Following a review on the research progress of diagenesis and micro-and nano-scale pore development of shale sequences

we take the Paleogene Shahejie Formation in Bohai Bay Basin and the Permian Lucaogou Formation in Junggar Basin as examples to analyze the diagenetic characteristics and reservoir space forming mechanism of organic-rich continental shale sequences.The study focuses on the effect of diagenetic (hydrocarbon-generating) evolution on reservoir space development.The results show that the porosity of the shale decreases significantly along with compaction at the early diagenetic stage; this is especially shown in the transformation from macro-pores to micro-pores

resulting in the relative increase of mesoporosity-microporosity.As of the end of early and the middle diagenetic stages

the porosity may reaches a new peak value at a depth of 3000m to 3700m in Dongying Sag

under the impact of early hydrocarbon generation of organic matters.And then as compaction comes to dominate again

the porosity decreases sharply

even though secondary micro-porosity increases due to acid fluid dissolution.The dissolution of acid fluids occurs in the Shahejie Formation

but it turns to be dominant in the Lucaogou Formation

a major reason for the formation of reservoir space.Geochemical analyses on rock samples from the Lucaogou Formation show that the samples with higher TOC generally have a lower content of δ

13

C in carbonate minerals

and a higher secondary dissolution porosity.Therefore

the diagenesis in organic-rich shale sequences is significantly different from that in ordinary shales.The organic acids and acidic CO

2

accompanying the hydrocarbon generation are of great significance to the modification and improvement of reservoir quality by dissolution

and also major factors controlling the distribution of shale oil sweet spots.