Discussion on genesis and geological significance of "low resistivity and low gas content" of Longmaxi Formation shale in southeastern Sichuan

Abstract

In order to clarify the genesis of "low resistivity and low gas content" in the shale of Wufeng-Longmaxi formations and effectively guide the next step of shale gas exploration in Sichuan Basin, a detailed study on the problem based on a large number of drilling surveys, dissections, and analysis of laboratory test results in the entire area was carried out herein. Low-resistivity wells of Longmaxi Formation shale in this area can be divided into two types based on their electrical and geological characteristics, which are less than 1 Ω·m and 1-10 Ω·m. Shale gas wells with resistivity less than 1 Ω·m are basically gas-free and mostly dry wells with relatively concentrated distribution areas, mainly located in southwestern Sichuan and western Changning. The resistivity curve of the shale exhibits a "thin neck" characteristic, with a Raman reflectance generally above 3.70% and a high amplitude graphite peak. In addition, the variation range of resistance values in rock electrical experiments under different states is small, and they all exhibit extremely low to low resistance characteristics, indicating that the conductivity of the rock skeleton caused by graphitization is the main factor affecting this type of shale. The drilling distribution area of shale with resistivity in the range of 1-10 Ω·m is relatively wider, mainly with slight gas content, distributed both inside and outside the basin. The characteristics exhibited by its electrical properties, laser Raman, and rock electrical experiments are significantly different from those of shale less than 1 Ω·m. The resistance curve of this type of shale exhibits a "gradual" characteristic, with a measured Raman reflectance of around 3.50% by laser Raman spectroscopy. The laser Raman spectrum also does not show obvious graphite peak characteristics. This type of shale undergoes a wide range of changes in rock electrical experiments under two states: dry water and saturated water, with a change in shale resistivity of 7 to 20 times before and after drying, and exhibits medium to high electrical resistance characteristics, revealing a significant impact of shale water content on shale resistivity. Based on actual drilling data, it is believed that the deterioration of shale gas preservation conditions and the increase in water content are the main reasons for this type of low-resistivity well.

Keywords

• low resistivity
• graohitization
• high water content
• preservation condition
• shale gas
• longmaxi formation
• southeastern