Tertiary Tectonic Compilation in a GIS for Indonesia

© IPA, 2006 - 28th Annual Convention Proceedings, 2002

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PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATION Twenty-Eighth Twenty -Eighth Annual Convention & Exhibition, October 2001 TERTIARY TECTONIC COMPILATION IN A GIS FOR INDONESIA James W. Granath* Janice Christ** Derek Fairhead Fa irhead*** *** William Dickson****

ABSTRACT

Petroleum exploration is often more efficient with a regional perspective in hand prior to focused seismic reflection studies. We have developed a low-cost method of geological spatial analysis that combines high-quality synoptic data coverage with localized geologic knowledge in a GIS format using common, off-the-shelf, PC-based software. The results are scaleable across at least two orders of spatial magnitude, are easily editable and highly portable. It is now a trivial matter to include public domain information within an exploration dataset. Equally, our results can readily be extended with commercial or exclusive data, allowing continuous updating. The cornerstone of our procedures is a merged regional grid of onshore, marine and high-resolution satellite gravity data, providing a unique means to unify previous tectonic interpretations and link and extend features into areas not adequately covered by seismic data or drilling. Digital elevation models, remote sensing datasets, and raster files created from the literature are feedstock for the generation of new interpretations. interpretations. A synthesis of the Banda Sea region using these techniques, for example, suggests a transform connection between the Sorong fault system and the Hamilton fault, separating Buru and Seram. INTRODUCTION

Wallace Pratt’s (1952) famous quote to the effect that oil and gas are found found in the minds of explorers is as apropos today as it was when he made it, but at least  _________________  ________ _____________________ ______________ ____ ____ ____ ____ ____ ____ __ * ** *** ****

Granath and Associates, Housto Houston n J-Sea Geoscience , Houston Geophysical Exploration Technology, Ltd., UK  Dickson International Geosciences, Houston

two things are different in the year 2001. Firstly, much of the world is more mature with respect to exploration; the sheer volume of information available to the explorer can be mind-numbing. Secondly, and fortunately, information technology tools are much more sophisticated than in Pratt’s day. The mental process to which Pratt referred is certainly the same, but the feedstock for it is quite different.  No longer longer must we depend depend on the tedious tedious prepar preparatio ation n of geographically accurate, hardcopy overlays of various maps to compare, contrast, and even store geological, geophysical, and business data. Compilation of those data can be a one-time affair today, and the overlaying of it an instantaneous event, leaving more time for the creative processes of imaginative imaginative thinking. Today, various kinds of data can be processed, combined, compared, analyzed, and displayed within the same context and be either  passed on to the co-wor co-working king explo explorers rers or or archived archived for for subsequent adventurous minds. Combining different types of data is particularly easier when the rigorous link of geographic location can be used. The data itself becomes a geographically accurate model of the real world. The idea that data can be digitally manipulated and stored to produce a map is not particularly particula rly new. GIS (geographic information systems, in which objects, data, and images are digitally linked within a database through geographic coordinates) coordinates) software is some s ome 30 years old now. With the development development of better better user interfaces, more user-friendly software, and links to simple databases, GIS has moved from an IT discipline to an analytical tool for scientists and decision makers. In  particu  particular, lar, spatial  spatial analysi s has become an objective of GIS advocates (Mitchell, 1999). Spatial analysis involves the use of queries of, and operations on, database entries to produce a new result. A familiar example in E&P would be to query a well database Vol. 1 - 345

for formation tops and produce an isopach map from the results. A more sophisticated use would be to compare results of database queries or operations with tectonic elements or alternative interpretations to test them and to generate a revised interpretation. To date, such uses of spatial data have been more common in industries that employ large, readily available, common datasets (like demographics, addresses, weather, etc.). Well and remote sensing data, in particular, are typically commercially available data sets while wells are also often maintained by oil companies on a proprietary basis. The wealth of geological data in the literature, however, has yet to be routinely available in GIS format. DIGs, GETECH and their scientific associates have been pioneering spatial analysis in the context of geological and geophysical data through the integration of GETECH's digital gravity, magnetic, and topographic/bathymetric data with value-adding geological information and interpretation from the literature. There are two projects in the Atlantic realm, one completed and one currently underway. Here we describe the tectonic component of a project in Southeast Asia, dubbed ‘SEAMAGIC’, and how we procedurally are effectively converting the literature to an editable exploration tool from its traditional role as a mental feedstock. WHY A GIS?

