特超低渗砂岩油藏储层非均质性特征与成藏模式--以鄂尔多斯盆地西部延长组下组合为例

Heterogeneity characteristics and accumulation model of ultra-low permeability sandstone reservoirs:a case study of the lower part of Yanchang Formation in the western Ordos Basin,China

针对陆相碎屑岩特超低渗储层油气运聚成藏研究中存在的问题,选择鄂尔多斯盆地西部延长组下组合砂岩油藏储层为研究对象,分析其沉积结构构造、成岩作用与致密化过程的非均质性,认识储集空间的非均质性分布规律与石油充注特征。研究发现,河湖相特超低渗含油储层表现出较强的非均质性,含油砂岩与不含油砂岩交互成层,其中不含油砂岩可分为3类:钙质砂岩、富软颗粒砂岩和含水砂岩。其成岩过程迥然,其中钙质砂岩和富软颗粒砂岩的发育多与中—低级构型界面有关。含油砂岩中可识别出3期沥青,分别对应于晚侏罗世、早白垩世中期、早白垩世晚期的石油充注。以这些沥青为时间标志可将含油砂岩中的成岩过程划分为3期,并在含水砂岩中识别出对应的3期成岩过程。在钙质胶结和富软颗粒致密砂岩中,成岩作用主要发生在第1充注之前,其构成了储层内的隔夹层,并且往往形成不同层次、不完全封隔的似网状结构,影响甚至控制了后期的流体活动,引起后期成岩作用和油气充注过程的非均匀性。在储层非均质性及沉积构造分析、成岩过程及其与油气充注关系研究、古物性恢复的基础上,提出了结构非均质性特超低渗砂岩油藏成藏模式。

To address problems in studies of hydrocarbon migration and accumulation in ultra-low permeability reservoirs of terrestrial clastic rock,this paper studies sandstone oil reservoir in the lower part of Yanchang Formation in the western Ordos Basin by analyzing heterogeneity in structure,diagenetic and tightening sequence of its sedimentary architecture,thus clarifying the heterogeneity distribution pattern and crude oil charging characteristics of reservoir space.This study reveals that fluviolacustrine ultra-low permeability petroliferous reservoir exhibits strong heterogeneity;petroliferous sandstone is interbedded with non-petroliferous sandstone,and non-petroliferous sandstone can be divided into three types,i.e.,calcareous sandstone,ductile grain-rich sandstone,and waterbearing sandstone.They differ dramatically in diagenetic sequence,where the formation of calcareous sandstone and ductile grainrich sandstone are mostly related to medium-low grade architecture boundaries.In oil-bearing sandstone,three phases of bitumen can be identified,corresponding to oil charging in Late Jurassic,middle Early Cretaceous,and late Early Cretaceous strata,respectively.With such bitumen as time markers,diagenetic of oil-bearing sandstone can be divided into three phases,and correspondingly these three diagenetic phases have been identified in water-bearing sandstone.In tight sandstone with calcareous cements and ductile grain-rich rock fragments,diagenetic mainly occurs prior to the first phase of oil charging,thus forming barriers and interbeds within the reservoir,and different layers of incompletely insulated quasi-network structures;it even has controlling influences on the post fluid movement,leading to heterogeneity during telodiagenesis and hydrocarbon charging.Based on analyzing reservoir heterogeneity and sedimentary structure,diagenetic sequence and its relationship with hydrocarbon charging,and paleophysical property restoration,this paper proposes a model for oil accumulation in structurally heterogeneous ultra-low permeability sandstone.