Jambi sub-basin, which is located in South Sumatra, Indonesia has enormous potential of shale gas play. Yet, detailed geological studies are rarely undertaken to understand this relatively new hydrocarbon play concept. This paper presents a combination of geochemical and geomechanical studies with the aim to better understand: (1) the maturity level of source rock; (2) the mechanical properties of shale; and (3) the quality of hydrocarbon source rock. This research began with determination of wells that penetrate the Talangakar and Gumai Formations that have shale in it. Source rock analysis was done by using TOC (total organic carbon), S1, S2, S3, Tmax, and Ro (vitrinite reflectance) data. Geomechanical evaluation was done by using XRD and well logs data. Brittleness index was obtained by using Jarvie et al. (2007) formula, based on the XRD data. S-wave and P-wave are used to calculate the rock strength, Young's modulus and Poisson's ratio with UCS-To methods.Source rock in the Geragai belongs to the of moderate-to-good category because it has more than 0.5% TOC and potentially forms gas because it has a type III kerogen. JTBS-2 well is the only well in the Geragai area which already mature and has been able to produce hydrocarbons, because it passed the oil and gas windows. Source rock in the Betara belongs to moderate-to-good category because it has more than 0.5% TOC potentially forms gas because it has a type III kerogen. Most formations in the Betara are not yet mature based on the value of Ro and Tmax. In wells that have not reached the oil window nor gas windows, the prediction line drawn on the Petroleum Source Rock Summary chart, estimated that they would pass the gas window at Lower Talangakar Formation or Lahat Formation at depth of more than 8000 feet. The results of XRD analysis showed that the Betara had a high brittleness index with an average of 0.809. Talangakar Formation has a higher rock strength values than Gumai Formation, both in Betara high and Geragai deep. The principle that say the rocks which have high TOC values will have a high value of BI can be proven in the study area, the rocks that have high Ro will have a high value of BI, cannot be identified in the study area. With sufficient high value of rock strength and low abundance of clay minerals, the rocks at Talangakar Formation is good for hydraulic stimulation.
The kerogen types at the origin of oil in the Jabung block are predominantly type-II and III based on Rock Eval pyrolysis, and are interpreted to originate from the fluvio-deltaic shale & coal of the Oligocene Talang Akar Formation (TAF). However, several outlying oils have been found in the wells NB-1 & NB-4 of the North Betara Field and indicate that kerogen type-I of lacustrine origin may also be in play. This scenario is further suggested by biomarker and carbon isotope ratios. In this paper, we infer that those oils are sourced from the Eocene Pre-Talang Akar Formation (Pre-TAF) section. This opens new exploration prospectivity for the Jabung area. Our analysis of selected petroleum system elements suggests that the lacustrine oils encountered in NB-1 & NB-4 originate from Eocene Pre-TAF source rock in deeper part and migrated into the younger Oligocene TAF sandstone as a sub-surface leak, or “a subsurface oil seep”. Oil migrated by fault vertically and then spread laterally to fill traps in TAF. A widespread unconformity at top pre-TAF may have provided an excellent seal at the origin a pre-TAF confined petroleum system, prevented the lacustrine oil from entering all the fields/structures in the Jabung block, This could explain the minimal distribution of the lacustrine oil at TAF level and above in the Jabung area. The Pre-TAF is associated with the early syn-rift phase in South Sumatra Basin. It also refers as Lahat or Lemat Formation in the basin and is a widely under-explored play, evidenced by the low reserve magnitude of fewer than 100 MMBOE. The distribution of Pre-TAF as source and reservoir rock is restricted to syn-rift depocenter area. From our latest interpretation, Pre-TAF in NEB Field, observed clearly from the 3D seismic data, is potentially well developed and of good quality, although no wells have penetrated the interval to date
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