Donald F. Saunders, C. Keith Thompson
Recon Exploration (Australia) Pty. Ltd.
Glenelg, S. Australia
EP-20 is located in the east-central portion of the Amadeus basin.
Petroleum in the basin has been found mainly in structural fold-related traps within Late Proterozoic to Late Ordovician marine to marginal-marine clastic and evaporate sequences.
The main source of oil production has been from the accumulation in the Mereenie anticline where oil, gas and condensate are reservoired in the Early Ordovician Pacoota and Stairway sandstones. Palm Valley is ranked as a giant gas/condensate accumulation producing from the Pacoota Sandstone. Dingo field gas is reservoired in the Arumbera sandstone of Late Proterozoic to Early Cambrian age.
A total of 50 exploration wells, to date, in the Amadeus basin had discovered the three fields and another 14 uneconomic accumulations of petroleum. Based on drilling experience, the most promising petroleum-bearing units are Early and Middle Ordovician Larapinta group clastic rocks with fracture and/or granular porosity. Cambrian and Proterozoic sandstones, siltstones and carbonates also offer promise as potential producers, but these have not been adequately tested by the drill.
INTEGRATED EXPLORATION
An integrated system of low-cost, direct or semi-direct oil-finding methods, combined with geomorphology and restricted, carefully focused seismic, is being applied to the exploration of EP-20 Concession in the Amadeus basin. The goals of this approach are (1) increased wildcat successes and (2) lower overall discovery costs than conventional methods.
The first principle of the approach to be described is to use the least expensive reconnaissance techniques over the largest area and then to focus on smaller areas for progressively more expensive detailing methods. Fig. 1 summarizes the philosophy underlying this specific approach in the Amadeus basin. The second principle is to tailor the exploration program to fit (1) the type of target sought, (2) the local geology, (3) the surface and environmental conditions, (4) the types and quality of available geochemical and geophysical data and (5) available funds. Based on recent discoveries in the United States and Australia, exploration programs including geomorphology, airborne hydrocarbon sensing, interstitial soil gas hydrocarbon analyses and detailed sub-surface geology have proved to be very cost effective.
Based on recent developments by Recon and others, soil magnetic susceptibility measurements also are a very useful adjunct for surface geochemical validation surveys. Most recent reviewers agree that petroleum accumulations at depth result in a variety of surface and near-surface chemical phenomena that can be detected successfully. The detection and measurement of micro-seeping light hydrocarbons in the atmosphere and soil gases, or adsorbed on soils or occluded in minerals, are direct indicators. Other methods classified as semi-direct or indirect indications depend on measuring other chemical, physical or biological changes in the altered zone over petroleum deposits. None of these methods can predict depth to production, the number of producing horizons or quality of production, but they do indicate whether hydrocarbon accumulations are likely to be found in specific sub-surface trapping localities, whether structural or stratigraphic. Under favorable conditions, soil gas hydrocarbon analyses may indicate whether gas or oil ir more likely to be responsible for detected anomalies. The presence of significant soil gas hydrocarbon micro-seepage anomalies provides better assurance against dry holes than drilling on structural information alone.
The use of multiple, independent (unrelated) methods such as (1) photogeology/geomorphology, (2) sub-surface geology, (3) aerial and/or soil gas hydrocarbon studies, (4) aeromagnetic and/or soil magnetic susceptibility measurements and (5) seismic can greatly increase the likelihood of success when compared with the use of any single method in isolation.
STATUS OF PROGRAM
The study of regional geology and the interpretation of satellite imagery have been completed and have defined 25 specific areas of geological and geomorphic leads for follow-up airborne hydrocarbon sensing. This has focused continued exploration on target areas totalling about 25 percent of concession area.
Preliminary airborne and surface surveys have tested the three 'benchmark' fields-Mereenie, Dingo and Palm Valley-and are currently testing the prospect areas. Typical initial results are described in the following sections. A big portion of the airborne and surface validation studies will have been completed by early 1989.
SATELLITE PHOTO ANALYSIS
The satellite photographs were viewed backlighted on a light table in a darkened room. Experience has shown this to be advantageous when annotating relatively faint anomalies, as are found in some parts of this region. Linear tonal features and stream drainage segments were annotated as possible indications of faults or fractures. Selected alignments of linear features may constitute lineaments which indicate fracture zones of major geological significance.
