Late Paleozoic Magmatism And Tts Significance To Reservoir Development In Santanghu And Tarim Basin, Xinjiang, China

Taking the Santanghu basin and Tarim basin, both in Xinjiang province, as typical examples, this paper studies some problems for the better understanding of the petroleum reservoir related to magmatic activity in this region. The study started with the evolution characteristics and laws of the formation of igneous. Moden analytical technology and methods are applied to carry out the systematic studies of petrology, mineralogy and geochemistry. The progress from this paper is as follows:1. By researching on the magmatism and volcanic reservoir characteristics of the Late Carboniferous to Permian volcanic rocks in Niudong area of Santanghu basin, some new understandings are gained as following.(1) The main volcanic rocks from Niudong area, Santanghu basin, eastern Xinjiang are defined as basic to intermediate volcanic rocks. The geochemical analysis of volcanic rocks shows not only the signature of within-plate setting, but also indicating some characteristics of subduction zone. It is inferred that the eruptive product can be generated in an extensional setting of the late stage of orogenesis. Two types of volcanics have been recognized: one is andesite and basaltic andesite with the geochemical characteristic of arc volcanics, and the other is intraplate basalt, which is shown that the basalt closely accrete in space with andesite and basaltic andesite.(2) Based on the lithology, composition, structure of volcanic rocks and their alteration in vertical, the lithofacies of the Late Carboniferous-Permian volcanic rocks from Niudong area, Santanghu basin, eastern Xinjiang are studied, the major volcanic rocks of the area are lavas, pyroclastic rocks. the volcanic rocks are divided into two facies (outburst and overflow facies) and eight subfacies (brecciated lava, upper vesicular lava belt, transitional lava belt, dense lava belt, basal vesicular lava belt, upper tuff subfacies, middle volcanic breccia subfacies, lower volcanic agglomerate subfacies).(3) The reservoir characteristics of the Late Carboniferous to Permian volcanic rocks in Niudong area of Santanghu basin were analyzed by observing core and thin slice with microscope and scanning electron microscope. The main factors affecting porosity and permeability of volcanic reservoir were also analyzed. The results indicate that the volcanic rocks reservoir are mainly composed of the vesicular volcanics and corroded volcanics. In the light of genesis the volcanic reservoir space is divided into primary pore, secondary pore and fracture. The most important types of pores are air hole, amygdale, intercrystalline pore and phanerocryst dissolved pore. The structural facture is the most important fracture types. The porosity and permeability of volcanic reservoirs were affected by the lithologic characteristics, lithofacies characteristics, eluviations and filling-replacement in the later stage. The pore spaces were filled by euzeolite, laumonite, the mixture of chlorite and smectite and quartz, secondary solution pores of the euzeolite is one of the most important types of reservoir space. It is considered that the subfacies of upper vesicular lava belt and basal vesicular lava are favorable reservoir development position. Based on the relationship between volcanic facies and the reservoir characteristic, a prediction and evaluation model is constructed and therefore the future exploration targets in Niudong area are designated.2. Based on the analysis of the Permian magmatic activity and its influence on the carbonate reservoir in Tarim basin, some conclusions and suggestions are presented.(1) The rocks of igneous intrusions from the wells in Tazhong-Bachu area are composed of gabbros and dolerites. Using K-Ar dating methods, the intrusion from Ordovician carbonates in Tazhong-Bachu area are products of Permian magmatic activity. The chemical charcteristics show that the rocks are typically within-plate environmental, and belong to alkaline series. Comparsion of intrusions with dykes and basalts of Permian outcrops in the western part of Bachu area, the similar character suggests they have the same magma source-enriched mantle.(2) The equation of heat conduction was used to calculate temperature profiles adjacent to thin igneous intrusions during the cooling processes. The zones of alteration by heat are related to the variation in the thickness of the thin igneous intrusions. The thermal effects of the sill intrusions including chemical reactions and mass transport on the surrounding carbonate rocks.(3) Using the micro-laser Raman spectrometer, on the basis of investigation of petrography, we analyzed pyroxene-hosted fluid inclusions from gabbro samples in Well He-3 of Bachu area. Fluid inclusions are primary and were trapped during growth of the host mineral. The results indicate thatCH4, C2H6, N2 and H2O are the dominant gas species. Two types of vapor compositions are distinguished that include: a) H2O -rich vapor, b) CH4+C2H6+ N2 mixing vapor. Therefore, fluid brought by the gabbro from the mantle source region is rich in H2O and reductive volatiles CH4, C2H6 and N2 etc. The carbon- and hydrogen-isotope composition of hydrocarbons in pyroxene is indicative of a magmatic abiogenic origin, and the results indicate Permian magmatism in the Bachu area of Tarim basin not only brings enormous thermal energy but also provides hydrocarbon and N2 etc.3. Based on systemic study of the magmatic activity from Niudong and Tazhong-Bachu areas from Santanghu and Tarim basins, Xinjiang province, we summarized the magmatic activity and its implications for petroleum reservoir. A conceptual model of the magmatism and its effects on development and alteration of petroleum reservoir is set up on the basis of the researches on the spatio-temporal coupling relationship with the process of magmatic activity.