靖边潜台西侧奥陶系马五4亚段岩相古地理特征

鄂尔多斯盆地中奥陶统马五₄亚段是靖边气田重要的天然气勘探层段，目前相关的岩相古地理图多以段和亚段为单位、以盆地为尺度，作图精度不能满足油气勘探开发的需要。以靖边潜台西侧为研究区，根据该区的古地貌特征、标志性矿物（硬）石膏的类型及其环境意义，分马五₄3、马五₄2和马五₄1三个小层开展岩相古地理工作。根据沉积背景和地层等厚图将该区中部厚度较大、地形略陡的区域解释为洼地，将洼地周边厚度较小、地形平缓的区域解释为坪。目的层段（硬）石膏有块状、球状结核和晶体等三种类型，块状硬石膏与暗色泥质藻纹层白云岩互层，代表了一种浅水水下蒸发、间或遭海水漫侵的潮上环境；球状硬石膏结核分散于浅黄色泥粉晶白云岩中，为准同生成因，代表了蒸发、偏氧化、变盐度的潮上环境；石膏晶体相对较少，多为柱状，或与球状硬石膏结核混生，代表的沉积环境与球状硬石膏结核基本相同。块状硬石膏主要分布于洼地中，为潮上带硬石膏洼地，根据硬石膏的含量进一步细分为含块状硬石膏白云岩洼地、块状硬石膏质白云岩洼地和白云质块状硬石膏洼地；球状硬石膏结核及石膏晶体主要分布于古地貌的坪中，结合其局限蒸发潮上带的背景，将其命名为球状硬石膏结核白云岩潮坪。马五₄3、马五₄2和马五₄1的岩相古地理格局基本一致，但从下向上，硬石膏洼地范围逐渐收缩，块状硬石膏的含量也逐渐下降，反映了沉积过程中水体逐渐蒸发变浅的过程。

Lithofacies Paleogeography Research on Ordovician Majiagou 54 Sub-member in the West Jingbian Platform

The fourth sub-member, the fifth Member, Majiagou Formation (hereinafter referred to as Ma54) of Middle Ordovician is the important gas exploration layer in Ordos Basin, but many lithofacies paleogeographic maps about this layer are based on member or sub-member unit and is with a basin scale, and their resolution is far away from the requirement of gas exploration and development.In this paper, according to paleogeomorphology, anhydrite or gypsum type and its paleoenvironment significance, lithofacies paleogeography will be mapped in the Ma54 divided into Ma534, Ma524 and Ma514 in west Jingbian platform, which fills the gap of lacking high-resolution maps.On the base of sedimentary setting and stratum thickness contour diagram, the thick and slightly steep area in the center is interpreted as depression, and the thin and gentle area around the depression is interpreted as flat.There are massive anhydrite, spherical anhydrite concretion and gypsum crystal in Ma45.Massive anhydrite interbeded with dark muddy algae laminar dolomite represents a shallow and subaqueous evaporative environment which is located in supratidal zone and is sometimes flooded by tide water.Spherical anhydrite concretions, which are dispersed in light yellow micritic and slit-sized crystal dolostone, are formed during penecontemporaneous stage and indicate an evaporative and oxidizing environment with variational salinity.Comparison with the two types of anhydrite, gypsum crystals are not very common, and most of them show a columnar shape.Some crystals associate with spherical anhydrite concretion, so their environment is the same as spherical anhydrite concretion's.Massive anhydrite is distributed in the depression, so the lithofacies paleogeographic name is called anhydrite depression.According to the content of massive anhydrite, anhydrite depression is divided into massive anhydritic dolomite depression, massive anhydrite dolomite depression and dolomitic massive anhydrite depression.Spherical anhydrite concretion and gypsum crystal are mainly distributed in flat.Combined their distribution and sedimentary setting of restricted evaporative supratidal zone, they are called spherical anhydrite concretion dolomite tidal flat.The basic lithofacies paleogeographies of Ma534, Ma524 and Ma514 are almost the same, but from Ma534 to Ma524 and then Ma514, the anhydrite depression becomes smaller and smaller, and the content of massive anhydrite decreases gradually, which shows the process that supratidal evaporative water becomes shallower and shallower.This paper makes some relative high-resolution lithofacies paleogeographic maps, which is very helpful to the gas exploration and development in Lower Paleozoic in Ordos Basin.