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

Abstract

The Tarim Basin is a significant petroliferous basin in the west of China. The Lower Cretaceous Bashijiqike Formation, whose buried depth exceeds 6000m and has a low matrix porosity(<10%) and permeability(<0.1mD), is the main gas producing interval in Kuqa depression of Tarim Basin. Structural fractures can improve reservoir permeability effectively. CT scanning results and pressure experiments indicate that although the fractures in tight sandstone can only cause low porosity (<0.5%), however they can improve permeability largely (to 10~100 times). Meantime microfractures can improve pore throat configuration. Microscope and SEM evidences indicate that connectivity of uncharged microfractures is proportional to fracture aperture. And microfractures can connect pore throats which are located 20~100 times around fracture aperture. In the study area, there are 3 stages of fractures which are controlled by palaeostress field and rock fabric. And 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 are located in anticline crest, fault transition zone and around secondary faults.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015