Qaidam Basin In The Eastern Part Of The Cenozoic Sedimentary And Bio-gas Accumulation Conditions Analysis

Qaidam basin is one of the most important oil-bearing basins in the northwest China, and begun to be explored in 1954. The north and west areas of Qaidam basin are mainly sections that exploit oil and gas so that the degree of exploration is very high in these areas, but in the middle and east areas the degree of exploration is very low. As the exploring field of gas is widen and some gas fields such as Sebei I ,II, the south of Taijnar, Yanhu, Taijnar, Tuofeng Mountain were found, people feel that there is large potential in searching gas fields in this area.In this paper the author analyzes the changes of Tertiary and Quaternary sedimentary facies and protracts sedimentary facies plans of seven periods according to research seismic facies and source materials of more than 60 important wells and paleobiology. Based on the study, I conclude the following fruits and opinions:1. Rock granularity of the reservoir is very fine at large in the researching zone, mainly is mealy sand-rock and mud-bearing mealy sand-rock, belonging to shore shallow lake environment.2. In the Quaternary system, Pleistocene series, lake basin in the researching area goes through expanding, stabilizing and shrinking. So shore lake, shallow lake, and deeper lake are joint each other vertically.3. The gas reservoirs in the researching area have the characteristic of generating itself, storaging itself and enveloping itself, and primary reservoirs exist in Ki-Kio. The thickness of continuous mud rocks can reach 120m so that they are not only good source rocks but also excellent cap rocks. These create the well conditions of storing gas.4. Diagenetic degree of the reservoir rocks is very low, so that can produce abundant primary porosity and have excellent permeability. The porosity of the area can reach 30%, and permeability is about 98.7 x 10-3 u m2.5. Source rocks of Tertiary system mainly distribute in the west section of researching area, and source rocks of Quaternary system mainly distribute in theeast sections. These source rocks mostly are sediment of shore lake-shallow lake; reservoir rocks have three types of sources, that is powder or fine sand-rock that deposit in the high-energy environment, sediments in the middle-energy environment and mud-bearing powder sand-rock. Cap rocks are mud-rock and salt-rock in the low-energy environment.6. In late Tertiary and early Quaternary sedimentary center migrated from West Taijnar to Dabson Lake and Southern and Northern Hibsan Lake as a whole that made ancient lakeshore line migrated from west to east.7. The author proposal that the northern slope belt and Tainan latent tenting belt are advantageous area that explore biology gas. In southern slope belt we should attach importance to find lithologic trap, and Yiliping depression area is advantageous exploring area of Tertiary.