Abstract

The Lalang and Mengkapan oil fields are located within the Malacca Strait production sharing contract area, Indonesia, some one hundred and eighty kilometres west of Singapore.

Exploration of the 2.9 million acre area commenced in 1970, but the Lalang oil field was not discovered until 1980, following extensive seismic surveys and the drilling of eight dry holes. The discovery of the Mengkapan oil field followed in 1981.

In both fields the oil is reservoired within sands of the Sihapas Group of Early Miocene age. Geo chemical data indicate that the shales interbedded within the Sihapas Group are the source rocks for the oil. Notwithstanding their young Late Tertiary age, these source rocks are mature for oil generation at a depth of burial of approximately 3,000 feet, due to the high regional geothermal gradient.

The trapping mechanism in both fields is controlled by the same regional north-trending left lateral wrench fault. The Lalang accumulation occurs in an asymmetric anticlinal fold, bisected by the controlling wrench fault, and showing continuous growth since at least the Early Miocene. Both sections of the Lalang field have been tested; the oil/water contact is slightly shallower in the up thrown section. This appears to be a characteristic feature of the basin. The net oil pay penetrated by the Lalang – 2 discovery well on the western section of the structure is 153 feet and by the Lalang – 3 well on the eastern one is 316 feet. Cumulative flow rates obtained by drill stem testing were 7,092 bopd in the No. 2 well and 10,353 bopd in the No. 3 well.

The Mengkapan accumulation is contained in an elongated upthrown half anticline to the east of, and paralleling, the controlling wrench fault. Four discrete oil pools totalling 234 feet of net pay were penetrated in the Mengkapan No. 1 discovery well. Cumulative flow rate from 6 drill stem tests was 12,017 bopd.

The discovery of the Lalang and Mengkapan oil fields opens a challenging new horizon to oil explorers, as the prolific Tertiary Central Sumatra Basin has now been proved to extend offshore from the island of Sumatra into the Malacca Strait.