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An Integrated Approach to Quantify the Contribution of Multiple Lacustrine Sources to Mixed-Oils in the Triassic Reservoirs in Slope Area of Mahu Depression, Junggar Basin, Northwest China


The phenomena of oil mixing of superimposed basins in northwest China is very common, while the theories and methods to quantify the contribution of different sources are still scarce, especially the sources are similar in their depositional environments and continuous in ages. Mahu depression, located in northwest Junggar basin, has recently achieved significant breakthroughs in Triassic Baikouquan formation(T1b) for oils in slope-sag area. While the oils have showed diverse features in biomarkers and isotope values, preliminary oil-source correlation studies have proved their sources are possible Middle Permian Wuerhe formation (P2w), Lower Permian Fengcheng formation (P1f) and Lower Permian Jiamuhe formation (P1j). In order to clarify their sources and their contribution to the oils, 40 DST-oil samples and 23 mudstone samples from P2w, P1f and P1j are collected and have been analyzed by GC-MS, GCMS-MS, CSIA(Compound-Specific Isotope Analysis). Firstly, we classified the oils into five groups by integrated molecular geochemistry, isotopes and elementary data and precise oil-source correlation were conducted, the first group oils sourced from P1f carbonates sediments formed in closed, hypersaline, reduced environment, the second group was from P1f semi-deep lacustrine mudstones, the third group oils originated from P2w shallow lacustrine mudstones, while the forth group oils was a mixture of P1f and P2w mudstones, the fifth group was mixed oils from the first group P1f oils and another P1j mudstones formed in saline, reduced environment. By establishing the binary mathematic models, the ratio of oil mixing and the contribution of multiple lacustrine sources to mixed oils were clarified. Then, Fluid Inclusion petrography, microthermometry and Micro-FTIR(Fourier transform infrared spectroscopy) were used to study on the thin sections of oil sands in typical reservoirs to illustrate the difference of oil sources. The Xinc,Xstd,ΣCH2/ΣCH3 and GOI data were calculated to rectify the contribution of diverse sources to the mixed oils. In a conclusion, the oils in T1b reservoirs of the slope area is mostly from P1f carbonates, the second important source is P1f mudstones, the P2w sourced oils are relatively limited and the contribution from P1j mudstones are negligible. This conclusion shed some light on the controlling factors of oil distribution in slope-sag area of lacustrine basins and could be used to predict the forming and distribution of mixed-oil reservoirs.