Abstract:

The borehole QK-1, one of the scientific drilling holes of gas hydrate, is located at Biluocuo area in the Qiangtang Basin of northern Tibet, in which the silty mudstones of Xiali Formation(J₂x) and limestones of Buqu Formation (J₂b) were drilled. The geochemical characteristics of the abundance, type and maturity of organic matters of the source rocks reveal the following aspects: the organic matter abundance of source rocks in Middle Jurassic is lower; the silty mudstones in Xiali Formation have reached be poor-medium source rocks with TOC of 0.26% to 0.38%, and the TOC values of limestones in Buqu Formation are between 0.02% and 0.08%, part of which has reached to be poor-hydrocarbon source rocks; the types of organic matters for source rocks in Xiali and Buqu formations are mainlyⅡ₂ type or Ⅲtype, which are beneficial to gas formation, and the source rocks of Xiali Formation is superior to that of Buqu Formation. The value of R_o for source rocks in this area is from 1.31% to 1.63%, showing that the organic matter has undergone a high-degree thermal evolution and is in a high-maturity stage, and the organic matter in Xiali Formation is even in an over-maturity stage. Combining all factors favorable for the genesis of gas hydrates, it comes to the conclusion that Xiali Formation is a potential exploration target.

Key words: hydrocarbon source rock, geochemical characteristic, organic matter, Xiali Formation, Buqu Formation , Qiangtang Basin