| The Qiongzhusi formation of Lower Cambrian is one of the most favorable reservoirs for shale gas exploration in our China, with an earlier sedimentation time, a higher Ro and a more complicated geological evolution history. And therefore it is very important to study the shale gas geological characteristicsfrom the point of view of accumulation. This thesis, making the Qiongzhusi formation be as the research subject, with the methods of data collection and arrangement, geological field investigation, core observation, experimental test and analysis, studied the shale gas accumulation characteristics form five respects: geological background, depositional environment, source rock and reservoir characteristics, accumulation process, occurrence characteristics and preservation conditions.The Qujing area is located in the south margin of Yangtze plate in the Yunnan Guizhou Uplift Zone. On the basis of studying regional geologic settings, and of the results of field geological survey, this thesis studied the evolution characteristics of the sedimentary and structure, dividing it into four stages: the primary stage of the Paleo Tethys Ocean, the Paleo Tethys Ocean stage, the Yanshan movement stage and the Himalayan movement stage.By using the theory of sequence stratigraphy, the lower Cambrian in this region can be divided into six three-level sequences, making an ordered transgression regression cycle. And the Qiongzhusi formation is in the Sq2 and Sq3 sequences. There are three sedimentary subfacies and six sedimentary microfacies. And the sedimentary environment has undergone four major evolutions. The shale with rich organic matter of the Qiongzhusi formation mainly develops in the transgressive systems tract of the Sq2 sequence, with 70~120 meters thick and 0~4500 meters deep.The type of organic matter in the the Qiongzhusi formation is mainly of I type, with an average Ro being 4.2%, and an average TOC being3.22%~3.55%. The main lithology type of the Qiongzhusi formation is carbonaceous shale, sandy shale, with a high content of quartz. The pore types are consisted of organic matter pore, intragranular pores, and intergranular pores. According to the mercury injection curve, the shale gas reservoirs can be divided into two types, and the second type of reservoirs is the main type. And their mercury injection curve of hysteresis loop is narrow. The pore size distribution is not continuous, and their porosity is low, with the average of 2.53%. Compared with the Longmaxi formation, the Qiongzhusi shale thermal evolution degree is higher, and the porosity is lower. This may indicate that a too high thermal evolution degree can be harmful to the pore developing.Using the research methods of ―ectonic movement- burial history– being heated-hydrocarbon generation‖, this thesis studied geological accumulation process of the shale gas of this area. The stratigraphic thickness trend analysis and the deposition rate shows that cumulative erosion of QD1 well has exceeded 5800 meters since the Indosinian epoch. Maximum buried depth of Qiongzhusi formation appeared in the Late Triassic in three, and it was deeper than 7000 meters. At that time the maximum heating temperature of Qiongzhusi formation could reach about 270 ℃. Most of the hydrocarbon generated during C to P.There are three main characteristics of the Qiongzhusi shale gas reservoir in this area: an advantaged sedimentary environment, a stronger diagenesis, and a more complicated accumulation process, with a ealier hydrocarbon generation period and a stronger denudation. The maximum adsorption capacity of Qiongzhusi formation is 2.95cm3/g, and total hydrocarbon content is 0.011%, and the average gas content is 0.54m3/t. Qiongzhusi shale gas preservation conditions in this area,which is studied from four aspect: tectonic conditions, groundwater conditions, the cover features, and the gas component, were a little poor in the shallow area and may become better in the deeper area. |
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