Abstract:

The diagenetic characteristics of deep natural gas reservoirs and the control of diagenesis on reservoirs in the lower part of the Fourth Member of the Shahejie Formation (Es₄x) in the north zone of the Minfeng sag in the Dongying depression haven’t been clearly understood, which have led to great risks to hydrocarbon exploration in this area. Based on a combination of thin-section identification, scanning electron microscope (SEM) observation, fluid inclusion analysis and an analysis of core properties, it has been systematically studied about the diagenetic characteristics and its influence on the reservoir property. Nearshore subaqueous fan glutenite is the main regional reservoirs, and the diagenesis is characterized by strong compaction, poor compaction, dissolution and cementation of many stages, complex metasomatism and lime-mud matrix recrystallization. The main cementation-dissolution sequence is feldspar dissolution/quartz overgrowth →carbonate cementation / quartz dissolution →carbonate dissolution /quartz overgrowth →pyrite cementation and bitumen filling. The difference of sedimentary characteristics in different sub/micro-facies of nearshore subaqueous fan controls the difference of diagenetic characteristics, which also determines the difference of reservoir physical property. The conglomerate in inner fan mainly experiences compaction and lime-mud matrix recrystallization, with weak dissolution, and its physical properties keep reducing. The lime-mud matrix recrystallization results in a rapid reduction of physical properties in middle-deep layers. The present physical property of inner fan becomes extremely poor and mainly develops dry layers. Because acid dissolution reconstruct the reservoirs and early hydrocarbon filling and abnormal high pressure can protect reservoirs, the reservoirs far away from mudstone layers in the braided channels of middle fan develop a great number of primary pores and a few secondary pores, have a higher porosity and permeability, and can be effective reservoirs for natural gas. The parts in interdistributaries of middle fan and outer fan develop intensively cementation, it forms dense cemented crusts at the top of the sandstone, and therefore they mainly act as cap rocks.

Key words: diagenesis, natural gas reservoirs, the lower part of the Fourth Member of the Shahejie Formation of the Paleogene;, glutenite bodies, Dongying depression, tight sandstone