利用能量半衰属性识别涠西南凹陷砂泥薄互层组合中浊积砂体

Recognizing turbidite sand bodies in thinly interbedded sand-shale reservoir in the Weixinan Depression by energy half-time attributes

涠西南凹陷是我国南海北部湾盆地重要的油气聚集地,浊积砂体是其重要的储量贡献地质体,显示了巨大的油气勘探潜力。随着勘探开发力度的不断推进,在砂、泥岩地震特征叠置严重的背景下,常规的地球物理技术手段难以识别真正的浊积储层。为此,基于浊积砂体强振幅反射形成的地质背景,剖析了非储层形成异常强振幅的两个主要成因:砂、泥岩薄互层组合容易形成强振幅反射;甜点储层与围岩响应差异小,低速泥岩强振幅响应干扰严重。因此提出了应用能量半衰时属性识别浊积储层的新技术:(1)确定研究区的等时地层反射界面,建立精细的等时地层格架,将五级层序地层作为一套等效体研究,并基于砂岩、泥岩密度比较稳定确立了异常体内部的振幅微变化主要体现在速度结构的变化上,即速度结构变化是识别真假储层的关键;(2)建立了砂岩、泥岩互层的楔状模型,论证了不同岩性组合对应不同的速度结构,介绍了不同速度结构引起能量半衰时属性值变化的情况,有效描述了砂泥薄互层内的速度结构变化;(3)基于研究区速度的精细分析,按照“同一岩性,同一物性”“同一岩性,不同物性”“不同岩性,不同物性”3种组合情况正演模拟,解读异常体振幅微变化的信息,由此判断引发阻抗变化的主因,综合预测异常体可能的岩性组合,极大地降低了岩性预测的多解性。结论认为:该技术运用于实例勘探中,岩性预测结果与钻井揭示相吻合,有力支撑了涠西南凹陷的勘探生产实践。

The Weixinan Depression is an important oil and gas accumulation area in the Beibu Gulf Basin of the South China Sea,and turbidite sand body is an important reservoir contributing geological body showing great oil and gas exploration potential.With the exploration and production constantly advancing,it is difficult to identify a true turbid reservoir by conventional geophysical techniques due to severe overlapping of sandstone and mudstone's seismic characteristics.For this reason,considering the association of strong amplitude reflection with formation of turbidite sand body,we analyzed the two main causes of the abnormal strong amplitude reflection in the non-reservoir formation,i.e.,easy production of strong amplitude reflection by interbedding of thin sandstone and mudstone,and severe interference in strong amplitude response of low velocity mudstone from indistinguishable responses of sweet reservoir and surrounding rock.On this basis,we put forward a new technology for reservoir identification based on energy half-time.Firstly,after the isochronous stratigraphic reflection interface is determined,and a fine isochronous stratigraphic framework is established,the fifth-order sequence is examined as a set of equivalent bodies.Because the densities of sandstone and mudstone are relatively stable,it is established that the slight amplitude variation inside the abnormal body mainly reflects the change of velocity structure.That is,the change of velocity structure is key to distinguish true reservoirs from false ones.Secondly,a wedge-shaped model of sandstone and mudstone interbedding is established,which demonstrates that different lithologic compositions correspond to different velocity structures,and different velocity structures cause the change of the attribute value when the energy half-time is changed.That is,the change of velocity structure in thin sand-mud interbedding can be effectively described by the property of energy half-time.Thirdly,based on the fine velocity analysis of the study area,slight amplitude variation inside the anomaly body can be interpreted by forward modeling according to three combination schemes:same lithology and physical property,same lithology but different physical properties,and different lithologies and physical properties.Hence,the main cause of the impedance change can be determined,and the possible composite lithology of the abnormal body can be predicted synthetically to reduce prediction ambiguity.Applying this technology to the case exploration,the results of lithology prediction agrees with the drilling findings,which strongly supports the exploration and production practice in the Weixinan Depression.