Dynamics Of Hydrocarbon Accumulation In The Yitong Basin

Yitong Basin is a Cenozoic slip-extensional rift basin that developed in in northeast section of the Tanlu Fault zone, and is located between the Changchun city and Jilin city of Jilin province.With the Daheishan horst, which adjacent by the Soliao basin, on the northwest edge of the Yitong basin and the Nadanhadaling terra on the southeast edge, the basin's form looks like a long belt with a length of 160km, width of 10-20km and the total area of 2200km2, and extend for the direction of NE45.Based on generally collecting, arranging and studying the present documents in geology, geophysics, geochemistry and analytical experiments and the previous achievements of the research area, guided by the theory of petroleum pool forming dynamics, applying the integrated technique that combining the static with the dynamic, the macroscopic with the microscopic and the qualitative with the quantitative, with the primary content about the dynamics of hydrocarbon accumulation in the Yitong basin, the fundamental reservoir forming conditions, the feature of the fluid dynamic field and the process of the dynamics of hydrocarbon accumulation in the Yitong Basin were systematically analyzed. And the carrier system, the main controlling factors of hydrocarbon accumulation and the evolution history of secondary migration of hydrocarbon in the Yitong Basin were deeply dissected and finally the migration and accumulation units of hydrocarbon were classified and evaluated. It will help to better understand the rules for the hydrocarbon accumulation in the Yitong Basin and the similar slip-extensional rift basin, and provide an example for predicting the favourable exploration targets. The thesis is divided into four parts as follows.1. Analysis on the geological elements of hydrocarbon accumulation in the Yitong BasinBased on the thickness of dark shale, organic matter abundance, organic matter type and organic matter maturity, the source rocks of secondary tectonic units in the Yitong Basin were comprehensively evaluated. The results indicate that there are three types of source rock named as the Eocene Shuangyang, Sheling and Yongji formations, and the Eocene Shuangyang is supposed to be the major source rock due to the wide distribution and great thickness of the dark shale, relatively richer organic matter abundance and higher maturity.Based on the analysis of the the sedimentary facies, lithological and physical properties of the Eocene and Oligocene reservoirs in the Yitong Basin, the results suggest that the siltstones, fine sandstones and sandy conglomerates are the major reservoir rocks. Although the physical properties of the reservoirs are gradually destroyed due to the increase of burial depth, the reservoir conditions of Shuangyang Formation in deep depth may be improved by possible development of the abnormal porosity and permeability in partial intervals in the Luxiang fault depression and the Moliqing fault depression. Reservoir conditions were mainly controlled by multiple factors such as sedimentary environment, diagenesis and tectonic movements. In a word, with the most favorable reservoir conditions, the Eocene Shuangyang formation is the major oil bearing interval in the Yitong Basin.The sealing conditions of cap rocks were intregratelly evaluated on two aspects:regional distribution characteristcs and seal mechanism of cap rocks, which indicates that the Yongji formation is the most important cap rock in theYitong Basin due to its highest shale content, greatest single thickness and stable regional distribution. Excess pressure generally exsits in cap rocks from the study area, therefore, the sealing condition is improved by abnormal high pressure (undercompacted) in the shale. Based on the comprehensive evaluation of the above geological elements of hydrocarbon accumulation, it can be concluded that the type of authigenic reservoir is the most important one in the Yitong Basin.2. The distribution characterictics and evolution history for the fluid dynamic fieldBased on the statistics of the geothermal gradient and heat flow at present all around the basin, it indicates that the average geothermal gradient within the shallow layers is higher than the deep ones. Laterally, the average geothermal gradient is relatively higher within the Moliqing fault depression and Luxiang fault depression but relatively lower within the Chaluhe fault depression. Within the different tertiary tectonic units in the same fault depression, the sag always has higher geothermal gradient than the structural high. The distribution and thermal conductivity of the major faults plays an important role on the lateral distribution of geothermal field at present. By using the technique of BasinMod, the evolution history of the geothermal field was rebuilt based on the representative wells from the different secondary tectonic units, the results suggested that the geothermal gradient gradually went down in the evolution history.By caculating the measured formation pressure data within the reservoirs and cap rocks, it can be concluded that the reservoir