中扬子区龙马溪组浊积岩沉积特征及地质意义

Sedimentary characteristics and geologic significance of turbidite of Longmaxi Formation in middle Yangtze region

为厘清中扬子区龙马溪组中部砂岩的沉积特征及其与油气的关系,通过野外地质调查和钻井资料分析,结合岩石薄片鉴定和分析化验资料,分析砂岩的沉积特征、测井响应特征和成因模式。结果表明,该砂岩位于龙马溪组下部富有机质页岩之上,以岩屑长石石英砂岩为主,具有典型浊流沉积的粒度特征;且具典型的鲍马序列,可见块状层理、水平纹理、包卷层理、递变层理和波状层理等沉积构造;在测井曲线上呈现明显的齿化现象。这些证据均表明龙马溪组中部砂岩具有典型的浊流特征。受区域构造活动和古地理背景的影响,浊积砂岩呈现出薄层大扇特征,从秀山凹槽往中上扬子盆地推进铺开,可识别出浊积内扇、浊积中扇和浊积外扇等3个亚相。该浊积扇既可作为下部页岩气封存的顶板,也可作为有利的致密砂岩储集体,其分布区可以作为下一步的重点综合勘探区域。

In order to discuss the sedimentary characteristics of turbidites in the middle section of the Early Silurian Longmaxi Formation in western Hubei to eastern Chongqing area and its relationship with oil and gas,based on outcrop observation and drilling data analysis,combined with slice identification and laboratory data analysis,the lithologic association, vertical sequence,granularity distribution,sedimentary structure and logging response characteristics of turbidites were analyzed,while the plane distribution rule and genetic model of turbidites were revealed. The results show that the turbidite sandstone developed above the organic-rich shale of Longmaxi Formation. The turbidites are mainly lithic feldspar quartz sandstone, with typical characteristics of turbidite deposition.The turbidites are characterized by massive bedding,horizontal grain,convolute bedding,graded bedding,wave bedding,and has a typical Bauma sequence. In addition,the log presentation shows jugged shape. Therefore,the evidences show that Longmaxi sandstone has the typical characteristics of turbidity. Influenced by regional tectonic activity and paleogeographic background,The turbidite sandstone shows a fan-shaped distribu-tion characteristics of thinning and increasing extent from southeast to northwest,with a massive debris and other material flowing through Xiushan to the grooves within the craton and spreading inside the Yangtze platform. The turbidite fan was divided into three subfacies including inner fan,middle fan and outer fan in the direction of fan propulsion. The turbidite sandstone in the study area has good petroleum geological conditions,which can be used as the roof of the lower shale gas storage and advantageous compact sandstone collective. The turbidite sandstone distribution area becomes the important oil-gas exploration target.