Modern GIS are structured well to represent geological and geophysical data. Potential methods data, satellite spectral, imaging radar, topography,  bathymetry, and any data structured in a ‘one data  point per one location’ format, can be represented in a raster format. A raster is essentially a grid of cells, each with definite geographic position and each cell containing a quantitative data point. The area of the surface of the earth represented by the cell defines the resolution of the data: the finer the cells, the more narrowly the data are defined geographically. SEAMAGIC uses to advantage that any hardcopy can  be converted to a raster file by scanning into an appropriately detailed image. The pixels in the image, equivalent to the cells in a raster, can then be georeferenced, and the image overlaid to any other dataset in the project. On the other hand, data with multiple attributes per database entry (e.g. wells, geological outcrop, fault and fold traces, etc.) are best formatted as vector data.

This is the heart of a GIS: each location is linked to a database, which can in principle contain any sort of  background, or attribute data - quantitative or qualitative. Modern systems usually are built around  provision for point, line, and polygon vector files, virtually covering the full range of possible mapping elements. In the system used in SEAMAGIC TM (ArcView 3.2   ) these vector files are called shape files. These are the interpretive data generated from  primary datasets, often imagery in SEAMAGIC. Figure 1 shows the area of eastern Indonesia between the eastern arm of Sulawesi and Irian Jaya as a set of stacked vector files portraying the crustal makeup of the area as polygon files and with overlays of various tectonic elements in line files and volcanoes as a point file. This is essentially a ‘screen dump’ from the GIS (with a north arrow and scale bar added) just as an explorer might work with it, a working file so to speak. ADVANTAGES OF A GIS

From the exploration point of view the main advantage of this approach to capturing regional geology, and the literature that supports it, lies in the ability to overlay the public domain information with  proprietary information to pursue the normal variety of exploration activities, e.g. generate plays, evaluate farm-in opportunities, direct new business ventures, etc. Seismic locations can be compared to geology as easily as any other data, thus linking the seismic interpretation process to any previous work in the area. Seismic data can be linked to the GIS either through hotlinks (pop-ups) of images or direct representations of SEG-Y data and compared to other cross-sectional information. Field and well data can  be handled similarly, with the added advantage that all attribute data collected on either of these can be used in spatial analytical projects.

Over time, the GIS has the potential to become a corporate repository of data that is readily at hand, a corporate memory of sorts. In addition, the repository is scale independent in that data can be accurately compared over two orders of magnitude. It is editable, eminently so in that the GIS itself is the manipulative as well as the archival platform. It is portable; indeed one of the rapidly advancing aspects of GIS is its maintenance over the web. It is ideally suited to corporate intranets. And GIS systems are Vol. 1 - 346

increasingly compatible with other data formats. CAD files in particular are easily imported to a modern GIS. Scripts and translators are written for a large number of software packages to connect them into GIS environments.

recognizable control points in the map itself. The result is a raster, usually a GEOTIFF file, that is compatible with the primary sources of information and can be overlain and compared to primary information.

