四川宣汉盘龙洞上二叠统长兴组沉积相与岩石地层精细划分

盘龙洞剖面长兴组与普光气田同属于川东北的碳酸盐台地边缘相礁滩相层序,其岩石地层和沉积相研究结果对于认识普光气田、龙岗气田的储层发育和分布规律具有很好的参考作用。根据对薄片的仔细观察分析以及对相关地层的认识,对剖面长兴组的结晶白云岩地层进行了原岩恢复。根据古生物面貌及岩石组构特征,将盘龙洞剖面长兴组自下而上划分四段:下生屑岩段、礁灰岩段、上生屑岩段、微生物岩段。下生屑岩段包括了下部含小型有孔虫的深水陆棚相泥晶灰岩、以及上部含腕足类和棘皮类的浅水陆棚相生屑泥粒岩。礁灰岩段厚达108m,由海绵骨架岩和障积岩组成。上生屑岩段厚约45.3m,已经白云石化为细晶白云岩,导致前人往往误划分为蒸发台地相。仔细观察后仍然可以进行原岩恢复,主要由生屑泥粒岩、粒泥岩组成,含棘皮类、有孔虫、粗枝藻、等组成的群落。微生物岩段厚约8.7m,不仅地层以不规则薄层状为特征,而且含有特征的微生物群落。与普光6井、湖北利川见天坝长兴组剖面对比后发现,它们有相似的沉积层序:下部的深水相灰岩、中部的生物礁岩、上部的开阔台地相或相似环境的生屑灰岩。这应代表台缘礁滩相带的典型层序,可作为认识台缘礁滩相储层的基础。前人研究未能识别长兴组顶部的微生物岩段。这直接导致了二叠系—三叠系界线的无法正确确定。本文提出可以把微生物岩段作为一个岩石地层单位对待,可以作为确定二叠系—三叠系界线的标志层,把界线放在微生物岩段的中部。

Revised Sedimentary Facies and Lithostratigraphy of the Upper Permian Changxing Formation at Panlongdong Xuanhan Sichuan Province

The Upper Permian Changxing Formation at Panlongdong, Xuanhan, Sichuan Province is a platform marginal facies sequence consisting of 4 lithologic parts in ascending order: the Lower Bioclastic Member, the Reef Member, the Upper Bioclastic Member, and the Microbialite Member. The Lower Bioclastic Member consists of the lower dark lime mudstone with small foraminifers below 1mm and the upper bioclastic packstone with abundant brachiopod and crinoidal bioclasts. The Reef Member is about 108m thick, consisting of framestone and bafflestone. The Upper Bioclastic Memberis 45.3m thick, composed of fine dolostone. Previous researchers interpreted it as evaporate facies. This study, however, determined it is open platform facies, consisting of bioclastic packstone and wackestone. The Microbialite Member is 8.7m thick, and contains microbial fossils. The Changxing Formation sequences of Panlongdong, Well Puguang-6, and Lichuan, Hubei Province have similar facies sequence: the lower deep water limestone, the middle reef rocks, and the upper open platform limestone. This sequence is typical of the platform marginal deposits in the Sichuan Basin, and is significant to understanding the platform marginal facies reservoirs. Previous researchers did not recognize the Microbialite Member, and the P-T boundary was not correctly determined. We propose in this paper to place the P-T boundary at the medium of the Microbialite Member.