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CONTENTS

Bengkalis Trough is a roughly 20-30 km wide, 100 km long depression trending north-south direction from around the southern coastal line of Bengkalis Island to the south, and eventually terminating on the Tigapuluh Mountain in the eastern central Sumatra (Cameron, et la., 1982). It is shown in Fig. 1. In many publications the Bengkalis Trough is often mentioned as Bengkalis Depression (Lee, 1982) or Bengkalis Suture (Tjia, 1989). Based on its tectonics and sedimentation setting, Bengkalis and its adjacent areas are part of the Central Sumatra Basin, one of the hydrocarbon rich basins in Indonesia. This circumstance principally can not be separated from what is called the Bengkalis Trough. How is the tectonic origin of this components, how its relation to the hydrocarbon accumulation in this region, will be slightly revealed mainly based on the gravity anomaly expression. Gravity values in this region (Fig. 2) are all low positive anomalies ranging from around 10-30 mGal. In this region, smaller values reflect the thicker and deeper granitic basement, and the relatively higher anomalies are vise versa. Anomaly patterns approximately along the Panjang Strait (9-13 mGal) are interpreted as the north end of the Bengkalis Trough. It is also as a weak zone about 20-30 km wide. The zone separates other anomaly patterns in the west and east side. Based on the two patterns, it can be predicted that both sides have somewhat moved to the right forming the dextral slip fault. Gravity models (Fig. 3 and 4) show that the gravity anomaly in this area is caused by mainly three elements involving upper mantle (r=2.92 gr/cc), granitic basement (r=2.65 gr/cc), and about 1000-2000 m thick overlying sediments (average r=2,5 gr/cc). Low anomaly shows deeper basement (thick sediments) and relatively higher values express basement high (thinner sediments). The shallower upper mantle controls the higher gravity values on both sides of the profile. The geothermal gradient anomaly above the average around Minas and Bengkalis Trough (Eubank & Makki, 1982) may be caused by the existence of that upper mantle. It can be interpreted that initially the Pre-Tertiary basement was separated along the north-south trending normal faults due to the rifting dominated tectonic, then was followed by sedimentation. In its development those faults become the major dextral slip fault due to the compression dominated tectonic occurred probably during Oligo-Miocene period. Wrench fault was developed resulted from the dextral slip movements, which finally from drag folds as hydrocarbon traps in the adjacent Bengkalis Trough.