The East Java-Makassar-Muara (EJMM) MegaProject consists of 98 2-D lines totaling more than 16,000 km (9,941 miles), covering the four hydrocarbon-bearing basins: East Java/North Lombok, South Makassar, the Kutei/North Makassar, and the frontier Muara basins.

It combines five separate long offset 2-D surveys acquired by PGS and MultiClient Geophysical between 2005 and 2010 into a single consistent time-, phase-, and amplitude-matched seismic dataset. The interpretation of the EJMM MegaProject dataset has been undertaken post-data merge to provide an improved, consistent, and comprehensive interpretation by tying wells to a common 2-D seismic database.

FIGURE 1. The black lines indicate the EJMM MegaProject. The gray lines are other PGS 2-D data. This Tertiary isochron map defines the North Lombok, South Makassar, North Makassar, and Muara basins. (Images courtesy of PGS)

The 2-D grid ties the basins together and intersects several key wells. It gives the ability to obtain a regional geological perspective to help evaluate proven and new plays, look at the petroleum systems at the regional scale, understand recent well results, and correlate key stratigraphic elements across the basins to better understand the lateral continuity of sequences and structural development of the basins.

Regional geologic setting

The EJMM MegaProject covers four Tertiary basins along the eastern margins of the Sundaland Craton in the tectonically complex area between the island of Java to the south and the islands of Borneo and Sulawesi to the north, all of which are affected by the collision of Eurasia with the Australian and Pacific plates.

From the Early Cretaceous to no later than the Early Eocene, the area experienced periodic episodes of extension and compression, which have resulted in the current configuration of the basins. This dataset has been constructed to study the back arc East Java/North Lombok basins in the southern zone of the data and the South and North Makassar basins, which extend northward to the contiguous Muara basin around the northeastern edge of Borneo.

All of the basins in the EJMM MegaProject contain thick Tertiary sections. An isochron map between seabed and top pre-rift (basement) defines the Tertiary depocenters. North Lombok, South Makassar, and North Makassar basins are clearly defined, whereas the Muara basin's complexity is shown (Figure 1). The stratigraphy of these basins is very similar, with both syn-rift and sag contributing to the deposition of Paleocene-Eocene clastics, which are overlain by Eocene-Oligocene reefal complexes buried by thick Miocene-Recent packages.

All of the basins have undergone various degrees of extensional and compressional tectonics, resulting in a range of possible exploration targets, from simple inverted basin floor fans or stratigraphic pinchouts to intricate thrust-fold complexes resulting from more intense deformation as the Australian continental plate collided with the continental Eurasian plate.

Seismic interpretation

Six key horizons were interpreted for this project. Five of these are major unconformities, regional seismic markers that have been tied to formation tops where well data is available. Due to the nature of these markers, interpreting horizons away from the well ties was generally straightforward over most of the project area except in zones of major compression and inversion where either the horizon has been eroded or the tectonic complexity prevents accurate imaging by the seismic data.

A simplified stratigraphic column has been prepared for the EJMM MegaProject area. This shows the key lithological descriptions of the four main basins (East Java/North Lombok, South Makassar, North Makassar, and Muara basins) and their relationship to the horizons picked (Figure 2).

The seabed horizon was picked as the first continuous trough on all the 2-D lines across the project area. The importance of picking this reflector is mainly for calculating time thickness maps. The Early Pliocene Unconformity, related to the collision of the Australian continental plate with the Eurasian continental plate, is the shallowest horizon picked. The Mid-Miocene Unconformity is an extensive marker that correlates to a major eustatic sea level fall and the spreading of the South China Sea. It reflects transpressional inversions related to a number of local and regional collisional events, primarily along basin edges. Post-rift thermal sag was initiated in the Late Eocene and continued through the Oligocene into the Early Miocene. The Top Oligocene horizon defines the end of this rift/sag period and the onset of inversion as the front edge of the Australian Plate makes contact with the Eurasian Plate. It marks the end of oil-prone lacustrine and deltaic petroleum systems and the beginning of the development of shallow to deep marine systems.

FIGURE 2. This simplified stratigraphic column shows the lithologies across the basins and the relationship of the horizons in this project.

The Eocene Unconformity is a rifting surface related to the initial collision of the Indian-Australian plate with Eurasia and the beginning of the spreading of the Celebes Sea.

The top pre-rift pick marks the base Tertiary. There are only a couple of well ties for this horizon in the South Makassar basin. The geological age of the pre-rift section is equivocal, usually labeled as Pre-Tertiary but postulated to be at least Mesozoic and likely to be even older, possibly ranging to Pre-Cambrian.

FIGURE 3. The North Makassar and South Makassar basins are shown here in the higher image in one continuous seismic section, while all four Tertiary basins of the EJMM MegaProject are shown in one continuous seismic section in the lower image.

Advantages of a MegaProject when exploring

The EJMM MegaProject gives new entrants and existing operators the opportunity to regionally evaluate the geological concepts and hydrocarbon play elements of these Tertiary basins on a single 2-D seismic dataset. It enables tying key wells, understanding recent well results, and correlating stratigraphy across the basins to help derisk decisions and high-grade prospective areas.

The main aim is to be able to evaluate basins at the "big picture" regional scale instead of "postage stamp" evaluations. A continuous 2-D section approximately 2,000 km (1,242 miles) long shows how the four basins can be seen in one section (Figure 3).

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