Analysis of shale pore characteristics and controlling factors based on variation of buried depth in the Longmaxi Formation, Southern Sichuan Basin

This paper is the result of oil and gas exploration engineering.

Objective With the maturity of exploration and development technology in shallow and medium-deep shale, deep shale has become a focus of next exploration and developmentin China. Clarifying the pore characteristics of shale under different burial depths and its controlling factors is conducive to promoting the selection and evaluation of deep shale gas.

Methods Methods such as nuclear magnetic resonance, field emission scanning electron microscopy and X-ray diffraction were used to comparatively evaluate the porosity and pore structure of Longmaxi-Formation in southern Sichuan Basin, and its influencing factors were also discussed.

Results With the increase of burial depth, the total porosity shows a downward trend, and the main porosity of shale below 3000 m, between 3000-3500 m and deeper than 3500 m are 4%-8%, 3%-7% and 3%-6%, respectively.There is no significant difference in pore types and size distribution of the Longmaxi Formation shale at different depths, and a large number of nano-scale organic pores, mineral intergranular pores, intragranular dissolved pores and micro-fractures are developed at different depth.

Conclusions A large amount of organic matter formed at high U/Th ratio and low Ti content is the key factor for the development of deep shale pores. High siliceous mineral content and high formation pressure coefficient could protect pores of shale reservoirs and slow down compaction of its overlying strata. However, excessive burial depth wouldalso lead to a reduction of shale pores and porosity.