Characteristics and significance of the penecontemporaneous karst in lacustrine carbonate, Da'anzhai Member, Lower Jurassic, Beibei area, eastern Sichuan Basin

There are few reports about the penecontemporaneous karst of the lacustrine carbonate of Da'anzhai Member which developed on the top of Lower Jurassic Ziliujing Formation,eastern Sichuan Basin.Based on the analysis of the macroscopic and microscopic characteristics of Heping Reservoir section in Beibei area,It is discovered that the penecontemporaneous karst in the lacustrine carbonate,Da'anzhai Member,and its characters as follows:(1)The dissolved fractures and karrens are common,which have been filled with vadose silts,bioclasts,terrigenous quartzes,crystal and intergrown calcites;(2)The penecontemporaneous karstification mainly occurs in the micrites and shell limestones of the middle and upper part of the shallowing-upward sequence,and the desiccation cracks occur in some shallowing-upward sequences;(3)Three meteoric-water diagenetic lenses are identified in this section.By further study we also find the factures,stylolite and burial dissolution pores occur along the karst system(tubes,conduits and caves of the karst).And the asphalt not only fills the stylolites and fractures,but also is mixed with the vadose silt in the karst system,although the pore space formed by penecontemporaneous karstification is filled with vadose material and cements.Therefore it is concluded that the development and modification of the tight lacustrine carbonate reservoir of Da'anzhai Member,to some extent,are relevant with the penecontemporaneous karstification,and the penecontemporaneous karst reservoir may develop in the buried Da'anzhai carbonate of the basin.