METHODOLOGY

The TIF raster images are processed within the GIS itself to ‘bleach’ the undesired colored pixels, usually the background colors, to transparent, thus rendering the desired information in a form that can be overlain on the primary source files. This allows the interpreter to recognize the geophysical or topographic signature of the geological elements of the image in the primary source files and to either extend the features beyond the area studied or to adjust their position. Interpretive files can be created within the GIS, in vector format, that integrate different data sets and produce a coherent, regional file to which any sort of attribute data can be attached. In the case of the tectonic maps compiled for SEAMAGIC, these attribute tables contain names of features, synonyms, references to the scanned source data, ages of activity, etc.

The foundation datasets for SEAMAGIC are: (1) GETECH’s comprehensive, high-resolution satellite gravity data, reprocessed using techniques proven by GETECH in the South Atlantic and Central Asia. The advanced  processing provides anomaly attributes that, when correlated to known structural ‘control’ data, inherently allows confident extension into unmapped areas. (2) Onshore gravity data merged into a regional grid, and adjoined to the marine data to form a continuous regional coverage. (3) Magnetic data.

AN EXAMPLE IN EASTERN INDONESIA

(4) Digital elevation models (DEM) for the onshore in as fine a resolution as possible (5) Any satellite or airborne radar imagery as we may find useful in compiling an interpretation. (6) Regional geological maps.  Note that, except for the geological maps, these are all raster datasets of primary information. This continuous coverage of geographically registered datasets provides a unique means to unify previous tectonic interpretations and extend, link, and rationalize features into areas not adequately covered  by seismic data, drilling, or the geological literature itself. Secondary sources of information, and the invaluably unique aspect of SEAMAGIC, are scanned and georeferenced illustrations from the literature. This amounts to a vast source of detailed geological information that has accumulated in the public domain, but is normally regarded as interpretive and is usually difficult to integrate with exploration type data. These scanned images are processed using specialized software that assigns geographic coordinates to the file through the identification of

The major barrier to linking the geological literature to an exploration program is the accurate location of mapping elements. Potential fields and topographic information are the critical link in this objective. A wealth of information on basement composition and its influence on the overlying Tertiary is a natural by product of the gravity and magnetic processing. The normally low-density and non-magnetic Tertiary sediments of Indonesia overlie various ages of  basement with generally higher density and more variable magnetization. Potential field methods are thus ideal for identifying and mapping the Tertiary  basins and for linking complex structural elements together. Although it is of a relatively small scale, Figure 2 shows a merged DEM and bathymetry dataset with the same line and point files of structural elements as are shown in Fig. 1. The major elements of the region are readily apparent in this figure as well as its companion dataset in Fig. 3, an isostatic gravity anomaly map. The collision of the Sangihe and Halmahera arcs is evident in the north with a dramatic gravity low. The sweep of the Banda Arc is identifiable around the Banda Sea with its line of volcanoes behind the trench. The interface of the Vol. 1 - 347

Seram Trough with Irian Jaya and the Timor Trough with the Australian craton in the southeast are easily identified. The controversial Ayu spreading center can be easily seen in both the gravity and bathymetric data, north of Irian Jaya in the southwestern Pacific. The Banda Sea crustal makeup is a matter of controversy. Various authors have attributed the Banda lithosphere to Mesozoic oceanic crust, Tertiary marginal sea basins, or pieces of distended continental crust. In Fig. 1, we subscribe for the time  being to a Mesozoic South Banda Basin (Pigram and Panggabean, 1984) and a Neogene North Banda Basin (Rehault et al. as reported in Villeneuve et al., 1994), and several continental crustal fragments (e.g. Silver et al., 1985, as shown in the oceanic crustal shape file. To illustrate the GIS techniques used in this compilation, we have expanded the size of some of the distended continental fragments on the basis of the topographic and gravity data. In addition, to unify the map in terms of its structural kinematics, we have had to add some structural elements in the central Banda Sea. The Seram Trough represents the convergence between the island of Seram and the Banda Sea plate with the Bird’s Head (Vogelkopf Peninsula) of Irian Jaya. Note the trough north of the Selat Manipa between Buru and Seram, where the Seram Trough terminates. Because of the quite different current tectonic settings of Buru and Seram that trough can be used to locate a left-lateral transform that branches off the Sorong Fault System to connect to and terminate the accretionary prism. There is an even more dramatic signature of the end of the Seram Trough evident in the isostatic anomaly data (Fig. 3). This transform also appears to offset Buru Island from Seram Island, and therefore its full southern extent must lie somewhere south of the islands themselves. Noting also the different signature in the isostatic dataset (Fig. 3) between the floor of the central Banda Sea and a higher and apparently younger northwestern part of the sea, the transform appears to continue towards the SSW to transect the central Banda Sea. If this is true, the transform may parallel a trend of diffuse seismicity across the central Banda Sea to connect to the east end of the Hamilton Fault (which lies along the north side of the Tukang Besi platform). Other data could in principle be added to these simple interpretations to test them or to develop alternatives. Detailed geological maps of Buru and Seram, for example, could be dropped into this study along with

