Digenetic Sequence of Low Porosity and Permeability Reservoirs from Nearshore Subaqeous Fan of Shahejie Formation in the Steep Slope Zone of Chezhen Depression, Bohai Bay Basin.

Based on the core, thin section, SEM and vitrinite reflectance data, the characteristics of lithology, physical properties, diagenesis, diagenetic stage and sequence, pore type, secondary pore for sand bodies of nearshore subaqueous fan of Shahejie Formation in the steep slope zone of Chezhen depression were discussed; formation condition and controlling factor of secondary pores ("sweet spot")in reservoir with low porosity and permeability were analyzed. The results show that the sand bodies of nearshore subaqueous fan of Shahejie Formation in the steep slope zone of Chezhen depression are consisted of feldspathic sandstone, mainly experiencing compaction, cementation, dissolution and metasomatism in the burial diagenetic evolution, and the reservoir is in the middle diagenetic stage now with the burial depth of 2 000-3 800 m, and the reservoir has the characteristics of low porosity and permeability; the dissolution plays a significant role in improving reservoir quality during the burial diagenetic evolution in the mainly depth; the feldspar dissolution by organic acid in the depth of 2 300-3 100 m and the intergranular and intragranular pores increase the porosity and permeability of reservoir and improve the reservoir quality, and the porosity is up to 25% and the permeability is up to 50 × 10-3 μm2 , so that the reservoir spaces contribute to petroleum accumulation and high yield. In general, dissolutions and secondary pore development zone are very important for predicting favorable reservoir. [ABSTRACT FROM AUTHOR]