致密气源层内沉积特征及与致密砂岩气藏关系--以川东北元坝地区须三段为例

川东北元坝地区须三段长期被认为是烃源岩层,在该层试气中获得120.8×108 m3/d高产工业气流井,表明以须三段为代表的致密气源层内勘探前景广阔。本文依据钻井取芯、测井、录井等资料,运用多信息综合研究方法,详细研究了川东北元坝地区须三段沉积特征、沉积演化及源储配置特征,进而探讨致密气源层内致密砂岩气藏的展布特征。研究表明研究区西区发育辫状河三角洲,东区发育曲流河三角洲;通过分析十多种沉积微相类型,结果表明辫状河道、水下辫状主河道及水下分流河道储集砂体为元坝地区须三段最有利的储集相带。同时,东、西区源储配置差异明显:东区底部为厚套的水下分流河道砂体,顶部为薄层的砂体与煤及炭质泥岩互层;西区水下辫状河道及辫状河道砂体分布范围广,多层砂体叠置连片,砂体与煤系烃源岩构成互层,似"千层饼"状特征。相对于东区,西区源储配置特征对于致密气成藏更为有利,因此建议将以后的勘探重点放在西部的致密砂岩勘探区域。

Sedimentary Characteristics and Their Relationship with Tight Sandstone Gas within the Source Rock of Tight Gas: A case study from Third Member of Xujiahe Formation in Yuanba area,NE Sichuan Basin

The third member of Xujiahe Formation has been considered the source rocks. However, the well obtained 120.8×108 m3/d yield in gas production of the member, showing the exploration prospect being bright within the source rock of tight gas, especially in the third member of Xujiahe Formation. This article aims to use the multi-information integrated research method, to elaborately analyze sedimentary microfacies, features and sediment evolution of different sand groups, and characterize their relation with tight gas sandstone reservoir, based on the data of drilling cores, loggings and analysis assay. The result shows that braided fluvial delta was developed in the west of the study area and meandering fluvial delta was developed in the east. Among those different microfacies, braided channel, subaqueous braided main channel and subaqueous distributary channel are relatively favorable reservoirs. Meanwhile, the difference of east and west sedimentary facies configuration are obvious. In the east, thick set of subaqueous distributary channel resevoirs are distributed at the bottom, and thin set of sandstone and coal bed superimposed each other at the top. In the west, the distribution of braided channel and subaqueous braided channel sandstone are widely and stacked. And carbonaceous mudstone and coal bed with sandstone superimposed each other, appearing as a "thousand layers cake" like stacking pattern. Comparing to the east, the source rock and reservoir are closely proximity, so the west has favorable accumulation condition. The present study suggested that the future exploration would be focused on the west.