Distribution and geologic significance of Girvanella within the Yijianfang Ordovician reef complexes in the Bachu area, West Tarim Basin, China

摘要

The distribution of Girvanella within the Yijianfang Ordovician reef complexes of the Bachu area, western Tarim Basin, is elucidated in order to reveal the effects of Girvanella on the formation of potential reservoirs. Three categories of Girvanella include rafts, intraclasts, and crusts. Both Girvanella rafts and intraclasts have been transported, whereas Girvanella crusts are preserved in situ. Each category has been preserved mainly in fore-reef and back-reef outer shoals, and in upper slope deposits. Trends in the abundance of in situ Girvanella are inversely proportional to the abundance of Calathium and bryozoans. In situ Girvanella is abundant primarily in comparatively low-energy deposits, whereas Calathium and bryozoans occur commonly in higher-energy deposits, inferring that the faunal variations within the reef complexes are highly controlled by the original sedimentary conditions. The abundance of Girvanella is inversely correlated with both porosity and permeability. This may be due to effects associated with Girvanella including: (1) Girvanella rafts and intraclasts composed of micrite are well preserved, and the micrite envelopes associated with Girvanella crusts may protect the skeletal grains, preventing them from being dissolved; (2) Activities of Girvanella may block up primary pores. Results of this study will be useful for understanding the relationship between microbes and sedimentary environments and provide critical information about the genesis of potential reservoirs within the Ordovician reef complexes.