西湖凹陷平湖组物源分析

Provenance analysis of Pinghu Formation in Xihu sag

西湖凹陷平湖组是否存在东侧物源存在争议,东侧物源的确定对油气勘探意义重大。综合利用钻井、地震、分析化验等资料,分析西湖凹陷平湖组沉积物的重矿物ZTR指数、砂岩类型、岩屑组分、砂岩百分含量以及地震相、同沉积构造特征等,发现平湖组沉积物的陆源重矿物ZTR指数及砂岩成分成熟度呈"凹陷两侧低、中间高"的特征,自生重矿物菱铁矿及砂岩百分含量则呈"凹陷两侧高、中间低"分布;由地震反射特征可知,平湖组存在明显的盆地边缘杂乱反射及指示东侧物源存在的前积反射构型;而且平湖组中晚期生长背斜的存在,从构造应力方面进一步印证了平湖组沉积时期东部隆褶带的存在。综合以上研究,认为东部隆褶带在平湖组沉积期就已经隆起并作为西湖凹陷的东侧物源,西湖凹陷中东部深层平湖组具有良好的油气勘探前景。

It is disputable about whether the eastern provenance exists for the Pinghu Formation in the Xihu sag. Its clear answer will be significant for hydrocarbon exploration. Based on drilling, seismic, and test data, the Pinghu Formation sediments were analyzed in respect of ZTR index of heavy minerals, sandstone types, debris composition, sandstone content, seismic facies and syndepositional structural features. It is fotmd that the ZTR index of terrigenous heavy minerals and the maturity of sandstone components in the Pinghu Formation are ＂low at both sides of the sag and high in the middle＂, while the contents （or percentages） of siderite （an authigenic heavy mineral） and sandstone are ＂high at both sides of the sag and low in the middle＂. According to seismic reflection features, the Pinghu Formation has apparent chaotic reflections at basin margin and prograding reflection configuration that indicates the existence of the eastern provenance. Moreover, the existence of growth anticlines during middle-late stage of the Pinghu Formation further proves, from the prospective of tectonic stress, that there was an eastern folded and uplifted belt during the sedimentation of the Pinghu Formation. It is concluded that the eastern folded and uplifted belt had already uplifted and become the eastern provenance during the sedimentation of the Pinghu Formation. The deep Pinghu Formation in central and eastern parts of the Xihu sag has good hydrocarbon exploration prospect.