The Krisna Field, located on the western flank of the Sunda Basin, constitutes one of the most significant petroleum discoveries by PERTAMINA-HAPCO in the Southeast Sumatra Production Sharing Contract Area. The oil accumulations found to date are associated with the â??Krisna Highâ?¿, an old paleotopographic basement high which in late-Oligocene time acted as a source of terrigenous material and later became a favorable site for the growth/sedimentation of reefal carbonates. Deposition of the reservoir rocks took place mainly under transgressive conditions, although some regressive pulses have been recognized. As the rate of transgression of the â??Krisna Highâ?¿ increased, reefal growth could not keep pace and ceased.

Hydrocarbon accumulations found on the â??Krisna Highâ?¿ are mainly of a stratigraphic nature; however, the structural component in some instances plays an important role.

Geochemical data indicate that Krisna hydrocarbons were generated in the Sunda Deep. Therefore, moderate-to-long distance migration is postulated via the basal unconformity and major fault zones.

A preliminary interpretation of the relationship between facies distribution and porosity development is presented in the discussion to follow. Porosity in the reservoirs is mainly secondary, and is associated with low energy back-reef facies.

Introduction

The â??Krisna Highâ?¿ is an old paleotropographic basement high which was part of the ancestral Sunda landmass. It is located on the more stable western flank of the Sunda Basin, approximately 85 miles northwest of Jakarta in the western part of the Java Sea, in an area operated by HAPCO Division of Natomas International Corporation. (Figure 1) illustrates the location of Krisna Field with respect to other PERTAMINA-HAPCO oil fields in the Southeast Sumatra Production Sharing Contract Area.

In late-Oligocene time, the â??Krisna Highâ?¿ acted as a local source of terrigenous material, which was deposited around the high in swampy and shallow marine environments. This period was followed by one of transgression, with conditions favorable for reefal growth resulting in the development of fringing reef complexes of the lower Batu Raja around the â??Krisna Highâ?¿ during early-Miocene time. At this point in time, further transgression produced slightly deeper marine depositional conditions, and the Batu Raja shale and most of the upper Batu Raja were deposited. The site of shallow water carbonate developmented shifted westward, toward the culmination of the â??Krisna Highâ?¿, where carbonate growth/sedimentation continued over a more restricted area. Subsequently, the rate of transgression of the â??Krisna Highâ?¿ increased, such that reefal growth could not keep pace, and, as a consequence, ceased. The deposition of dominantly fine terrigenous sediments continued throughout the remaining early-Miocene time.

Since sedimentary facies, diagenetic processes, and distribution of porosity are closely interrelated, a preliminary interpretation of these parameters as they relate to the lower Batu Raja carbonates is presented herein. The best porosity development is secondary, and appears confined to low-energy, back-reef facies. Primary porosity in the reef core has been almost totally obliterated by diagenetic processes. Intertidal to supratidal carbonate deposits have also been heavily affected by cementation and recrystallization. Thus, the reef core generally appears as poor quality reservoir rock, and the intertidal to supratidal carbonates appear as a permeability barrier adjacent to the strandline.

History of the Discovery

The Krisna-1 well was drilled in early 1976 to test the crest of the structure. It tested at a rate of 2686 BOPD from a carbonate section found resting on basement. However, it was not until late 1979 that the drilling of Krisna-3, demonstrated the large hydrocarbon potential of the Krisna area. Drilling of this well resulted from reinterpretation of both subsurface and seismic data, which indicated the presence of additional stratigraphic section on the flanks of the Krisna High, which had not been encountered in Krisna-1.

Krisna-3 was followed by a successful confirmation well drilled directionally to the south of the Krisna-3 surface location, and development of the area was confirmed as economically viable. Eleven months then elapsed from the date of determination that a commercial discovery had been made, to production start-up from the first platform in November, 1980. Currently there are four platforms on production in the Krisna area, plus one in the Yvonne area, approximately one and one-half miles to the north of the Krisna â??Aâ?¿ platform.

Regional Setting

The southeast Sumatra Production Sharing Contract Area includes the Sunda Basin, its related sub-basins and the Asri Basin.

The Sunda Basin is a north-south oriented, asymmetrical basin, with a broad, gently dipping, stable western flank. It is bound in the east by the Seribu fault system and the Seribu Platform. The Cinta-Rama arch forms its southern boundary and separates it from the Kitty-Nora Sub-basin. (Figure 2) illustrates the main geologic features of the Contract Area.

The deepest part of the Sundra basin received a thick sequence of clastics, viz. the Talang Akar Formation. The Talang Akar section has a maximum thickness in excess of 8500', and rapidly thins to the north, west and south. The basin flanks correspond to more tectonically stable areas on which considerable carbonate sedimentation/growth took place during Miocene time.

The â??Krisna Highâ?¿ is an old pre-Tertiary basement high located on the western shelf of the Sunda basin. This high area played two important roles:

  1. as a provenance area for clastic deposition; and

  2. with continued transgression, as a favorable location for the growth of reefal complexes.

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