Abstract:

     The carbonatite karst reservoir of Huanglong Formation in the Upper Carboniferous is one of the most important natural gas reservoirs in northern Chongqingeastern Sichuan area Based on the structural features of karst rocks,we divide the paleokarst rock series of Huanglong formation into four types:slightly corroded; medium corroded and porous; strongly corroded and brecciated; strongly corroded and metasomatic secondary greypaleokarst rock; and differentiate two kinds of cements:limniccalcite and limnicdolomite Compared with matrix rocks,we have made a synthetical analysis of the geochemical characteristics of carbon,oxygen and strontium isotopes for different carbonate rocks, karstificated dolostones, karst breccias and carbonate cements The results suggest that the carbonate reservoirs in the formation were formed as a paleokarst in the Late Carboniferous,with different karst products showed by different stable isotopic geochemical characteristics, and that the evolutionary patterns of the isotopes are obviously controlled by the palaeogeographic background,fluid property,fluid sources,as well as isotopic fractional effect Based on the structure and the differences of geochemical characteristics of paleokarst rock series,we sum up their four basic features as follows:(1)with enhancement of karstification,the δ¹³C shows weak negative deviation and the δ¹⁸O shows strong negative deviation; (2)the ⁸⁷Sr/⁸⁶Sr ratio presents an evolutionary trend from the positive deviation for the infiltrating fluid belt and the active underflow belt gradually transforming to the strong negative deviation for the inactive underflow belt; (3)corresponding to the evolutionary trend of carbon,oxygen,strontium stable isotopes,the paleokarstreservoir properties changed from bad to good and to worse again; (4)limnicdolomite may have strong ability of “capturing” ⁸⁷Sr leading to the Sr isotope positive deviation effect in the course of crystallization and deposition

Key words:

northern Chongqingeastern Sichuan area;Huanglong formation;paleokarst reservoir;carbon,oxygen,strontium stable isotopes;geochemistry;karst fluid