The Reservoir Heterogeneity Characteristics and Affecting Factors of Tight Lacustrine Dolomite Reservoirs

Lacustrine argillaceous dolomite reservoir has been found the Ⅲ-5 Member of Shahejie Formation, Tanggu District, Bohai Bay Basin, China, which buried depth is 2 800-3 600 m and keeps oil in the fault block-lithologic argillaceous dolomite reservoir. Considering structure and sedimentary background, the research summarizes reservoir characteristics mainly includes lithologic character, reservoir property and hydrocarbon distribution. At the same time, the paper provides the controlling factors and pattern of hydrocarbon accumulation through integrated analysis of source-reservoir-seal combination and evolution of tectonics, diagenesis and hydrocarbon accumulation. The results show that reservoir lithologic is mainly dolomite and argillaceous dolomite and belongs to fracture-pore reservoir of ultra low porosity and permeability, whose reservoir space, to a large extent, includes intercrystalline pore, secondary dissolved pore, dissolved fractures and fractures. Overpressure is closely correlated with the hydrocarbon distribution. The overpressure wells show medium-high production and their water cuts are under 20% with a continuing downward trend; while instead, the normal wells show low production and water cuts are above 55% owning a smooth or upward trend. Mudstone is high-quality source rocks and can supply enough oil to dolomite reservoir. The quantity of hydrocarbon generation and reserves capacity of dolomite are both closely relevant to abundant algae laminae. Fault sealing controls the hydrocarbon accumulation and distribution of overpressure, whose one of the main driving force is hydrocarbon generation. The overpressure has been the significant mark of hydrocarbon enrichment. The hydrocarbon-charging time is behind the fault sealing, thus hydrocarbon-charging is effective.