CHARACTERISTICS AND INFLUENCE OF GRAPHITIZATION OF ORGANIC MATTER OF QIONGZHUSI SHALE IN LESHAN-LONGNÜSI PALEO-UPLIFT AND THE PERIPHERY

The lower Cambrian Qiongzhusi Formation is one of the favorable beds of Shale gas exploration in China, but this layer has the problem that the shale maturity is too high to result in graphitization of organic matter. Therefore, based on three key wells in the Sichuan Basin, the W201, N206, and GS17 wells, we used resistivity logging and Raman spectroscopy to analyze the graphitization of organic matter in the Lower Cambrian Qiongzhusi shale in Leshan-Longnüsi paleo-uplift and the periphery. On this basis, we predicted the depth limit and distribution range of graphitization of organic matter in shale, and discussed the effect of graphitization of organic matter on the physical properties of shale reservoirs. The main conclusions were as follows: (1) The Raman spectra of shale samples show that the thermal evolution in the three regions is the highest in the N206 Well, followed by the GS17 Well, and the lowest in the W201 Well. Correspondingly, the graphitization degree of organic matter is the highest in the N206 Well area, followed by the GS17 Well area, and the W201 Well area is not graphitized. (2) The degree of graphitization of organic matter revealed by the electrical characteristics is consistent with that obtained by Raman spectra, and the depth of 5200m is the depth threshold for graphitization of shale organic matter in the Moxi-Gaoshitai area, and GS17 Well just spans the depth threshold. It shows that graphitization of organic matter occurs under 5200m in GS17 Well, but the degree of graphitization is lower than that in Changning region. In the region of W201 well, no graphitization of organic matter has occurred. The resistivity has a good negative correlation with GR and TOC, indicating that N206 well region has undergone severe graphitization. The shale distribution area of the Lower Cambrian Qiongzhusi Formation in Leshan-Longnüsi paleo-uplift and the periphery is dominated by the organic graphitized area, and only two non-graphitizing areas appear in the main structural parts, namely Weiyuan-Ziyang region and Moxi-Gaoshiti region, and the area is 7300km² and 7400km² respectively. (4) The effect of graphitization of organic matter on the shale reservoir space is mainly manifested in the massive reduction or even disappearance of intercrystalline pores of organic matter and clay minerals, resulting in the porosity of the matrix falling to 1/3 to 1/2 of the normal level, and makes the shale reservoir with no or only a small amount of gas.