川东北元坝地区上三叠统须家河组天然裂缝发育特征与主控因素

Fracture development characteristics and main controlling factors of natural fracture in the Upper Triassic Xujiahe Formation in Yuanba area,northeastern Sichuan Basin

川东北元坝地区上三叠统须家河组致密砂岩储层物性较差,天然裂缝的发育改善了储层物性。天然裂缝是主要的储集空间和渗流通道,对天然气的运移、成藏和高产有着重要的影响。本文根据地表露头、岩心、薄片和成像测井等资料,对元坝地区须家河组天然裂缝成因类型及发育特征进行研究,并分析控制天然裂缝发育的主控因素。结果表明,研究区主要发育构造成因的低角度和高角度剪切裂缝,主要方位为NW-SE(300°±10°)和NEE-SWW(70°±5°)向,有效性好;裂缝充填程度较低,有效性好。构造部位、岩性和岩层厚度为影响裂缝发育的主要因素。鼻状构造部位及断层上升盘距离断层面400 m以内、垂直断距在60~120 m之间、北西向断层延伸线拐点附近为裂缝发育的有利部位,有利于天然气高产;中细粒石英砂岩和细粒长石岩屑砂岩中裂缝最为发育;高能量环境规模砂体(高石英含量,低泥质含量)更有利于裂缝发育;裂缝发育程度与岩层厚度呈明显的负相关关系,当岩层厚度小于1 m时,裂缝密度大,发育程度高。

The Upper Triassic Xujiahe Formation in Yuanba area,northeastern Sichuan Basin is a typical tight reservoir with poor physical properties,whilst the development of natural fractures improves the reservoir properties.Natural fractures provide the main reservoir spaces and seepage channels thus have an important impact on the natural gas migration,accumulation and high yield.Based on outcrop,core,thin section and image log data,this paper investigates the genetic origin of natural fractures and analyzes the characteristics and main controlling factors of natural fracture development in the study area.Natural fractures are mainly low-and high-angle shear fractures of tectonic origin and oriented NW-SE(300°±10°)and NEE-SWW(70°±5°),with low fracture filling.Fracture development is mainly influenced by structural location,lithology and rock thickness.The favorable sites condusive to high natural gas production have the nose-like structure,within 400 m from the upthrown side of the fault,with vertical fault length between 60120 m,in the vicinity of the inflection point of the NW fault extension line.Fractures are most developed in medium-fine quartz sandstone and fine-grained feldspar lithic sandstone,with the high-energy,large-scale sand body(high content of quartz and low content of shale)more conducive to fracture development.There is a significant negative correlation between the degree of fracture development and the thickness of the rock layer,where high fracture density and high fracture development are achieved when the rock layer is less than 1 m thick.