Abstract:

With multi-period of acid-alkaline fluid evolution as well as multi-period of hydrocarbon charging during the burial, the reservoir physical property went through a complex evolution in the upper part of the fourth member of the Shahejie Formation (Es^s₄) in the north zone of the Bonan sag. With the comprehensive analysis of burial history, the thermal evolution of organic matter and dehydration of gypsum-salt bed, combined with characteristics of different hydrocarbon inclusions and salt-water inclusions, the diagenetic sequence of Es^s₄ in the north zone of the Bonan sag is studied. During the deposition of Es^s₄ , the original formation-water is of weak alkalescence because of the dry climate; from the beginning of the deposition ofEs^s₄ to 42.5 Ma，the formation fluid keeps weak alkaline with immaturate organics, and the main type of diagenesis is compaction, and early spherulitic pyrites are formed. From 42.5 Ma to 30 Ma，the formation fluid becomes acid for the existence of organic acid, and feldspar dissolution and quartz overgrowth take place. From 30 Ma to 24.6 Ma, the dehydration with alkaline water of gypsum-salt bed in the top of Es^s₄  occurs as well as decarboxylation of organic acid, thus the formation water becomes alkaline, and carbonate cementation and quartz dissolution simultaneously take place. During this period the first phase of hydrocarbon filling takes place from 28.8 Ma to 24.6 Ma, and from 24.6 Ma to 6 Ma, the organic acid is generated for the second time resulting from the uplifting stratum, and the formation water becomes acid, so the second phase of acid dissolution and quartz overgrowth begins, and the second phase of hydrocarbon filling take place in the period of 10 Ma to 6 Ma. From 6 Ma till now, the dehydration occurs again in gypsumsalt beds of Es^s₄  and the formation fluid becomes alkaline once more, so a few of later pyrite cementation can be found.

Key words: diagenetic fluid, fluid inclusion, hydrocarbon fillingl Es^s₄, Bonan sag