TECHNICAL PAPERS

SPECTRAL AND MINERALOGICAL CHARACTERISATION OF ALTERATION ASSOCIATED WITH HYDROCARBON SEEPAGE USING GEOSCAN AMSS MKII DATA OVER PALM VALLEY, AUSTRALIA

Presented at the Thirteenth International Conference and Workshops on Applied Geologic Remote Sensing, Vancouver, British Columbia, Canada, 1-3 March 1999.

ABSTRACT

A Landsat TM spectral anomaly over part of the Palm Valley Gas Field in Central Australia has been known for some time.  However, earlier work was unable to characterise the alteration mineralogy associated with that anomaly which appeared to be the product of increased carbonate in soils and geo-botanical effects.

Geoscan data flown over the anomaly in 1989 recognises the same large scale feature as detected by Landsat TM but also discriminates a second spectral anomaly in which the mineralogical alteration can be clearly mapped.  The second spectral feature is tightly zoned and structurally controlled at the intersection of two important lineaments.  The multi-spectral data was processed to analyse spectra over the different zones of the anomaly and determine the mineralogy which was then verified by field mapping and laboratory spectral analysis.

The multi-spectral data identified a zone of silicification around a central strongly ferruginiesd zone.  Using these characteristics as a model, a third spectral anomaly was recognised.  Clear mineralogical differences between the three spectral anomalies point to complex movements and evolution of hydrocarbons within the area and the ability of multi-spectral instruments to map such mineral zonations confirms the potential of spectral remote sensing in the search for hydrocarbons in sedimentary basins.

1.0 INTRODUCTION

The Palm Valley Gas Field is situated in the central northern part of the broad, east-west trending Amadeus Basin, in Central Australia (figure 1).  The gas field is located in the Palm Valley anticline, some 120km south west of Alice Springs.   The Devonian Hermannsburg Sandstone crops out over the whole anticline and has been deeply dissected.  The climate in the area is arid and soils are poorly developed, red in colour consisting of coarse quartz sands with secondary iron oxides developed from the underlying sandstone.  Vegetation in the area is sparse, dominated by spinifex grasses and scattered shrubs and mulga trees (figure 3).

Figure 1: Location of the Amadeus Basin, the Palm Valley Gas Field and the area of this study (in part after Simpson et al. 1989) with extract of the Hermannsburg and Henbury 1:250, 000 scale geological map sheets.

The Hermannsburg Sandstone is a coarse grained, well bedded, red quartzose sandstone which, at Palm Valley forms the entire outcrop of the Palm Valley Anticline.  This structure is an arcuate, east - west trending fold which extends over some 40km. Matrix porosities and permeability within the anticline are low to very low and the principal conduit for gas is controlled by NNW and ENE fractures (Do Rozario & Baird, 1987).

The Palm Valley Gas Field was selected for study as part of an international research program to test for signs of hydrocarbon gas seepage in an area where none were known (Huntington & Simpson, 1985). The Palm Valley site was thought particularly suitable because outcrop comprised only one single lithology.  The program used the Thematic Mapper Simulator (NS001) and data were acquired in a single flight line on October 21 1985.

The study identified a strong colour anomaly in the northern flank of the anticline which transgressed bedding and which was not recognisable in colour air photography (Simpson et al., 1989). Fieldwork noted subtle differences in the colour of the sandstone, the vegetation, soil pH and the presence of calcrete within the anomaly.   Spectral analysis using an InfraRed Intelligent Spectroradiometer (IRIS) was carried out in order to determine the spectral character of the Hermannsburg Sandstone, the sandstone within the anomaly and the calcrete.  The spectra suggested that the unaltered Hermannsburg Sandstone is dominated by haematite and contains significant kaolinite.  The sandstone in the anomaly however appeared to contain very little clay and to be more magnetite rich, observations which were supported by both XRD and magnetic susceptibility analysis.

