Ultra-Deep Fractured Tight Sandstone Gas Reservoirs: Characteristics and Quantitative Evaluation of Fractures in the Lower Cretaceous, Keshen Gas Field Tarim Basin, China

Abstract

Abstract

The Tarim Basin is a significant petroliferous basin in the west of China. The Lower Cretaceous Bashijiqike Formation, whose buried depth exceeds 7000m and has a low matrix porosity(<10%) and permeability(<0.05mD), is the main gas producing interval in Keshen Gas Field of Tarim Basin. Structural fractures can improve reservoir permeability effectively. CT scanning results, analysis on FMI and pressure experiments indicate that although the fractures in tight sandstone can only cause low porosity (<0.1%), however they can improve permeability largely (to 10∼100 times). Meantime micro fractures can improve pore throat configuration. According to the CLSM (Confocal laser scanning microscope) and STM(Scanning tunnel microscopy), there are three types of configuration between micro fractures and matrix pore throats, which are controlled by fracture development stage and reservoir diagenetic stage. Microscope and SEM evidences indicate that connectivity of uncharged micro fractures is proportional to fracture aperture. And micro fractures can connect pore throats within a certain range, which are located 20∼100 times around fracture aperture. In the study area, isotopic dating of fracture fillers analysis indicate that there are 3 stages of fractures which are controlled by palaeostress field and rock fabric, microscope evidences show different fracture fillers components in different stages. According to that, the third stage fractures play an important role in improving tight sandstone permeability. Isotopic dating of fracture fillers and burial history analysis indicate that some fractures developed before gas emplacement, but most of fractures probably developed in the same time with peak gas generation and charge, which suggests that gas probably migrated along fractures and accumulated in the sandstone reservoirs. Build-up of pressure data during producing test indicates that high fracture permeability areas controlled by fracture strike and width are located in anticline crest, fault transition zone and around secondary faults.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016