开鲁盆地陆西凹陷下白垩统层序地层学和油气评价

Lower Cretaceous Sequence Stratigraphy and Hydrocarbon Evaluation of Luxi Depression in Kailu Basin

开鲁盆地陆西凹陷是一个典型的中生代单断箕状盆地 ,充填了厚达 30 0 0余米的下白垩统并表现出良好的沉积旋回特征。依据钻井取心资料、测井和地震资料以及分析化验资料 ,将下白垩统划分成周期为 4～ 6Ma的 3个沉积层序并总结了层序边界和体系域边界的识别标志。考虑箕状盆地结构、构造活动特征以及沉积物供源方式 ,分别论述了盆地陡坡、深洼和缓坡的层序地层样式 ,止出不同地形区的层序地层叠置样式主控因素是有差异的。依据盆地缓坡地形变化带 ,可以将一个沉积层序细分成低位、湖侵和高位体系域。研究区层序Ⅲ高位和低位体系域富含油气资源 。

Luxi depression is a typical Mesozoic faulted down basin with more than 3 000 m lower Cretaceous in thickness. The fillings of lower Cretaceous were deposited in semi saltwater lacustrine and were characterized by clastic cycles in vertical profile. According to the characteristics of sequence boundary and flooding surface from core, well logging, seismic data and results of example analysis in laboratory, the lower Cretaceous in the study area could be divided into three depositional sequences with period 4～6 Ma. There was a special topographic break on gentle slope in Luxi depression during Cretaceous; so, a sequence could be made up of lowstand, transgressive and highstand systems tracts like Vail's marine sequence model. Because of the difference of basin texture, fault activity and material supply, the main controlled factors of sequence style were different in steep slope, deep water area and gentle slope. The major controlled factor of sequence style on steep slope was fault activity and material supply, and the nearshore subaqueous fan was main sedimentary type; the factor controlled sequence style in deep water area was relative change of lake level, the sedimentary facies was characteristic of shallow deep lake; the factor controlled sequence style on gentle slope was relative change of lake level and material supply, fan delta or river dominated delta was major sedimentary type. Therefore, the sequence models were established separately on steep slope, deep water area and gentle slope. A sequence was made up of lowstand, transgressive and highstand systems tracts with topographic break on gentle slope of Luxi depression. Lastly, authors analyzed the condition of hydrocarbon accumulation in different sequence framework, and pointed out that the lowstand and highstand systems tracts of sequence Ⅲ were rich in hydrocarbon resource, and were the main section for finding new hidden traps.