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The Application of Mobile Metal Ion Soil Geochemistry To

Oil Exploration, Perth Basin, Western Australia

DATE: October 1998 CSD001

EXECUTIVE SUMMARY

Within the Perth Basin, Western Australia, a soil sampling survey collected surface soil samples along a selected line designed to traverse four previous seismic targets. Two targets had producing oil wells, one had been drill tested and abandoned and the forth remained untested. The soil samples were then treated using a mobile ion geochemical process whereby loosely bound element ions are detached from the soil, held in an extractant solution and then measured in a laboratory using an ICP- Mass Spectrometer. The outcomes can be summarized as follows:

1. A number of species showed variation in geochemical soil response in the vicinity of known oil traps;

2. Where the Group - Species A,B,C only were present on the traverse, they appear to

corresponded to the seismic targets:

3. The Group – Species E,F,G,H,I,J correspond to locations with producing oil wells:

4. Samples with the highest combined response ratio levels, were from the closest locations to the two producing oil wells along the traverse selected (the odds of this occurring as the result of a random event, are infinitesimally small);

5. At the eastern end of the traverse the area is a designated seismic target and is anomalous in

most species from the two groups and would be considered a highly prospective drill target.

6. Uniform background has been obtained on that portion of the traverse (SW) away from oil and seismic targets , and for which no change in surface soil type is apparent.

1.0 INTRODUCTION "Mobile Metal Ions" is a term used to describe ions which have moved in the weathering zone and which are only weakly or loosely attached to surface soil particles. Studies from Australia and overseas have shown that such Mobile Metal Ions are useful in locating buried mineralization. Mobile Metal Ions are generally at very low concentrations in the soil. To successfully interpret these weak signals, a series of very carefully quality-controlled steps have been developed which, when put together, constitute an integrated package - the MMI Process. Specially prepared and proprietary digests have been developed to "release" the adsorbed ions from the soil material. It has been found that there is an optimum leach solution for each metal. Consequently, a number of digestions are required on each sample. Digestions for all samples in the batch process are carried out under identical conditions. Results for the most effective digest are reported for each species. Analysis of the digest solutions for the target element is undertaken using equipment and techniques that routinely operate at the parts per billion level. Rigorous quality control is undertaken to minimize cross contamination between samples. With the aid of custom computer interpretation packages, the analysis results are integrated. For each species, a background and a peak to background ratio at each sample position (the Response Ratio) is calculated. The data may be presented as stacked bar charts, sectional (line) plots, or as colored image plans. Where the data are presented as stacked bar charts:

1. each species is represented as a unique colour; 2. the Response Ratio for each species at each sample point is shown as the height of that

colour in the stacked bar at that location; 3. at a location which has a "background" response, each species has a bar height of one, and

for a nine species survey the total bar height is therefore nine Response Ratio units; and 4. for significant locations, one or more species would be expected to show a Response Ratio

greater than two units. 2.0 BACKGROUND Geophysics, principally seismic techniques, has and will continue to provide the primary methods for discovery of sub-surface oil and gas. Seismic is without peer for high-resolution structural mapping over a depth range measured in kilometres (Seveme, 1991). It is however, not strongly influenced by the presence or absence of petroleum. Surface geochemistry is an additional tool which may be an aid in determining the presence of hydrocarbons at depth. Detection of hydrocarbon accumulations by surface geochemistry is discussed in a number of publications (e.g. Collins et. at., (1992), Horvitz (1985). The fundamental theory behind using surface geochemistry to indicate the presence of petroleum at depth is not well understood, but is predicated on the empirical observations that hydrocarbons migrate to the surface through seemingly impervious barriers and leave their signatures in soils.

Empirically, these same observations are now taking place in relation to new Russian (CHIM), American (Enzyme Leach), and Australian (MMI) geochemical methods for conventional mineral deposits, such as those of base and precious metals. Again, whilst the underlying transport mechanisms are at best poorly understood, the empirical observations, and the potential benefits accruing from application of these techniques is undeniable. In the case of petroleum, direct methods involving detection of hydrocarbons is based on the concept that C1-C6 hydrocarbons migrate from reservoirs at depth and can be detected by, for example, gas chromatographic techniques. However, it is also well known that sampling soil gases for organic constituents introduces a number of reproducibility problems. Richers et. at., (1991) devised a soil pulverizing method which was found to be superior to free soil gas analysis. Saenz and Pingitore (1989) found that samples collected at 30 m depth provided far better discrimination between producing and barren reservoirs than samples at 3m depth. One method of potentially overcoming this problem is to implement a sampling technique which desorbs accumulated material from one or more of the soil phases, e.g. clay surfaces or organic matter. This is essentially the MMI approach, used for obtaining an accurate record of sub-surface sources of metals from traces of adsorbed metals in the A horizon of soils. Two research programmes, sponsored by companies such as B.H.P., M.I.M., Normandy Poseidon, Pasminco, Aberfoyle, Great Central, Delta, Acacia and others, have been undertaken at the Geochemistry Research Centre, Technology Park, W.A. in order to investigate the fundamentals of this method, and to provide information relevant to its implementation into routine exploration. The first of these programmes, M219, entitled "the Mechanism of Formation of Mobile Metal Ion Anomalies" ran from 1993 to 1995. The second, M267, entitled, "Soil Geochemical Anomalies - their dynamic nature and interpretation" ran from 1995 to 1997. They achieved very successful outcomes. Both projects ran for two years, with budgets in the vicinity of$500,000. There is increasing evidence that inorganic species associated with petroleum reservoirs can also be detected in surface soils. Collins et. al., (1992) report significant concentrations of iodine in soils in the vicinity of petroleum reservoirs in Colorado. Mississippi oil field brines were noted (Saunders and Swann (1990)) to contain high concentrations of Pb, Zn, and Fe. A range of species, including Pb, Zn but also Ni, V, Br, and I have been observed in soils in the vicinity of gas reservoirs. In 1998, Wamtech received a W.A. State Government W AISS grant to develop new leachants, and extend the range of species and commodities for which the MMI technique could be used. The present document describes the application of the MMI-H leachant, developed recently within that programme, to the Dongera Area in Western Australia. 3.0 GEOLOGY Production from the Dongera Area began in 1984, following the discovery by WAPET in 1965. It lies in the Perth Basin; production is from various sands in the Jurassic-age Cattamarra coal measures. There are distinct pools, with the main production coming from the Cockleshell Gully formation. At Mt. Homer the structure consists of a tilted fault block with a rollover anticline on the downthrown side of a major fault. Further details of the geology, including structural details and oil production history were not sought by, nor made available to the MMI practitioners before carrying out the orientation survey, or before attempting this preliminary interpretation of the results. The only facts relating to oil production which were available and utilized were the surface positions of beam pumps.

