--> A Sequence Stratigraphy-Based Diagenetic Study with the Sha-3 Member Sandstones in the Central-South Parts of the Raoyang Depression as an Example

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A Sequence Stratigraphy-Based Diagenetic Study with the Sha-3 Member Sandstones in the Central-South Parts of the Raoyang Depression as an Example

Abstract

Abstract

Based on the thin sections combined with well logging and drilling data, this study documents the diagenetic characteristics of the sandstones in the Sha-3 Member of the Shahejie Formation in the central-south parts of the Raoyang Depression. It was conducted within the sequence stratigraphic framework from two aspects: control of sequence boundary on diagenesis and diagenetic evolution of sandstones in different systems tracts. The results indicate that the Shahejie Formation comprises one second-order sequence and three third-older sequences. Each of them can be subdivided into three system tracts: low stand (LST), transgressive (TST) and high stand (HST) systems tracts. The sequence boundary controlled carbonate cementation and dissolution in the underlying sandstones. However, it did not act as the preferential migration pathway of fluids which facilitated the dissolution in the sandstones overlying the sequence boundary. Sands in the LST tend to be mixed with clays and were less resistant to compaction so that primary porosity was most likely to be lost due to compaction and precipitation of early carbonates from the over-saturated alkaline solutions. Although sandstones in TST are of small size, they are well sorted. As a result, considerable primary porosity could be preserved at the early diagenetic stage. Furthermore, secondary porosity could be developed since sandstones in TST were in direct contact with source rocks and could be easily get accessed by organic acid. Sands in HST consist of well-sorted large grains and were more resistant to compaction. Their relatively shallow burial made it possible for primary porosity to be preserved more easily. In addition, the leaching by the atmospheric waters in the early diagenetic stage and the subsequent dissolution of organic acids led to the significant development of secondary porosity, which improved the reservoir quality of the sands considerably. Thus, sandstones in HST possess the best reservoir quality. This study proposed the diagenetic evolution models for sands in different systems tracts at syngenetic, eogenetic, and telogenetic stages. It characterized the diagenetic features within the sequence stratigraphic framework. The study results made it possible to delineate the high quality reservoirs.