The Study On Sand Causes And Distribution Of Chang81of Zhen Bei Area In Ordos Basin

Zhenbei Oil Field, belogging to the southern part of Eastern region of Gansu province, islocated in the southwest of Ordos Basin. Its main oil-producing layer Chang81is developed inthe Yanchang Formation of the Triassic series. With the high development of its exploration,the existing achievements and research findings can be hard to meet the productionrequirements, so the cause of sand body formation need to be further analysized with higherprecision. The analysis of distributing characteristics of sand bodies of different types couldguide the next employment of exploration and development. For this purpose, on the basis ofsurveying and studying the previous researching results systematically, under the instruction ofthe principles of modern sedimentology and combining with the applying of the techniquessuch as cathode luminescence, heavy mineral and grain size analysis, grain fraction tracing,regional lithological correlation, systematical core analysis and sand body exquisite dissection,this thesis conducts a systematic research on source direction, sedimentary facies, the formingreseasons of sand body and its distributing rules of the oil layer group Chang81of Zhenbei oilfield.From the plaeocurrent direction of the glutenite, the partition of clastic rocks componentsand provenance, the light and heavy mineral association, grading analysis of sandstone underthe regional tectonic and sedimentary background, it is considered that during the Chang81forming period, the sand layer was in the control of southwest sedimentary system, Thecarbonate debris is mainly scattered in Chang7and its above layers of Eastern region of Gansuprovince, but few in layer Chang8. Due to the large quantity of carbonate debris inKongtongshan Conglomerate, which was considered previously as the provenance for theChang8of zhenbei area, it can be drawn the conclusion that the provenance of layer Chang8isfar more than Chang7in distance and Kongtongshan Conglomerate is the provenance ofChang7and its above layers not Chang8.Through analyses of grain-size and its distributing characteristics, sedimentary tectonics,plaeo-biomarkers and logging facies, braided river delta in shallow water was the main deposittype during the Chang81forming period. The lake shoreline, determined by the water andunderwater tokens, was consistent with the line of well zhen126-zhen9-zhen84. The sedimentary subfacies included delta plain and delta front. The delta front can also be dividedinto inner-front and outer-front.Elaborate research is conducted on the forming reasons and characteristics of Chang81sand body. It is considered that there are two kinds of sand bodies developed in the researchingarea, i.e. single type and overlay type. The single sand type incorporates distributary channelsand body of delta plain, underwater distributary channel sand body of inner delta front, sheetsand body of inner delta front, under water distributary channel sand body of outer delta frontand sheet sand body of outer delta front. However, the overlay type includes continuoussuperpositioned sand body of multi-periods of delta plain, muddy intercalation sand body ofmulti-periods of channels outlay of inner delta front.The distributions of Chang81sand body and Chang73high resistant mudstone havecertain correlationship. In vertical direction and flat surface, the western part, where the Chang81sand body is thicker, is corresponding to the thinner Chang73high resistant mudstone. Inthe middle part, however, the moderate thickness of Chang81sand body counterparts with arelatively stable thickness of Chang73high resistant mudstone. In the eastern part ofresearching area, the single and overlap Chang81sand body are relatively thinner, but Chang73high resistant mudstone is highly developed. It is regarded that the type of paleo-lake basinis the key point for the sand body distributing of researching area and even to the Easternregion of Gansu province.