GEOCHEMICAL MODELING ON LEMBAK HALF GRABEN OF SOUTH SUMATRA BASIN

• Main Author: AGUS SARDJONO, EKO
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/69594
• Institution: Institut Teknologi Bandung

South Sumatra Basin occupies the southern part of Sumatra Islands as one of Tertiary back arc basin series. This basin is also known as a productive hydrocarbon sedimentary basin in Western Indonesia and it contributes significant oil and gas discoveries. Many studies carried out in this basin such as litho-stratigraphic division, stratigraphic sequence of Abab – Raja - Pandan, N6-N7 sequence boundary and its effect on reservoir rock potential, South Sumatra tectonic, syn-rift study and the tectono-stratigraphy of Lembak half graben. The objectives of this study are to figure out the potential hydrocarbon accumulation on Lembak half graben, especially on the source rock of Talang Akar Formation and to describe the hydrocarbon generation history. Both results will play an important role for reserve determination in this area. This study uses deductive methods for generalization the relatively similar condition areas and it applies 1D and 2D geochemical modeling for prospecting Talang Akar Formation source rock. The potential reserve of Lembak is calculated by the Schmooker’s formula. The results of geochemical modeling and volumetric calculation are: • Geochemical analyses on 7 well samples showed the fairness of source rock quality of Talang Akar Formation and it would be a potential oil and gas producer. • Shale compaction curve showed three segmentations. The segmentation between Lahat Formation and Talang Akar Formation is predicted to be different rate of sedimentation, while the segmentations among Talang Akar Formation and Baturaja Formation are due to the different rock compositions. Another segmentation occuring on Baturaja Formation and Gumai Formation are interpreted to be an overpressure zone. • Using plotting sonic transit time method, erosional zone occuring on Muara Enim Formation is predicted to be 65 – 90 m of thickness. i • Paleo heat flow and present day heat flow calculation result are between 1.07 and 2.08 HFU and it indicates that the Lembak half graben has relatively normal heat transfer. • Geochemical modeling 1D showed that oil generation from the source rock of Talang Akar Formation started at Early Miocene time (19 mya) and the peak oil generation occured during Middle Miocene time (14 mya), whereas gas generation started at Midle Miocene time (11.5 mya) and reached the peak of generation at Late Miocene time (5.5 mya). • Cummulative hydrocarbon expulsion curve showed that the oil expulsion started at Midle Miocene time (13.5 mya) while gas expulsion started at Late Miocene time (6.5 mya). • Two dimensional geochemical modeling showed the half graben geological section represented morphological, structural and sub-surface geological aspects. This geological section also showed vertical and lateral hydrocarbon migration and lateral source rock potential. • Total generated hydrocarbon volume is 5442 MMBO. Assuming that 5% of total volume generated hydrocarbon reached and accumulated in the reservoir rock, the hydrocarbon potential volume on Lembak half graben is 272 MMBO.