Hydrogeochemistry Of Geothermal Water, Thermal Effects Of Magma Intrusion And Their Significance Of Petroleum Resource

It has been proved that the accumulation of petroleum in sedimentary basins is normally controlled by regional tectonic activities, which mainly presented as the influences of deep fluid/magma intrusions on the thermal evolution of sedimentary rocks and geochemical alteration of fluid substances. The impact of magmatic activities are generally manifested in two aspects:(1) The relaeased heat by the cooling and crystallization of intruded magma can increase the geothermal gradient of the basin, lead to contact metamorphism in the wallrocks, and accelerate the hydrocarbon generation of the organic-rich argillaceous wallrocks.(2) Volcanic rocks, as an important part of the sedimentary basin-filling sequences, has potential to be reservoir rock or seal rocks. As for the deep fluids derived from the lower crust and/or mantle often contain a certaion amount of nobel gas, which can migrate through faults and locally enriched in special strata to be non-hydrocaybon gas resources. The thermal effects of magmatic intrusion and volcanic rocks as potential reservoir rocks or seal rocks were investigated by taking typical Mesozoic magmatic activities in southeastern China as examples, and the geological constraints on the hydrochemistry and spatrial distribution of geothermal water in the Weihe basin in central China was studied. In this dissertation, the following three aspects of geological study are reported:Firstly, based on the geochemical analysis of geothermal water in eastern Weihe basin, the significant control of faults on water sources and spatial distribution are disclosed. The deep faults covered by aquitard layers is favor for the enrichment of noble gases in aquifers. Geochemical analyses are crucial for geothermal evaluation and exploration. Geothermal waters in the eastern Weihe Basin of Central China have been widely exploited, but the geological factors that control their water source and distribution have not been clearly defined. The geothermal source in the basin is affected by local heat flow anomalies and fault systems, which serve as transport channels, as well as material and energy sources. In this study, the chemical composition, trace element concentration, and isotopic compositions of H and O in water samples from29geothermal wells were analyzed. Q-mode multivariate statistical analysis and R-mode factor analysis were performed to classify the samples and determine their sources. Water-bearing stratigraphic and regional faults dominantly influence the spatial distribution and chemical composition of geothermal waters in the basin. Geothermal waters emanating far from faults commonly have the same chemical composition as formation water, whereas those emanating near faults exhibit characteristics of formation water mixed with substantial recharged meteoric water or minimal deep runoff. The water pumped from the wells in Chang’an, Lintong and Tangyu generally occur as meteoric water, whereas that pumped from wells in Xi’an and Xianyang exhibits a few characteristics of deeply derived fluids, in addition to those of formation water.Secondly, based on a survey of typical geological section and numerical analysis of heat transfer model, the thermal evolution of wallrock affected by granitic intrusions was studied, and then the impacts of the granitic magmatism in southeastern China on sedimentary strata was investigated. When magma invasive into the muddy wallrocks, the thermal effect will lead to thermal contact metamorphism, changed diagenesis and promot the hydrocarbon generation of the wallrocks. The heat transfer model is one of the important tools in investigating the magma intrusion and the thermal evolution of its wall rock. In this study, we selected two typical granite-mudstone contacts in eastern Guangdong to do the geological survey and analysis of2-D heat transfer model. Our simulation results are consistent with the geological survey analysis. We found that the scale of rock is the key factor controlling the heat-affected area. Magma intrusion of tens of square kilometers may cause prominent heat effect in area, and hence lower depth of the oil threshold by5km. The range of heat-affected area in lateral direction is about2times of the intrusion rock. During magma condensation and crystallization process, the main direction of hear transfer is upward, and lateral direction transfer is secondary. Furthermore, the numerical simulation of the thermal evolution of potential hydrocarbon source rocks in the southeast China provides a basis to determine the oil and gas exploration targets in this area.Thirdly, through systematic analysis of the typical basaltic volcanic edifice and thermally metamorphic reservoir rocks, the potential reservoir space system connected by fractures and lithologic boundaries consisted by primary and secondary pores and cracks was identified. Meanwhile, the reservoir spaces in a diabase-horfels system in the Zhanhua depression was analyzed as well as the sealing ability of acid volcanic rocks. As a special type of oil and gas reservoir rocks, volcanic rocks have been successfully explorated and developed both in China and other countries. However, for its small size and relatively low oil reserves than sedimentary rocks, and complex reservoir spaces, the researches on the physical properties of volcanic rocks have been carried out only in the last decade. As most of the previous studies are depending on the description of drilling cores and ananlysis of well log data, field investigations on the cracks, pores and contact zone of volcanic layer in Niutoushan edifice located in the eastern Fujian province was carried out in order to obtain the spatial distribution of the pores and cracks with various genesis. The porosity and permeability of typical basaltic rocks have been analyzed. Three effusive facies basalt layers and two volcanic plugs with rock outcrops can be identified. The porosity of effusive basaltic volcanic rocks changes greatly from5.34%to23.73%, however the permeability is low (average0.173×10-3μm2). Joints and cracks in the whole edifice are well developed. In addition to columnar joints in volcanic plugs, crack explosion and condensation joints in the volcanic eruption phase, the contact between the lava and underlying rocks (high porosity zones), mantle xenoliths cracks, explode holes and post-volcanoe tectonic or weathering fractures, also play important roles in the accumulation of petroleum. These facial spaces in different volcanic faces compose a whole network, which maks volcanic reservoir high potential. In addition, the pore characteristics of contact metamorphic hornfels near diabase sill in the Zhanhua depression is well analyzed and it is disclosed that the intrusive rocks-argilliceous wall rock in contact zone can provide ideal reservoir spaces.