| The sealing capacity research of Tertiary caprocks in Lenghu structural belt is weak, which restricted the fine exploration of the study area. Based on the data of well drillings, drilling cores, loggings,2D seismic and tests, this passage aims to establish caprock evaluation standard by studying the macro and micro characteristics and dynamic analysis the effectiveness of the sealing capacity combined with the evolution of hydrocarbon expulsion and structure, behind the selection of evaluation parameters. And then study the comprehensive evaluation of study area cap layers and predict the favorable zone of caprocks. A comprehensive analysis of the caprocks led to the following considerations:1. Four sets of argillaceous caprocks(E1+2、E31、E32、N1) have been recognized. The macro characteristics of caprocks were showed:The deposition period from E1+2 to E31 are a complete sedimentary cycle consisting of lacustrine transgression and regression. The E32 caprock which stays at the end of lacustrine transgression has the highest deposition thickness. And the argillaceous caprocks thickness from E1+2 to N1 migrated spatially from north to south. The faulted thickness of N1 and E32 caprocks are thicker than others. The thickness of E1+2 caprock is followed. And the thickness of E31 caprock is the minimum thickness. The faulted thickness of Lenghu No.4 structural belt and culmination 1 in Lenghu No.5 structural belt are thicker than other areas. The depositional environment of study area caprocks are mainly include sheet flow, flood swamp, distributary interchannel, underwater distributary interchannel and shore shallow lake environment. The diagenetic stage of study area caprocks are from A of early diagenetic to B of middle diagenetic. And the diagenesis of argillaceous caprocks gradually enhance with increasing buried depth.2. The micro characteristics of caprocks:The microscopic pore structure of the study area caprocks is parallel planar shape pore. The BET specific surface area of study area caprocks are distributed in 8～43m2/g. The pore diameter of Lengdong area and Lenghu No.4 structural belt caprocks are distributed in 4.21～32.94nm. The pore diameter of Lenghu No.5 structural belt caprocks are less than 10nm. The breakthrough pressure and formation pressure are gradually increase from N1 to E1+2. And from Lengdong area to culmination 4 in Lenghu No.5 structural belt, the breakthrough pressure and formation pressure are gradually increase. Differences in the formation are not as significant as in the structural belt. Among them, it is approximately, about 60%, which the brittleness index of Lengdong area and Lenghu No.5 structural belt. The brittleness index of Lenghu No.4 structural belt is lowest, about 47.33%.3. The effectiveness of caprocks evolution:The hydrocarbon sealing ability of caprocks from E1+2 to N1 in culmination 4 in Lenghu No.5 structural belt had formed between 19～40Ma, which is between the end of first oil and gas accumulation stage(4.5Ma) and the original of second oil and gas accumulation stage(24.6Ma). The hydrocarbon sealing ability of caprocks from E1+2 to E32 in Lengdong area, Lenghu No.4 and culmination 4 in Lenghu No.5 structural belt, and the oil sealing ability of N1 caprock in Lenghu No.4 structural belt, had formed between 5.1～50Ma. The gas sealing ability of N1 caprocks in Lenghu No.4 structural belt and the hydrocarbon sealing ability of N1 caprock have not been formed.4. Comprehensive analysis of caprocks:In N1 caprock, there are four type caprocks which are class Ⅳ to Ⅰ. The E32 caprock mainly developed class Ⅲ to Ⅰ. In E31 caprock, there are mainly class Ⅲ and Ⅱ. In E1+2 caprock, there are mainly class Ⅱ and Ⅰ. Therefore the E32 and E1+2 caprock have more superior caprock type. The next is E31 caprock. And the N1 caprock has the worst quality of caprock type. The types of caprocks are gradually better from Lengdong area to culmination 4 in Lenghu No.5 structural belt in the plane.5. The caprock from class Ⅲ to Ⅰ are capable to seal commercial hydrocarbon flow. And the natural gas production area is corresponding to the class Ⅱ and above. The comprehensive analysis showed:There are some stratum has a favorable caprock conditions for industrial oil and gas, which are the stratum of E1+2 in Lengdong area, Lenghu No.4 structural belt and culmination 1 in Lenghu No.5 structural belt, the stratum of E31 in Lenghu No.4 structural belt and culmination 1 in Lenghu No.5 structural belt, the stratum of E32 in culmination 1 in Lenghu No.5 structural belt. And there are some stratum has a favorable caprock conditions for industrial oil, which are the stratum of E31 in Lengdong area, the stratum of E32 in Lengdong area and Lenghu No.4 structural belt. The stratum from E1+2 to N1 in culmination 4 in Lenghu No.5 structural belt have favorable caprock condition for industrial gas. The stratum of N1 in Lenghu No.4 structural belt and Lengdong area are lack of the ability to seal the industrial oil and gas. |
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