A research on characteristics of paleo-krast of Lower Cambrian Shilongdong Formation in Western Hubei-Eastern Chongqing-Western Hunan-Hubei Provinces, China

Dissolution is one of the most important factors to control the development of high-quality reservoirs. In this research, it is the first time to identify the paleo-krast characteristics by field profiles, micropolariscope,... [ view full abstract ]

Dissolution is one of the most important factors to control the development of high-quality reservoirs. In this research, it is the first time to identify the paleo-krast characteristics by field profiles, micropolariscope, cathodeluminescence, SEM and geochemistry. The field characters are as follows. 1. The stalactite layer and the belt of weathering clay are developed. 2) The top strata successively develops basement rock, weakly-weathered layer, intensely-weathered layer and solution-collapse layer. 3) There are carneule and sandstone filler. 4) There are angular-subangular dolomicrite rubbles. Calcsparite cement and detrital material of terrigenous origin fill among rubbles.In micropolariscope, the observations are as follows. 1) Dedolomitization and zonal structure are developed. 2) There are dolomicritic rubbles with uniform size and random spread. 3) Among the rubbles, the detrital material of terrigenous origin including banded mudstone, angular quartz sandstone and arenopelitic clast are filled. 4) There are angular-subangular quartz rubbles of terrigenous origin.By researching on cathodeluminescence, the dolomicrite of sample 11-CSD-4 has kermesinus light. But the cement has no light. The dolomicrite and the calcsparite cement of sample 9-MLZ-4 respectively show orange-red light and kermesinus light. By the analysis, Mn element contents both are 0.03%. Fe element content of 11-CSD-4 is 1.03%. And 9-MLZ-15 has 0.83%. This result indicates the reason of the calcsparite cement without light in 11-CSD-4 is that Fe element content is too high leading to cathodeluminescence cancellation.By analysis of 12 samples of C, O isotopes of Shilongdong Formation in Dingzhai-Miaoliangzi profile, the value of δ13C and δ18O respectively are 0.8%~-7.5% and -5.7%~-10.6%. The average value of δ13C and δ18O respectively are -2.67% and -8.84%. The value of C and O isotopes both are negative. By the analysis of the cement in pores and cavities and the matrix, the value of δ13C of the cement is -1.2%.The value of δ18O is -10.6%.The value of δ13C and δ18O of matrix respectively are -0.8% and -5.7%. This result indicates that stratum undergoes two-stage diagenetic fluids. Based on the results of the studies above, we conclude that the top strata of Shilongdong Formation underwent a large-scale marine regressing which may influences the researches of the oil and gas exploration and stratigraphy.