Turkish Journal of Earth Sciences




The Yingshan Formation, located on the Tazhong Northern Slope, contains oil- and gas-rich layers with the reserves of about 700 × 10^6 TOE. The high-resistivity inner layers isolate the hydrocarbon bearing zones and form the sequential sets of reservoir bed-seal assemblages in a vertical direction within the Yingshan Formation, which is directly bound above by a micritic carbonate cap rock that is overlain by the 3rd to 5th members of the Lianglitag Formation. The sealing capability of the cap rock and inner barrier layers was evaluated macroscopically and microscopically in terms of the core breakthrough pressure and thin-section identification. The evaluation parameters were extracted from the statistical analysis of drilling and logging data. The 3rd to 5th members of the Lianglitag Formation are more shaly, but the inner barrier layers in the Yingshan Formation are more dolomitic. Argillaceous limestone is more capable of sealing oil and gas zones than micritic limestone. The 3rd to 5th members of the Lianglitag Formation, of which the gamma ray response and core displacement pressure are greater than 20 API and 14 MPa, respectively, provide good sealing with thicknesses of more than 100 m and have better sealing with thickness of more than 200 m. For the same porosity, dolomite has lower coreflood displacement pressure than limestone. The difference in coreflood displacement pressure between the barrier layers and the underlying reservoir bed is 6 MPa, the cutoff value for sealing capability. Carbonate sealing was controlled by early sedimentation and was influenced by late diagenesis. The direct cap rock is dense and has cement content of more than 10%, up to 31%. The reservoir bed has cement content of less than 10%. Generally, the direct cap rock and the inner barrier layers are relatively stable on the lateral distribution.

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