STUDY ON OIL SHALE CHARACTERIZATION OF SANGKAREWANG FORMATION ON TL-1 AND TL-2 WELLS IN OMBILIN BASIN, WEST SUMATRA

• Main Author: Hidayat, Rahmat
• Format: Theses
• Language: Indonesia
• Subjects: Geologi, hidrologi & meteorologi
• Online Access: https://digilib.itb.ac.id/gdl/view/77695
• Institution: Institut Teknologi Bandung

Oil shale is a fine-grained sedimentary rocks which contain very rich organic matter called kerogen, the portion of organic matter that is not soluble in common organic solvents. Oil shale is essentially a petroleum source rock which has not undergone complete thermal maturation required to convert organic matter to oil. As one of alternative energy sources, petroleum like-liquids yielded from retorted oil shales can be used as a complement to conventional oil. In Indonesia, oil shale is still considered as non-renewable new energy which its utilization is not optimal yet. In the future, oil shale is expected to give contribution to national energy mix that can reduce dependence on conventional oil consumption. Ombilin Basin is an intermontane basin which formed structural and sedimentary basin associated with tensional and thrust faults along Sumatran Fault System. Oil seeps discovery is surface manifestation and prospector for hydrocarbon occurence. Significance of Sinamar-1 drilling in 1983 proved that there is an active petroleum system in the area and regarded that Sangkarewang Formation as the primary hydrocarbon source rock in the basin. Source rock evaluation-based oil shale characterization was conducted on fine clastic rocks obtained by drilling from TL-1 and TL-2 wells in Talawi area, West Sumatra. Source rock evaluation was examined with considering that there are source rock intervals on Sangkarewang Formation having low thermal maturity. Based on evaluation, analysed fine clastic sediment can be classified as oil shale. Geochemical and petrographic assessment revealed that oil shale have very good generative potential (TOC > 2%) and tend to be oil prone (Type I and II kerogens) although these rocks are thermally immature (Ro <0,65%, pyrolisisTmax ranging from 4340 to 4430 C, CPI values >1, NSO fraction domination and some biomarker maturity parameters). Fischer retort assay analyses shows that there are correlation between carbon organic contents to shale oil that is formed during pyrolisis. Based on kerogen and maceral analyses, organic matters in oil shales are derived from algae with contribution of terrestrial high plant that were deposited in lacustrine environment. Abundant fungal spore found in samples suggesting humid and wet climate. Macerals identification show that oil shale consists of abundant lamalginite and classify it as Lamosite-type oil shale. Biodegradation in oil is detected from absence of n-alkana peaks, suggest that light stage of biodegradation has already begun. Oil-source rock correlation by using saturated fraction show that they have similarity in organic matter origin and depositional environment but have difference in thermal maturity. Low thermal maturity in rocks can be related with formation of Neogen graben in Late Oligocene-Early Miocene times, where most of the northwestern parts of the basin (Talawi Sub-basin) were uplifted and eroded. This event made burial and further diagenesis of Sangkarewang Formation and others older strata was disturbed. According to facies differentiation by Humphrey’s et al. on Sangkarewang Formation, oil shale on TL-1 well represent the second facies which is characterized by alternations of fine-grained sandstone and siltstone that were deposited around basin margin or lake edges, whereas oil shale found on TL-2 well represent the first facies which consists of organic rich shale interbedded with fine-grained sandstone and siltstone that were deposited in anoxic waters of lake. With low thermal maturity, good generative potential fine-grained rocks on TL-1 and TL-2 wells can be chategorized as oil shale. Artificial pyrolisis is required if its potency would be utilized as one of energy sources.