Characteristics of Middle Permian Chihsia Dolomite in Western Sichuan Basin, China: A Case Study of Hydrothermal Dolomitization

Hydrothermal alteration has become one of the most significant factors in carbonate hydrocarbon reservoir development. The study of dolomite of the Middle Permian Chihsia Formation in Western Sichuan Basin sheds new light on the mechanism of hydrothermal dolomitization, and would provide the dolomite research with a Chinese case study. In Chihsia carbonates, preexisting sedimentary fabrics have been strongly and patchily overprinted by medium- to coarse-grained crystalline dolomite phases. Therefore the dolomitic limestones or calcareous dolomites are the dominant rock types in outcrop and hand specimen scale. The dolomitized parts contain planar and non-planar crystalline dolomite phases, and occur with authigenic minerals (illite, fluorite, quartz, pyrite and fluorapatite). The macro- and micro-petrographic studies of the dolomitized facies and microthermometric analysis of fluid inclusions have revealed the presence of four major dolomite phases and one post-dolomitization calcite phase. The first dolomite phase of small and euhedral to subhedral crystals is volumetrically minor and has the lowest homogenization temperature (T_h) range in dolomitizing paragenetic sequence (89 to 109°C). The following dolomite phase grows on the precursor dolomite crystal, and always occurs as the clear rim of a cloudy dolomite core. The clear dolomite rims have higher T_h values than the core parts. The third and non-planar dolomite phase plays a very important role in Chihsia Formation, and contains the fluid inclusions with average T_h of 140°C. The fourth dolomite phase often occurs as void-filling dolomite cements, and consists of characteristic saddle dolomite cements that contain fluid inclusions with average T_h of 151°C(The maximum is 243°C). A lower temperature calcite phase fills some voids and replaces some dolomite cements, indicating a temperature drop in diagenetic system. The relationship between stylolites and dolomites suggests that the dolomitization event occurred in the shallow burial environment (less than 1000m). The geochemical analysis indicates that the dolomitizing fluid was a combination of heated marine-derived water and possibly a modicum of extraneous hydrothermal fluid. On the basis of existing evidences in this study, we propose that the dolomitizing mechanism of Chihsia Formation in Western Sichuan Basin has some connections with ELIP (Emeishan Large Igneous Province) associated thermal events.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California