湖泊三角洲前缘砂体内部构成及不均一性露头研究

AN OUTCROP STUDY ON INTERNAL ARCHITECTURE AND HETEROGENEITY OF LACUSTRINE DELTA-FRONT SANDBODIES

本文论证了河口坝和水下分流河道砂体的内部构成是产生储层砂体不均一性的根本原因,成岩作用(富铁钙质胶结作用)使沉积不均一性更加复杂化,河口坝砂体和水下分流河道砂体是三角洲前缘的主要组成部分,口坝砂体中沉积事件间的不连续面和沉积构造产生了垂向不均一性,水下分流河道砂体的基本构成单位是河道单元,河道单元被一系列底冲刷面或内冲刷面(第三/第四级界面或第五级界面)区分开,储层岩性相又是河道单元的基本构成单位,在河道单元中,沉积构造、粒度、填隙物和分选性等参数的不均一性变化取决于古水流强度的变化,砂体内部构成和不均一性严格地控制着储层内部高孔隙度、渗透率和流动单元的分布。

The research on the internal architecture of reservoirs is the key to the oil and gas fields development in inland lake basins. A study on modern or ancient outcrops is helpful for us to comprehensively understand the similar underground reservoir constructions. In northeast margin of Ordos Basin, 2 - D or 3 - D outcrop sections of lacustrine delta system are preserved well in the Yan'an Formation of Middle Jurassic. This paper suggests that the internal architectures of the mouth bar and subaqueous distributary channel sandbodies , the main elements of delta front, make the reservoirs heterogeneous , which is complicated by diagene-sis (calcareous cementation rich in Fe). The mouth bar sandbody occurs in sheetlike ,and was formed by several depositional events whose uncontinuous surface as well as the sedimentary structures result in the vertical heterogeneity in the mouth bar sandbodies. The channel u-nits, whose basic architecture element is reservoir lithofacies , separated by bottom scour surface and/or inner scour surfaces (third/fourth or fifth-order boundary) are basic architecture elements of subaqueous distributary channel sandbody. The heterogeneity variations of a sedimentary structure; grain size, grain-to-grain filler and sort etc. within channel unit are con- trolled by paleocurrent energy changes , and the internal architecture and heterogeneity of sandbodies controlled the porosity, permeability and the distribution of fluid-flow unit in reservoirs.