Abstract

The Pungut and Tandum oil fields are located in Riau Province, Central Sumatra, approximately 65 km northwest of the city of Pekanbaru. The oil, which is trapped in structural closures, is produced from Lower Miocene sandstone reservoirs.

Although both producing structures are anticlinal highs aligned along opposite sides of a major north-south trending fault they differ considerably in physical characteristics. The Pungut structure, which is located on the southwestern side of the fault, is a low-relief, open, elongate anticlinal high; the Tandun structure, which is located on the northeastern side of the fault, is a high-relief, tight, small, round anticlinal high. The relative movement across the major fault changes from normal to reverse southward. The apparent change from tensional to compressional faulting, the development of secondary tensional faults along the northern part, as well as the configuration of the fields suggests the presence of horizontal right-lateral movement along this major fault.

The Pungut and Tandun fields are broken-up by a complex system of faults into individual blocks. Since the faults are sealing, each block has its own oil-water contact which tends to complicate development of the fields. Well spacing must, therefore, be designed to drain all of the oil contained within each block. Development of the Tandun field is also complicated by onlap and pinchout of basal Lower Miocene reservoir sands around the Paleogene topographic high located beneath the highest portion of the present day structure. This provides for a combination of structural and stratigraphic entrapment around this high.

Even though the geology of the area is very complicated, where both structural and stratigraphic conditions effect the entrapment of oil, the geological environment is favorable for further exploration for oil. Successful resolution of the complex structures has resulted in extensions of the fields and has highlighted additional areas of exploratory interest. Enechelon folds developed at an angle to the major fault, structural closures located along the major fault, structural highs located between sealing faults, as well as onlap and pinchout of sands against older topographic highs provide additional excellent play opportunities in the area.