CHALLENGE OF TERRESTRIAL SHALE GAS FROM YANCHANG FORMATION IN ORDOS BASIN COMPARING TO MARINE SHALE GAS FROM LONGMAXI FORMATION IN SICHUAN BASIN, CHINA

In order to explore the challenges and existing problems of the terrestrial shale gas exploration, the key factors that restrict the exploration of terrestrial shale gas are found out by comparing the geological characteristics of terrestrial shale from 7^th Member of Triassic Yanchang Formation (called Chang 7 shale) in Yan’an area, Ordos Basin and marine shale from Lower Silurian Longmaxi Formation (called Longmaxi shale) in Weiyuan area. It is summarized through research that:

① The terrestrial shale has various lithologic facies, rapid changes in thickness, lack of thick organic - rich shale, stronger heterogeneity comparing to marine shale, as well as the thickness of single layer of shale is thin.

② Clay minerals content is higher in Chang 7 shale (about 40%~50%) than Longmaxi shale (about 30%~40%), lack of carbonate minerals and biogenic silicon, resulting in lower rock brittleness and not conducive to artificial fracturing.

③ Gas content in Chang 7 shale is lower than Longmaxi Shale, with the field analytic result as 0~4.39m³/t (average 1.16 m³/t) in Chang 7 shale and 0.3 ~ 5 m³/t (average 1.89 m³/t) in Longmaxi shale, because organic matter abundance is high in Chang 7 shale (TOC mostly 3%~6%) while maturity is low(Ro = 0.8-1.1 %), basically in the stage of “oil window” and the main product is liquid oil and associated gas.

④ Organic pores and interlayer bedding fractures provide most reservoir space in Longmaxi shale which are rare in Chang 7 shale. Sandy lamina improve the reservoir property in Chang 7 shale.

⑤ Free gas is the main composition of Longmaxi shale gas while absorbed gas may be the majority gas content in Chang 7 shale.

⑥ Longmaxi shale gas is preserved better because well-enclosed roof and floor, relatively stable tectonic background make most shale gas trapped in the strata, while Chang 7 shale has good hydrocarbon migration conditions.

⑦ According to the experience of shale gas exploration in Longmaxi shale, high production wells are usually located in areas where the strata is overpressure, with pressure coefficient higher than 1.2, while Chang 7 shale is generally low - normal pressure.

Above all, many difficulties are to be solved if terrestrial shale gas is about to achieve further success. The study of terrestrial shale gas has a long way to go.