Differences Of Fluid Activities And Their Influences On The Hydrocarbon Migration And Accumulation Between Typical Sags In Continental Rift Basin

Jizhong and Jiyang Depressions in Bohaiwan Basin are typical cenozoic continental rift basin.There are great differences in sedimentary and tectonic features which lead to the differences in fluid activities between different sags.Hydrocarbon generation,migration,accumulation and preservation take place in the environment of formation water.Therefore,the differences of fluid activities will lead to the differences in oil accumulation between different sags.The types of sags is firstly classified and then the differences of fluid activities and their influences on the hydrocarbon migration and accumulation have been investigated by synthesizing formation water chemistry field,pressure field,temperature field,fluid dynamic field and differences in hydrocarbon accumulation conditions and hydrocarbon distribution characteristics in different types of sags.The sag can be divided into four types based on the thickness percentage of Ek-Es₄,Es₃-Ed and N-Q,which are early,early-middle,middle,middle-late types,respectively.From early sag to early-middle,middle and middle-late sag,the average thickness and percentage of N-Q gradually increased.However,the average thickness and percentage of Ek-Es₄ gradually decreased.The depth of Es₄ sedimentary water became shallow and the depth of Es₂,Es₁ and Ed sedimentary water became darken.The thickness of weathered denudation at the end of Ed gradually decreased while the thickness of weathering denudation at the end of Es₄ gradually increased.The deposition rate and fault activity in N-Q gradually increased and the deposition rate and fault activity in Ek-Es₄ gradually decreased.From early sag to early-middle,middle and middle-late sag,the overall characteristics of formation water salinity,percentage of CaCl₂ type water,r?Cl-Na?/rMg have a tendency of decreasing first and then increasing.However,percentage of NaHCO₃ type water and rNa/rCl have a tendency of increasing first and then decreasing.The geochemical characteristics of formation water in paleocene are similar to the previous overall characteristics.These suggest that early-middle and middle sags are dominated with sedimentary water,early sag is dominated with atmospheric water and middle-late sag is dominated with mixed water.According to the source,the formation water in Ng can be divided into 4 types,which are original sedimentary water,affected by Es₁,Es₃ and Es₁,Es₄ and Es_3.From the first type to fourth type,the characteristics of formation water salinity,percentage of CaCl₂ type water,r?Cl-Na?/rMg have a tendency of increasing and the percentage of NaHCO₃ type water and rNa/rCl have a tendency of decreasing.The formation water in buried hill can be divided into 4 types according to burial period,which are early buied,early-middle buied,middle buied and late buied.From the first type to fourth type,the characteristics of formation water salinity,percentage of CaCl₂ type water,r?Cl-Na?/rMg have a tendency of decreasing and the percentage of NaHCO₃ type water and rNa/rCl have a tendency of increasing.From early sag to early-middle,middle and middle-late sag,the distribution layer of overpressure gets more and newer,the start depth of overpression gets deeper and pressure structure has the characteristics of single overpressure to double overpressure.The middle Baxian Sag and middle-late Raoyang Sag are characterized by low water head,early Langgu Sag is characterized by low and middle water head,early-middle Dongying Sag and middle-late Zhanhua Sag are characterized by middle-high water head.The vertical hydrodynamic zones can been divided into trongly,smoothly and steadily alternate aqueous zones,from early type to early-middle type,middle type and middle-late type depressions.From early sag to early-middle,middle and middle-late sag,the boundary depth of different hydrodynamic systems has gradually become deeper.From early sag to early-middle,middle and middle-late sag,the distribution layer of source rock,reservoir and regional cap rock gets more and newer,the combination of source rock,reservoir and cap rock is from self-generating and self-presering to below generating and upper presering,the major hydrocarbon accumulation period becomes later.The oil reserves of early and middle-late sag are mainly in lower salinity formation water,the oil reserves of middle sag is mainly in lower-middle salinity formation water,and the oil reserves of early-middle sag are mainly in middle-high salinity formation water.The oil reserves of early sag is mainly in weak overpressure and middle water head zone,the oil reserves of others are mainly in normal pressure and low water head zone.The oil reserves of early and middle-late sag is mainly in strongly alternate aqueous zones,the oil reserves of early-middle sag is mainly in smoothly and steadily alternate aqueous zones,the oil reserves of middle sag is mainly in smoothly alternate aqueous zones.