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A new gamma ray data acquisition and interpretation technique
was used to select locations for 10 successful oil wells in Hart County, Ky.
The method uses potassium 40 surveys coupled with remote sensing imagery and
subsurface geological mapping, which provide information about the
hydrocarbon production potential of the field.
Several surface techniques to find oil and gas deposits prior to drilling
have been developed during the last century. Magnetic, gravity, geochemical,
iodine, telluric, radon, radiometric and seismic surveys have their
respective strengths and weaknesses in various geologic settings. No single
method can reliably find oil and gas in all geologic settings; even 3-D
seismic has had its share of dry holes. There appears to be no direct
relationship between the amount of money spent on surveys and the amount of
oil and gas found. Indeed, if 1/10 of the money spent on seismic research
were spent on researching other surface methods, the industry would be
drilling better wells and fewer dry holes.
Radiometric or gamma ray surveys, which relay on the
surface measurement of natural gamma radiation, have been popular since the
1940’s. Many empirical studies have shown that there is often a low total
gamma measurement above oil and gas deposits. This low is surrounded by high
total gamma values, forming a classic "halo effect." Such surveys
are often called "high and low maps." Generally, there is a total
gamma high above natural fracture zones.
For decades, explorationists have been using single
channel gamma ray spectrometers and scalers to search for total gamma highs
and lows. Excellent correlations have been found in known fields, such as
Golden Spike field in Cheyenne County, Colo., and Albion-Scipio field in
Hillsdale and Jackson Counties, Mich. There have been some great discoveries
using conventional radiometrics in the Bowling Green Consolidated field and
the Salt Lick Creek field in Warren County, Ky.
When asked to review reports of radiometric highs and
lows, the first question should be, "High and low compared to
what?" Neither the clients nor the service companies have ever given a
satisfactory answer based on radiation physics. Natural radiation,
particularly gamma radiation, is not fixed or constant but highly random and
variable. Many purveyors of radiometrics do not consider the randomness and
variability of the gamma spectrum in their interpretation. A geostatistical
analysis is needed to put the measurements into prespecify.
Potassium 40
Potassium 40 is one of the most abundant naturally occurring radioactive
substances on the earth. As in the case of total gamma measurements, ample
empirical evidence exists showing that potassium 40 lows occur at the
surface above hydrocarbon deposits, although the exact mechanism is unclear.
One hypothesis for this phenomenon is that hydrocarbons like ethane and
methane seep upward through the caprock and are eaten by native microbes
similar to those used for soil bioremediation. High microbe and iodine
concentrations are often found in potassium 40 lows, so geochemical testing
of the soil can supplement potassium 40 data during analysis.
KUTT Radiometrics
A new acquisition and interpretation technique has been developed for
radiometric data that is like going from 2-D to 3-D seismic. Called KUTT
radiometrics, the technique breaks the natural gamma ray spectrum into
individual components: potassium 40 (K), uranium 214 (U), thorium 208 (T)
and total gamma (T). Using KUTT radiometrics results in a fourfold increase
in data, which translates into greater accuracy in interpretation.
Several downhole logging tools separate the natural gamma
radiation into potassium 40, uranium 214, thorium 208 and total gamma
spectra so log analysis can determine the types of clays and lithology. This
is because potassium 40 is found in higher concentrations in clay minerals
associated with shales. The KUTT surveys use the same approach, plotting the
results over a 2-D area instead of along the single dimension of depth.
Other geochemical tests results, such as soil analysis for iodine and gas
concentration, can supplement the KUTT data for increased confidence in
selecting where to drill for hydrocarbons. The KUTT method is similar to
remote sensing gamma radiation methods, but offers considerable more detail,
particularly for deciding where to drill on a 50-acre lease.
KUTT radiometric data cannot determine how deep the
hydrocarbon-bearing formations may be. The measurement is a surface
measurement and can only detect gamma radiation emanating a few feet below
the surface. In addition, hydrocarbon seeps may not be generated directly
below the potassium 40 low area, but may travel along natural fractures
until reaching the surface. Consequently, seismic data can be used in tandem
with KUTT data to locate the actual hydrocarbon trap.
The appropriate tool for KUTT radiometrics is a multi
channel gamma ray spectrometer, such as the Exploranium GR410 or its
equivalent. This instrument uses a 21-cu-in. sodium iodine crystal detector.
To reduce atomic particle counting errors and obtain statistically valid
counts, each channel must take measurements for at least 2 minutes. All
readings must then be corrected for Compton’s scatter. Because of the
volume of data collected by the KUTT method, computer programs that can
generate 3-D contour plots are required for data processing and analysis.
