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Assessment of Hydrocarbon Prospectivity in Terai and Siwaliks, Exploration Block 2, Western Nepal



Author(s)

Dharma Raj Khadka

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DOI:10.17265/2328-2193/2017.03.003

Department of Mines and Geology, Kathmandu, Nepal

BasinMod 1D software with faulting module is used to model two synthetic wells taken from a geoseismic section in Exploration Block 2 in western Nepal to understand the burial and thermal history of exterior belt (Terai) and Siwalik fold and thrust belt. The study focuses the measured inputs of source and reservoir rocks of Surkhet Group consisting of Swat shale (2%), TOC and Melpani sandstone porosity (10%). The geohistory curves show rapid sedimentation and tectonic subsidence. The thermal history is constrained using a 20 °C/km geothermal gradient for the exterior belt, whereas for the Siwalik fold and thrust belt, a two-step geothermal gradient is proposed using a 20 °C/km for the upper 2,000 m and 23 °C/km below this depth. The modeled values for maturity show that the Surkhet Group lies in the mid mature oil window in the exterior belt, but for the Siwalik fold and thrust belt, the hanging-wall Paleogene wedge is in the early mature stage, whereas the footwall Paleogene wedge is in the late mature stage. Oil generation for the Swat shales started at 6.3 Ma at 3,988 m depth with peak oil generation 2.4-1.3 Ma at 5,435-5,782 m depth in the exterior belt. However, the Siwalik fold and thrust belt modeling shows that the footwall Swat Formation has no oil generation capacity after the faulting episode, whereas it had been producing oil since about 8.5 Ma at 3,800 m with main phase of oil generation at about 7 Ma at 4,600 m. The hanging-wall Swat Formation has been in the early mature stage of oil generation since faulting. The timing of structural trap formation window is set to 4.1-1.8 Ma based on geological evidence from the literature. The results show trap formation is more or less contemporaneous with hydrocarbon generation and expulsion and timing will be critical for assessments of the prospectivity.

Siwaliks and Terai, basin modeling, hydrocarbon prospectivity.