The origin of formation water and the carbonate cement precipitation mechanism of the Xujiahe Formation tight sandstone in the western Sichuan foreland basin

Abstract

Carbonate cement is the most important cement type of the Xujiahe Formation in the western Sichuan foreland basin. In particular, the Xu-2 Member has a much higher content of dolomite cement, and the Xu-4 Member predominately contains calcite cement. It is concluded that the formation water contained in the Xujiahe Formation in the western Sichuan foreland basin was originally sourced from meteoric water, the formation water in both members was heavily influenced by the release of water from the compaction of mud shale and coal beds, and the present-day characteristics of the water are the result of concentration and water-rock interaction during the burial process. The Xu-4 Member has a relatively open water-rock system that results in the formation water exhibiting heavier oxygen isotope values and the carbonate cements exhibiting lighter carbon isotope values, dominated by calcite. The Xu-2 Member has a more closed water-rock system, and the significantly heavier carbon isotope values of the carbonate cement in it are related to the source of cementitious material, the lithology of its underlying source rock, and the tectonic uplift erosion.