Abstract:

Five gas fields have been located in East Sichuan since the discovery of Carboniferous gas pools there in October,1977.It has been established from studies that they are formed of carbonate elluviums between two erosion surfaces, with good combination of source-reservoir-cap rocks.Paleotectonics is believed to be the controlling factor at the time of deposition,mainly in the intertidal zone Solution pores which afford reservoir space and intercrystalline openings which form channels for intercommunication are well developed in the dolomites.Reservoirs with effective porosity are presumed to the best and the thickest around the center of the eluviums in the middle part of east Sichuan,and poorer in the northern and southern parts.Belated migration of oil and gas,apparent dissection of topography by fossil faults,absence of large scale lateral migration of hydrocarbons and gas accumulation controlled mostly by local traps are the salient features. Structures that were formed relatively earlier in the western and northern parts are of three types:higly-inclined,lowly-inclined and hidden;most of them are gas prospects.They are mostly anticlinal,though fault traps,stratigraphic overlaps and monanocks are not uncommon.Traps with good preservation deserve prior consideration,and in the opinion of the author,exaloration efforts should be directed in the following order of priority:the north-west of Liangping-Tianjiang line and the south-east of the line,no work in the south of Changshou-Fengdu for the present;anticlines should be tapped first.