detail of the Selat Manipa topography to test for a left-lateral signature. Proprietary seismic data could  be interpreted and linked to the same area. Multiple working hypotheses (Chamberlain, 1897) are as easily investigated as editing the file of the strike-slip elements on the fly, and carrying separate interpretations in separate shape files. The same  principles can be brought to prospect and play definition, and hence the best practices of the geotechnical side of risk analysis are less time consuming than doing things by hand. CONCLUSIONS

Geographical Information Systems (GIS) afford an inexpensive, relatively easy way to compile, organize, store, and analyze geological and geophysical data for hydrocarbon exploration and to merge different types of datasets. Modern techniques allow the integration of the literature with more conventional exploration datasets in a detailed, accurate way.

REFERENCES

Chamberlin, T.C., 1897. The method of multiple working hypotheses, Journal of Geology, v. 5, p. 837-848. Mitchell, A., 1999. The ESRI Guide to GIS Analysis, V. 1 Geographic Patterns and Relationships. ESRI Press (Redlands, CA), 186 pp. Pigram, C.J., and Panggabean, H., 1984. Rifting of the continental margin of the Australian continent and the origin of some micro-continents in eastern Indonesia, Tectonophysics, v. 107, p. 331-353. Pratt, W. E., 1952. Toward a philosophy of oilfinding. AAPG Bulletin, v. 36, no. 12, p. 2231-2236. Silver, E.A., Gill, J.B., Schwartz, D., Prasetyo, H., and R.A. Duncan, R.A., 1985. Evidence for a submerged and displaced continental borderland, north Banda Sea, Indonesia, Geology, v. 13, p. 687691. Villaneuve, M., Cornee, J.J., Martini, R., Zaninetti, L., Rehault, J.P., Burhanudin, S., and Malod, J., 1994. Upper Triassic shallow water limestones in the Sinta Ridge Banda Sea, Indonesia , Geo-Marine Letters, v. 14, p. 29-35. Vol. 1 - 348

FIGURE 1   – Shape file presentation of the Banda Sea area of eastern Indonesia. Crustal types are classified on the basis of oceanic (and its broad age), continental named terrains, miscellaneous continental crust, and accretionary prisms. Line file overlays include the tectonic elements of a conventional tectonic map, and point files for volcanoes. This  presentation is identical to the screen presentation of the GIS. NBB North Banda Basin, SBB South Banda Basin, B.Buru, S Seram, TBP Tukang Besi Platform, sm Selat Manipa, hf Hamilton Fault, sfs Sorong Fault System.

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FIGURE 2 – Raster file of the digital elevation data (DEM) of the same are as shown in Figure 1. Line and point shape files are the same as those of Figure 1 and are given in the legend for Figure 1.

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FIGURE 3   – Raster file of the isostatic anomaly derived from satellite and onshore gravity, merged into a single file. The area is the same as shown in Figure 1. Line and point shape files are the same as those of Figure 1 and given in the legend for Figure 1.

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