The major structures of the Amadeus basin are easily mapped using the newly available Russian satellite photos. The excellent resolution of the images (5-10 m) allows identification of dips and strikes of beds along many of the major anticlines and synclines. Fig. 2 shows part of a typical image, and Fig. 3 illustrates its interpreted photogeologic and geomorphic features. No attempt was made to duplicate the available geologic maps, however; efforts were directed towards indicating favorable zones to be surveyed using the airborne hydrocarbon sensor, including apparent closed structures or geomorphic anomalies along the anticlinal axes (Fig. 3, nos 5, 6, 7). Special emphasis was placed on locating zones of possible concealed structures by extrapolating mapped structures out under the sand cover using geomorphic features as guidance.
Circular to subcircular drainage or tonal patterns were annotated as possible concealed positive structures such as anticlines, domes or other antiforms. Recent observations of drainage deflection anomalies over giant stratigraphic traps of the Alabama Ferry Field in East Texas have led to the conclusion that these features may be a by-product of hydrocarbon micto-seepage-related secondary carbonate mineralization in the near-surface sediments. Bacterial oxidation of hydrocarbons creates carbon dioxide, which can react with calcium silicates to produce carbonates and silicification. The resulting induration of the sediments creates local zones of higher erosion resistance and consequent topographic highs which may be detected as geomorphic anomalies. Four features possible caused by this process are shown in the synclines mapped in Fig. 3 (nos 1, 2, 3, 4).
Some of the earliest observations thought to be related to this form of near-surface alteration include a report describing several instances where high seismic interval velocities exist over structural petroleum accumulations but not off them. The coincidence of secondary mineralization, such as caliche, with microseepage in the vicinity of some Texas oil fields have been reported. High seismic velocity in the Mesozoic-Tertiary sediments of the Spitsbergen basin have been related to porosity reduction due to cementation. It has been suggested that high cohesion causing more difficult drilling over sub-surface productive structures was often related to silicification in the strata above production. The high cohesion and chemical stability is also responsible for the topographic highs related to fields in some areas. For over 40 years Geochemical Surveys, Inc., of Dallas, Texas, has been measuring a special form of carbonate mineralization in the soils and has been relatively successful in using it to prospect for petroleum deposits.
The occurrence of carbonate mineralization derived from bacterial oxidation of hydrocarbons has been reported by several investigators. There is isotopic evidence that the carbon in the carbonates originated from petroleum at depth in the Cement and Davenport fields, Oklahoma. The presence of digenetic carbonate mineralization over the Cement field in Oklahoma is undoubtedly responsible for the name of the field.
It is suggested here that a most reasonable explanation for these effects is digenetic carbonate mineralization and/or silicification caused by bacterial oxidation of microseeping hydrocarbons over oil or gas deposits. The carbon dioxide produced can react with calcium silicates in the sediments to produce both calcium carbonate and silica by reactions such as the following.
CaAl2Si6O16 + CO2 = Ca CO3 + 6SiO2 + Al2O3
Several features which may be of this type were found on the Wild Eagle Plain, the broad syncline south of the James Range (Figs 2, 3), and in the Middle and Levi Range synclines. It is suggested that if these are found to show evidence of current hydrocarbon microseepage, all the broad synclines should be surveyed for similar geomorphic anomalies that might not be strong enough to be obvious on satellite photos. It should be noted that the Dingo field is a local positive structure in the Orange Creek syncline and was not seen as a geomorphic feature on the satellite images. The drainage anomalies over Alabama Ferry were detectable easily only on maps at 1:24,000 scale showing much drainage detail. The airborne hydrocarbon sensor can be utilized cost effectively to search directly for evidence of hydrocarbon gas micro-seepage in these areas, and may detect traps not discernible on either aerial or satellite photos.
Light tonal anomalies were annotated because they may be caused by patches of caliche or other surface evaporites possible related to gas microseepages and their alterations. Care was taken to eliminate those light tonal anomalies that appeared to be caused by playas or patches of sand dunes with little probable relationship to petroleum. The final copy of the anomaly map was prepared as an overlay to the 1:250,000 scale base and geologic maps.