The mineralogical differences between the fresh Hermannsburg Sandstone and the sandstone in the spectral anomaly could be explained as the chemical effect of hydrocarbon seepage a model supported by a soil gas survey which identified anomalously high soil free-alkane-gas (C1 to C4) content over the anomaly.

In spite of distinct spectral differences and the ability of the Thematic Mapper Simulator (NS001) to discriminate the spectral anomaly, the instrument had insufficient spectral resolution to map and characterise the mineralogical changes which brought about the spectral anomaly.  It was decided to revisit the site with the Geoscan AMSS MKII instrument to determine if a higher spectral resolution might discriminate that mineralogy and develop a method for the identification and understanding of alteration associated with hydrocarbon seepage.

2.0 GEOSCAN AMSS MKII DATA

Table 1. Geoscan MK2 band positions and band widths.

Geoscan Mk2 Airborne Multispectral Scanner (AMSS) data were acquired over the Palm Valley Gas Field on November 1st 1989 but, for reasons beyond the control of the author, the data was not analysed and field evaluation not possible until many years later.  Nevertheless, the data not only confirm the earlier work but localise an additional and more striking zoned colour anomaly not previously recognised (figure 2).   The Geoscan AMSS MKII recorded 24 channels within the range of  0.49 mm to 12.0 mm. These spectral bands comprised 10 in the visible/near infrared (VNIR), 8 in the shortwave infrared (SWIR) and 6 in the thermal infrared (TIR) portions of the electromagnetic spectrum (Table 1). The Palm Valley data were collected from an altitude of 14,090 feet above ground level for a ground resolution of 9m and a swath width of 8.92km.

2.1  PRELIMINARY DATA PROCESSING

After calibration using the Internal Average Relative Reflectance (IARR) Calibration routine, display of the data using simple algorithms enabled a first pass characterisation of the known anomaly.  The principal mineralogical effect appeared to be an increase in carbonate content, in keeping with the findings of Simpson and others.  However, the Geoscan data also recognised a NE trending fault structure which marked the western limit of the known anomaly and, further along its length to the north east, was associated with a second, clearly zoned spectral anomaly located at the intersection of the NE trending structure with one of a NNW trend (figure 2a-d).   This latter anomaly has a core shaped like a musical quaver which clearly transgresses lithological boundaries, appeared to be enriched in iron oxides and to be surrounded by an extensive siliceous zone (figures 2 and 5). 

Figure 2: a) Geoscan band 17 8 2 RGB display (equivalent to Landsat TM 7 4 1 RGB) showing the strong Quaver colour anomaly.	Figure 2: b) Geoscan TIR band 20 21 22 RGB display showing silica as blue.  Note the strong indication of silica around the Quaver anomaly.
Figure 2: c) Grey scale image of Geoscan Fe-Oxide index demonstrating the strongly ferruginous nature of the core of theQuaver anomaly.	Figure 2: d) SWIR band difference image, 12-11 14-17 14-18 RGB showing carbonate as white.  Note the lack of carbonate in the area of the Palm Valley anticline and known anomalies and the strong indications in the broad plains to the north.

3.0 FIELD EVALUATION

Figure 3: a) - left - The dark ferruginous ridge which forms the core of the Quaver anomaly (background) with the surrounding silicified shallow dipping sandstones (foreground) and b) - right - the nodular ferruginised sandstone core of the anomaly.

The quaver anomaly was visited in June 1993 and found to consist of a low ridge of dark brown - black altered sandstone which transgressed bedding in the Hermannsburg sandstone (figure 3a).  The black sandstone weathers to a rubbly surface with the appearance of a conglomerate.  However, on closer inspection, the sandstone was found to be massive but highly ferruginised, the rubbly weathering picking out harder, more ferruginised nodular shapes (figure 3b).  The ferruginisation had obliterated bedding and was restricted to a narrow vertical zone within shallow northerly dipping horizons (figure 3a).  The sandstone immediately surrounding the central ferruginised zone consisted of very hard, highly siliceous grey-white sandstone (figure 3a).  Surprisingly, no calcrete or other forms of carbonate were recognised in or immediately around the quaver anomaly.

