鄂尔多斯盆地西南缘奥陶系平凉组改造砂沉积特征及意义

doi:10.11743/ogg20180217

摘要/Abstract

摘要： 为研究鄂尔多斯盆地西南缘奥陶系平凉组改造砂沉积特征，运用沉积学相关理论和方法，基于露头、古水流恢复及地球化学等资料对研究区，重点开展了改造砂沉积特征、形成过程、主控因素及油气勘探意义研究。认为改造砂在研究区较为发育，其影响因素包括物源、相对海平面、地形及构造运动。结果表明，平凉组岩性单一，沉积构造丰富，可分为3种典型岩相。岩相1为水平层理泥岩相，代表深水原地沉积；岩相2为交错层理砂岩相，反映浊流沉积；岩相3为双向交错层理砂岩相，为改造砂沉积。改造砂典型沉积特征为：①颗粒分选、磨圆相对较好，存在多个粒径分布总体；②具有两个古水流优势方向，一个为大致沿斜坡向下，代表浊流运动方向，另一个大致平行斜坡，为等深流运动方向；③生物扰动发育；④概率累计曲线1~3段式，兼具重力流和牵引流特征；⑤下粗上细正粒序，顶部见侵蚀特征；⑥B，Cr，V等微量元素相对富集。研究区等深流向西大致平行斜坡运动过程中，可对南东向沿斜坡向下运动的浊流沉积进行簸选、搬运、改造及再沉积，进而提高改造砂的储集性能，其与粗粒等深流沉积可作为良好的油气储集体。同时，细粒的深水原地沉积和等深流沉积是较好的烃源岩。改造砂、等深流沉积及原地沉积组合出现可形成理想的生储盖组合而成为潜在的油气储集场所。

关键词: 浊流, 等深流, 改造砂, 平凉组, 奥陶系, 鄂尔多斯盆地

Abstract: The principles of sedimentology were used to study the sedimentary characteristics,forming process,dictating factors and petroleum exploration significances of reworked sands in the Ordovician Pingliang Formation at southwestern margin of Ordos Basin based on outcrops,restoration of paleocurrent and geochemistry data.The result shows that reworked sands are relatively well developed in the study area and they are mainly affected by factors including provenance,relative sea level,topography and tectonic activities.The Pingliang Formation in the study area is unitary in lithology and various in sedimentary structures and can be divided into three typical facies.Facies 1 of mudstone with horizontal bedding represents deep water autochthonous sediments.Facies 2 of sandstone with cross bedding indicates turbidity sediments.Facies 3 of sandstone with bimodal cross bedding points to reworked sands.The typical characteristics of reworked sands are as follows:1) relatively well-sorted and -rounded grains of bimodal grain-size distribution;2) two predominant directions of paleocurrent,with one going down along slopes,indicating the direction of turbidity current movement,and the other running parallel to slopes,showing the direction of contour current flowing;3) obvious bioturbation;4) one to tripartite divisions of cumulative frequency curves,showing both gravity and tractive current features;5) upward-fining normal grading with erosive top;and 6) relatively abundant in trace elements (B,Cr,V,and etc).While flowing westward roughly parallel with the slope,the contour current in the study area could have helped to improve reservoir properties of the reworked sands through winnowing,transporting,reworking and re-deposition of the southeastward running turbidity sediments down the slope.Both turbidity sediments and coarse-grained contour current sediments could serve as good oil and gas reservoirs.At the same time,fine-grained deep water autochthonous sediments and contour current sediments could be quality source rocks.It is very likely for the reworked sands,contour current sediments and autochthonous sediments to combine with one another to form ideal hydrocarbon generation-accumulation-sealing assemblages in the study area.

Key words: turbidity, contour current, reworked sand, Pingliang Formation, Ordovician, Erdos Basin

中图分类号:

TE121.2

李华,何幼斌. 鄂尔多斯盆地西南缘奥陶系平凉组改造砂沉积特征及意义[J]. 石油与天然气地质, 2018, 39(2): 384-397. doi: 10.11743/ogg20180217 Li Hua,He Youbin. Sedimentary characteristics and significance of reworked sands in the Ordovician Pingliang Formation,southwestern margin of Ordos Basin[J]. Oil & Gas Geology, 2018, 39(2): 384-397. doi: 10.11743/ogg20180217

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