DISCUSSION OF ANOMALIES
The geology mapped by the Australia Bureau of Mineral Resources indicates that most of the anticlinal features in EP-20 show exposed beds of Early Ordovician age (Pacoota and/or Stairway formations), thereby minimizing the chances of finding unbreached reservoirs similar to those of Mereenie and Palm Valley associated with the anticlinal axes. The synclines, however, contain unexposed beds of those formations where both low-relief structures (such as the Dingo field) and stratigraphic traps could constitute promising prospects. Geomorphic anomalies mapped in the synclines will be given equal weight with anticlinal structures in follow-up studies. The entire lengths of anticlinal trends in EP-20 will be surveyed using the airborne hydrocarbon sensor, with concentrated efforts on the indicated anomalies. Pre-Ordovician reservoirs, including the Arumbera Sandstone of the Dingo gas field, are most likely to be encountered along these trends. Several geomorphic tonal and stream drainage anomalies are indicated as possible prospects in the sand-covered areas. Figure 4 presents an index map of favorable areas to be investigated during the continuing exploration program.
SYNCLINAL LEADS (FIG. 4)
Anomalies 1 through 6:
Drainage deflection anomalies located within a broad syncline south of the James Range. These might indicate local low-relief antiform structures or more erosion-resistant areas of secondary mineralization related to microseeping hydrocarbons over stratigraphic oil accumulations.
Anomaly 7: Drainage deflection anomaly in the Middle Range syncline.
Anomaly 8: Drainage deflection anomaly in the Levi Range syncline.
ANTICLINAL LEADS (FIG. 4)
Axis A-A': Mereenie Axis.
This axis is well defined on the west end and shows one apparent small drainage deflection anomaly just outside EP-20 (Anomaly 9). As shown on the Anomaly Map overlay, this axis includes the Mereenie Field. Beds dipping to the north suggest the extension of the axis to the east as indicated through the cluster of geomorphic anomalies (Anomaly Group 10). It is suggested that the entire covered portion of the indicated axis be surveyed for micro-seeping hydrocarbons.
Axis B-B': James Ranges Axis.
This axis passes through small segments of EP-20 along the northern edge, and projects into EP-20 under cover (Anomaly 1). Anomaly 12 is an apparent domal feature at the east end of the James Ranges.
Axis C-C': Petermann Creek/Hills axis.
A narrow anticlinal axis with two obvious closures (Anomalies 13 and 14), a possible faulted nose (Anomaly 15), and an extension under the sand cover (Anomaly 16).
Axis D-D': Wallera Ranch trend.
The axis is defined west of EP-20 and projects under sand cover along a north-dipping exposed bed to the Wallera Ranch geomorphic anomaly (Anomaly 17). It is suggested that the entire covered axis be surveyed for micro-seeping hydrocarbons.
Axis E-E': Seymour Range axis.
This axis extends from a geomorphic anomaly on the west end (Anomaly 18) through the Seymour Ranges where it is well defined by both north- and south-dipping beds (Anomaly 19).
Axis F-F':
This plunges to the west-southwest under the sand cover (Anomaly 20).
Axis G-G': Mill Ridge axis.
The west end of this axis appear to plunge westward and extend eastward under cover towards a light tonal (LT) anomaly and thence to Mill Ridge where it is defined by both north- and south-dipping beds (Anomaly 21).
GEOMORPHIC LEADS (FIG. 4)
Anomaly Group 22:
A group of geomorphic drainage and tonal anomalies in the Stone and Dead Bullock Plains.
Anomaly 23:
This is a large geomorphic feature that appears to be structurally high, based on apparent dips of nearby outcropping beds.
Anomaly Group 24: A zone of geomorphic anomalies centered on the Chandler Range.
Anomaly Group 25:
A group fo relatively weak geomorphic features.
AIRBORNE HYDROCARBON SENSOR SURVEY
The helicopter-borne gas sensor system is a direct detector of the presence of hydrocarbon gases escaping at the surface of the earth. The following model has been presented to explain the phenomenon. The transmitter sends out radio pulses of specific frequency from a rotating antenna. A portion of the transmissions is absorbed by the hydrocarbon gas molecules which are excited to a higher energy state. In the time between the emitted pulses, the excited gas molecules return to their original energy state, emitting energy of a different frequency from the incident-exciting frequency. A receiver is tuned to detect this characteristic signature or echo from the gases.
The source location and relative intensity of the return signal is displayed during the survey on a cathode ray tube video-screen and recorded on maps of the surveyed area. Helicopter location is determined by reference to mapped topographic and/or cultural features supplemented, where necessary, by appropriate radio location aids.
