Diagenetic characteristics and their control on porosity of sandy conglomerate reservoirs in faulted basins

The petrography and diagenesis of sandy conglomerate reservoirs from the Shahezi Formation of the Xujiaweizi area in the Songliao Basin were studied, and the controls of diagenesis on porosity evolution were analyzed based on thin section identification, cast thin section observation, SEM and X-ray diffraction. A quantitative model was established. The results showed that the sandy conglomerate reservoirs in the Shahezi Formation are characterized by complex components, low maturity and poor physical properties. Various diagenetic effects took place in the sandy conglomerate reservoirs, among which the compaction, cementation and dissolution effects are very common and obviously influence porosity, while fracturing, metasomatism and recrystallization effects have little control on porosity. The quantitative evaluation of the relationship between diagenesis and reservoir porosity showed that compaction greatly reduced porosity, by 24.7% in average, which was the main reason for reservoir densification. Dissolution increased porosity by 4.2% in average, which delayed the densification process. Cementation filled residual intergranular pores, reducing porosity by 5.3% in average, which was also a key factor for reservoir densification. The porosity of sandy conglomerate reservoirs is the comprehensive result of early compaction, medium erosion and late cementation.