The Application Of The Thermometric Indicators To The Study Of Thermal Evolution In The Lower-Yangtze Region

The Lower-Yangtze region is one of the important regions with exploration prospect of oil-gas. Combine with the out-crop research, several methods are applied to the study of thermal evolution in the Lower-Yangtze region and several aspects of cognitions are obtained as introduced as follow. These methods include the reflectivity of the glass plastid, the study of clay, the fission track and the temperature-measure of inclusion and so on. Through the analysis of reflectivity data it is could concluded as follows: Firstly, there is relativity among the R0 values, the depth and the stratigraphic horizon of sample. Secondly, the R0 values of the source rock from the upper–Palaeozoic to the Triassic are clearly lower one step than those of the Lower-Palaeozoic. Thirdly, there is provincialism for the R0 values, it is increased swiftly with the increasing of the depth in the Wuwei area in south part of Anhui province but it is increased slowly both in south and in north of Jiangsu province. Fourthly, from the southwest to the northeast in the study area the devolutionary stacking pattern in the R0-depth section shows that there is provincialism for both the changing law and the feature of the R0 values. Fifthly, the existence of the nappe can result in inversion phenomenon of R0 values. The research results of the apatite fission-track demonstrate several geological features. Firstly, there is large-scale rising-denuding event from the late Indochina Period to the early Yanshan Period. Secondly, the different-degree anneal of the apatite fission-track for the samples from the strata before the Pukou Fm. indicates that there is large depositional thickness and the buried depth for the Pukou Fm.. Thirdly, the rapid buried process and the hereafter rapid rising lead to the absent of the Tertiary deposits in the south part of Jiangsu. Fourthly, the time is different for the last of the large-scale rising-denuding event both in the south and the north of Jiangsu, in north of Jiangsu it is very late (38Ma)　and corresponded to the Sanduo Event but in south of Jiangsu it is very early (65～90Ma) and corresponded to the Yizheng Event. Fifthly, the source region of sediments from the Mesozoic to the Cenozoic is different in times and in space, there are three sets of time-span (68-94Ma,139-177Ma and 118—234Ma)for the start rising-cooling time of the source region and these three sets of time-span respectively correspond to the time (K2,J2-3 and the end of T2) that the rising region transits the isotherm of the complete anneal zone. The study on the inclusion shows that it can be discovered the local regions with low-evolutionary extent and the exploration foreground of oil and gas in the Lower-Yangtze region that is marked a widespread high-evolutionary extent. The study on clay minerals shows that the strata from the Silurian to the Permian in the Tangshan area are turnover. Because of the tectonic thrusting, a set of rock sheet superimposes the strata from the Silurian to the Permian. This set of rock sheet is raised and denuded after the late of the Cretaceous. According to above materials it can be made some conclusions for the evolution of source rocks in the study area. Firstly, the high evolutionary extent of the lower-Palaeozoic source rocks in the Wuwei area of the south Anhui province is genetically relative to the abnormal base-thermal flux. Secondly, in the south Jiangsu the thermal-evolutionary extent for lower-Palaeozoic source rocks is very high but the thermal-evolutionary extent for upper-Palaeozoic to the Mesozoic source rocks is very low. Thirdly, the thermal-evolutionary extent for the four sets of source rocks in each depress of the north Jiangsu is very high and become descending from the depressed to the uplift region. Fourthly, the thermal evolution for the marine source rocks in north Jiangsu is end in the end of the Tertiary hut those in south Jiangsu and in south Anhui is end in the end of the Cretaceous. Fifthly, different cognition for the invert phenomenon of the R0 values and the strata that are opened out at the Shenke well, i.e. there is twin hydrocarbon-formation in the North-Jiangsu Basin that the strata from the Cretaceous to the tertiary, the late formation of the oil-gas is occurred from the late Cratacerous to the tertiary.