The Relation Between Diagenetic Characteristics Of Shallow Reservoirs And CO₂ In Yingehai Basin

Yingehai basin is rich in natural gas with different contents of C02 . In order to evaluate resource objectively, it is very important to find out the origin and the main source of CO2 and to study the distribution pattern of hydrocarbon, nonhydrocarbon gas and probe the controlling factors,Diagenetic evolution of different reservoirs (rich in CO2 or rich in CFLO and the relation between diagenetic features and CO2 have been studied systematically by using conventional and advanced techniques and the integrated the methods of petrology and geochemistry. Based on the petrologic and geochemical data as well as dynamics reactions of carbonate梒lay minerals, the origin and distribution of CO2 have been synthetically discussed. The results have shown as follows:1. Abnormal evolution of shallow reservoirs has related to the upward movement of hot fluid. Influenced by the upward movement of hot fluid, shallow reservoirs have shown the characters of late diagenetic stage B (or A). With the intermittent activity of hot fluid containing CC^, ferrous carbonate minerals dissolved and precipitate repeatedly many times. In addition, dissolution of shells and feldspars, decrease of chlorite and increase of kaolinite could be found. On the contrary, the reservoirs that were not influenced by hot fluid just were in early diagenetic stage B.2. Based on the synthetic analysis of varied data, the mechanism of repeated dissolution and precipitation of ferrous carbonate minerals has been discussed. When diapir fractures opened, entering of hot fluid into reservoirs may cause dissolution of carbonate minerals, and when diapir fractures closed, carbonate minerals precipitated in high temperature condition.3. The correlation of isotopic constitution and the relations between diagenesis and CO2 indicate that CO2 mainly derived from Tertiary Ca-Mud rocks. The carbonate cements in reservoirs under the affection of hot fluid may form CO2, but the yield of CO2 is low. There is less possibility that COi originated from pre-Tertiary carbonate rocks.4. The dynamics study of carbonate-clay minerals reactions shows that high temperature will benefit the reactions process and high pressure will be disadvantageous to the reactions. After all, the negative effect of pressure is far less than the positive effect of temperature.5. The distribution pattern of gas has certain regularity vertically and laterally. The distribution and accumulation of COa are controlled by mud-diapir development.