利用铀的测井响应恢复鄂尔多斯盆地古水深

RESTORING PALAEO-DEPTH OF THE ORDOS BASIN BY USING URANIUM RESPONSE FROM GR LOGGING

沉积时期的古水深分析是沉积相研究的基础。以鄂尔多斯盆地孟坝地区延长组为例,利用自然伽马能谱曲线获得的钍(Th)、铀(U)曲线,根据Th/U比值与氧化—还原条件的关系、氧化—还原条件与水深的关系,探讨了用Th/U比值恢复古水深相对变化的可能性。研究表明:①Th/U比值的大小与古水深具有密切关系,即Th/U比值小趋向于还原环境,沉积时期水体相对较深;Th/U比值大则趋向于氧化环境,沉积时期水体浅。②Th/U比值曲线可以近似地看作是古水深相对变化曲线,能够反映出古水深相对变化的旋回性。分析认为,研究区延长组沉积时期水深周期性变化非常频繁,每个油层组都可以划分出多个水体突然加深再逐渐变浅的旋回,这是造成砂层较薄、砂泥岩频繁交互沉积的主要原因,同时也说明盆地的沉降具有振荡性。

Palaeo-depth analysis in sedimentary period is the base of study on sedimentary facies.Taking Yanchang Formation in Mengba area,the Ordos Basin as an example,making use of Th/U curves by GR spectral log curves and in light of the relationship between Th/U ratio and oxidation/reduction condition and/or relationship between oxidation/reduction condition and water depth,this article discusses the possibility to restore palaeo-depth by Th/U ratio.It is shown by research as following:①the Th/U ratio is closely related to palaeo-depth,namely,if the value of the Th/U ratio is smaller,it is closer to oxygen free environment and water is deeper in sedimentary period;if the value is larger,it is closer to well oxygenated environment and water is relatively shallower in sedimentary period;②the Th/U ratio curve can be approximately regarded as the relate change curve of palaeo-depth which can reveal the cycling of palaeo-depth relate change;it is also regarded that water depth has frequent changes in a circle in Yanchang formation in sedimentary period and each oil layer can be divided into several cycles that water turns from sharp deep to increasing shallow,which is the main cause to thinner sand and alternating deposit between sand and mud and can show that basin deposition is characterized by oscillation.