Interpretation Methods
Radiation anomalies above oil and gas reservoirs can manifest themselves
as a total gamma low or potassium 40 low. A low is defined mathematically
and statistically as value lower than the mean minus the standard deviation.
This statistical treatment accounts for the random variations of natural
gamma radiation. The more readings in a given area, the more statistically
valid the results. These interpretation methods have proven successful
individually, as well as in combination. Contour maps of potassium 40 and
total gamma data are constructed using computer programs to aid in
interpretation. The best indicator of hydrocarbons in KUTT radiometric
survey is a potassium 40 low, followed by a total gamma low. KUTT data
collected and processed this way show the shape and extent of hydrocarbon
pools, but cannot predict with certainty the formation depth or the volume
of hydrocarbons that may be encountered.
Crossplots of thorium/uranium vs. uranium/potassium have
been useful in fields with numerous existing wells, preferably more than 50
for statistical purposes (Fig. 1). On these plots, productive wells cluster
in one area, while non-producers cluster in another, forming what is called
a "productivity window" (Fig. 1). In cases where various wells tap
different pay zones, wells producing from the same pay zone tend to cluster
together on these plots. In one case, where there were only 12 existing
wells with production rates between 5 b/d and 50 b/d, the T/U vs. U/K
crossplots showed production level windows with the 50 b/d wells clustered
together and the 5 b/d wells clustered together. A prospective well site was
selected based on KUTT data that indicated it would fall in the 50 b/d
cluster, and sure enough, that well was a good producer after it was
drilled.
More Success Stories
Since 1990, KUTT surveys have been used to select sites for successful
wells. One successful 10-well program was completed in May 1996 by Majestic
Management Corp. of Glasgow, Ky. More than 250 KUTT readings were taking,
processed and evaluated for the Blue Hole drilling program in Hart County,
Ky. Ten well sites were selected based on contour maps of potassium 40 low
measurements (Fig. 2). All 10 wells encountered moderate to excellent oil
shows in the Brownsport formation at about 850 ft, with initial production
ranging from 10 to 50 b/d. Ten more well sites were selected using this
method, eight of which found oil and two lost circulation.
KUTT radiometric successes are not limited to Kentucky or
to oil fields. In Pickett County. Tenn., NCM Energy drilled the Wayne
Williams No. 1 well in a potassium 40 low defined by KUTT measurements, and
the well blew out at 400 b/d. Alamco’s big gas discovery in Whitley County
displayed both potassium 40 lows and total gamma lows (Fig. 3). The Warsaw
field in Hamilton County, Ill., has associated potassium 40 and total gamma
lows, even at 4,200 ft. Successful oil and gas wells have also been located
in West Virginia using potassium 40 surveys.
Surveying Known Fields
Several surveys have been conducted near existing fields or wells to
verify the method and further outline the extent of the discoveries. KUTT
surveys conducted around the famous Ferguson and Jones wells in Clinton
County, KY., exhibited strong potassium 40 lows, as did the Neal, Burchett
and John Poor wells. The Pine Grove field in Edmonson County, Ky.,
discovered by Robo Enterprises, has striking potassium 40 and total gamma
lows that define the field. Most of the successful wells in the nearby Rhoda
field are also in potassium 40 or total gamma lows.
Cheaper Than 3-D Seismic
The KUTT method is fast and inexpensive. About 100 acres/day can be
tested at a cost of about $650/day. As one field in Illinois, a KUTT survey
took 2 ½ days to cover 18 sq miles at a cost of less than $2,000. As with
any data collection system, closer spacing between measurements yields more
accurate results. A maximum spacing of 200 ft is recommended for most
situations. In evaluating the potential of an already selected drill site, a
grid of 15 KUTT readings should be made on a 50-ft or less spacing. Readings
may be taken in a grid, along a line, at random or any pattern desired.
KUTT radiometric surveys can pre-screen areas for 3-D
seismic surveys, reducing costs for these more expensive surveys by limiting
them to a smaller area known to have hydrocarbon seeps. Conversely, KUTT
surveys can be performed after 3-D seismic surveys to verify the presence of
hydrocarbon and avoid drilling dry holes.
Each new well planned according to potassium 40 data
brings new insights and refined interpretation techniques for various
geologic settings. Unfortunately, there are no formal research studies in
progress at universities or extensive field tests being conducted to define
the mechanisms or constraints of this new technology. The only research
carried out to date has been funded entirely by clients. Given the proper
amount of scientific attention, KUTT radiometric methodology could play a
major role in selecting well sites prior to drilling.
While not foolproof, the results to date have been good.
Prudent operators seeking new ways to improve their chances of success are
turning to potassium 40 surveys to reduce risk and achieve greater
production.
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