Thin Section-Based Geochemical Dissolution Experiments of Ooid Carbonates

Fang Yang; Xie Shuyun; He Zhiliang; Liu Yin; Bao Zhengyu; Wo Yujin; Zhang Dianwei

doi:10.3799/dqkx.2016.066

Abstract：To explore the law of carbonate dissolution evolution to facilitate oil and gas exploration,the dissolution process and kinetic mechanisms of oolitic limestone and oolitic dolomite in sulfuric acid medium under normal temperatures are discussed in the paper.In-situ micromorphology of the reaction surface was observed by environmental scanning electron microscope (ESEM)and the elemental distribution was analyzed by fractal and multifractal theory using the X-ray mapping capabilities of the ESEM.The different dissolutions observed in the experiment indicate that the dissolution of the mineral starts at points of structure defects (as cleavage or fracture),and that the dissolution rate of oolitic limestone at the point with high Ca content is faster than that with low Ca or high Si content,which results in an uneven corrosion pits and pore spaces formed on the reaction surface.Moreover,the sulfuric acid cracks the ooid dolomite into granular and intergranular fractures and pore space in or between particles.Analysis of the weight loss by dissolution of thin section shows that the weight loss of oolitic dolomite is about 80% higher than that of oolitic limestone,indicating that the sulfuric acid has a stronger dissolution ability for oolitic dolomite.It is found by micromorphology observation that the granular surface of oolitic dolomite formed by sulfuric acid dissolution is more conducive to deepening of dissolution,which further improves development of pore space and connectivity.The fractal and multifractal analyses indicate that the dissolution process is controlled by differences between the element distribution heterogeneity of Ca,Mg and Si.Additionally,large difference of composition,strong heterogeneity of element distribution can promote the dissolution reaction.