东爪哇盆地抱球虫灰岩浮游有孔虫组成及指相意义

Microbial Composition and Significance of Lower Pliocene Sedimentary Globigerinid Limestone Facies in the East Java Basin

印尼东爪哇盆地上新世发育的抱球虫灰岩,已成为该地区主要的勘探目的层之一,目前已发现了多个油气田。该地区发育的抱球虫灰岩具有浮游有孔虫颗粒含量高达50%,粒间孔和浮游有孔虫体腔孔发育的特征,其岩芯分析孔隙度最高可达52%,是该区域优质储层,但对其沉积相研究因缺乏可靠的指相证据而认识不一,从而影响了对此类储层控制因素及展布的研究,限制了该区的油气勘探。通过对该套灰岩浮游有孔虫属种组成及生态环境分析,发现研究区抱球虫灰岩发育的浮游有孔虫种类众多,水深上包括浅层水种、中层水种及深水种,区带上包括热带、副热带及温带属种,为不同深度、温度带浮游有孔虫的混合。同时发现了指示存在上升流的属种Globigerina bulloides和Globorotalia Menardii(s),并且大量发育,指示研究区抱球虫灰岩的形成与上升流有关,这对研究区该套储层的沉积相识别有重要指导意义。区域古沉积环境及海流分析表明研究区具有发育上升流的条件,进一步佐证了该套灰岩的形成受上升流影响。提出研究区抱球虫灰岩的形成主要是生活在陆坡及半深海的浅、中、深层水种的抱球虫死亡后,由海底洋流和上升流等水动力运移至陆棚边缘,因受离岸流的影响,能量减弱,而在陆棚边缘沉积,并经受波浪等水动力的淘洗、筛选,从而形成了不同生态环境抱球虫属种组成的抱球虫灰岩。

Globigerinid limestone is well developed in the Pliocene Series of the East Java Sea Basin,and has be⁃come one of the main exploration targets in this area.Many oil and gas fields have been discovered in Globigerinid limestone reservoirs.Globigerinid limestone is dominated by Foraminifera tests,particularly of the planktonic genus Globiegerina,which may form up to 50 percent of the limestone.Pore space in the limestone comprises foraminiferal test pores and intergranular pores.The porosity of the grainstones may exceed 50 percent,which represents a high⁃quality reservoir.Due to the lack of reliable facies markers,however,opinions differ as to its origin and depositional environment,seriously affecting knowledge of the control factors and distribution of such reservoirs,and limiting ex⁃ploration in the area.This study examined the depositional environment of Globigerinid limestone based on thin sec⁃tions of casts,core observations and microbial data,together with the ecological environment of planktonic Foramin⁃ifera.It is concluded that the Globigerinid limestone consists of different species,including shallow⁃water species,middle⁃water species and deep⁃water species,as well as tropical⁃,subtropical⁃and temperate⁃water species.Globige⁃rina bulloides and Globorotalia menardii were found in large quantities,indicating the existence of upwelling.The combined characteristics of the paleosedimentary environment and paleocurrents indicate that the formation of Globi⁃gerinid limestone in the study area was closely related to upwelling,and occurred in two stages:firstly,the plankton⁃ic foraminiferal species living in the shelf slope and middle ocean were deposited on the sea bed after their death.The bottom current and upwelling current carried the different species to the shelf margin where the energy is weakest and they accumulated there,influenced by offshore currents.Secondly,they were winnowed and transported by waves or storm⁃associated currents,eventually forming one of the primary high⁃quality reservoir exploration targets in the East Java Bain.