3ppb with a 9:30am peak on window. On July 7 toluene pulsed at 10:30am and 4pm at the center of the 45-degree window. Ethyl benzene followed toluene (Figure 10).
The neap tide (Figure 11) on February 19, 2002, shows much less contrast between high and low tides. High tides are smoother, less disrupted curves. The less abrupt breaks from one condition to the opposite are conducive to more uniform gas flow to the surface. Uneventful curves show few concentration changes. A hard copy of tides from March 3, 2002, to April 27, 2002, using tide force program is available for your study. The instrument used to measure BTEX gases is the MSI-301, routinely deployed in oil exploration.
An actual field test was conducted using 38 stations scattered over several sections in Dawson County, Texas. The test shows how tidal disturbances can be recognized and discarded. Figure 12 is the benzene response to the 9:30am and 4:15pm pulse windows. Toluene and ethyl benzene pulses also match the 45-degree windows (Figures 13 & 14). Truly anomalous areas on benzene are obvious.
Many of the pulses of the hydrocarbon gas occur at the 45-degree window, and the length of the gas concentration pulse averages about an hour. This pattern suggests that hydrocarbon gases are expelled at a faster rate during earth tide forces over a short time and are then lowered back to their normal pressures.
A possible explanation for the intermittent nature of gas pulses, that is, the apparent failure of some 45-degree windows to cause a gas pulse, is that the volume of gas from hydrocarbon accumulations is not sufficient to provide enough gas to supply a pulse at each tidal peak. A short period of reflux to a critical pressure in the fractures may be necessary before a subsequent tidal flux triggers another gas pulse. Other factors involving fracture-filling rate are seal integrity, overburden composition, and depth of burial.
While most of the tidal windows do not trigger pulses of soil gas migration, a significant number do. There can be little doubt that geochemical techniques which take instant readings, such as alkanes in soil gases, the BTEX gases, radar, and radiometric methods, are susceptible to measurement error resulting from tidal flux. NPE has revisited projects where erroneous predictions were made and applied the factor of tidal forces to the data. In at least one project, the errors coincided with tidal disturbances. The predictive accuracy of commercial success can be improved merely by avoiding periods of tidal change--primarily at new and full moon phases.
By revisiting anomalies at different times during the project, the danger of false readings is minimized. NPE personnel now take graphs of tidal forces to the field on each project. This information alerts the field technician to possible times of erratic concentration and guards against potential errors in interpretation. Every instrument used in these experiments has recorded these brief gas spikes, so the bursts do not result from instrument malfunction.
Finally, the question arises of how fractures that originated in the Cambrian Era, for instance, can find expression through time and reach today’s surface. Rejuvenation of ancient tectonics has long been the explanation for this condition. For a fracture to be most effective in transferring hydrocarbon gases from reservoir to detector, it should have vertical pneumatic continuity. The billions of earth tide expansions and contractions through geologic time offer a strong mechanism to perpetuate such fractures. Cannon (Cannon 1998) proposes that tidal flux provides a four-times-per-day bending that, over millions of years, fatigues and reinforces the old zones of weakness.
In summary, earth tides affect hydrocarbon gas migration, a relationship that may be a strong indication of pathways of vertical migration. Alton Brown (Brown 2000) made great strides in quantifying rates of vertical gas migration and in documenting the role of fractures in migration.
This paper provides additional evidence that vertical fractures are the primary conduit through which oil traps communicate clues of their presence to the surface. While evidence from this small sample is intriguing, follow-up studies are essential to document further details of timing tidal and migration events. It may be possible to identify which tidal event is more prone to trigger migration pulses. There may be measurable differences between migration rates during full moon conditions as compared with new moon conditions. Other basins and other techniques are fertile fields for future study.
Author Unknown, Our Restless Tides, National Oceanic and Atmospheric Administration, 1990, out of print.
Brown, Alton, Evaluation of possible gas microseepage mechanisms, AAPG, v. 84, No. 11, November, 2000, pp 1775-1789.
Calhoun, G.G. , Hawkins J.L., BTEX detector’s results good in oil identification, Oil & Gas Journal, March 30, 1998.
Cannon, Jan, Support for a Terrestrial Model of Gravity Tide Tectonics, AAPG Southwest Convention Transactions, March 29-31, 1998.
Schenewerk, Mark, personal communication, January 3, 2000.
Zervice, Chris, NOAA, personal communication, March 2002.