The airborne gas sensor can evaluate approximately 30,000 acres (121 kM2) per day and is flown at low altitudes in all open fields and cleared areas. This method has proved successful in this area and in many other areas with similar surface conditions.
PRELIMINARY RESULTS
An estimated 20-25 per cent of identified target areas have been covered by the airborne hydrocarbon sensor at the time of writing, and 14 micro-seepage anomalies have been detected. Palm Valley, Mereenie and Dingo fields have been studied as 'benchmark' examples. All three fields show airborne evidence of active hydrocarbon micro-seepage. The airborne anomaly over Mereenie Field and an associated interstitial soil gas hydrocarbon profile are presented as examples of these types of data (Fig. 5). The central part of the airborne anomaly is rated as 'significant' in that its strength is estimated at greater than three times the background. It is surrounded by a "moderate" anomaly (2 to 3 times background) and a "minor" anomaly (1 to 2 times background).
INTERSTITIAL SOIL GAS HYDRO-CARBON SURVEY
In a final geochemical step, interstitial soil gas hydrocarbon analysis is used to confirm the presence, extent and composition of hydrocarbon seepage over the best airborne hydrocarbon anomalies. This provides quantitative and truly direct evidence of petroleum at depth.
Soil gas hydrocarbon analyses have been used in petroleum prospecting for many years, and developments in this area have been reviewed by several authors.
SAMPLING
Reconnaissance soil sampling is generally undertaken as profiles over each selected anomaly with a sample spacing of 0.2 km. If it is desired to obtain more detailed information on the extent of the most promising anomalies, that can best be done using a "staggered" grid pattern.
Samples are obtained by probe from a depth of about 75 cm. Several have described the patented probe collection unit equipped with an integral slide hammer that drives it into the soil. When it has reached the desired depth, a special valve is opened on the tip of the probe and a gas sample is drawn through the probe into a hypodermic syringe. The tip of the needle is sealed with a silicone rubber stopper and the syringe is placed in a protective metallized envelope.
ANALYSIS
At intervals during the survey, samples are taken to a nearby field laboratory and analyzed by sensitive hydrogen flame ionization gas chromatography. The samples are analyzed for seven hydrocarbon constituents: methane, ethane, ethylene, propane, propylene, iso-butane and normal-butane at sub-part-per-million levels. The data are processed using special Recon Exploration proprietary statistical computer programs to eliminate many types of false anomalies, to greatly enhance sensitivity, to predict whether oil or gas is more likely to be found under each anomaly and to identify possible fault-related seepage.
DATA INTERPRETATION
The following analysis procedure is used for final interpretation of these data; it has proved very valuable in analyzing and comparing petroleum prospects in other areas. The basic methods apply equally well to soil gas hydrocarbon data, and have been further developed (by Recon Exploration) to attack some specific problems in petroleum prospecting.
BACKGROUND CHARACTERIZATION
Separating genuine anomalies from random variations in measurements due to experimental errors, sampling differences, normal fluctuations in background levels, etc. requires establishing a realistic background level and a measure of the normal spread of measurements around that value. In the absence of anomalies, the background population may be described adequately in terms of the mean value and the standard deviation. If some anomalous values are mixed with the background, a skewed population with a larger mean and larger standard deviation results. If used in anomaly evaluation, they will result in anomaly thresholds which are too high, and some valuable anomalies may be missed. A relatively simple method has been found easy and effective in rectifying this problem for many different kinds of data. It assumes a normal (gaussian) distribution of background values, and involves the elimination of extreme values in computing a useful approximation of the true background mean and standard deviation.
First, the arithmetic mean and standard deviation for the data set are calculated. Then, all values exceeding the mean plus the standard deviation are eliminated, and a new mean and standard deviation calculated for the remaining data. Again any values exceeding the new mean plus the corresponding new standard deviation are eliminated. This process is repeated until the mean value and the standard deviation are not appreciably influenced by any remaining small anomalous values. Finally, the remaining values are used to calculate an approximate 'background' mean and standard deviation to serve as the basis for evaluating potential anomalies in the original data set.
ANOMALY IDENTIFICATION
Table 1 summarizes the probabilities that particular single-point anomalous values are due to normal random variations in the background population and thus may constitute false anomalies.
