| The Ordos Basin is located in the western part of North China Craton with abundant oil and gas resources.With the discovery of the Lower Paleozoic Gas Field(Jingbian Gas Field),the exploration principle has changed from "mainly exploring the lower Paleozoic and simultaneously exploring the upper Paleozoic" to "basing on the upper Paleozoic and exploring the lower Paleozoic",and many large gas fields have been discovered successively.But the exploration of Benxi Formation is relatively lagging behind.Benxi Formation is unconformly deposited on the Majiagou Formation,and the formation thickness is strongly controlled by palaeogeomorphology.Because of the great variation of formation thickness and the unclear spatial distribution characteristics of sand bodies,the study on the development law of favorable sand bodies in Benxi Formation is relatively weak.Based on the provenance analysis and palaeogeomorphological restoration of the study area,analysed the distribution law of sand bodies.And combined with the characteristics of reservoir sandstone,further explored the favorable sand bodies' development law of Benxi Formation,which provides a certain geological basis for oil and gas exploration in Benxi Formation.In this paper,based on the analysed of field outcrop,drilling and logging data,the provenance of Benxi Formation in Ordos Basin is analyzed by means of sedimentological,geochemical and zircon U-Pb chronological analysises.Based on the slice observation of rocks,the contents of conglomerate and sandston,sandstone debris and heavy minerals are counted,and then compiling the regional distribution maps of conglomerate,debris and heavy mineral assemblages,it is preliminarily determined that the source supply mainly comes from three directions: north,southwest and south of Ordos.Through the determination of trace and rare earth elements in mudstone and rare earth elements in zircon,standarding the data and compiling the standardized diagrams.From the standardized diagrams of trace and rare earth elements in the north and the south samples,it was found that there were differences in Provenances between the north and south in the study area.According to zircon dating results,there are three peak ages of zircons in the north samples: 315 Ma,1874 Ma and 2411 Ma,and four in the south samples: 443 Ma,945 Ma,1697 Ma and 2457 Ma.And in the South zircon samples,some zircons exist,which age ranges from 500 to 1500,however it is non-existent in the north samples.Based on the above analysis results,the provenance of Benxi Formation samples in the north and south of the study area does not own the same provenance.The provenance of the samples in the north comes from the Xingmeng orogenic belt,while the provenance of the samples in the South comes from the South Qinling Mountains.There are coal seams on the top of Benxi Formation,which are widely developed in the study area and play a role of filling up the palaeogeomorphology.Combining with the actual situation of Benxi Formation development in the study area,we choose the impression method with formation thickness as the core to restore the palaeogeomorphology before the deposition of Benxi Formation.The palaeogeomorphological restoration map show that the main ancient trench areas are located in the Yuling-Zizhou-Zhidan area,and the distribution of paleo-trenches in the north,southwest and south of the study area indicates that there are paleo-rivers in the south and north of the study area.And it is consistent with the results of provenance analysis.From the result of provenance analysis,the material is imported from the north,southwest and south direction to form delta sand bodies in the north and south of the study area.The middle part belongs to the mixed source sedimentary area,affected by tidal action,the delta sand bodies are transformed into banded tidal bar,especially,the southern sand bodies has lost its original shape.From the palaeogeomorphological restoration map,the palaeogeomorphological characteristic of the study area from north to South is palaeo-highland to palaeo-trench to palaeo-highland,which results in the greatest thickness and the best connectivity of sand body in the central region.The thickness of sand body gradually thins to the north and South sides,and the connectivity also gradually becomes worse.Based on the study of petrological,pore,physical and diagenetic characteristics of reservoir sandstones,the diagenetic facies in the study area can be divided into six types: dolomite,kaolinite cementation-dissolution secondary pore facies;calcite,illite,kaolinite cementation-dissolution secondary pore facies;quartz,kaolinite cementation-primary secondary pore facies;quartz,illite,kaolinite cementation-secondary pore facies;calcite,kaolinite cemented-compacted secondary pore facies and compacted compact facies.In those diagenetic facies,dolomite,kaolinite cemented-dissolution secondary pore facies and calcite,illite,kaolinite cemented-dissolution secondary pore facies are the most favorable ones for the development of reservoir sand bodies.According to the above analysis results,the favorable sand body distribution area in the study area is depicted,and the most favorable area is located in Yulin-Zizhou-Zhidan area in the central part of the study area.The sand body in this area is strongly transformed by tide,owns high maturity,good connectivity and large thickness.The diagenetic conditions of Shenmu-Shuangshan area on the north side and Fuxian-Yichuan area on the south side are slightly worse than those in the central,and these are sub-favorable favorable area. |
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