Abstract:

As one of petroleum exploration frontier formations in Qaidam Basin, Carboniferous has great shale gas source potentialities but at a relatively immature exploration stage presently. Based on field geological survey and drilled well analyses, the distribution of Carboniferous shale for the Delingha depression in Qaidam Basin was clarified. The characteristics of organic geochemistry, porosity and permeability for Carboniferous shale samples were tested. We also made isothermal adsorption tests and seepage experiments for studying the shale accumulation conditions. Combined with basin modeling analysis of Well Decan 1, this paper evaluated Carboniferous shale gas system for the Delingha depression in Qaidam Basin. Carboniferous shale is widely distributed in the depression with various thickness and buried depth. Most shale has higher organic matter richness and organic matter types of Ⅱ2 and Ⅲ, and is at the thermal stage of maturity and high maturity. Upper Carboniferous shale has large thickness and organic matter richness than that of Lower Carboniferous. Carboniferous shale shows the reservoir characteristics of low porosity of 1.89%5.48% and low permeability of (0.1152.7)×10-7 μm2. Thick shale of overlying formations provides favorable cap condition for Carboniferous shale gas. Basin modeling indicates that from the end of the Paleogene to present is the critical moment for the Carboniferous shale gas system in the Delingha depression. Natural gas shows of Keluke Group of Upper Carboniferous indicate that Carboniferous shale gas system exists in the Delingha depression. In light of shale thickness, buried depth, TOC, thermal maturity and other geological conditions comprehensive evaluation, this paper divided the Delingha depression into two shale gas systems, i.e., Delingha shale gas system and Aibei shale gas system.

Key words: shale gas system, Carboniferous, Delinghe depression, Qaidam Basin