Bernard C. Duval, Ghislain Choppin de Janvry, Bernard Loiret
Total
Paris
Extensive reinterpretation of geological and geophysical data from the Mahakam delta of Indonesia has recently been completed, allowing the petroleum system of the entire area to be clearly understood.
The Mahakam delta is in Kalimantan, Indonesia, on the east coast of the Island of Borneo.
Handicaps related to the huge volume of data and scarcity of accurate stratigraphic information were overcome by using specially designed data sheets and by applying sequence stratigraphy and a number of specific and specially designed geological tools that proved very effective.
The Mahakam delta is seen to still have substantial remaining potential, and this kind of study appears applicable to similar deltaic settings.
MAHAKAM EXPLORATION
Mahakam is a Tertiary delta on the east coast of Kalimantan, the Indonesian part of Borneo island (Fig. 1).
The sedimentary cover extends from Upper Eocene to Quaternary with a maximum thickness of about 12,000 m developed directly beneath the present delta.1 Large amounts of oil and gas from a number of fields have been found in a Middle to Upper Miocene deltaic sequence.2
Exploration started as early as 1888 with field geological investigations in the vicinity of oil and gas seeps. It resulted in the oil discoveries of Sanga Sanga in 1897 and Samboja in 1909 in uplifted Middle Miocene sands at relatively shallow depths.
In the 1970s several giant gas and oil fields were first discovered offshore, later in the delta area, mainly because of major improvements in acquiring seismic through mangrove swamps.
By 1985 proven and probable initial reserves amounted to about 2.6 billion bbl of oil and 20-30 tcf of gas.
Prospects for new discoveries had become poor, but this was a core operating area for Total. When demand for gas for a nearby liquefaction plant increased, it was decided that exploration should be renewed.
The purpose of this article is to show how, with a suitable technical and management approach, exploration could be revitalized and lead to new discoveries, including a giant gas/condensate field.
EXPLORATION ANALYSIS
A major basin analysis study was initiated to unravel the geological model and identify the functioning of petroleum systems over an area covering 40,000 sq km.
Total Indonesia's acreage position extends over three distinct geological units within the outer Kutei basin, the North Mahakam area, the South Mahakam area, and the carbonate Paternoster platform (Fig. 2).
The North Mahakam area, centered on the modern delta, is limited outwards by a belt of listric faults at the front of the Mahakam deltaic system. It contains the bulk of hydrocarbon reserves.
In the 4 year analysis special attention was given to new conceptual models based on sequence stratigraphy" to look for stratigraphic traps.
The main objectives of this comprehensive study were to evaluate the remaining exploration potential of the permits after 20 years of rather intensive structural exploration followed by field delineation and development.
This type of study in a mature area involves handling huge amounts of disparate data in a complex deltaic environment characterized by alternations of sands and shales with only subtle lithology variations.
DELTAIC SYSTEM INTRICACY
The number of potential hydrocarbon reservoirs is high: as many as 550 different reservoirs for a single oil field, Handil.
Thin stacked lithologies like these do not make it easy to calibrate with seismic.
In order to overcome the complexity of the deltaic complex and to help the transfer of well data on to seismic lines, synthetic documents were created by special data treatment methods resulting in sliding averages for simplified descriptions of alternating sand-shale lithologies.
The use of sequence analysis from seismic and well data permitted the definition of a precise chronostratigraphic framework at basin scale. It provided a new geological model with improved significance for paleogeographic, structural, and hydrocarbon evaluation.
About 250 control wells and 35,000 km of seismic were incorporated in this recently completed extensive reinterpretation.
GEOLOGICAL MODEL - STRATIGRAPHY ANALYSIS
The main deltaic and marine environments were redefined in terms of sequence analysis.
Sequential log analysis of key wells was undertaken, combined with the regional interpretation of selected lines in terms of seismic stratigraphy (i.e., identifying the geometric relationship between seismic reflections, onlaps, downlaps, truncations, etc.).
Vail's chart was used intensively, and workers recognized characteristic stratigraphic signatures of the Neogene.5
To calibrate the thin stacked lithologies, the team used a scale for the well data comparable to seismic. Sliding averages of basic rock components like sand percentages, coal occurrences, and carbonates were computed, providing a suitable and efficient tool for the study.
An east-west chronostratigraphic chart (Fig. 3) obtained from combined seismic and well data covering a time interval of 13.5 million years ago to 5 million years ago reveals the following points of interest:
- Abundance of coals in the delta plain facies. This is a key point because these are the most prolific source rock for oil and gas, together with some organic shales.
