鄂尔多斯盆地镇原北部三叠系延长组长7—长6油层组古地貌与砂体分布特征

Palaeogeomorphology reconstruction and sandbody distribution of the Chang 7-6 intervals of Triassic Yanchang Formation in northern Zhenyuan area,Ordos Basin

结合岩心、测井等资料与Shanmugam重力流观点,将鄂尔多斯盆地镇原北部地区延长组重力流沉积物分为浊积岩、砂质碎屑流沉积和滑塌岩3类。以成因层序地层学为指导建立精细地层格架,采用优化后的回剥法定量恢复研究区古地貌。发现镇原北部地形以坡折带为主,长7底部湖侵范围最大,坡折坡度约0.45°,坡折下砂质碎屑流沉积和浊积岩连片展布,滑塌岩发育较少。长73—长72沉积时期,基准面下降,坡折带和沿岸线分布的河口坝砂体向湖区推进,坡折带逐渐变宽缓。长71沉积晚期,基准面上升,坡折带和砂体向陆退缩;长63—长61沉积时期,基准面略下降,三角洲向湖盆填充,坡折带坡度约0.1°,坡折下发育砂质碎屑流沉积和滑塌岩,浊积岩少见。

Triassic Yanchang Formation is one of the main targets of oil and gas exploration in northern Zhenyuan area. The types and distribution of sandbody are controlled by the palaeogeomorphology. Based on cores,well log analyses,and sandy debris flow viewpoints proposed by Shanmugam,the gravity flow deposits can be divided into turbidites,sandy debris flow sediments and slump rocks. After building the high-resolution sequence stratigraphy framework,an optimized backstripping method is used to restore the deposited terrain,and the palaeogeomorphology map of the studied area is drawn by short-term base level unit. The study finds that the north of Zhenyuan situated near the depositional slope break zone,Chang 73 was of the maximum transgression,the degree of the slope is 0.45,turbidites and sandy debris flow sediments developed in the deep lake in the northeastern part of the study area. From Chang 73 to Chang 72 depositional periods,the base level decended,the mouth bar and the slope break zone gradually advanced to the lake,and the angle of the slope break zone began to decrease;from Chang 72 to Chang 71 depositional periods,the base level began to rise,the slope break and the sandbody gradually retreated to the shore. From Chang 63 to Chang 61 depositional periods,the base level decended,sediments deposited in the basin,the slope decreased to 0.1°,sandy debris flow sediments and slump rocks developed under the slope break.