Shale Gas Potential of Lower Paleozoic Strata in China Southern Region

Xian-ming Xiao and Hui Tian
State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou,China

Organic-rich shales developed widely in the Lower Cambrian and Lower Silurian strata in China southern region. The shales are characterized by a very high thermal maturity, and experiencing an early deep burial and a late strong uplifting. The Lower Cambrian and Lower Silurian shales arrived to a burial depth of 7000-9000m and 6000-7000m at the end of early Cretaceous, with current EqVRo (equivalent vitrinite reflectance) values of 3.0-4.0% and 2.5-3.0%, respectively. The uplifting, companying with a strong folding in some areas, occurring during later Cretaceous and Cenozoic eroded the upper part of Mesozoic strata of 3000-4000m in the Sichuan Basin and 4000-6000m in the folded areas. This geological background would have a significant influence on the reservoir characterization of the shales as well as their gas storage capacity. The present study focuses on two key issues related to the above characters of the Lower Cambrian and Lower Silurian shales in China southern region. One is the gas generation potential of the shales matured to a very high maturation stage and their reservoir characterization, and the changing of their stored gas with the uplifting and erosion of the overlying strata. The following research was conducted and recognitions were achieved.

(1) A lower Paleozoic organic-rich shale sample with a lower maturation level (EqVRo=1.1%) was artificially pyrolyzed in sealed gold tubes to obtain its gas generation kinetic parameters. Combining these parameters with the thermal history of the Lower Cambrian and Lower Silurian shales in the Sichuan Basin, a geological model of the gas generation was established. This model indicates that the main stage of the gas generation of the shales occurs from 1.5% to 2.5% of EqVRo, the gas generation dead line occurs at about 3.0% of EqVRo, and about 10% of the total gas yield was generated within the maturation stage from 2.5% to 3.0% of EqVRo. This means that the shales can still generate a significant amount of gas when it was matured to a very high maturity (i.e. 2.5% of EqVRo) with further increasing the burial depths, and this gas is a supply for the increase of gas storage capacity of the shale.

(2) Different types of pores occur in the lower Cambrian and the Lower Silurian shales from the China southern region. The porosity and TOC content showed a significant positive correlation, but the relationship between the porosity and maturity (EqVRo values of the studied samples are in the range of 2.0-4.0%) is not clear. The porosity of the lower Cambrian shales (2-8% of TOC) is mainly in the range of 3-5%, including a matrix inorganic porosity of about 1.5%. The Lower Silurian shales (2-10% of TOC) have a principal porosity range of 3-6%, and their matrix inorganic porosity is about 1.9%. Organic nanometer pores developed well in two sets of the shales, becoming a dominant pore type. Therefore, two sets of the shales even in very high maturity stages still hold a certain porosity, which provides space for shale gas storage.

(3) A geological gas storage model of the lower Cambrian and Lower Silurian shales was established for the Weiyuan area in the Sichuan Basin. The gas storage capacity of the Lower Cambrian and Lower Silurian shales at their deepest burial (Early Cretaceous) reached a maximum, respectively up to 1.98 - 2.93 m³/t and 3.29~4.91 m³/t. During the uplifting, the free gas content in the shales decreased constantly, and the adsorption gas content increased. In the current burial depth conditions (an average depth of 3000 m of the Lower Cambrian and 2000 m of the Lower Silurian), the average gas storage capacity is 1.98 - 2.93 m³/t and 2.94 - 4.42 m³/t, respectively. Therefore, although the Lower Cambrian and Silurian shales in the Weiyuan area have experienced a strong uplifting, their gas content is similar with the Marcellus shale of the United States, still with industrial shale gas potential.

(4) The shale gas potential of the lower Cambrian and Lower Silurian shales in the China southern region are also restricted by the structure deformation. The pilot development wells indicate that the shale gas yields are high enough for the commercial development in the tectonic stable Sichuan Basin and its surrounding areas, but much lower in the strong folded areas where the faults and fractures formed by tectonic activities connect to the shale pore-fracture system, resulting in a significant loss of the shale gas. Gas in-place content in the lower Paleozoic shales in the China Southern region are mainly controlled by the preservation conditions, and a favorable area will be uplifts within the Sichuan Basin or a large syncline in the fold zone with a burial depth of 2000-4000 m.

AAPG Datapages/Search and Discovery Article #90180©AAPG/SEPM/China University of Petroleum/PetroChina-RIPED Joint Research Conference, Beijing, China, September 23-28, 2013