Research On Reservoir Characteristics In Lacustrine Gas Shale:Example From The 7^th Member Of Yanchang Formation In GanQuan In The Ordos Basin

The proved success in shale gas exploration and production in North America has attracted the eyes of China which has become the biggest resource consuming country of the world. No one knows whether the legend could be repeated in China since the source rock there is distinguished by its lacustrine origin. However, the recent discovery of gas logging abnormal in the shale sequence of the upper Triassic Yanchang Formation, Ordos Basin no doubt gain the explorers’confidences in continental shale gas. It has been studied that the fine sediments of semi-deep and deep lake facies in Chang 7 member, named as the Zhangjiatan Shale is source rock of high quality with some other favorable conditions for shale gas production, say relatively shallow buried and large area. Therefore, the research on reservoir characteristics of the shale become the first consideration for reserve estimation.In this paper, the study area is the 7th member of Yanchang Formation in GanQuan in the Ordos Basin. Many kinds of geological experiments, such as XRD component analysis, rock pyrolysis analysis, SEM observation after Argon ion polished, mercury injection, gas adsorption and He-porosimetry and permeability experiment, are used to research on reservoir characteristics in lacustrine gas shale. The research content includes the spatial distribution characteristics of shale gas reservoirs, mineral composition and geochemical features, pore types and reservoir physical property characteristics. The results show that:(1) The mineral composition of shale gas reservoir in the study area are mainly quartz, feldspar and clay, also containing small amounts of carbonate, siderite and pyrite, brittleness mineral content is less than the shale gas reservoir in America;(2) Organic matter type of shale gas reservoirs in the study area is type Ⅰ-type Ⅱ1, TOC range in 2%～6%, Ro range in 0.51%～0.51%.All of them makes a significant contribution to the gas in the reservoirs.(3) Pores in gas shale can be divided into three basic types, including interparticle pores, intraparticle pores and organic matter pores. Interparticle pores, which can be divided into pores between grains and pores between clay minerals, are the main pore in the shale. Intraparticle pores, which can be divided into dissolution pores and intercrystalline pores, significantly increases the effective pore of reservoir rock. Organic matter pores to be related to maturation of organic material.(4) The majority pores in the study area range in 2-100 nm. Under 2 nm pores and more than 100 nm pores accounts for a smaller pore volume. Shale with silt laminae have better petrophysical properties than pure shale reservoir.