Controlling of cements and physical property of sandstone by fault as observed in well Xia503 of Huimin sag, Linnan sub-depression

Abstract

Cement content of carbonate in tight sandstone near section is much higher than that of the normal sandstones far away from the fault of well Xia503, in the Huimin sag in Linnan sub-depression. In order to understand the origin and its impact on fault sealing, analyses of the whole-rock minerals, casting thin sections, cathodoluminescence, isotope and physical properties are conducted on cores from well Xia503. It is found that δ ¹³C varies from 0.1‰ to 0.6‰ with the average value of 0.42‰, δ ¹⁸O varies from ∼13.5‰ to ∼12.3‰ with the average of ∼13.1‰, and C-O isotope plotting points are distributed in the low to moderate temperature area of the hydrothermal dolomite. According to the occupied relationship, cathodoluminescence, and C-O isotope feature, the carbonate cementation could be divided into four stages: calcites, dolomite, ankerite, and ferrocalcite. It is discovered that the carbonate cementation is negatively related to reservoir physical property, with the porosity of 4.8%, permeability of 0.37 mD, and displacement pressure of 1.97 MPa in the tight sandstone, which have increased by almost one order of magnitude compared to the porosity of 14.3%, permeability of 3.73 mD, and displacement pressure of 0.27 MPa in the normal sandstone, which is far away from the fault. Regardless of the lithology of the counterpart wall of the fault, only the displacement pressure difference caused by carbonate cementation between the tight sandstone and the normal sandstone could seal 41 m high oil column.