The object is to provide a subterranean formation processing composition which has a highly stable viscosity against change in temperature and a high water stopping property. A subterranean formation processing composition comprising cellulose fibers wherein the composition contains ultrafine cellulose fibers having 0.14 to 2.5 mmol/g of a substituent is used. The substituent is preferably an anion group, particularly a phosphoric acid group. The present invention relates to various fluids which are used in well processing. The present invention relates to a method for processing a subterranean formation, for example, drilling of an exploratory well or a wildcat, an appraisal well, an exploratory well or an exploration well, a delineation well, a development well, a production well, an injection well, an observation well, and a service well; cementing; fracturing; and a method for producing petroleum resources. The present invention provides also a cellulose fiber-containing composition which is not limited to the subterranean formation processing use.