Re-Evaluating Petroleum Play of the Late Triassic Sediments to Find Oil in the Upper Yangtze Region

The upper late Triassic Xujiahe Formation (T3x) is the most important terrigenous stratum gas province, with accumulated gas production over 3TCF, technical recoverable resources at 18TCF (500BCM) and geological reserve close to 50TCF (1TCM), in the Sichuan Basin (SCB). In the Southern Plateaus of SCB (SP-SCB), Upper Yangtze Region also has the matching T3x, i.e., Baiguowan/T3b, often located in the folded smaller basins, i.e., Xichang Basin in this paper, which has not been explored as much. Past study of source rocks in T3x already found extracts to have non-terrestrial-humic biomarker values, and this work further adds the gas composition and isotope data from the T3x formations already producing and the T3b formation in two exploratory wells in the SPSCB, i.e., the Xichang basin/Zhaojue sub-basin, where oil stains and bubbled oil and bitumen are found in cores. Based on the geochemical analysis results of gas isotopes and biomarkers T3x/T3b (in SP-SCB) formations, this paper discusses the sedimentary environment and thermal evolution of source rocks of T3x & T3b, and possible transformation of the oil and gas generated, and the potential to find oil and gas in the T3x and T3b formations. We use latest isotope-maturity–wetness–Gas to Oil Ratio (GOR) templates for gas isotope data interpretation, and show the difference between T3x and T3b, and discuss the possibility of finding liquid in T3x and T3b. The conclusion is that T3b in Xichang basin should have oil and gas, and estimated Gas to Oil ratio is about 3~5:1 on a barrel of equivalent (BOE) basis.