| The original reservoir-forming conditions of marine strata in Qiannan-Guizhong depression were superior, however, it was seriously destroyed after several tectonic movements, such as Caledonian,Hercynian,Indosinian,Yanshan and Himalayan. It is now typical residual basin, which is characterized by multistage hydrocarbon generation and migration, formation of reservoir, reconstruction and dispersion and so on. Based on the analysis of cap rock features in Qiannan-Guizhong area and the influences of tectonism and magmatism, the relationship between hydrological-geochemical characteristics and hydrocarbon preservation conditions was systematically studied through the view of fluid chemical-dynamics. Therefore, the evolution discipline on hydrocarbon preservation conditions of marine strata in Qiannan-Guizhong depression was reverted. Moreover, the comprehensive evaluation theory and method in regard to hydrocarbon preservation conditions of residual basin were improved, and a practical evaluation index system for hydrocarbon preservation conditions was proposed to assess that of Qiannan-Guizhong depression, so that the beneficial storage area was found, giving direction for future exploration of oil and gas. Results as following were drawn during this research:(1) The sealing ability of cap rock in Qiannan-Guizhong depression was evaluated synthetically through both macroscopic and microscopic study. Results show that the Lower Cambrian Niutitang Formation, Middle- Lower Silurian Wengxiang Formation and Middle- Lower Devonian argillaceous rock are regional cap rocks of Qiannan-Guizhong depression; The Lower Cambrian Niutitang Formation, Middle- Lower Silurian Wengxiang Formation argillaceous cap rock distributed continuously and deeply buried in Qiannan depression, especially the Changshun sag, areâ…¡-â…¢type cap rock, with middle-good sealing capability; The total thickness of Middle- Lower Devonian argillaceous cap rock in Changshun sag of southern Qiannan depression and the northwest and eastern areas of Guizhong depression is above 600 m. Middle- Lower Devonian argillaceous rock also areâ…¡-â…¢type cap rock, and with middle-good sealing capability; Lower carboniferous cap rock is of residual distribution, however, it can form an effectiveâ… -â…¡type local cap rock in coordination with Upper Devonian argillaceous cap rock.(2) The Qiannan-Guizhong depression, especially the southwest of Qiannan depression and the middle-west of Guizhong depression, is tectonic stable area with no deep faults. low tectonic deformation strength, and small Yanshan and Himalayan erosion thickness of 3500m accumulatively, recoveryed by vitrinite reflectance(Ro) and homogenization temperature of fluid inclusions. So the hydrocarbon preservation conditions were not obviously destroyed by tectonic activity. Also they were not obviously impacted by earthquake, and hardly influenced by magma and ore-forming hydrothermal activities.(3) Based on the study of faults development, distribution and temperature of hot spring, the present meteoric water infiltration depth can be estimated. That of Qiannan-Quizhong margin, e.g. Qianzhong uplift, northeast of Guizhou, northeast, southeast and southwest of Guangxi, is above 1000m, with high hydrogeology open degree, while the present meteoric water infiltration depth in depression interior is below 800m. and with low hydrogeology open degree. Combining with the analysis of drilling well formation water and hydrocarbon geochemical test data, following conclusions are proposed:Strata of depression interior, which was shallower than 800m, was strongly impacted by meteoric water infiltration, and should be free water exchange zone, having poor hydrocarbon preservation conditions; Influence on strata deeper than 800m, called exchanging-retarded zone, became weaker. The hydrocarbon preservation conditions turn into good gradually as the depth increase; The exchanging-stagnant zone should be deeper than 2000-3000m. Therefore, the superficial strata of Qiannan-Guizhong depression have poor hydrocarbon preservation conditions, and deep strata are the target of exploration.(4)The formation and evolution process of paleofluid in marine strata of Qiannan-Guizhong depression was impacted by meteoric water infiltration. Through the analysis of stable carbon and oxygen isotopes of calcite veins and homogenization temperature of fluid inclusion, the ancient meteoric water infiltration depth was recovered. Calculated results show that the infiltraion depth in Qianxi-Pingba-Nandan-Hechi area was about 1600m, and gets deeper in peripheral area; the infiltration in east of Duyun-Sandu-Luocheng and the south area of Xiangzhou-Du'an was deeper, with the value between 2200 and 3000m.(5) The evolution process of paleofluid dynamic field and hydrocarbon accumulation-preservation conditions was reconstructed by the deeply study of deposition, tectonic evolution history of Qiannan-Guizhong depression. During the Caledonian and Hercynian, source rock of Qiannan-Guizhong depression began to generate oil in large quantities. Caledonian and Hercynian were the main oil accumulation stages for Qiannan-Guizhong area, while Indosinian was the main gas accumulation stages as the persistently weak tectonic activity. Caledonian and Yanshan-Himalayan movements were all intense, seriously destroyed the oil and gas reservoir of study area, however, as the size of Silurian oil generation was small, the Early Paleozoic reservoir in Qiannan depression was damaged to a certain extent by Caledonian movement, but that of Qiannan-Guizhong depression was mainly destroyed during Yanshan-Himalayan.(6) The type of southern marine hydrocarbon preservation unit was classified through the study of tectonic activity, geofluid chemistry-kinetics features of Qiannan-Guizhong depression, combining with the characteristics of hydrocarbon preservation in Sichuan Basin, Nanpanjiang, Shiwandashan, middle and lower Yangtze region. The Qiannan-Guizhong depression is now a typical residual basin destroyed by tectonic activity, while some part belongs to uplifting-meteoric water infiltration damaged type or denudation exposed type.(7) The coupled relationship between hydrogeological and geochemical characteristics and the hydrocarbon preservation conditions in Qiannan-Guizhong depression was studied based on the systematical analysis of the present hydrogeology and geochemistry-dynamics, the geochemical characteristics and dynamic field evolution of paleofluid. Then combined with caprock development situation, tectonic and magmatic activity in research area, a practical evaluation index system for hydrocarbon preservation conditions was proposed. Based on that index system, the hydrocarbon preservation conditions in Qiannan-Guizhong depression was comprehensively assessed:Changshun sag in the southwest of Qiannan depression and Xincheng area in the middle-west of Guizhong depression, have relatively good hydrocarbon preservation conditions; The strata deeper than 2000-3000m vertically also have nice hydrocarbon preservation conditions; The deeply buried gentle anticline and low amplitude structure in depression may be suitable for accumulation and preservation of gas. |
