鄂尔多斯盆地中奥陶统马家沟组沉积环境模式

中奥陶统马家沟组是鄂尔多斯盆地的主要天然气储集层之一。奥陶纪由于西边贺兰裂谷和南面秦岭裂谷发生扩张裂离,在盆地西缘和南缘产生裂谷肩翘升,形成 L 形隆起带。在均衡补偿作用下,隆起带东侧伴生一西缓东陡不对称的(内)陆架盆地。研究建立的东西向穿越(内)陆架盆地中心的沉积模式表明:当高海平面时期,L形隆起带沉没于海平面之下,(内)陆架盆地海水环境正常,气候湿润,石灰岩沉积遍及整个盆地;包括隆起带上,仅(内)陆架盆地风暴浪基面之下中心位置有少量灰质白云岩和白云岩沉积。低海平面时期,气候干旱,L 形隆起带接近或有时出露海平面之上,(内)陆架盆地海水补给主要来自东方,(内)陆架盆地处于半局限和局限环境,盆地中心沉积了硬石膏岩和白云岩,盆缘硬石膏岩白云岩坪为白云岩沉积区。极低海平面时期,气候极为干旱,L 形隆起带成为剥蚀区,(内)陆架盆地仅能从东面获得少量已浓缩的海水补给,盆地海水中 CaSO_4和 NaC1高度浓缩,盆地中心沉积石盐岩,盆缘白云岩硬石膏岩坪则沉积硬石膏岩,夹石盐岩和白云岩。L 形隆起带以西毗邻贺兰海槽陡斜坡带,发育各类重力流碳酸盐岩沉积环境,海槽内沉积碳酸盐和硅质碎屑混积型浊积岩。L 形隆起带以南,即渭北隆起地区,整个马家沟期发育为末端变陡的缓坡沉积环境。

Depositional Environment Model of Middle Ordovician Majiagou Formation in Ordos Basin

Ordos basin,which is located in the west of North China Plat- form,covers an area of about 250,000km~2.Helan Trough and Qinling Trough are respectively to the west and the south of the basin.The Middle Ordovician Majiagou Formation is one of the most important natural gas reservoirs in the basin.Due to spreading of Helan and Qinling rift valleys during the Ordovician period,a L-shaped swell belt formed in the western and south- ern parts of the basin.At the same time,an unsymmetrical compensation basin(or depression),named inner shelf basin, generated in the eastern part of the basin as a result of equilib- rium activity.A depositional environment model that extends from the east to the west and passes through the center of in- ner shelf basin is proposed as follow.During the highstand, seawater encroached into the inner shelf basin came from not only the east but also from the west.Owing to moist climate, the seawater cycle was normal.Limestone deposited in the whole inner shelf basin, including the swell belt,excepting a small mass of dolostone or dolomitic limestone could deposit in the center of basin. At the Lowstand,L-shape swell belt reached closely to the sea level or above it.Seawater supply- ing to inner shelf basin came from the east,occasionally little quantity of sea water flew across the swell belt into the basin. Dry climate ted to seawater concentration, and the deposition- al environment turned into restricted. Anhydrock mainly de- posited in the center of the basin,and the area of dolostone deposition was in the basin marginal flat.At the Lowestand, the swell belt was above the sea level and became the ero- sional area.Only a little quantity of sea water came from the east, dryer climate led to a great deal of CaSO_4 and NaCl con- centration.Halilyte deposited in the center of inner shelf basin and anhydrock deposited in the basin marginal flat.On the steep slope of Helan Trough that lies on the west of L-shape swell belt,carbonate gravity flow environment developed,the deposits include rockfall,slump,slid,debris flow and tur- bidite.However turbidity rock containing carbonate debris/sili- con debris mixture developed in submarine trough.On the south of L-shape swell belt,a tip-steep carbonate ramp devel- ops,in which the depositional system is quite different from those mentioned above.