莺歌海盆地黄流组浅海重力流砂体物源分析及油气地质意义

Provenance Analysis and Petroleum Geological Significance of Shallow-Marine Gravity Flow Sandstone for Huangliu Formation of Dongfang Area in Yinggehai Basin,the South China Sea

莺歌海盆地中央带上中新统黄流组一段近年新发现一套浅海重力流沉积体系,夹持于海南古隆起和越南昆嵩古隆起之间,探讨其物源供给对掌握砂体展布规律以及"源-汇复合体系"的研究具有重要意义.结合东方气田新钻井资料和岩石测试资料,如砂岩碎屑组分、重矿物组合特征及元素地球化学特征对研究区黄流组物源进行了分析,结果表明:(1)西部昆嵩隆起物源是本区浅海重力流沉积体系的主要物源区,浅海重力流砂岩长石、岩屑含量较高,结构成熟度高,泥质含量低(平均3.8%),以岩屑石英细-极细砂岩为主;源自东部物源浅海砂坝砂岩的长石、岩屑含量较低,成分成熟度高,但泥质含量高(平均18.6%),以岩屑石英粉砂岩-石英粉砂岩为主;(2)源于西部物源的浅海重力流砂岩锆石、电气石含量低,磁铁矿、石榴石含量高,源于东部物源的浅海砂坝锆石、电气石、白钛矿含量高,磁铁矿、石榴石含量低.此外,区域古水流方向主要为南东-东西向,反映其可能受到西部越南水系的影响;(3)岩心样品相似的稀土元素配分模式表明浅海重力流砂岩具有一致的物质来源,稀土元素含量及比值、Th-Sc及Co/Th-La/Sc图解均显示中性-长英质源岩特征,La-Th-Sc三角图、Th-Sc-Zr/10三角图以及主量元素判别图解均显示样品落在主动大陆边缘或大陆岛弧区域,说明浅海重力流砂岩源区为挤压构造背景,与昆嵩隆起相符合;(4)中央底辟带西侧的浅海重力流砂体规模大,物性好,含气丰富,为下一步向西部勘探的主力砂体,而中央底辟带东侧的浅海砂坝储层规模较小,储层质量较差.

The reservoir in Upper Miocene Huangliu Formation of Yinggehai basin, which is located between Kuntum paleo- uplift and Hainan paleo-uplift, is shallow-marine gravity flow deposits with burial depth ranging from 2 600 to 3 500 m. Analy- zing the provenance of the sandhody is significant in understanding its distribution and ＂source to sink system＂, hence for the oil-gas exploration and development in this area. The provenance of sediments in Upper Miocene Huangliu Formation was ana- lyzed by integrating the components of the sandstones, assemblage styles of the heavy mineral, paleo-current direction and geo- chemistry characteristics of the sandstones. Results show follows. （1） There are two provenances, one from west and the other from east, in the study area. Western sourced sandstones form the shallow-marine gravity flow which are mainly suhlitharenite with high feldspar （average is 6.1%） and lithic （average is 11.7%）contents,low shale content（average is 3.8%）,However eastern sourced sandstones, which are mainly subarkose-quartz siltstone with low feldspar and lithic contents with averages of 4.6% and 2.7%, respectively, high compositional maturity, high shale contents （average is 18.6%）, form the neritic sand bar. （2） The shallow-marine gravity flow sandstones exhibit low zircon, tourmaline contents, and high magnetite, garnet contents, while the neritic sand bar sandstones exhibit high zircon, tourmaline and leucosphenite contents, and low magnetite, garnet contents. The direction of paleo-current in study area is from southeastward to eastward. （3） The similar rare earth element （REE） patterns of 28 sandstones from three western wells indicate that they were sourced from the same provenance. The dia- grams of Th-Sc and Co/Th-La/Sc and elemental ratios of these sandstone samples show that they were derived from intermedi- ate to felsic source rocks. The measured geochemistry data of these core samples fall into the fields of active continental margin or continental island arc in the diagrams of La-Th-Sc, Th-Sc-Zr/10, Ti/Zr-La/Sc and major element discrimination, indicating that the source rocks of the shallow-marine gravity flow sandstones were formed under the tectonic setting of an convergent en- vironment. （4） The integrated method for provenance analysis indicates that the shallow-marine gravity flow sandstones were derived from the western Kuntum uplift. The shallow-marine gravity flow sandstones in western mud diaper belts are large in scale, rich in gas, better in reservoir properties and the main favorable exploration target towards the west, whereas the neritic sand bar sandstones in eastern mud diaper belts are small in scale, Door in reservoir auality.