Study On The Thermal History And Hydrocarbon Accumulation Periods Of Sartai Depression In The Yabrai Basin

Yabrai Basin is a small Meso-Cenozoic graben basin which located in the northern Alashan Block of Inner Mongolia,China.At present, the basin is in a less degree of exploration that the research contents only limited to the seismic prospecting, strata, sedimentary facies, tectonic characteristics and simplex geochemical characteristics.And the study on the thermal evolution of source rocks, the characteristics of paleotemperature and present-day geothermal and the periods and process of hydrocarbon accumulation in the basin is still very weak. Thermal evolution not only directly restrict and affect the generation, migration and accumulation of oil and gas, but also has an important guiding act on petroleum resources evaluation and exploring the hydrocarbon region. In this study,different kinds of paleotemperature scales such as vitrinite reflectance(Ro),apatite fission track(AFT) and fluid inclusion were used to discuss the paleotemperature evolution and reconstruct the geothermal history of different sub-sags on the basis of regional geological structure background research, previous existing research results and the exactly burial history models. In this study, the highlights of paleotemperature and present-day geothermal field,thermal history, maturity evolution of source rocks, hydrocarbon generation history and hydrocarbon accumulation period of Sartai depression in Yabrai Basin were discussed. From the above research the following conclusions could be drawn:(1) It was the first time to use the method of apatite fission track dating in Yabrai Basin based on the regional geological tectonic setting,and the AFT dating results show that the study area may has experienced three times of uplift process,they are in the end of Jurassic(138.0±10?135.8Ma),late stage of Early Cretaceous(116.0±8?109.3Ma) and middle-late Paleogene(52.0-29.4Ma). And the first two geothermal events should have a greater influence on the petroleum accumulation because of the Yanshan orogeny.(2) In study area, the temperature of current-geothermal field is low and conductive with the geothermal gradient of 2.76?/100m. The paleogeothermal field of each secondary tectonic unit in Sartai depression was different:The paleogeothermal field of Yanchang and Xiaohuzi Sub-Sag was conductive with the paleo-geothermal gradient of 2.8?3.4?7100m and the paleotemperature was 81.2?128.1?;The paleotemperature and paleo-geothermal gradient in Heisha Low Uplift were 103.2?140.2? and 4.2?4.8?/100m, much higher than that in the sedimentary depression because of the thermal polymerization and thermal anomaly of the low uplift. The maturity evolution of source rocks was mainly controlled by paleogeothermal field, because the paleotemperature of Middle Jurassic was much higher than present.(3) The analysis results of geochemical features and maturity evolution of Middle Jurassic source rocks in Sartai Sag,Yabrai Basin, and its relationship with hydrocarbon accumulation show that the source rocks from J2q and J2x1 in depression center are medium-good source rocks in medium-high maturity.can be a good oil (or gas) source; The source rocks from each atrata in Heisha low uplift have a poor oil-generating potential because of the low abundance of organic matter;The source rocks from J2x1 in the whole Sartai dpression are generally poor with low maturity.(4) The analysis of hydrocarbon generation and expulsion show that there were 1-2 hydrocarbon generation processes and the peak stage happened in the late stage of Late Jurassic (139.5?144.1Ma) and the late stage of Early Cretaceous (102.2-98.6Ma). The hydrocarbon-generating density and hydrocarbon expelling intensity in depression center are much higher than that in slope zone and Heisha low uplift, and it is the main source area of Yabrai Basin.(5)The information from AFT data and thermal simulation results show that the Sartai depression experienced a fast buried-warming and uplift-cooling event in the early stage and gradually weakened with the shrinking evolution of basin.The J2x1 strata in Yanchang sub-sag and Xiaohuzi sub-sag have the similar thermal evolution process in time, but the temperature of J2x1 in Xiaohuzi sub-sag was generally higher than the other, especially in the Early Cretaceous that Xiaohuzi sub-sag had a much better geothermal field environment for hydrocarbon generation. The temperature of J2x1 warmed up to 72.0?140.0? during 180.0?146.7Ma and 130.5?100.0Ma,and the two warming processes were the crucial stages of the source rock's maturity evolution and hydrocarbon generation in study area.(6)The analysis results of thermal history simulation and hydrocarbon accumulation period show that the hydrocarbon filling process of the three second tectonic units are different. However, for the whole Middle Jurassic reservoir in Sartai depression, there were two times of hydrocarbon filling process happened at early stage of Late Jurassic (145.0?152.0Ma)and Early Cretaceous(100.0?134.0Ma)about the same time of peak paleogeothermal which reflected the importance of geothermal evolution in the process of hydrocarbon accumulation.Moreover, the results of fluid inclusion and illite K-Ar dating indicated that there was the possibility of late hydrocarbon accumulation in Xiaohuzi Sub-Sag.