Research On Organic Fluid And Migration Of It In Dabashan Foreland Structure

Organic fluid in foreland structure is original records of evolution of foreland structure and hydrocarbon migration. The research methods of fluid inclusion and organic geochemistry are used in this paper to analyze the formation of organic fluid, temperature and pressure conditions, fluid properties and composition of it. Formation and migration process of organic fluid,especially abnormal overpressure of organic fluid effected on the organic fluid migration are discussed to provide information to oil and gas exploration in Dabashan foreland structure.Through field investigation and careful observation to the slices,it showed that there are three kinds of organic fluid inclusions in Dabashan foreland structure, which are records of formation and migration process of organic fluid in Dabashan foreland structure. The first kind is liquid hydrocarbon inclusion which distributed in the early fractures or holes, with yellow or weak yellow fluorescence, representing the early formation and migration of hydrocarbon. The second kind is gas inclusion which distributed in late fractures or cleavage cracks, with no fluorescence, representing the migration of natural gas. The third kind is bitumen inclusion which distributed in the intergranular fractures in the calcite faults, black, solid, with no fluorescence, representing the destruction of natural gas reservoirs or hydrocarbon reservoirs. The first two kinds of organic fluid inclusions are mainly analysed in this paper.Composition of fluid inclusion indicate that gas composition of fluid inclusion in Dabashan foreland structure is CH4-based, CO2 and CO in middle. Liquid composition of fluid inclusion is dominated by H2O, CH4 levels are generally low. Statistical results of the inclusion analysis showed that homogeneous temperature in Dabashan thrust belt mainly ranged from 130℃to 280℃with two temperature peaks respectively 180℃and 210℃, and homogeneous temperature in Dabashan foreland belt mainly ranged from 130℃to 200℃with two temperature peaks respectively 150℃and 180℃, additionally homogeneous temperature in Dabashan foreland depression mainly ranged from 100℃to 190℃with two temperature peaks respectively 110℃and 150℃. Homogenization temperature test results showed that the Dabashan thrust belt experienced a higher thermal evolution, correspondingly Dabashan foreland belt and foreland depression experienced a lower thermal evolution. Salinity peak in Dabashan thrust belt and foreland belt was 14-16wt%NaCl, correspondingly Dabashan foreland depression with the salinity peaks of 10-12wt%NaCl and 14-18wt%NaCl. High salinity of fluid inclusions indicated that fluid inclusion was in a closed system, and with no join of low salinity fluid. Fluid property was dominated by CaCl2-H2O system, meaning the formation water.Tt curve method showed that the Dabashan foreland structure experienced three times of formation and migration of organic fluid. The first time occured in 280Ma (equivalent to early Permian), when large hydrocarbon formed from the Lower Paleozoic source rocks. The second time occured in 180Ma (Early Jurassic), when gas reservoirs which were formed in Lower Paleozoic were destroyed due to orogenic movement during the Triassic. The third time for the Early Cretaceous (136Ma-128Ma), present the late migration of organic fluid related to the Yanshan tectonic movements.By mean of measurement contained in rock cements and mineral veins filled in fractures in Dabashan foreland structure, pressure and pressure gradient of paleofluid were calculated using data of tempreture-salinity. Results indicated that pressure in Dabashan thrust belt mainly ranged among 200-230MPa with paleopressure gradient of 2.2MPa/100m, compared with pressure of 180-210MPa and paleopressure gradient of 2.3MPa/100m in Dabashan foreland belt, while pressure of 150-190MPa with the paleopressure gradient of 2.0MPa/100m in Dabashan foreland depression. Paleopressure gradients in Dabashan foreland structure were obviously higher than normal hydrostatic pressure gradient, which revealed the existence of abnormal highpressure in Dabashan foreland structure.Paleofluid pressure in Dabashan foreland structure decreased from northeast to southwest, which resulted in paleogeofluid migration from north to south. Dabashan foreland belt fit with "overpressured fluid compartments",which was limitted between Dabashan thrust belt and Dabashan foreland depression, sealed by a serious of thrust faults. After a comprehensive analysis on regional stratigraphy, lithology and soure-reservior-roof characteristics, it is considered that there are two "overpressured fluid compartments" in Dabashan foreland structure. The upper compartments, consisted by Permian-the Upper Triassic, was tectonic destruction. The lower compartments, consisted by the pre-Triassic was kept completely, and can be a good potential exploration target in Dabashan foreland structure.