The Evolution Of Geotemperature-Geopressure Fields And Hydrocarbon Accumulation In Deep Reservoirs Underlying Gypsum-salt Strata In Dongying Sag

The distribution and evolution of geotemperature-geopressure has a close relationship with hydrocarbon accumulation. It plays an important role in studying hydrocarbon reservoirs. The very thick paleogene gypsum-salt strata vertically divided Dongying sag into two relatively independent petroleum systems. The formation underlying gypsum-salt strata has advantageous hydrocarbon accumulation conditions, and it is an important goal for further exploration. Based on conventional geological analysis, such as tectonic activities and sedimentary burial, this paper mainly studies the evolution characteristics of geotemperature and geopressure fields of hydrocarbon accumulations below gypsum-salt strata in Dongying sag. Furthermore, aimed at the hydromcarbon reservoirs, we discuss the hydrocarbon accumulation process, differentiation causes and the controlling function of gypsum-salt strata on hydrocarbon accumulation. Combined with conventional analysis and some simulation methods, fluid inclusion analysis was mainly used to analyze paleopressure of reservoirs.This thesis concluded the origin of the gypsum-salt strata with trace elements and REE analysis. The halite of gypsum-salt strata mainly came from deep hot brine, and the gypsum mainly came from the eroding product of terrestrial ancient gypsum layers. Furthermore, transgression of sea played an important, but not major, role in providing salt resource for gypsum-salt strata. When the climate was dry in the early paleogene, the transpiration was very strong. As a result, the lake water oscillation led to the formation of rhythmic layers with clastic rocks and gypsum-salt rocks. Later, when the climate was a little humid, the layering effect of salinity in deep water resulted in deposition of gypsum-salt rocks.Using the data of vitrinite reflectance, this thesis calculated geothermal gradient evolution of Dongying sag. The geothermal gradient, which was high in early stage, decreased at a stable rate in medium stage, and decreased sharply in late stage. The evolution curve tends to be arc-shaped. It analyzed the applicability in pressure calculation of SDT, basin simulation and fluid inclusions. The first two methods are suitable in the interior of basin/sag where is filled with thick mudstone and lack of faults. They can correctly calculate the geopressure of source rocks. The method of fluid inclusions shows the fluid pressure of reservoirs correctly.The deep reservoirs of Dongying sag are abundant in fluid inclusions. Large quantities of hydrocarbon inclusions are found in F8 well, FS1 well, FS2 well, T165 well and DF3 well. FS3, FeS4, FS6, Y559 and HK1 are rich in brine inclusions and lack of hydrocarbon inclusions.An experiment studied the cryogenic Raman spectroscopy of the system NaCl-H2O. It clarified two types of water molecules and different vibrations in each type of water molecules in hydrohalite. Hydrohalite would differentiate with the cooling process. This decreased the reliability of using cryogenic Raman spectra of hydrohalite at one single point to determine the salinities of fluid inclusions. Instead, it is more reliable to use statistical values of Raman peaks of hydrohalite at multiple collecting points. The brine inclusions and hydrocarbon-bearing brine inclusions in the deep reservoirs in Dongying Sag mainly are NaCl-H2O system. This illustrates that the type of formation water along with oil-gas migration in hydrocarbon accumulation process mainly is NaCl-H2O.The experiments provided homogenization temperatures, ice points/salinities of brine inclusions and homogenization temperature, vapour/liquid ratios, composition of hydrocarbon inclusions. PVT simulations of fluid inclusions were completed with PIT simulation software. The capturing temperature and pressure are calculated using PVT simulation of hydrocarbon inclusions and homogenization temperature-salinity method of brine inclusions. Combined with geothermal history and burial history, this thesis illustrated the evolution of reservoir geopressure. According to simulation results, it divided the geopressure history into five stages and identified three stages of hydrocarbon accumulation.The uncompaction, hydrocarbon generation and tectonic activities were the main three factors of the overpressure development in Dongying Sag, and gypsum-salt layers played an important role in preserving the pressure. The principal element of pressure increase in stage I is undercompaction, and the badly activity of Chennan Fault released overpressure. The pressure increase in stage II mainly result from generation and accumulation of hydrocarbon. The uplifting and eroding of formation chiefly lead to the pressure decrease in stage III, and the diminution of hydrocarbon activity also weakened the pressure. The stage IV and V of geopressure evolution were equivalent with the second and third stages of hydrocarbon accumulation, and the fluid pressure varied with the intensity of hydrocarbon activity.Large quantities of hydrocarbon inclusions exist in the halite of DF3 well. This halite layer at least experienced two stages of hydrocarbon activities, because the homogenization temperature concentrated in two intervals of 60~80℃and 90~100℃. This suggests that the sealing ability of gypsum-salt strata is not absolutely effective. Discordogenic faults may communicate the two independent hydrocarbon systems under and upon gypsum-salt strata, and lead to material exchange. The formation upon the gypsum-salt strata along with deep fault and/or salt structure is the important zone for hydrocarbon accumulation.This thesis improved the hydrocarbon accumulation model of deep reservoirs underlying the gypsum-salt strata in northern Dongying sag. There are three hydrocarbon accumulation stages of deep reservoirs in Minfeng area, Dongying Sag: embryo stage of ancient reservoir (35.0-24.6 Ma), shaped stage of ancient reservoir (14.0-5.0 Ma) and forming stage of present reservoir (5.0-0 Ma). After three stages of hydrocarbon injection, the present distribution characteristic of oil and gas reservoirs was formed under the control of temperature, pressure, tectonic position and so on.