- Several transgressive-regressive cycles are developed within the deltaic interval, and maximum flooding surfaces are identified in conjunction with the main delta front encroachments. These surfaces, as well as some major sequence boundaries, can be correlated at a basin scale and provide effective internal seals in the system, and most of them can be correlated at basin scale.
- Some carbonates help underline the shelf boundary, and lowstand seismic wedges appear with some proven traps in Sisi field. Basin floor fans or slope fans are often well characterized from seismic, and located downdip from the mouth of incised valleys.
- On the southeast side of the basin are massive slope shales, strongly overpressured by lack of sufficient interconnected sands within their mass.
GEOCHEMICAL MODEL
Migration was thought, based on earlier models,6 7 to be essentially vertical (Fig. 4).
Gas was believed to be generated deep below within the overpressured shales, migrating upwards, and extracting heavier compounds from the source rock during migration through the oil window. The heavier hydrocarbons were progressively released by retrograde condensation while pressure and temperature dropped with decreasing depth.
In the current model (Fig. 4), the existence of deep source rock has been ruled out because the organic content decreases drastically as marine environments are reached.
The base of the effective kitchen is considered to be the top of the overpressured zone. The top of the kitchen was derived from projecting vitrinite reflectance data. The usual value of .6 was chosen. That yielded a good cross section of the kitchen.
This exercise, performed in three dimensions, resulted in an isopach map that showed that the most effective kitchen is restricted to the inner part of the offshore area.
The best area for oil and gas generation is located at the intersection between coaly delta plain facies and the thickest kitchen, and it is not a very large area. Some hydrocarbons have been found near the thinner eastern kitchen that eventually will be producing, but the source is not that good and the sand content is low.
The pattern of occurrence of hydrocarbons is closely linked to the identified kitchen, and most dry holes are 10-20 km from the kitchens, regardless of the structural configuration and the reservoir developments.
A 2-D geochemical model for 1 million years ago confirmed that migration paths are mainly lateral, because of a compressional rather than tensional structural style, and because it is limited or guided by good internal regional seals such as maximum flooding surfaces.
HYDROCARBON TYPES
Differences in fluid composition between the fields are explained by the effective volumetric and qualitative potential of the source rock and the expulsion-migration efficiency, which is of course closely related to the presence of drains.
For example, westward migration from coaly delta plain facies will result in the accumulation of large amounts of oil together with gas, which is the case of Handil.
Eastward migration from coal-poor distal deltaic environments will be more difficult due to conflicting facies evolution - less sands, less drains, less connections and it will result only in gas and condensates such as in Tunu field. This does not answer why Bekapai and Attaka oil fields exist along the same trend as Tunu.
The explanation is that vertical migration, in spite of being a subsidiary process, is an important factor and occurred late in these faulted structures. Whereas Tunu is not faulted, Bekapai is explained by dismigration through those active faults of a previous gas/condensate pool in deeper horizons,
The migration process is continuous and is still occurring, so there is no question about the timing of migration vs. the timing of structures.
Fresh water is present in the uppermost section over large areas. Such input of meteoric waters is clearly related to strong uplift of the hinterland where erosion during the Pliocene has exposed most of the Upper and Middle Miocene section close to the coast.
The knowledge of the subsurface temperature distribution and anomalies was essential to feed the geochemical model with proper thermal values. This provides a flavor of the complexities involved in such a comprehensive study.
REMAINING POTENTIAL
All significant fourway dip closures have already been drilled, and the only remaining prospects are minor closures on faults or potential traps with a strong stratigraphic component.
Here is a list of targets from speculative to promising:
Incised valleys are risky targets because of a strong risk of leakage through sands beyond and below the erosional contact of the valley.
Basin floor fans or slope fans have good entrapment potential due to sealing by surrounding shales but are handicapped by their unfavorable geochemical context within overpressured organic poor shales and by burial depth.
Low stand wedges have good potential because of the depositional framework. However, the actual pinchout is difficult to map with accuracy, and risk of leakage is difficult to evaluate.
These wedges are considered most prospective where incorporated within a structure or at least with counter dip as is the case in Sisi field. However, the distal reservoirs are fine grained and not so good.
Channel fills of the deltaic plain, regardless of the systems tract involved, constitute the main reserves in the inner deltaic areas. Entrapment results from the intersection of a channel with a Structural high.
The potential of the remaining structural noses was evaluated with reference to the estimated head of passes of the delta and the orientation of the distributaries at the corresponding time.
RECENT EXPLORATION
The proposed petroleum system model enhances the interest of the western flank of the existing structural highs, whenever stratigraphic traps are possibly identified.
