Mechanism and Distribution of Calcareous Interbeds in Songtao Uplift and Its Periphery of Qiongdongnan Basin

Peiyuan Zhu,Qingtao Yuan,Li You,Jia Zhong,Aiqun Liu

doi:10.4236/ojms.2018.83020

Abstract

Based on core and casting slice observation, well drilling and logging data, the source of carbonate materials, lithology together with electric properties, types, genesis mechanism and distribution of calcareous interbeds in Songtao uplift and its periphery of Qiongdongnan Basin have been thoroughly analyzed. Results show that typical features have been appeared from well logging curves, containing low gamma-ray, low acoustic travel time, low neutron value, high density, as well as bright white calcium nodules or bands in electrical imaging well-logging curves. Drilling results reveal that calcareous interbeds developed mostly in high position of paleostructures and their distribution was controlled by the combined effects of macroscopic and microscopic factors. Macroscopically, calcareous interbeds relate to paleogeomorphology together with the combination of sandstone and mudstone. They are also controlled microscopically by the source of carbonate cements and pore space. Under normal circumstances, with regard to the same sand, the closer to the mudstone and the thicker of mudstone, the more conducive to the formation of calcareous interlayer. Low compaction strength, high content of rigid particles, coarse grain size, well-sorted sandstone and large pore space during carbonate cementation are favorable for the development of calcareous interbeds.

Highlights

IntroductionInterlayers refer to impervious bed or low-permeability layer within the sand
TypeIis formed by the integration of Ca2+ with CO32− during the shallow burial stage, usually filling in the primary intergranular pores
While under deep burial conditions, the maturation of organic matter releases a large amount of organic acids, which facilitates the precipitation of calcium carbonate again by increasing the dissolution of carbonate and pH of the fluid, type III comes into being

Summary

Introduction

Interlayers refer to impervious bed or low-permeability layer within the sand. The morphology and distribution of interlayers are not stable due to the change of the micro-facies or phase of sand bodies caused by transient and local water flow changes. With the success of S structure, it became the first potential commercial discovery in northern depression belt of Qiongdongnan Basin, but the widespread development of physical interlayers (mainly calcareous interlayers) in the area brought difficulties to the exploration. Calcareous interlayers refer to tight sandstone with carbonate content over 10% formed inside sand body. As a seepage barrier of the reservoir, it increases the heterogeneity of the reservoir, resulting in the thinning of the effective reservoir and the deterioration of physical properties, which restrict the later exploration and development

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Results

Discussion

Conclusion