PA97 - PO - 16

INDONESIAN PETROLEUM ASSOCIATION Proceedings of the Petroleum Systems of SE Asia and Australasia Conference, May 1997

APPLICATION OF THE PETROLEUM SYSTEM CONCEPT TO RECONNAISSANCE ASSESSMENTS OF MATURE AND EMERGING PRODUCING BASINS,

WITJ3 EXAMPLES FROM INDONESIA

L. Rogels Hardy* S h d a Muchsin*

La Ode Ichram* Luki Samuel**

Eddy Pumomo * *

INTRODUCTION

Companies engaged in new venture oil and gas exploration commonly conduct basin evaluations as an initial effort to decide where to focus in-depth studies. A traditional method to assess remaining exploration potential for mature and emerging producing basins is to compare measures of exploration maturity to certain indicators of ultimate potential, such as source rock voiume, thermal maturity, and charge and trapping efficiency (Kingston et d., 1983, Demaison and Huizinga, 1991). To be performed rigorously, these traditional basin evaluation methods require a large amount of data, and are very time consuming. They require subsurface data, such as well logs, seismic lines and sample analyses to be interpreted to delimit the extent of an active petroleum system or coalescing systems. For a company without a presence in a basin, these data are usually difficult to obtain.

Surface indications of oil and gas, such as seeps, fields, and reports of successful DST's are also indicators of an active petroleum system or coalescing systems. This information, however, can usually be quickly acquired in the public domain or purchased from consulting firm databases. Synthesis of this information using the method outlined below can quickly give an initial indication of the size and richness of the petroleum system or systems of mature or emerging producing basins.

* Unocal Indonesia Co ** Pertamina

METHODS AND LIMlTATIONS

The following method is used for this type of assessment :

Indications of an active petroleum system at the surface, such as of all known seeps, fields, and DST's for a basin, are plotted on a map.

A line is then drawn around all these locations, forming a closed loop.

This closed loop encompasses an area herein defined as the Known Petroleum Systems Area (KPSA), measured in square kilometers.

This area can be mapped to show the approximate limit of the known petroleum system or systems in most mature or emerging basins. This, when combined with the number of new field wildcats drilled in the basin, can give a relative indication of the "richness" of the basin, and future potential for reserve additions. These initial relative indications can determine which basins to study further, integrating regional geologic and reserve information commonly available for purchase.

This method assumes that surface indications of oil and gas approximately delineate the subsurface extent of the petroleum systems of a basin (Magoon, 1987). This assumption is not always valid, therefore the method is only appropriate for "first pass" scoping evaluations for basins with sufficient exploration and production. Basins with few wells, inaccessible areas, or super seals may have large areas of active

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© IPA, 2006 - Proceedings of an International Conference on Petroleum Systems ofSE Asia and Australasia, 1997

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petroleum systems with no known surface indications, hence the KPSA would be underestimated. Areas with no petroleum system may exist within the K P S A of large basins with multiple petroleum systems determined in this manner, hence the KPSA could be

DISCUSSION OF APPLICATION OF THIS METHOD TO THE PRODUCING BASINS OF INDBNES

This method of evaluation was applied to all producing basins in Indonesia, with the exception of the Bula Basin in Eastern Indonesia. Also, the East Natuna and Bintuni Basins were added because of their significant discovered gas resources. The KPSAs are shown in Figure 1. To place the KPSAs in geologic context, probable petroleum systems areas are also shown, estimated from the Total Sediment Thickness Map (Figure 2) (modified after CC 1991). South Sumatra, followed closely by Northeast Java and Kutei, have the largest KPSAs in Indonesia (Figure 3). The KPSAs of the less mature basins, especially East Natuna and Bintuni may, of course, grow with more drilling.

The most new field wildcats (NFWs) have been drilled in Central Sumatra, followed closely by Northwest Java (Figure 4). As an indicator of exploration maturity, the number of NFW-s can be divided by the KPSA to give NFW density in wells per 1000 sq km. (Figure 5). The most densely explored basin in Indonesia is Central Sumatra, followed by Northwest Java and North Sumatra. Relatively few wells have been drilled in the South Sumatra, Kutei, and Northeast Java basins, considering the size of their KPSAs (Figure 6).

Estimated ultimate oil, gas, and oil equivalent are shown in Figures 7-9 (modified after Sujanto et a]., 1993, and Howes and Suherman, 1995). ("Remaining" oil and gas include proven reserves and estimates of probable oil and gas in or near known accumulations). Central Sumatra is by far the most dominant oil basin, and East Natuna leads for gas, followed closely by Kutei. On an oil equivalent basis, Kutei ranks a close second to Central Sumatra.

A cross plot of ultimate oil equivalent and NFW density is shown in Figure 10. The East Natuna, Kutei, and South Sumatra Basins stand out as being

lightly explored with significant resource discovered, therefore these areas should have excellent remaining exploration potential. (The East Natuna Basin is so lightly explored, as are other frontier basins, that this evaluation method is not rehable).

The Kutei and South Sumatra Basins, however, have sufficient data for this method to be valid. A relativc estimate o f KPSA "richness" can be shown by combining these indices of ultimate resources discovered to date with expioratio2 derasiq (Figure 119.

CONCLUSIONS

Applying petroleum system principles to the study sf mature and emerging basins can quickly determine which basins to study further. Using this method, the basins most worth further study in Indonesia are the Kutei, East Natuna, an South Sumatra, based 0x1

large KPSA, low exploration density, and large reserves discovered to date.

REFERENCE

Committee for Coordination of Joint Prospecting (CCOP), 1991, Total Sedimentary Isopach Map Offshore Southeast Asia, Technical Bulletin, 23.

Demaison, G.T., and Huizinga, .T., 1991, Genetic Classification of Petroleum System, AAPC Bulletin, 75/10, 1626-1643.

Howes, J.V.C., Suherman Tlsnawijaya, 1995, Indonesian Petroleum Systems, Reserves Additions and Exploration Efficiency, Proceedings of the Indonesian Petroleum Association, 24/2, 1-1 7.

Kingston, D.R., Dishroom, C.P., and Williams, P.A., 3, Hydrocarbon Plays and Global Basin

Classification, AAPG Bulletin, 67, 2184-2198,

Magoon, L.B., 1987, The Petroleum System A classification Scheme for Research, Exploration, and Kesource Assessment, U.S. Geological S w e y Bulletin 1870.

.9 Kartanegara, L, Sum Gultom, k., 1993, An Assessment on Xndonesian Natural Gas Reserves and Resources, Proceeding Indonesian Association cf Geologist, 22/2, 638-652.

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