Mahakam delta, located in Tertiary Kutei basin, is one of the most prolific oil and gas producing area in Indonesia. Many oil and gas fields have been discovered and developed in this area so far.
Sediments in Mahakam delta is composed of Middle Miocene to recent deltaic sediments. Maximum thickness of these deltaic sediments so far drilled is ca. 4, 500m in the central part of the present day Mahakam delta.
Oil and gas fields are located on NNE-SSW trending anticlinal trends formed by Late Miocene to Pliocene compression. Main reservoirs are deltaic sands of Middle to Upper Miocene. Both oil and gas fields are located on same anticlinal trends. On BADAK-HANDIL and ATTAKA-BEKAPAI anticlinal trends, anticlines forming oil fields are always associated with large scale normal faults whereas anticlines forming gas fields are not.
Coals, lignites and shales in Middle to Late Miocene deltaic sediments are recognized as potential oil source rocks in this area. These sediments have high TOC and high hydrocarbon potential yield values with kerogens being type III.
Studies on extracted hydrocarbons (C
1-C
35) from source rocks at HANDIL oil field suggests that HANDIL oil must have been generated from these Miocene source rocks.
Tmax value of 435°C (
R0 0.6% equivalent) corresponds to top of oil window and
Tmax of more than 445°C (
R0 of more than 0.75% equivalent) corresponds to peak oil generation at HANDIL field.
Based on various geological and geochmical aspects, it is possible to say that BEKAPAI oil might also have been generated from the same source rocks as HANDIL oil.
Primary migration pathways are not yet well defined in Mahakam delta. However, as oil accumulations are associated with large scale normal faults and gas accumulations are not, these faults may have acted as important oil migration pathways.
Future geochemical studies in Mahakam delta shall be directed to solve the questions on the origin of oils in different fields, primary migration pathways as well as effects of high pressure zones, commonly recognized in the area, on migration mechanisms.
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