Hydrocarbon Possibilities of Concealed Mesozoic-Paleogene Sediments Below Himalayan Nappes--Reappraisal

Abstract

Hydrocarbon exploration of the Ganga (Ganges) and Sub-Himalayan basins indicates the presence of upper Neogene-Quaternary continental molasse and the absence of Paleogene sediments regarded as hydrocarbon source beds. Marine to brackish water Paleogene-lower Neogene sediments, closely fringing the Sub-Himalayan foothills, increase in thickness northward. Their presence and facies in the frontal schuppen zone and in the Lesser Himalayan windows, commonly in close association with upper Mesozoic shelf sediments, indicate a formerly greater width for the late Mesozoic-early Neogene shelf-miogeosynclinal basin. The pre-Tertiary rocks of the Lesser Himalayas are in nappes floored by the flat-lying sole thrust redesignated the Main Boundary thrust (MBT). Schuppen structure and ateral shortening complicate the geometry of the MBT. Downfaulted klippen and decken also are present in grabens of pre-Tertiary rocks in the Sub-Himalayan schuppen zone. Pre-Tertiary tectonic grain and metamorphism, ubiquitously developed within the Lesser Himalayan cover rocks, are typically absent in the subjacent Paleogene-lower Neogene window-zone sediments. Deformations within the latter are much less than in pre-Tertiary cover rocks, with which they are commonly intersliced owing to the development of schuppen structure. Tertiary deformations are unlikely to affect adversely the possible hydrocarbon resource within the thick Mesozoic-lower Neogene autochthonous sediments or those which have migrated to the Lesser Himalayan cover rocks. This opinion is corroborated by a few oil and gas shows from the Eocene limestone outcrops close to the MBT and also from the Lesser Himalayan pre-Tertiary metamorphic cover rocks.