Formation fluid characteristics and hydrocarbon accumulation in the Dongying sag, Shengli Oilfield

Abstract

Based on the analysis of geochemical characteristics and distribution of different formation waters, a new origin identification standard was established for the formation water in the Paleogene Shahejie Formation in the Dongying Sag. The migration paths of the connate water expelled from source rocks are similar to those of the hydrocarbons coming from the same rocks, so the geochemical characteristics of the connate water coming from the source rocks can be used as auxiliary indexes to study hydrocarbon migration. Considering fluid pressure and formation water distribution, the Paleogene fluids are divided into three fluid systems: normal-pressure open fluid system, overpressure confined fluid system, and transition fluid system. The differences in hydrocarbon-bearing fluid characteristics, hydrocarbon migration dynamic, hydrocarbon-bearing fluid flow type between these fluid systems were studied. According to characteristics of the hydrocarbon-bearing fluid charging into traps, and formation water drainage pattern, three typical oil accumulation models were classified. In the overpressure confined fluid system, the main oil accumulation mode is high oil saturation fluid entering traps and displacing formation water, and “continuous” reservoirs are prone to form, and fluid oil saturation, fluid charging pressure and fluid seeping conditions affect reservoirs’ oil accumulation extent. In the normal-pressure open fluid system, the main oil accumulation modes include low oil saturation fluids enter a trap while formation water is overflowing out of the trap, and low oil saturation fluids enter a trap while formation water is seeping out of the trap. The amount of hydrocarbon-bearing fluid, fluid oil saturation, trap height, and caprock sealing ability affect reservoir's oil accumulation extent. Key words: formation fluid system, hydrocarbon-bearing fluid, formation water, connate water, non-Darcy flow, hydrocarbon accumulation model