Figure 4: Ground spectra collected from the Quaver anomaly and environs.

Several rock samples were collected from within and around the quaver spectral anomaly for spectral analysis.  Reflectance spectra were collected from rock samples taken from in and around the quaver anomaly using an IRIS.   The fresh, unaltered sandstone has deep absorption features at 0.53 and  0.88 mm and an overall appearance in the Near Infra Red which is very similar to haematite (figure 4).  An absorption feature at 2.2 mm could be indicative of montmorillonite or kaolinite.  However, in the silicified samples, there is a clear indication of the kaolinite doublet (figure 4).  The silicified sandstone appears much the same as the fresh although the absorption band at 0.53 mm is broader and shallower than in the fresh rock and in haematite (figure 4).  The effect of secondary silica can also be seen in the asymmetric widening of the 2.2 mm absorption band (figure 4).  The ferruginised sandstones of the core of the anomaly show the most change.  The characteristic haematite absorption at 0.53 mm and 0.88 mm is replaced by absorption at 0.49 mm and 0.97 mm respectively, to produce spectra much more typical of limonite and goethite (figure 4).   Furthermore, in the ferruginised samples, the Short Wavelength Infra Red absorption at 2.2 mm has gone altogether (figure 4).  The spectra are compared and appear to show the haematitic quartz sandstone being depleted of the ferrous iron oxides in the silicified zone and enriched with ferric iron oxides in the core.  

4.0  ADVANCED IMAGE PROCESSING

Simple band difference algorithms had proved successful in identifying mineral alteration associated with the passage of hydrocarbons through sandstones.   Field observations coupled with spectral analysis was able to characterise the nature of the alteration.  However, to be useful to exploration, it was important to determine whether the airborne instrument used could not only discriminate the alteration but also characterise it spectrally.  Detailed spectral evaluation of the airborne data was attempted but instrument noise superimposed on the data made spectral matching and spectral feature fitting difficult to quantify.  It was not possible to remove this noise from the data by dark subtraction because the data contained several pixels in each band with absolute zero digital values and several pixels in each band which were saturated.  A distinct patch of ground within the data for which an accurate ground spectrum had been obtained was selected within the data and the instrument noise removed by making the mean spectrum for that area fit the ground spectrum.  The instrument noise removed data were then compared spectrally with other ground spectral data and found to be consistent.

A spectral feature fitting routine was then applied to the total data set so as to search for similar spectral and hence alteration anomalies.  The spectral signature of the ferruginised core of the quaver anomaly could be mapped in other discrete areas as also could the minerals goethite and limonite using a USGS library reference spectrum in each case.  In addition to the original Palm Valley spectral anomaly described by Simpson and others and the quaver anomaly described here, there exists another spectral anomaly with associated iron oxide enrichment and surrounding silicification to the west of the quaver anomaly located south of Kaparilja Spring along what appears to be a faulted northern margin of the anticline (figures 2 and 5).

Figure 5: a) Greyscale image of the spectral fit for goethite showing the strong match over the Quaver anomaly and south of Kaparilja Spring.	Figure 5: b) Greyscale Geoscan silica index showing the strong silicification observed around the Quaver anomaly and additional silicification to the south of Kaparilja Spring.
Figure 5: c) Greyscale image of the spectral fit for calcite showing the predominance of carbonate in the plains to the north of the anticline and in the original TM anomaly.	Figure 5: d) Geoscan AMSS spectra from a west - east traverse across the Quaver anomaly.
Figure 5: e) Field Spectra taken from the Quaver anomaly and its environs convolved to the specifications of the Geoscan AMSS.

Interestingly, carbonate in the form of calcrete along fracture plains is only seen in association with the original anomaly (Simpson and others 1989).  There is no evidence for carbonate either in the field or in the airborne data at the quaver anomaly nor is there any indication in the data over Kaparilja Spring.   There is no evidence for silicification in the original anomaly although it has been observed both in the field and in the data at the quaver anomaly and is strongly indicated in the data over Kaparilja Spring.

