鄂尔多斯盆地西部奥陶系风化壳岩溶作用模式

鄂尔多斯盆地下古生界马家沟组顶部遭受长期风化剥蚀，形成了以膏云岩为主的风化壳岩溶储层。通过偏光显微和超微扫描电镜深入观察了（含）膏云岩的孔隙类型及充填规律，分析了其岩溶作用特点，研究了以膏云岩为主的岩溶作用特征与传统碳酸盐岩岩溶作用特征的区别，探讨了膏云岩发育区的岩溶作用模式。结果表明:硬石膏结核和石膏晶体以其强亲水性和远高于白云石、方解石的溶解度，极易率先吸水发生溶解形成组构选择性溶蚀孔隙，导致膏云岩层蜂窝状溶孔的形成。硬石膏的高溶解度和力学不稳定性使得研究区以膏云岩、白云岩和灰云岩互层的风化壳储层以膏模孔、扩溶膏模孔及与之伴生的胀缩微裂缝为主要储集空间，孔隙大小具有明显的自限性。岩性—（含）膏云岩和沉积微相—海平面低位期潮上带（含）膏云坪沉积是储层形成的先天物质基础和环境条件，并因此直接导致（含）膏云岩储层的成层分布特征。表层膏云岩首先遭受淡水淋滤形成蜂窝状溶孔，多层成层分布膏云岩这种独特的物理化学性质使风化壳内的岩溶水以弥散性渗透为主，这是膏云岩发育区岩溶作用特征和模式与碳酸盐岩产生重要差异的根本原因。

Model of Weathering Crust Karstification in the Ordovician,Western Ordos Basin

The top of the Majiagou Formation in the Lower Paleozoic of the Ordos Basin experienced long-term weathering and denudation and formed a weathered crust karst reservoir dominated by gypsodolomite. In this study， the pore type and filling behavior of the gypsum dolomite were examined， and the karstification properties of the gypsodolomite were analyzed. The difference between the karstification characteristics of gypsodolomite and traditional carbonate karstification was studied， and a karstification model of the gypsum dolomite development area is discussed. Due to their strong hydrophilicity and solubility higher than either dolomite and calcite， anhydrite nodules and gypsum crystals readily absorb water， then dissolve to form selective pores in the fabric. This leads to the formation of honeycomb dissolution pores in the gypsodolomite layers. Due to the high solubility and mechanical instability of anhydrite， the weathered crust reservoir space led to interbedded gypsodolomite， dolomite and calcite dolomite in the study area， consisting mainly of dissolution pores in the anhydrite nodules and crystals together with associated expansion microcracks. The pore sizes are self-limiting. The lithology of gypsodolomite and sedimentary facies-anhydrite in a tidal flat of the upper tidal zone were the essential materials and environmental conditions for the formation of the reservoirs， and thus led directly to the strata-controlled distribution of gypsodolomite gas reservoirs. The surface gypsodolomite was firstly leached by fresh water to form honeycombed pores. Due to the unique physical and chemical properties of gypsodolomite， the karst water in the weathering crust was mainly permeated by diffusion.Karstification in the gypsodolomite development area was due to the dissolution of gypsodolomite by diffusion permeation， which is the fundamental reason why the karstification properties and model differ from those for carbonate rocks.