Influence of sedimentation and diagenesis on reservoir physical properties: a case study of the Funing Formation, Subei Basin, eastern China

Abstract

The sandstone of the third member of the Funing Formation (E1f3) in the northern slope zone of the Gaoyou Sag has the typical characteristics of high porosity and ultralow permeability, which makes it difficult for oil to flow. In this study, the lithological characteristics, sedimentary facies, diagenetic characteristics, pore structure, and seepage ability of this sandstone are characterized in detail. Correlation analysis is used to reveal the reason for the sandstone high porosity-low permeability phenomenon in the study area. The results indicate that this phenomenon is controlled mainly by the following three factors: 1) the sedimentary environment is the initial affecting factor, whereby the deposition of a large number of fine-grained materials reduces the primary pores of sandstone. 2) The Funing Formation has undergone strong compaction and cementation, which have led to the removal of most of the primary pores and a reduction in size of the throat channels. 3) Owing to fluid activity during the later stage of diagenesis, sandstone underwent intense dissolution and a large number of particles (feldspar and lithic debris) formed many dissolution pores (accounting for nearly 60% of the total pore space). Among these factors, dissolution has contributed the most to the development of high porosity-low permeability phenomenon. This is mainly attributed to the inhomogeneous dissolution process, whereby the degree of particle dissolution (e.g. feldspar) exceeds that of cementing minerals (clay and carbonate minerals). The secondary dissolution pores have increased the porosity of sandstone in the study area; however, the pore connectivity (permeability) has not been significantly improved, thus resulting in the special high porosity-low permeability characteristics of this sandstone.