4.0 GEOCHEMICAL PROGRAMME The geochemical sampling programme was carried out in 1998. It consisted of one line of samples oriented approximately NE-SW, and passing approximately 50 m to the south of:

1. two producing wells. 2. two abandoned production wells 3. dry exploration hole 4. seismic target yet to be tested

Samples were at 50 m intervals in the centre of the line, and at 200 m spacing on the extremities (see plates 1- 2)

Plate 1 - Producing well (45-63 bbl/day) Plate 2 - Soil Sampling

Samples were taken from a depth of 15 cm below surface and collected pale marine sands (see Plates 3-4). Samples were digested in MMI-H extractant, and analyzed for ten species at the ALS Perth laboratory. The results were returned to Digger for interpretation.

Plate 3 - surface marine sands Plate 4 - sample hole

The sampling material was in most cases, coarse, grey sand, as shown in Plate 4. The only exception to this was the material encountered on the NE section of the sands overly a lateritic duricrust, for example that shown in the distance of Plate 2. Here the clay and iron content of the samples was visibly higher.

5.0 RESULTS A background level for each species was calculated by determining the arithmetic mean of the lowest quartile of data for that species – called the background level for that element for that sample group. By dividing the backgrounds calculated for each species into the individual element levels (ppb assay data for each species) the Response Ratios for each species at each sample site were calculated. This is a method of gauging the relative level of anomalism for individual sample positions. 6.0 DISCUSSION AND RECOMMENDATIONS The HDRG Response Ratio chart for all species of the study area’s traverse is shown in Figures 1 and 2. Also shown are the relative positions of producing oil wells (W) and existing seismic targets (S). To the left (SW end) of the traverses, the Response Ratios expected over barren ground are indicated; ten samples on the end of the line are uniformly low, with a combined response ratio for ten species less than 20 units. In comparison, samples at several other locations on the traverse have distinctly anomalous multi-species responses, with combined stacked bar charts in excess of 60 times background. Specific samples include MH5-8, MHl8, MH26- MH29 and MH34-38. With some of these samples adjacent to long term oil producing wells, the outcomes can be summarized as follows: Sample Number

Groups Species Group 1 Species A,B,C

Species Group 2 Species E,F,G,H,I,J

Comments

MH 5-8 Present Moderate Not Present Dry Well Seismic Target MH16-19 Sporadic weak Present strong Producing well Seismic Target MH26-29 Present Moderate Present V Strong Producing well Seismic Target MH34-38 Present Moderate Present Strong No Drilling Seismic Target The outcomes can be summarized:

1. A number of species showed variation in geochemical soil response in the vicinity of known oil traps;

2. Samples with the three highest combined response ratio patterns, were from the closest locations to the two producing oil wells along the traverse selected (the odds of this occurring as the result of a random event, are infinitesimally small);

3. Where the Group - Species A,B,C only were present on the traverse, they appear to corresponded to old seismic targets:

4. The Group – Species E,F,G,H,I,J correspond to locations with producing oil wells: 5. At the eastern end of the traverse the area is a designated seismic target and is anomalous in

most species from the two groups and would be considered a highly prospective drill target. 6. Uniform background has been obtained on that portion of the traverse (SW) away from oil,

and for which no change in surface soil type is apparent.

The issue of possible contamination needs to be considered. Two types of contamination are possible, agricultural and as a result of exploitation of the oilfield. Agricultural contamination can be eliminated because of the asymmetric distribution of the anomalism toward sample locations in proximity to oil. Fields subjected to fertilizer application would contain very even distributions of trace species supplements, if they were used. Some of the species noted as anomalous would not be used in agriculture. Two possible types of contamination from the oilfield operations can be considered. Spillage of oil products was not visually evident, and would be expected to influence results at more than one or two sites around each well. Samples were taken sub-surface. Formation water, apparently used to water the tree farms around each well, are accordingly, and by inference low in dissolved solids, including anions common in brines such as bromide and iodide, and accompanying trace metals. However, whilst further work would be required to exhaustively eliminate both of these possibilities; the tentative conclusion is that contamination is unlikely to explain the results. These empirical observations in relation to surface mobile ion geochemistry for oil are similar to those which occurred in the testing of MMI for base and precious metal occurrences in the early 1990s. What followed, successfully and logically in that case was (a) further follow up case history work and (b) a MERIW A sponsored research programme to examine the fundamentals of the process, and to expand its application to other commodities and locations. The recommendations flowing from the present study therefore follow a well-established precedent: