PETROLEUM AT THE ROOF OF THE WORLD: The geological evolution of the Tibet (Qinghai‐Xizang) Plateau Part I

The Tibet (Qinghai‐Xizang) Plateau is underlain by many thousands of mters of Ordovician (and possibly Cambrian) through Middle Eocene, gently‐dipping, essentially undeformed, stable platform sequences. These are largely qf shallow‐water, marine‐shelf origin in the Ordovician‐Triassic sections, but are of mixed continental and marine facies in the Jurassic through Midle Eocene sections. Two east‐west fault troughs ‐ the Banggong Co ‐ Nu Jianand Indus River ‐Yarlung Zangbo ‐ contain thick sections of Triassic, Jurassic, and/or Cretaceous marine flysch associated with ophiolites. The only severe deformation to affect the plateau was during the Mesozoic, and it was concentrated close to the through‐going east‐west fractures. Post‐early Eocene beds are entirely continental, and are concentrated nsmall‐to‐large grabens. One of these, the Lunpola Basin, has been moderately explored. Drilling here during the 1970s recovered small amounts of heavy, biodegraded oil of non‐marine origin, from a sectionthat ranges in age from Middle Eocene through Quaternary Plateau‐wide stratigraphic studies reveal that the Ordovician through Early Carboniferous, and Late Permian through Middle Eocene, sections were deposited on a monolithic stable block. All were deposited in essentially warm tropical seas in close proximity, as demonstrated by the, facies, faunas, and floras. These same studies, plus related tectonic investigations, reveal that the through‐going east‐west fracture zones, popularly interpreted as “sutures,” are not sutures at all. Rather, the facies belts in rocks of all ages studied in detail routinely cross the so‐called “sutures,” commonly at very sharp angles, proving that these east‐west fault zones exerted no control overthe general lithofacies deposition patterns of the Himalaya‐Tibet block. The oniy facies control exerted by the fracture zones was in the small, narrow troughs that developed along them. Thus, the Himalaya‐Tibet block behaved as a single structural unit through time, and was never segmen red in to various microcontinents. Although some potential for commercial petroleum discoveries exists in the post‐middleEocene grabens. the greatest potential of the Himalaya‐Tibet block is in the Paleozoic‐Mesozoicsection. with its promise of multiple source‐beds. reservoirs, traps, and seals. We predict that, one day, this “roof of the world” will host many petroleum Jields.