川西地区中三叠统雷口坡组四段古海湾特征及其油气地质意义

Characteristics and hydrocarbon geological significances of paleo-bay in the fourth member of Middle Triassic Leikoupo Formation,western Sichuan Basin,SW China

基于四川盆地西部(简称川西)地区11条重点露头剖面与盆地内部400余口钻井资料,通过沉积学、岩石学和测井分析等手段,结合微生物岩、膏盐岩、风暴岩等特殊岩相标志,明确川西地区中三叠统雷口坡组四段(简称雷四段)的沉积相类型,刻画川西地区雷四段海湾相的展布特征及演化,并探讨其油气地质意义。研究表明:①川西地区雷四段沉积期为“海—湾—坪”沉积模式,自东向西依次为蒸发潮坪相、局限潮坪相和海湾相3个相带,其中海湾相发育海湾边缘、潮下海湾和海湾斜坡亚相;②海湾边缘主要发育微生物礁和颗粒滩微相,潮下海湾发育微生物坪、深水海绵礁和静水泥微相,海湾斜坡则主要为上斜坡的风暴和垮塌沉积;③雷四段1亚段沉积期由于海平面下降,海湾边界位于广元—梓潼—都江堰一带;雷四段2亚段沉积期,海湾边界退至上寺—江油—都江堰一带以西;雷四段3亚段沉积期,发生小规模海侵,海湾边界扩展至盆地中北部的上寺—梓潼—阆中—武胜—盐亭—成都一带;④川西地区雷四段沉积期古海湾呈现出一隆两凹的古构造地貌格局,形成源储平面间互、纵向叠置的配置关系,海湾边缘发育微生物礁和颗粒滩储层,海湾两侧凹陷内发育优质烃源岩,由此构建了旁生侧储与自生自储两种高效的源储配置模式,这些新认识有望开启雷口坡组油气勘探的新局面。

The depositional facies types of the fourth member of the Middle Triassic Leokoupo Formation(Lei-4 Member)in western Sichuan Basin are examined through the methods of sedimentology,lithology and analysis of well-logging data,as well as the special lithofacies indicators such as microbialite,gypsum-salt rock and tempestites,using the data of about 400 wells and 11 outcrop sections.The distribution and evolution and its hydrocarbon geological significances of the bay facies have been discussed.The Lei-4 Member in western Sichuan Basin has an ocean-bay-flat depositional model,with the presence of evaporated tidal flat,restricted tidal flat and paleo-bay facies from east to west.The subfacies such as bay margin,subtidal bay and bay slope are recognized within the paleo-bay,with microbial reef and grain bank microfacies in the bay margin,microbial flat,deep-water spongy reef and hydrostatic mudstone microfacies in the subtidal bay,and tempestites and collapsed deposits in the upper bay slope.The bay boundary covered the Guangyuan-Zitong-Dujiangyan area in the period of the first submember of the Lei-4 Member(Lei-4-1)with falling sea level,regressed westward into the Shangsi-Jiangyou-Dujiangyan area in the period of Lei-4-2,and expanded to the Shangsi-Zitong-Langzhong-Wusheng-Yanting-Chengdu area in the northern part of central Sichuan Basin in the period of Lei-4-3 along with a small-scale transgression.The topographic pattern of“one high and two lows”is confirmed in the Lei-4 Member,corresponding to a configuration of source rocks and reservoir rocks alternated horizontally and superimposed vertically.Two efficient source-reservoir configuration models,i.e.side source&side reservoir,and self-generating&self-storing,are available with the microbial reef and grain bank reservoirs at the bay margin and the high-quality source rocks within the sags on both sides of the bay.The research findings will inevitably open up a new situation for the hydrocarbon exploration in the Leikoupo Formation.