Study On Characteristics And Origins Of Favorable Lacustrine Carbonate Reservoir In QHD428E Structure

Petroleum prospecting and exploration examples show that lacustrine carbonate rock is an important petroleum reservoir in the Bohai Bay Basin. Lacustrine carbonate rocks are characterized by various types of rock types, complex textures, and huge vertical and horizontal variations, in which oil and gas accumulate in favorable reservoir with relatively high porosity and permeability. For studying lacustrine carbonate reservoir in QHD428 E Structure of Bohai Bay Basin,the author used microscopic thin section, scanning electron microscope(SEM), cathodoluminescence, X-ray diffraction, pressure mercury analysis, physical properties analysis, geochemical analysis and other test analysis methods to study petrology, reservoir and diagenesis characteristics systematically.Based on establish reservoir evaluation standard, the author defined favorable reservoir and summed up the favorable reservoir characteristics.By analyzing the origins of favorable reservoirs,the main control factors for favorable reservoir development are concluded clearly.Lacustrine carbonate rocks mainly develope in carbonate beach(dam), less develope in carbonate flat,and criticaly consists of dolostone. Grain structure is the major type of rock texture and bioclastic dolostone spread most extensively. According to the main diagenesis phenomenons, authigenic mineral precipitation relative order, clay mineral evolution characteristics and organic matter maturity, reservoir diagenetic evolution sequence is established and reservoir mainly underwent three diagenetic period including syngenesis, eodiagenesis and mesogenetic stage. Research on reservoir quality shows that reservoir space types are mainly residual intergranular pore, visceral foramen and less secondary pores and fractures,the major throat types are flaky and shrink neck throat and most reservoirs are equipped with high porosity and high permeability.According to mercury injection data and curve patterns, reservoir pore structures are divided into three types such as I(good), II(moderate), III(bad).Based on the relationship between porosity and permeability, the reservoir evaluation criteria based on permeability is established, the high permeability and medium permeability reservoirs are classified as favorable reservoirs, and the favorable reservoir characteristics are summarized. Through polarizing microscope observation and analysis of the authigenic carbonate mineral isotope geothermometer and inclusion homogenization temperature, reservoir pore evolution characteristics are inversd. Loss of porosity due to compaction and cementation were 10.7% and 10%, dissolution contributed 1.6%.As a result, porosity evolved from 48.5% of initial porosity to 29.4% today. There are two distinct phases in the process of favorable reservoir pore evolution. Porosity increased considerably in syngenesis and decreased obviously between the end of A stage and B stage of eodiagenesis.By means of carbon and oxygen isotopes, electron microprobe analysis, microscopic section and fluorescence thin section observation, the author analyzed the origins of favorable reservoir and summarized major controlling factors of favorable reservoir. Favorable reservoirs mainly formed on underwater uplift beachs(dams) which are less affected by terrigenous materials, are mainly conprised of bioclastic beachs and have high energy and clean water. Visceral foramen in a great quantity of bioclasts has a great contribution to reservoir porosity. Early pore-lining dolomite and early petoleum injection play the most important parts for saving reservoir space. Overpressure is beneficial to porosity preservation in local areas.Dissolution is too weak to contribute porosity but plays a positive role in improving permeability.