川东及邻区上二叠统生物礁的白云岩化

DOLOMITIZATION OF THE UPPER PERMIAN ORGANIC REEFS IN EASTERN SICHUAN AND ITS ADJACENT AREAS

白云岩化是川东及邻区上二叠统礁体的重要成岩事件。根据矿物岩石学、X光衍射、流体包裹体、碳氧稳定同位素、微量元素及阴极发光等项目的研究,笔者认为,礁体白云岩主要是埋藏交代成因,礁顶有少量微-粉晶白云岩为潮坪成因。本文着重讨论礁体围岩为生屑灰岩的埋藏压实流体白云岩化模式,简述礁控白云岩储集特征及其成因。在此研究基础上,提出了一个白云石结构成因分类方案。

In eastern Sichuan Province and its adjacent areas, the Upper Permian Chang-xing Formation patch and pinnacle reefs have been dolomized. Reef dolomiteis an important reservoir rock in this area. Dolomite is mainly distributed in theupper and flank parts of the reef. From the flank to core of a reef, dolomitizationweakens. According to the occurrence, mode of formation, crystal morphology etc.,the reef dolomite is classified into five types: (1) tidal-flat dolomicrite; (2)percolaling brine dolomicrite; (3) burial fine-medium crystal dolomite, accountingfor over 90 percent of reef dolomite; (4) idiotopic dolomite cement; (5) saddledolomite cement. This paper puts forward a new textural-genetic classification of dolomite. Powder X-diffraction analysis indicates: tidal-flat dolomicrite has a low de-gree of ordering. In the burial dolomite, the degree of ordering of euhedral andsubhedral dolomites is higher than that of anhedral dolomite. Homogenizationtemperatures of inclusions of saddle dolomite and euhedral dolomite cements arefairly high. The freezing method analysis suggests that formation water rich inMg^(2+) and Ca^(2+) and with high salinity tend to precipitate saddle dolomite. δ^(13)C ofburial dolomite is obviously higher than that of tidal-flat dolomite. Sr and Navalues of reef limestone, nonreef limestone, tidal-flat dolomite and burial dolomiteare different. Under the cathode lumines-cence, tidal-flat dolomite has no lumines-cence and burial dolomite has strong dark-bright orange and yellow lumines-cence. Mineralogical, petrological and geochemical studies demonstrate that 2-7-m-thick dolomicrite at the reef top in eastern Sichuan province is of tidal flatorigin, while the bulk part of reef dolomite-fine-to medium-crystal dolomite is ofmetasomatic origin in the burial diagenetic environment. This paper mainly dis-cusses the mechanism of distribution of reef dolomite and the favorable thermody-namic and kinetic conditions for burial dolomitization. The authors believe thatwhen country rocks of the reef are bioclastic limestone, in the burial diageneticenvironment, compaction fluids affluxed towards and through the reef and causedit to be dolomitized.