COMPREHENSIVE EVALUATION ON GAS-BEARING PROPERTIES OF THE LOWER CAMBRIAN NIUTITANG FORMATION SHALE: A CASE STUDY FROM THE CENGONG BLOCK, NORTHEASTERN GUIZHOU PROVINCE, SOUTH CHINA

To gain a better understanding of the gas-bearing properties of the Lower Cambrian Niutitang Formation shale, various approaches to obtaining data of the shale gas content were adopted in this work, which referred to the water immersion test, ignition test, field desorption instrument method, adsorption isothermal experiment and logging data interpretation. Gas-bearing factors from shale petrological facies, sedimentary environment, stratigraphic cyclicity and structural condition were analyzed. Results of the water immersion and ignition test suggested that the Niutitang Formation shale has good gas-bearing properties because of the continuous dense clusters of bubbles and long flame. Lost gas contents (QLG) of the shale calculated by different methods were quite distinct, and QLG calculated by the linear regression were less than values obtained by the multiple regression and diffusion model, and QLG of each sample calculated by the diffusion model showed huge difference. There exists a strong positive correlation between the adsorbed gas content obtained from adsorption isothermal experiment and TOC content, and on the basis of this the predicted adsorbed gas content could be calculated by the multivariable linear regression model of the logging data interpretation. The sedimentary environment not only controls the distribution of shale reservoir but shale petrological facies types. The shale formed in the deep water shelf environments, in which siliceous and argillaceous shale facies are developed, tends to be characterized by high TOC content, abundant organic pores, and high gas content. According to consequences of the frequency spectrum analysis using the GR, DEN, LLD and SP curves, the significant frequencies were identified as three groups of the low, medium and high which is 0.008~0.037, 0.058~0.104 and 0.115~0.162, respectively, and the corresponding average values are 0.022, 0.082 and 0.139, respectively, which could be considered that these variation in the shale were driven by the Milankovitch cycle. Furthermore, a similar change tendency of both gas content and logging data with burial depth was revealed. The tectonic controlling effect influencing the high gas content in this area is recognized that the broad axis of the box syncline is the favorable location for shale gas accumulation.