楚雄盆地下侏罗统磨拉石楔沉积与含油性研究

EARLY JURASSIC MOLASSE WEDGE SEDIMENTS AND OIL POTENTIALS IN THE CHUXIONG BASIN,YUNNAN

关键词:  磨拉石楔  沉积相  下侏罗统  楚雄盆地

Key words:  molasse wedge  sedimentary facies  Lower Jurassic  Chuxiong Basin

摘要:

楚雄盆地为一中生代周缘型走滑前陆盆地。通过对下侏罗统冯家河组沉积特征的详细研究与对比,作者认为早侏罗世是楚雄前陆盆地强烈坳陷与沉降的主要发展时期,从早期到晚期经历了一个突然变深又变浅的典型前陆盆地充填过程。在前陆冲断块体的不断逆冲和加载条件下,楚雄盆地西部早侏罗世沉积物的来源主要为哀牢山造山带隆起,其沉积环境以滨、浅湖或湖泊三角洲为主。位于盆地东北部的永仁中和、大姚龙街等地区的早侏罗世沉积物则来源于元谋古隆起,沉积特征亦显示其为滨、浅湖或湖泊三角洲相。位于盆地中部的楚雄会基关地区(楚参一井),下侏罗统沉积厚度急剧增加,测井曲线、录井资料以及岩心薄片鉴定、粒度分析均显示其沉积环境为深湖央浊积砂体。在新平叠社莫一带也发现有深湖相泥灰岩或灰泥岩沉积。由此表明,早侏罗世楚雄前陆盆地的沉降与沉积中心已经从晚三叠世的古哀牢山前缘向NE迁移至楚雄会基关以及新平叠社莫一带,盆地内的沉积物呈明显的前陆楔形体充填。大姚龙街地区冯家河组第三段中首先发现的沥青砂岩进一步增强了楚雄盆地产油气的可能性。根据产状,沥青砂岩分为缝隙型和粒间孔型两类,它们同产于逆韵律结构发育的湖泊三角洲相沉积层序。因此,对该区下侏罗统缝隙型圈闭以及岩性圈闭构造应予以足够的重视。

Abstract:

The Chuxiong Basin is a Mesozoic strike-slip peripheral foreland basin.The authors contend,in the light of the detailed study and comparison of the sediments from the Lower Jurassic Fengjiahe Formation,that the Early Jurassic is an important stage of intense downwarping and subsidence of the basin,spaning from early to late stages the foreland basin filling processes of suddenly deepening and then shallowing.Because of the continue thrust loading of the foreland thrust blocks,the Early Jurassic sediments in the western part of the Chuxiong Basin were derived from the Ailaoshan uplift in the southwest,and their sedimentary environments are dominated by littoral and shallow lakes or lacustrine deltas;those in Zhonghe of Yongren and Longjie of Dayao in the northeastern part of the basin originated from the Yuanmou uplift,and were deposited in a similar sedimentary en-vironment,and those in the Huijiguan region,Chuxiong in the central part of the basin rapidly increase in sediment thickness and lay down in the deep-lake sedimentary environ-ments with turbidite sandstone bodies on the basis of well logs,thin sections and grain-size analysis.The discovery of the deep-lake marls or lime mudstones in the Dieshemo zone, Xinping has revealed that the subsidence and depositional centres of the Early Jurassic Chuxiong Basin were migrated during the Late Triassic from the frontier of the Ailao Mountains northeastwards to the Huijiguan-Dieshemo zone,where the sediments were in-filled in the form of foreland wedges.Furthermore,the first discovery of asphaltic sand-stones in the third member of the Fengjiahe Formation in the Longjie region,Dayao has al-so disclosed the oil potential in the Chuxiong Basin.The asphaltic sandstones may be clas-sified into two types in terms of occurrence:the crack and pore-space filling type and inter-granular pore-space filling type,all of which occur in the lake-delta sequences where the reversed rhythmic stratifications are well-developed.It follows that care should be taken to distinguish the Lower Jurassic crack and pore-space filling type traps from the lithologic traps in the study area.