Unless it is very strongly anomalous, a single point usually is not judged to be significant in most geochemical data. Only when several adjacent points on a profile or grid of samples are simultaneously anomalous is the grouping accorded potential importance. If a useful anomalous threshold of the mean plus 3.0 standard deviations is adopted, statistical theory indicates that two adjacent points will satisfy the same criterion if both exceed the mean by 1.8 standard deviations (see Table 2).
Thus, if one accepts only anomalies with two or more adjacent points that exceed the mean by two or more standard deviations, one may assume with reasonable safety that they are due to causes other than random background fluctuations (assuming, of course, the sampling interval is appropriate).
COMPOSITE ANOMALY MAPPING
The data for each profile are processed separately to suppress effects which may be due to differences in sampling conditions that can modify the overall sensitivity of the measurements. This also compensates for any general changes in the background levels in different areas due to local conditions.
As a first step, the analytical data are statistically evaluated to determine the background mean and standard deviation for each hydrocarbon component: methane, ethane, ethene, propane, propene and the butane. The results are used to evaluate each component of each sample in terms of its degree of "anomalousness". This is indicated by each component's "standardized" value, based on the number of standard deviations above or below the background mean. The values are combined for each sample to determine an average degree of "anomalousness" for all components heavier than methane. These are referred to as the 'C2-C4 Sum' or 'C2-C4' values.
PRELIMINARY RESULTS
The three benchmark fields and size of the airborne anomalies have been studied by interstitial soil gas hydrocarbon profiles during the preliminary stage of surface validation surveys. Fig. 5 shows the strong hydrocarbon micro-seepage anomaly located over the Mereenie Field.
PLANNED MAGNETIC SUSCEPTIBILITY SURVEY
Grassroots-depth soil samples, which can be conveniently collected along with each of the soil gas samples, are analyzed for magnetic susceptibility.
Several investigators have reported shallow-depth aeromagnetic anomalies over oil and gas fields. These 'micromagnetic' anomalies are attributed to digenetic magnetic iron minerals in the near-surface rocks and soils. These are believed to be formed by chemical reactions of sedimentary iron minerals with traces of hydrogen sulfide produced by sulfate-reducing bacteria during their consumption of hydrocarbon gases seeping upwards from petroleum deposits. Thus, the shallow-depth (short-wavelength/high-frequency) magnetic anomalies provide the possible basis for a 'semidirect' petroleum detection technique.
Soil magnetic susceptibility is a relatively new tool which was developed by the authors to validate aeromagnetic 'micromagnetic' anomalies caused by shallow-depth sedimentary accumulations of digenetic magnetic minerals over petroleum deposits. A principal problem with airborne 'micromagnetics' prospecting methods has been difficulty in separating significant petroleum-indicating anomalies from 'cultural' anomalies due to pipelines, tanks, casings and other iron or steel objects. Results of recent studies demonstrate that inexpensive magnetic susceptibility measurements on soil (or well cuttings) samples can be used to verify questionable aerial anomalies. These data can prove the presence of anomalous amounts of digenetic magnetic materials in shallow-depth soils and rocks over petroleum accumulations.
Recent preliminary tests over 15 oil and gas fields and one gas storage area compared soil magnetic susceptibility anomalies and soil gas hydrocarbon anomalies using samples collected along the same profiles. The two data sets were found to be complementary, filling in the gaps in each other's anomalies, and thus providing more complete guidance to the productive areas than could be derived from either data set alone.
SAMPLING, ANALYSIS
Samples are obtained at the same sites used for soil gas sampling. They are taken at grass-roots depth, and placed in plastic vials. The sites are chosen to avoid inclusion of any contamination not native to the residual soils.
The samples are analyzed using a modified magnetic susceptibility meter.
CONCLUSIONS
The photogeologic/geomorphic study of relatively high resolution Russian satellite photography has identified 25 target areas for further exploration by airborne and surface geochemical methods. The region to be explored by airborne hydrocarbon sensing has been reduced to about 20 per cent of the original study area.
The airborne hydrocarbon sensor has disclosed hydrocarbon micro-seepage anomalies over three known fields-'benchmarks'-as well as several prospects. The airborne anomalies have been validated by near-surface interstitial soil gas hydrocarbon measurements which have characterized them as to the types of microseeping hydrocarbon gases.
It is planned to continue the airborne and surface studies to cover the further photogeologic/geomorphic leads with the addition of soil magnetic susceptibility measurements to complement the soil gas hydrocarbon data.
Copyright 1990 Oil & Gas Journal. All Rights Reserved.
