地震反射特征对风化壳发育带的指示意义:以东营凹陷永北地区沙三上亚段为例

东营凹陷永北地区T1不整合面之下风化壳发育带的的预测是地层油藏勘探的重点与难点。综合利用高精度三维地震解释、岩心观察、薄片鉴定、测录井分析及物性分析等方法,结合构造发育史、埋藏史等研究成果,对东营凹陷永北地区T1不整合面下沙三上亚段地层中特殊的地震反射特征进行研究,并对其地质意义进行了探讨。研究表明:T1不整合面下发育典型埋藏风化壳,包括风化粘土层和半风化淋滤带2层结构;在大气淡水风化、淋滤作用下,半风化淋滤带内部地震同相轴消失,表现为空白反射特征;半风化淋滤带物性得到改善,与未风化岩石之间形成一个物性差异界面,表现为不连续的角度不整合面;未受明显风化、淋滤作用的地层,则表现为典型的角度不整合;地震反射特征的演化过程可划分4个阶段:沙三上沉积后阶段、沙三上抬升后阶段、风化壳形成早期阶段、风化壳形成阶段。地震空白反射带的发育范围是风化壳发育带的保守范围或最小范围,向湖盆方向逐渐过渡为正常地层。风化壳的发育大大改善了原岩的储集性能,可作为良好的油气储集空间和运移通道,是下步地层油气藏勘探的重点目标。

Investigation of Weathered Clastic Crust Based on Its Seismic Reflection Characteristics: the upper Layer of Es3 in Yongbei Area of Dongying Sag

Prediction of the developing zone of weathered clastic crust under the T1 unconformity is one of the key and critical issues in the exploration of stratigraphic reservoirs in the Yongbei area, Dongying sag. Based on integrated analyses of high-precision 3D seismic interpretation, core observation, thin section observation, logging interpretation and physical property testing, combined with the setting of structural and burial histories, we investigate the special characteristics of seismic reflection of the upper layer of the Es3 Formation under the T1 unconformity surface in Yongbei area, Dongying sag, and further address their geological implications. Results clay rocks and semi-weathered rocks. In the semi-weathered rocks, seismic event disappears to show blank reflection due to meteoric weathering and solution. This results in an improvement of reservoir properties and an interface with underlying unweathered rocks. A discontinuous angular unconformity surface was formed. In contrast, the unweathered and undissolved strata shows typical angular unconformities. The evolution of seismic reflection characteristics could be divided into 4 phases, e.g., deposition of the Es3 Formation, uplifting of the Es3 formation, early stage of the formation of the weathered clastic crust, and the main formation stage of the weathered clastic crust. The development of the seismic blank zone is not smaller than that of the weathered clastic crust. It changes to normal strata to the basinward. Development of weathered clastic crust improves reservoir properties largely, which could act as resrevoir space and migration path for oil and gas. Therefore, such crusts are important exploration targets of stratigraphic reservoirs in the study area for the next step.