Geological conditions for shale gas accumulation and favorable area prediction of Luofu formation of Devonian in Tian'e-Nandan area

Based on field geological survey analysis, drilling data analysis, organic geochemical analysis and field gas content analysis, the study on accumulation conditions, reservoir conditions, enrichment regularity and preserving conditions of shale gas in Luofu formation of Devonian in Tian'e-Nandan area of northwestern Guangxi was carried out. The results indicated the depositional period of Luofu formation in this area was semi-deep-water interplatform basin of depositional environment and it had obvious vertical zoning. The upper lithology of Luofu formation was siliceous mudstone, shale and dark shale were 40~80 m, mainly of limestone construction. The overall maturity of Luofu formation was high, the average maturity(Ro) was greater than 2.0%, and the average TOC was 2.36%. The organic type was a typical mixed kerogen, the upper part of Ⅱ1 was humic saprophy and the lower section of Ⅱ2 was sap humus. The average content of shale quartz was 30.5% and the carbonate mineral content was 24.6%. The clay mineral content was 42.2% and the reservoir space was mainly composed of interglacial pores, dissolution holes and organic matter holes. Therefore, the region was qualified with good shale gas accumulation and enrichment conditions. It was concluded the preserving condition was the key to the shale gas enrichment in the Tian'e-Nandan region, and the Gengliang-Kunli region was predicted to be favorable exploration area.