Stratigraphic Distribution And Basin Evolution Process During Cambrian In Ordos

Ordos basin,which is the second largest oil and gas sedimentary basin in China, located in the north of Qilian and Qinling orogen of tethys tectonic domain, is the favorable potential prospecting area of marine oil and gas exploration in China. Marine carbonate strata are widely distributed with great sedimentary thickness and superior petroleum geology condition,which has great potential of exploration and development in the early palaeozoic era in Ordos basin.In the present stage, researches of the early palaeozoic era are mainly concentrated on the middle and upper Ordovician, which is enriched of natural gas, and whether there is oil reservoir in the underlying Cambrian and deeper Proterozoic still remain unclear. Therefore, it is necessary to in-depth research on the basic geological problems of Cambrian, such as structure, sediment distribution and basin evolution process, etc.Based on previous studies, and taking the "configuration control basins, pots control the deposition, the deposition control combination" as the guiding ideology, combined with sedimentology, sequence stratigraphy, lithofacies ancient geography theory and geochemistry,physics, etc., we used the data of 53 wells and 16 outcrops to research the distribution of Cambrian strata and basin evolution process, and achieved the following conclusions:1) During Cambrian period, the relative position of North China Plates contains Ordos Basin Yangtze Plate contains Sichuan Basin and Tarim Plate were generally consistent, both in the low latitudes in the southern hemisphere equator.So their palaeogeography,paleomagnetism, paleoclimate are basic similar.2) Cambrian is widely distributed in the basin, with the characteristics of "Southwest thick, central thinning, northern missing". The lower Cambrian presented a "L" type characteristics in the west and south of the basin. The middle and upper Cambrian were widely distributed in the basin with similar thickness.3) According to previous researches and the latest drilling data, Cambrian was a complete transgressive process with the development of littoral, platform sedimentary facies in the basin. Shore deposits had a banding distribution mainly along the archicontinent.Platform slope facies developed along the west and south of the basin presented "L" type.4) We studied the lithofacies palaeogeography of Cambrian in the basin accurate to Formation. During the Mantou age of the early Cambrian, the sea level of the west and south of the basin was relatively consistent and the sedimentary sequence can be well correlated. Shoreface clastic sedimentary facies mainly developed along the ancient land,while platform slope facies developed in the western and southern margin of the basin presented "L" type. During the middle Cambrian, with the sea transgression covering the entire basin, forming extensive carbonate platform sedimentary facies. Sanshanzi age of the Late Cambrian period, open platform environment developed widely and the central ancient land reappeared, which marked the ending of transgression and the beginning of the regression.5) On the basis of the new understanding of the lithofacies palaeogeography of Cambrian, combined with single factors analysis of various wells, stratigraphic section and mudstone, sandstone, limestone, dolomite thickness data, we determined three hydrocarbon generation sags developed in the different periods, including Helan sag, Yaoxian sag, Shenmu sub-sag and Zhongyang sub-sag.