Microfossils from the Chert in the Lower Cambrian Shuijingtuo Formation at Miaohe, Zigui, Hubei Province

This paper reports for the first time on the microfossils discovered in the black cherts of the basal part of the Lower Cambrian Shuijingtuo Formation at Miaohe village, Zigui County, Hubei Province, all of which were observed in thin sections. Nine genera, 9 species and 3 indeterminate species (including a new species) are described. According to these microfossils, the conclusion is made that the microfossil zone may be equivalent to the small shelly fossil assemblage zones 2—3 of the Meishucun stage. They have provided another important line of evidence for the correlation of the Sinian-Cambrian boundary strata with those of the adjacent areas.

Evaluation of Shale Reservoir Quality by Geophysical Logging for Shuijingtuo Formation of Lower Cambrian in Yichang Area, Central Yangtze

Taking the shale of Shuijingtuo Formation of Lower Cambrian in Yichang area as the research object,the shale reservoir characteristics are comprehensively evaluated and classified by fitting regression and formula calculation method in this study,using laboratory testing and geophysical logging data.The results show that the interpretation data of ECS(elemental capture spectroscopy)logging has a high correlation with the measured minerals data,which can be a good method to evaluate the minerals component of the shale.The calculated content of brittle minerals at the lower part of Shuijingtuo Formation is the relatively highest,generally more than 40%,which is the most favorable segment for fracturing.The correlation coefficient between the interpretation data of CMR(combinable nuclear magnetic resonance)logging and the result of laboratory porosity test is 0.97,which can effectively and accurately evaluate the reservoir porosity.The evaluation results show that the porosity of the lower member of Shuijingtuo Formation is generally greater than 3%,while that of the upper member is generally less than 3%.The lower segment is with the relative optimal physical conditions.There is a good correlation between the acoustic logging data and the gas bearing content testing results.A gas bearing content evaluation model is established.The results show that the gas bearing content of the lower 20 m shale is generally more than 2%,indicating that the lower part is a shale gas enrichment segment.Mechanical parameters such as Young modulus,Poisson ratio and brittleness index of shale reservoir are evaluated by using the logging data of P-wave time difference and S-wave time difference.The continuous 15 m shale at the lower part is with the relatively optimal low Poisson ratio,high Young modulus and high brittleness index,developing the optimum brittle condition.Based on the evaluation and classification of above parameters,the shale is divided into three types.The TypeⅠis the optimal,mainly located at the bottom.Its thickness is 8.5 m in total.The TypeⅡmainly develops at the middle part.The TypeⅢis the worst,mainly at the upper part.