The Evaluation And Contrast Of Caprock Seal Ability In The Foreland Basins Of Central-Western China

Based on amount of datum of oil geology, geochemistry and geophysics, this paper evaluates and compares the caprock seal ability in foreland basins of central-western China combining with analysis and researching.Using the data of logging and production, combining the structure feature, This paper researches chiefly on macro-growth feature and micro-seal ability of caprock in three typical kinds of foreland basins of central-western China including Southern Junggar superimposed foreland basin, Western Sichuan reconstructed foreland basin and Northern Qaidam neogenic foreland Basin. The regional caprocks of three kinds of foreland basins grew chiefly in depression basin period. In early foreland basin period, only Western Sichuan grew a caprock. In early regeneration foreland basin period, three kinds of foreland basins all grew a caprock. But in later regeneration foreland basin period, they didn't grow caprock and form effective association of caprock and reservoir.Based on contrast of macro-growth feature, in the caprock of Western Sichuan foreland basin, the caprock is plastic, the lithology is chiefly mudstone and gyprock, the thickness of individual layer is great, the ratio of mudstone thickness and bed thickness is very big, consistency is good, the seal ability of physical property is powerful, the abnormal pressure is widespread, the seal ability of hydrocarbon concentration is weak, the quality of caprock of Western Sichuan is very good. In Southern Junggar foreland basin, the caprock is plastic, the lithology is chiefly mudstone and gyprock, the thickness of individual layer is great and even more than 100m, the ratio of mudstone thickness and bed thickness is very big, consistency is good, the seal ability of physical property is powerful, the abnormal pressure is powerful in Anjihaihe, the seal ability of hydrocarbon concentration is weak, the quality of caprock of Southern Junggar foreland basin is good. In Northern Qaidam foreland basin, the difference of caprock of different structure is distinguished, the lithology is chiefly sandy shale, the individual layer is very little and chiefly is between 0m and 5m, the consistence is very bad, the physical property is weak, the formation pressure of the caprock below Lower Ganchaigou formation is powerful, the seal ability of hydrocarbon concentration is moderate, the quality of caprock is bad.Using the data of logging, combining the growth feature, and adopting the triangular diagram of fault sealing ability, the faults seal ability of three basins is researched. According to the result, the faults seal ability of Southern Junggar foreland basin is good, except that the big faults near mountain is unsealing because of activity time and again, the fault in Anjihaihe formation and Tugulu group is sealed, and the seal ability of the fault in Sangonghe formation and Badawan formation is a bit weak. The seal ability of faults of Western Sichuan foreland basin is very good except several big exceedingly high faults. In Northern Qaidam foreland basin, the faults in Upper and Lower Ganchaigou formation is sealed, other is unsealed.