De-convoluting The Marine Crude Oil Mixtures In The Tabei Uplift, Tarim Basim, NW China

Research of marine crude oils in the Tarim Basin was summaried. Selection of oil-source correlation indexes and endmember is the main reason for the current understanding of the oil source. Most geochemical parameters have multiple meanings and are affected by many factors. Univariate or bivariate techniques treat only one or two parameters and consequently ignore information from these parameters. Chemometrics techniques can simultaneously treat almost all parameters that reduce the uncertainty in geochemical conclusions from one or a few parameters and provide a more comprehensive understanding of geological samples.Mixing different types of oils results in complex changes in the properties and compositions of the mixtures. Laboratory mixing experiments using three typical crude oils as endmembers showed that the concentrations of individual compounds in the mixed oils varied linearly with the contribution proportions of each of the endmember oils, but the ratios of the biomarkers included in each the oil with different concentrations varied non-linearly as the amount of each of the end-member oils during mixing. Alternating least squares using biomarker concentrations(ALS-C) is an excellent way to calculate the number, fractional contributions, and compound compositions of endmember oils in mixed oil, regardless of whether the end-member oils are known. The biomarker ratios of end-member oils cannot be directly obtained by ALS, but can be calculated using compound concentrations computed by ALS-C. ALS-C avoids the uncertainty associated with selecting end-member oils.To a great extent, the accuracy of the fractional contributions of end-member oils to mixtures using ALS-C depends on the numbers of mixed samples in the data set. The accuracy of the calculated compound compositions in the end-member oils is related to the concentrations of the compounds themselves.Geochemical characteristics of 60 crude oil samples collected from Palaeozoic reservoirs in the Tarim Basin were analyzed. The samples proved to be mixed oils of different maturity from diverse source rocks. Concentrations of 40 biomarkers and carbon isotopic compositions for the whole oils were analyzed by ALS to de-convolute the mixtures. Three endmember(EM) oils were identified. EM1 is the minimum contributor to the mixed oils, which originated from Cambrian-Lower Ordovician source rocks in the early to peak oil window stage and experienced two phases of mixing and biodegradation. EM2 is the secondary contributor, which originated from Middle-Upper Ordovician source rocks at the early oil generation stage and underwent two phases of mixing and one stage biodegradation in the reservoirs. EM3 is the major contributor, which was generated from Middle-Upper Ordovician source rocks at the late oil generation stage and mixed with earlier emplaced mixtures in the reservoirs. The final mixtures were not biodegraded further, but other secondary processes(such as evaporative fractionation or gas washing) cannot be excluded, and can result in different distributions of alkanes. Crude oil mixtures from this final phase are now being produced from the Palaeozoic reservoirs in the Tabei Uplift.Fifty-two crude oils recovered from the Ordovician, Carboniferous and Triassic reservoirs of the Tahe oil field are mixtures of oil having different maturity from source rocks of marine facies with different sedimentary facies. Three endmember oils were identified by ALS using a dataset of 38 biomarker concentrations. The endmember oil I was generated from Cambrian-Lower Ordovician source rocks in the peak oil generation stage. The endmember oils II and III originated from Middle-Upper Ordovician source rocks at different thermal evolution stages with the maturity of II lower than III oil. The endmember oil I migrated into reservoirs during the late Caledonian orogeny and subsequently was severely biodegraded. The endmember oil II filled reservoirs during the late Hercynian orogeny and mixed with the residual oils of the first charge, and subsequent uplift exposed the mixtures to biodegradation. The endmember oil III migrated into reservoirs where it mixed with the previous residual mixtures during the Himalayan orogeny. The second and third crude oil filling direction is from the south to the north and from the east to the west. The north or northwest is the hydrocarbon filling point. Crude oil in the Ordovician reservoirs of the Tahe oil field is mainly charged by horizontal or short distance vertical migration. However, the third phase of oil in the east region of the Tahe oil field is mainly migrated with long distance vertically. In the eastern region, the oils from the upper reservoirs(Carboniferous or Triassic) have relatively higher contributions of the endmember oils I or/and II than III; while the oils from the Ordovician reservoirs were just the opposite. This may be because the Ordovician reservoir has been destroyed, and then the mixed oils of the endmember oils I and II migrated upward to the upper reservoir before the third phase of oil charged. It is also possible that the differential accumulation between the third phase of crude oil and the previous mixed oils.There are good correspondences between the three endmember oils of the mixed oil from the Tahe oil field and from the Tabei Uplift. The parameters related to source show that both the endmember oils I have similar types of organic matter. Both endmember oils II and III are the same. There are certain differences between some parameters based on hopanes related to depositional setting. This is mainly because the difference of the sedimentary facies of source rocks in different regions and the degree of the secondary effects to the oils. In a word, the de-convolutions of the mixed oils from the two regions have obvious similarities on the whole(such as marine hydrocarbon source rock, three phases of charging, two phases of mixed and degradation), while have differences in the local(such as the source rochs sedimentary facies of the endmember oils are not exactly same).