geopressure is mostly normal in the Yitong Basin, and the overpressure mainly develop within the Shuangyang famation of the Chaluhe and Moliqing fault depressions. The deep shales usually have the overpressure while the shallow shales show the normal compaction trend. The uncompacted zone in the shale is mostly distributed in the Eocene formation, and there are three types of reservoir-seal energy assemblage in the Yitong basin. By using the technique of BasinMod, the evolution history of the excess pressure was rebuilt based on 5 wells and 13 lines, the results indicated that the excess pressure was mainly distributed within the Eocene Sheling and Shuangyang formations. The 1D and 2D modeling results both showed that the formation pressure evolution in the study area has obviously undegone cycles of accumulation and expulsion, the mechanism for pressure accumulation was undercompaction in the early period but hydrocarbon generation in the late period. The tectonic uplift was the main mechanism for pressure release.Base on the modeling of excess pressure evolution history for the above 13 2D lines, combined with filling history, the evolution chatacteristics of fluid potential was analyzed in the Yitong basin. The results indicated that the current distribution of fluid potential is characterized by "gravity flow" feature, and the high-value area of fluid potential is within the Shuangyang formation of the hydrocarbon generation kichen. The evolution history of fluid potential can be featured by the continuing developing high fluid potential within the Eocene formations in northwest of the hydrocarbon generation kichen, the fluid potential gradually decreased along the basement and fault towards the uplift in the Southeast area, which suggested that the large-scale lateral migration mainly happened within the Shuangyang Formation. And the fault should be the major vertical pathway for hydrocarbon migration. As a whole, the hydrocarbon migration always happened in at the same transport layer from the depression to slope high, and the structural high should be the favorable area for hydrcarbon migration during all the stages. In the critical period for the hydrocarbon accumulation, the fluid potential is relatively higher within the Shuangyang Formation, and the hydrocarbon generation kichen within the Chaluhe fault depression had a remarkable higher fluid potential than the depression within the Luxiang and Moliqing fault depression. Futhermore, the development of paleo-fluid potential was controlled by the evolution of palaeo-overpressure, both of them have cycling, Volating and decreasing trend.3. The study on the dynamics of hydrocarbon accumulation in the Yitong BasinBased on the results of erosion thickness restoration for the Qijia and Wanchang formations in the Yitong Basin, the burial history of 10 wells was rebuilt and the modeling results indicated that the tectonic-sedimentation development in the Yitong Basin was characterized by multiphasic evolution. Generally, the Yitong Basin had a higher sedimentation rate and tectonic subsidence rate in early period compared with the late period. The tectonic-sedimentation development in different tectonic units gradually became more and more similar.Based on the model of transit heat flow, the geothermal evolution history was rebuilt based on the 10 representative single wells in the Yitong Basin, the result showed that formation temperature rate was obviously controlled by sedimentation rate and they have proportional relationship. The basin sedimentation rate and formation temperature rate both decreased in the geological period, which are correspondent to the structural evolution of the Yitong Basin.In this study, chemical kinetics model was used for hydrocarbon generation history modeling of the main hydrocarbon source rocks of each representative single well in fault depression. Simulation results showed that, source rocks of Shuangyang Formation had the highest hydrocarbon-generating intensity, Sheling Formation took the second place, Yongji Formation had weak hydrocarbon generation capacity. Generation intensity of gas in Chaluhe fault depression was higher than oil, hydrocarbon source rocks of Shuangyang Formation and Sheling Formation had high thermal evolution degree, while product was dominated by gaseous hydrocarbon. Conversely, Moliqing fault depression and Luxiang fault depression had moderate thermal evolution degree, with liquid hydrocarbon as the main product. The simulation result of hydrocarbon expulsion history was consistent with hydrocarbon generation history and manifested as Shuangyang Formation had the highest gas expulsion intensity and oil expulsion intensity, which was the major hydrocarbon expulsion horizon. gas expulsion intensity of source rocks was higher than oil expulsion intensity in Chaluhe Fault Depression, while product was dominated by gaseous hydrocarbon, while Luxiang Fault Depression and Moliqing Fault Depression took liquid hydrocarbon as the main product. the source rocks of Shuangyang Formation began to expell hydrocarbon ealier than Sheling Formation, mostly in middle Oligocene. the