Diagenesis and physical properties evolution of turbidite reservoirs in Es3z of Niuzhuang sag, Dongying Depression

Key words:  diagenesis  diagenetic evolution  physical properties evolution  turbidite  Niuzhuang sag

Abstract:

A comprehensive analysis was conducted to evaluate the diagenesis and property evolution of Es₃^z in Niuzhuang sag, by combining core observation, thin section identification, image analysis, fluid inclusion analysis, carbon and oxygen isotope analysis and physical property test. The burial, thermal and hydrocarbon charging history of the reservoirs were also taken into consideration. The results show that the turbidite reservoirs of Es₃^z are characterized with low porosity and permeability, and may have experienced diagenetic environment changes through the sequence of weak alkaline, acidic, alkaline, and weak acidic conditions. With various diagenesis, the main dissolving-cementation sequence was:early siderite and calcite cementation; feldspar dissolution/quartz overgrowth/authigenic kaolinite precipitation; first hydrocarbon charging; ferrocalcite/ankerite cementation/quartz dissolution; second hydrocarbon charging; a small amount of feldspar dissolution/quartz overgrowth/pyrite cementation, and finally compaction existing throughout the whole burial history. According to the matching relation of reservoir physical properties and hydrocarbon accumulation, both conventional reservoirs and tight reservoirs coexisted in the turbidites, and the tight reservoirs could be further divided into two types, one became tight reservoirs during hydrocarbon accumulation and the other became tight before hydrocarbon accumulation. It is concluded that the best exploration targets are the conventional reservoirs followed by reservoirs becoming tight during hydrocarbon accumulation, and then the reservoirs becoming tight before hydrocarbon accumulation.