GEOLOGICAL AND GEOCHEMICAL STUDY ON GAS-CONDENSATING RESERVOIR OF DAANZHAI FORMATION, BAJIAOCHANG SICHUAN PROVINCE

Taking the Daanzhai formation of the condensate gas reservoir in Bajiaochang gas field which in the central Sichuan Basin as an example, this paper initiates with the description of its geological characteristics summarised below: 1.The producing formation is a typical inland freshwater lacustrine sediment which consists of lacustrine carbonates and interbedded with various thicknesses of shale layers. Coquina is a main reservoir rock. The black shale is a main source rock of hydrocarbons. 2. The reservoir rock is overlow permeable and unconvertional tight rock with the inhomogeneity. Through test to the core samples, it is proved that the porosities are mainly within the range of 1.0-1.5%, and the permeabilities are less than 0.5md (0.49 × 10^-4) μm²) 3. The main types of the organic matter (kerogen) in the source rock are sapropelic and mixed ones. The evolution of the organic materials is in the mature-early supermature stages. The reflectance of vitrinite (Ro) is about 0.8-1.1. 4. The gas reservoir of Daanzhai formation consists of several gas-producing layers and different fracture systems. The well interference exists distinctly in the same systems, but independence among different ones. The distribution of oil and gas in the anticline is controlled by both the diagenesis of reservoir and the structure. 5.The fluid pressure of the producing layer exceeds the hydrostatic pressure. The pressure coefficient is about 1.56-1.70, belonging to high abnormal pressure. The formation temperature is about 70-75℃. Then this paper deals with the gas sources and the possibility of being influenced by other sources, according to the components of light hydrocarbons (,C⁴- C⁷) series in the condensate, the paraffin index, the correlation of fingerprint compounds of light hydrocarbons and the carbon isotopic characteristics. Differing from the conventional point of view, a conclusion is drawn that the oil and gas are not only from the sources in Daanzhai formation, but also from that of the deeper Xiangxi group. This idea may have some significance in studying the distribution of the inhomogeneity of oil and gas, and forming mechanism of abnormal pressure, and so on. Moreover, it discusses the changes of light hydrocarbons in the condensate in regard to their content and composition.