5.0  DISCUSSION & CONCLUSIONS

Assuming that both the quaver and Kaparilja Spring spectral anomalies represent mineral alteration associated with hydrocarbon seepage,  the distinct differences in alteration mineral associations between these two and the original spectral anomaly would suggest that hydrocarbon seepage in the Palm Valley anticline was a complex process.  It is not the purpose of this paper to attempt to explain these differences but rather to point out that multi- and hyper-spectral data can play a role in mapping surface alteration caused by hydrocarbon seepage and hence aid in unravelling complex movements of hydrocarbons within sedimentary basins.

Simpson and others showed that even coarse spectral and spatial resolution instruments such as Landsat TM (both airborne and satellite) could recognise alteration associated with hydrocarbon seeps.  This work demonstrates that a moderate spectral resolution instrument, the Geoscan AMSS MKII, can not only recognise such alteration but can also characterise the alteration mineralogy.

This work was carried out over hydrocarbon seeps in a monolithic geological succession, the Hermannsburg Sandstone.  In more complex geological environments comprising multiple lithologies and in prospective basins where the geology is less well known, hyperspectral data are capable of discriminating and characterising the host rocks and their altered products and of providing detailed structural information.  Thus, spectral remote sensing can contribute significantly to the exploration of hydrocarbon bearing sedimentary basins.

6.0 REFERENCES

R.F. Do Rozario and B.W. Baird, "The Detection and Significance of Fractures in the Palm Valley Gas Field," APEA Journal, Vol. 27, pp 264-280, 1987

J. Huntington and C.J. Simpson, "Major New Australian/US Remote Sensing Initiative," The Australian Geologist, Vol. 57, pp. 34-35, 1985.

C.J. Simpson, J.R. Wilford, L.F. Macias and R.J. Korsch, "Satellite Detection of Natural Hydrocarbon Seepage: Palm Valley Gas Field, Amadeus Basin, Central Australia," APEA Journal, Vol. 29, pp 196-211, 1989.

7.0 FIGURE CAPTIONS

Figure 1: Location of the Amadeus Basin, the Palm Valley Gas Field and the area of this study (in part after Simpson et al. 1989) with extract of the Hermannsburg and Henbury 1:250, 000 scale geological map sheets.

Figure 2: a) Geoscan band 17 8 2 RGB display (equivalent to Landsat TM 7 4 1 RGB) showing the strong Quaver colour anomaly.

Figure 2: b) Geoscan TIR band 20 21 22 RGB display showing silica as blue.  Note the strong indication of silica around the Quaver anomaly.

Figure 2: c) Grey scale image of Geoscan Fe-Oxide index demonstrating the strongly ferruginous nature of the core of theQuaver anomaly.

Figure 2: d) SWIR band difference image, 12-11 14-17 14-18 RGB showing carbonate as white.  Note the lack of carbonate in the area of the Palm Valley anticline and known anomalies and the strong indications in the broad plains to the north.

Figure 3: a) - left - The dark ferruginous ridge which forms the core of the Quaver anomaly (background) with the surrounding silicified shallow dipping sandstones (foreground) and b) - right - the nodular ferruginised sandstone core of the anomaly.

Figure 4: Ground spectra collected from the Quaver anomaly and environs.

Figure 5: a) Greyscale image of the spectral fit for goethite showing the strong match over the Quaver anomaly and south of Kaparilja Spring.

Figure 5: b) Greyscale Geoscan silica index showing the strong silicification observed around the Quaver anomaly and additional silicification to the south of Kaparilja Spring.

Figure 5: c) Greyscale image of the spectral fit for calcite showing the predominance of carbonate in the plains to the north of the anticline and in the original TM anomaly.

Figure 5: d) Geoscan AMSS spectra from a west - east traverse across the Quaver anomaly.

Figure 5: e) Field Spectra taken from the Quaver anomaly and its environs convolved to the specifications of the Geoscan AMSS.