The 1 Pegah well drilled in the 1970s had some shows in the lower section and poor sands in general. The 1 Peciko well, drilled years ago downdip from 1 Pegah, found more sands and some gas.
The recent major discovery of gas at 1 Northwest Peciko, 7 km downdip from 1 Peciko, confirms the high remaining potential of the area. The well had a thick column of gas and condensate (Fig. 5).
Significant amounts of gas were found in thin sands belonging to various system tracts (mainly low stand and transgressive system tracts) in relation with a general southeastwards shaling-out and pressure enhancement from the undercompacted shales. Similar thin sands had already been found in giant Tunu gas/condensate field.
High productivity and sustained rates were observed during the first two years of production at Tunu and strongly support the economic potential of these partly stratigraphic plays.
The eighth delineation well has just been successfully drilled in Peciko. The field will undoubtedly be declared a giant with 3-5 tcf of reserves and possibly more.
CONCLUSIONS
In spite of intensive exploration in the past 20 years, the Mahakam area is seen as having substantial remaining gas potential, particularly in traps where favorable geochemical and sedimento-logical ingredients warrant further exploration for stratigraphic traps.
The definition of these new plays was possible owing to a renewed geological and geochemical model that led to better comprehension of the overall petroleum system.
Sequence stratigraphy enabled a precise chronostratigraphic framework at basin scale to be constructed for the first time and was the key to a better regional understanding of the sand and source rock distribution.
Early identification of tools and methods is needed, particularly finding the right tool to describe a complex lithology.
Time is of essence. It takes time and a lot of work to generate new ideas in the middle of established conventional wisdom. The best people should be involved.
Modeling is vital, but it must be carefully validated by geology and vice versa, with attention to the important role of regional seals. Hydrodynamic components should not be neglected.
More attention should be paid in the future exploration of deltaic provinces to the relationships between facies variations, overpressures, and stratigraphic accumulations.
A historical and forecast production profile of the area indicates that the bulk of an expected increase in gas production in the near future will come from Tunu field. A further production increase from Tunu together with the development of the significant discovery of Northwest Peciko will allow new production peaks to be reached onwards from 1994.
Future gas production in oil equivalent terms will surpass the previous peak rate of 230,000 bid in 1977 from Handil and Bekapai fields (Fig. 6).
ACKNOWLEDGMENTS
The authors thank Pertamina, Inpex, and Total for permitting this publication. They gratefully acknowledge the help received from Y. Grosjean and J.L. Piazza of Total in participating in the study and gathering data and illustrations. Comments and advice were kindly provided by A. Johnson of Total and G. Demaison of Petroscience.
REFERENCES
- Magnier, P., Oki, T., and Kartaadipura, L.W., The Mahakam Delta, Kalimantan, Indonesia, 9th World Petroleum Congress, 1975.
- Gerard, J., and Oesterle, H., Facies study of the offshore Mahakam area, Proceedings Indonesian Petroleum Association, 2nd annual convention, June 1973, pp. 187-194.
- Vail, P.R., Mitchum, R.M. Jr., and Thompson, S., III, Seismic stratigraphy and global changes of sea level, in Stratigraphic interpretation of seismic data, AAPG Memoir 26, 1977, pp. 83-97.
- Vail, P.R., Colin, J., Du Chene, R.J., Kuchly, J., Mediavilla, F., and Trifilieff, Y., La stratigraphie sequentielle et son application aux correlations chronostratigraphiques dans le Jurassique du Bassin de Paris, French Geological Society Bull., Vol. III, No. 7, 1987, pp. 1,301-21.
- Vail, P.R., Audemard, F., Bartek, L.R., Bowman, S.A.,Coterill, K., Emmet, P.A., Liu, C., Perez-Cruz, G., Ross, M.J., and Wu, S., The global stratigraphic signature of the Neogene, in Sea-level changes at active plate margins-processes and products, Mac Donald, London, 1989, p. 12.
- Durand, B., and Oudin, J.L., Exemple de migration des hydrocarbures dans une serie deltaique: le Delta de la Mahakam, Kalimantan, Indonesia, 10th World Petroleum Congress, panel discussion 1, 1979, pp. 1-9.
- Oudin, J.L., and Picard, P.F., Genesis of hydrocarbons in the Mahakam delta and the relationship between their distribution and the overpressured zones, proceedings Indonesian Petroleum Association, 11th annual convention, June 1982, pp. 181-202.
Copyright 1992 Oil & Gas Journal. All Rights Reserved.
