伊拉克HF油田Mishrif组碳酸盐岩米级旋回的识别及意义

Identification and significance of meter-scale cycle of carbonate rocks in Mishrif Formation,HF Oilfield, Iraq

伊拉克HF油田上白垩统Mishrif组发育于新特提斯洋被动陆缘的碳酸盐缓坡,沉积相受海平面变化的影响尚不明确。通过对岩心、薄片的观察和分析,对HF油田Mishrif组中的储层发育段MB1-2层(厚度约100 m)的岩性及沉积微相特征进行了研究。结果表明:MB1-2层段发育了中缓坡生物碎屑滩与内缓坡泥晶灰岩交互相沉积,岩性变化频繁,具有向上沉积水体变浅、颗粒变粗、岩层变厚的特征,表现为较典型的潮下型米级旋回。应用米兰科维奇周期原理,根据MB1-2层的单层厚度(0.1～4.8 m),通过Matlab软件计算,将自然伽马曲线转换到频率域以捕获米氏旋回信息,得出旋回周期为2～48 ka,并推断研究区MB1-2层的交互式沉积环境受岁差旋回与黄赤交角旋回相互作用所控制。

The Upper Cretaceous Mishrif Formation in the HF Oilfield of Iraq developed in the carbonate gentle slope of the passive continental margin of the Neo-Tethys Ocean. It is still not clear that the sedimentary facies are affected by sea-level change. Based on the observation and analysis of cores and thin slices, the lithology and sedimentary microfacies characteristics of MB1-2 layer in the Mishrif Formation of HF Oilfield are studied. The results show that the interaction between bioclastic limestone on the middle gentle slope and micritic limestone on the inner gentle slope are developed in MB1-2 layer with a thickness of about 100 m, and the lithology changes frequently. This layer is characterized with shallowing water body, coarsening grains and thickening strata upward, which is recognized as a typical subtidal meter-scale cycle. Based on the Milankovitch cycle principle and the thickness of single layer of 0.1~0.8 m, the natural gamma ray log curve is converted to the frequency domain to capture the information of meter-scale cycle by using Matlab software. It is calculated that the cycle period is 2~48 ka, and it is inferred that the interactive sedimentary environment of MB1-2 layer in the study area is controlled by the precession cycle and the obliquity of ecliptic cycle.