Microscopic characteristics and geological significance of tight dolomite in well Xike-1, Xisha Islands, China

A recent study suggests that the deep dolomites in well Xike-1 located more than 1000 m deep have a very low porosity, and no permeability for most of the samples. Given that the largest oil and gas fields in the South China Sea are all composed of biohermal dolomites while the deep dolomites of Xike-1 serve as the caprock formations rather than the reservoirs, this represents the first discovery and is of great theoretical importance. In this paper, core photos of the tight dolomites are presented. Hand specimens description and systematic porosity-permeability reveal a very low porosity, and no permeability for most of the samples. PLM, SEM and XRPD analysis discovered that the dolostones in the deep tight dolomites are quite large in size and euhedral or subhedral in shape. Alizarin red-stained sections are mostly pink in color and have a full coverage, which are considered as evidence for multiple calcite intrusions after the formation of ankerite dolomites. Biological relict textures are developed. The order degree is medium to low. The dolomites could be the product of penecontemporaneous dolomitization.