source rocks of Shuangyang Formation had the highest hydrocarbon expulsion efficiency, the oil expulsion efficiency and gas expulsion efficiency was mostly larger than fifty percent.The results of the apatite fission-track chronology for the Chaluhe Fault-Depression and Luxiang Fault Depression in the Yitong Basin showed that Chaluhe Fault Depression had experienced two stages of tectonic movement (uplift-cooling) since the Oligocene, respectively corresponding to the tectonic uplift event at the sedimentary end of Wanchang Formation and that of Chaluhe Formation. Luxiang Fault Depression had experienced at least one stage of tectonic uplift event since Oligocene, corresponding to the tectonic uplift event at the sedimentary end of Wanchang Formation.Study on hydrocarbon filling history of the Yitong Basin based on oil and gas bearing inclusion showed, Chaluhe Fault Depression in the Yitong Basin developed three stages of hydrocarbon accumulation, and mostly concentrated in Shuangyang Formation and Sheling Formation. Moliqing fault depression and Luxiang fault depression both had two stages of hydrocarbon accumulation, which mostly developed in the Shuangyang Formation.4. The classification and evaluation for the migration and accumulation units of hydrocarbonCharacteristics of hydrocarbons and oil-source correlation has been analyzed in the study, the results suggested that hydrocarbons in different secondary tectonic units have different characteristics and origin. The natural gas is the main product within the Chaluhe fault depression, whose oil is dominated by light crude oil. The oil and natural gas both originated from the source rock of the Eocene Shuangyang Formation. By contrast, the oil is the main product within the Moliqing and Luxiang fault depressions. The oil mainly originated from the source rock of the Eocene Shuangyang Formation as well and was dominated by light and normal crude oil.The hydrocarbon carrier system within the Yitong Basin mainly includes sand body, fault and basement unconformity, and different secondary tectonic units have its unique configurations of hydrocarbon carrier system, which result in obvious different migration styles. Because of the lack of faults within the Moliqing fault depression, this area is dominated by lateral hydrocarbon migration. By contrast, the hydrocarbon is vertically wide distributed within the Chaluhe fault depression, indicating that vertical hydrocarbon migration intensively developed in this area. The Luxiang fault depression has the most developed hydrocarbon carrier system and favorable configuration, therefore, the oils from the two hydrocarbon-generation kitchens could continuously accumulated towards the traps within the Wuxing structural high through the lateral and vertical hydrocarbon carrier system. Finally the biggest hydrocarbon reservoir formed as Changchun Oil Field in the Yitong Basin.The main controlling factors for hydrocarbon accumulation within the Yitong Basin are fault, basement unconformity, sandstone reservoir, seal and temperature-pressure system. These characters have different influential actions in different areas, which lead to different hydrocarbon accumulation patterns.In this study, the module of BasinFlow and Basin View was used as a simulator for recovering the evolution of hydrocarbon secondary migration within the E2S—>E2S source-reservoir system in the Yitong Basin, it can be concluded that The hydrocarbon secondary migration became the most intensive during the main period of oil and gas accumulation and in this phase oil and gas was sufficiently provided by hydrocarbon-generation kitchen, the hydrocarbon secondary migration can be characterized by the big density of flow line, the wide distribution of hydrocarbon migration and the strong affect of hydrocarbon migration. Therefore, the wide distributed lithologic, structural and complex traps, which are located within or nearby the hydrocarbon-generation kitchen, will be the most favorable targets for hydrocarbon migration and accumulation in the Yitong Basin.According to the concept for dividing trough of hydrocarbon migration and the source-reservoir system (E2S—>E2S) of the flow chart during the main period of oil and gas accumulation (10Ma), the Yitong Basin can be divided into 11 hydrocarbon migration-accumulation units. and the best units are the hydrocarbon migration-accumulation unitⅠin the Moliqing fault depression, the hydrocarbon migration-accumulation unitⅠin the Luxiang fault depression and the hydrocarbon migration-accumulation unitⅡin the Chaluhe fault depression. These hydrocarbon migration-accumulation units have some common features:the favorable hydrocarbon-generating kitchens were developed and sufficient oil source was supplied, the high-quality sand body in the Shuangyang Formation was highly developed and good reservoir condition was provided, the favorable complex conduit system of fault-sand body-basement unconformity were developed within these areas. Besides, the lack of tensing faults in the late period, the good sealing condition was provided, so the preservation condition for hydrocarbon is perfect.