渤海海域渤中凹陷中生界火山机构地质模式及建造过程

Geological model and construction process of the Mesozoic volcanic edifices in Bozhong Sag,Bohai Sea

精细刻画火山机构对火山岩储层预测十分重要。渤海海域中生界发育多类火山机构,且受后期改造强烈,目前缺乏对该区火山机构类型、结构及建造过程的系统研究。利用岩心、测井及地震资料,将渤海海域中渤中凹陷生界火山机构分为熔岩穹丘、复合层火山、火山碎屑岩席及盾状火山4种类型。分别刻画了这4种火山机构内部喷发单元及岩性、岩相构成,建立了地质模型,并概括形态特征及内部结构建立了4种火山机构对应的建造过程示意图。其中,熔岩穹丘具有高纵/横比,由厚层酸性熔岩构成,内部一般为块状构造;复合层火山在该盆地内分布范围广、规模大,外部形态为丘状,但纵/横比小于熔岩穹丘,内部岩性复杂,多为熔岩和火山碎屑岩互层构成的层状结构;盾状火山为平缓的低丘状外形,主要由基性岩组成,其岩浆黏度低、流动范围较大,所以形成的机构规模大,内部多为席状构造;火山碎屑岩席为席状及充填状,纵/横比极小,内部为多期喷发叠加形成的层状结构。此外,通过对比不同火山机构的物性,发现研究区熔岩穹丘及复合层火山的储层物性最好,火山碎屑岩席次之,盾状火山最差;熔岩穹丘及复合层火山机构可作为后续有利油气勘探目标。

Fine depiction of volcanic edifices is of great value to mapping volcanic reservoirs.A variety of volcanic edifices under fierce later modification are developed in the Mesozoic of the Bozhong Sag,Bohai Sea.There is,however,a lack of documents on the types,structure and construction process of the volcanic edifices in the area at present.The data of core observation,logging and seismic interpretation are applied to divide the Mesozoic volcanic edifices in the Bohai Sea into four types,that is,lava dome,stratovolcano,pyroclastic sheet and shield volcano.The study serves to depict the eruptive units,lithology and lithofacies within the four types of volcanic edifices,with a geological model established and based on the morphological characteristics and internal structure,the diagrams of four types are built to show the volcanic construction processes.Among others,the lava dome characterized by a high aspect ratio is composed of thick-bedded acidic lava of massive structure within;the stratovolcano widely seen in the Bohai Bay Basin and large in scale is mound-shaped in terms of external morphology,smaller than the lava domes in terms of aspect ratio,and characterized by complex lithological types,most of which are of alternating lava and pyroclastic rocks;and the shield volcano is of gentle low-mound shape,and mainly composed of basic rocks,featuring low-viscosity magma and large flow range,resulting in large-scale volcanic edifices of sheet-like structures inside to a large extent.The pyroclastic sheet is in the shape of sheet and filling,with very small aspect ratio and layered structure caused by multi-stage eruption and superimposition.In addition,the comparison of the physical properties of different volcanic edifices shows that the reservoirs of lava dome and stratovolcano types in the study area are of the best physical properties,which can be viewed as favorable targets for future oil/gas exploration,followed by the pyroclastic sheet and then the shield volcano coming at the worst.