LAUSR

Abstract The volcanogenic tuff layers are widely developed in the organic-rich shale systems of unconventional oil and gas resources. The tuffaceous shale composed of terrigenous detrital components and water-laid tuff is characterized by the complex porous structure related to tuff and organic matter (OM), which hampers the prediction of surface wettability and in-situ oil movability for the tight reservoir. Moreover, the roles of two components in effecting the spontaneous imbibition (SI) behavior remain poorly known. In this current work samples were collected from the Upper Triassic Yanchang Formation of Ordos Basin to set up a novel SI scaling model of tuffaceous shales with the consideration of tuff-hosted and OM-covered pores. The contact angle experiments were conducted to test the wettability of the samples, and spontaneous imbibition (SI) experiments were performed to study the tight oil movability in tuffaceous reservoirs with n-decane. The contents of residual hydrocarbons in the tuffaceous shale were calculated via the Rock-Eval method. Incorporating the above parameters, we proposed a modified scaling model of the SI data considering two specific factors for the tuffaceous shale samples: Factor I of the tuff-hosted pores indicated by the tuff components and Factor II of the organic matter (OM)-covered pores indicated by the residual hydrocarbon contents. The results show that all the tested tuffaceous shale samples are oil-wetting. The imbibition slopes increase with the development degree of the tuff-hosted and OM-covered pores. Notably, zeolite-type minerals, the primary porous components in tuffaceous tight oil reservoirs, contribute the most to the tuff-hosted pores. The long-term interactions between the zeolite mineral surface and residual hydrocarbons notably enhanced the oil wettability of tuffaceous reservoirs and led to a high in-situ oil movability.