Characteristics And Its Petroleum Geological Significances Of Permian Basalts In Southwestern Si-chuan Basin

The research object of this paper is the Permian basalts in southwestern Si-chuanbasin. This paper systematically discusses the characteristics and petroleumgeological significances of Permian basalts. Permian basalts in research area can bedivided into lava basalts, volcanic tuff, and volcanic breccia. According to verticalcyclicity and plane zonality of basalt lithofacies, Permian basalts are divided intoeruption facies, the overflow facies and volcanic sedimentary facies from the crater tosedimentary province.Thus Space-time distribution pattern of Permian basaltlithofacies is established.The main controlling factors of the reservoirs are lithology, diagenesis, tectonismand organic matters, especially the fractures. Basalts in study area are dense,and theprimary pores are less.After reformation of fractures, the basalts can form cracks-holeand hole-fracture reservoirs. Palaeogeomorphology of the development of reef flatfacies was controlled by the tectonic settings of basalt eruption. Synchronous fracturesprovided channels for fluids migration. And the fluids supplied magnesium ions forlimestone dolomitization of Maokou formation in lower Permian. Basalts domal upliftcaused Maokou formation exposed, which was advantage to the development of karstreservoirs. The development of basalts were favorable to the formation of lithologicupdip pinchout reservoirs, and with basalts as the barrier layer,it was conducive to theformation of different petroleum system.Analysis of gas geochemical parameters shows that the natural gas source ismainly from coal related gas of Ⅲkerogen of upper Permian, or may be form themixing source gas of lower Permian. And according to the distribution ofhydrocarbon source rock, reservoirs, gas source and tectonic conditions, the model ofbasalt gas accumulation is established.On the basis of favorable rock types, diagenesis, physical parameters andexisting oil and gas shows, the paper has carried out the forecast of favorableexploration area of basalt reservoirs in Sichuan basin.And the most favorableexploration area is mainly the overflow facies with the fracture-pore reservoir, wherethe thickness is very thick. The gas generating conditions, cover layer andpreservation conditions in this area are better.