Research On Geochemical Characteristics Of The Source Rocks And Crude Oils From Abyed-1Well In Yemen Exploratory Area

Based on the geological backgrounds, such as sedimentary characteristics and structural evolution for the Marib-Shabwah Basin, the hydrocarbon potential for the source rocks of different formations from the first risky well-Abyed-1in this studying area have been evaluated. In addition, combined the compositional characteristics of the biomarkers with carbon isotope values, the depositional environment the type and the maturity of organic matter in the source rocks from different formations and the origins of crude oils have been analyzed, and the relationship between the oils and the source rocks have been discussed. The main results as follows:According to the evaluating standard of marine source rocks, the four sets of source rocks in Abyed-1well have been evaluated from the abundance、type and maturity of the organic matter by the rock-eval data of the source rocks, vitrinite reflectance ratio (Ro) and the maturity parameters of biomarkers and aromatic hydrocarbon compounds. The results show that the organic carbon content in Nayfa and Sa’ar formation varies greatly, the mean value is up to2.0%above, but the hydrocarbon potential is lower, the types of organic matter in the source rocks are varied, but the type Ⅱ is dominant, the maturity is evidently low, indicating it belongs to the potential middle-good source rocks. Both the organic carbon content and hydrocarbon potential in the source rocks from Lam formation is relatively low, and the organic matter type is Ⅲ, and its maturity of organic matter has already entered the maturity stage, it should be a kind of effective bad source rocks. Both the organic carbon content and hydrocarbon potential for the source rocks from Meem formation is obviously high, the mean values are up to4.99%and13.48mg/g respectively, the type of organic matter is humic-prone caused by the high maturity, showing that it belongs to the best source rock. At the same time, there are some differences in the same formation source rocks between the Abyed-1well and AlShuaib-1well because of the lateral variation of the sedimentary facies.Based on the characteristics of the distributions and compositions of the biomarkers, it is found that diasteranes are rich in the four sets of source rocks, but there are big differences in the distributions and compositions of other biomarkers. For example, varied Pr/Ph ratio, C19-26tricyclic terpanes, high abundance of the tetracyclic terpanes, odd carbon number predominance in alkane and high abundance of C27sterane in the source rocks from Sa’ar and Nayfa formation indicate the shallow and suboxic depositional environment with the terrestrial higher plants and algae contributions inputs. For the homohopane series in the source rocks from Lam and lower Nayfa formation, its C35/C34ratio is more than1.1, indicating the strongly reduced depositional environment. However, gammacerane abundance for the former is obviously higher than the latter, the gammacerane index is up to0.76, showing that the salinity of the depositional environment for Lam formation is obviously higher than the lower Nayfa Formation. Meem formation is different from other formations in the extremely high abundance of the neohopanes, and their Ts/Tm and C29TS/C29H ratios are up to17.5and3.3, respectively, indicating a kind of the shallow, oxidizing and strong bacteria activity in the depositional environment. And the lower C24Te/C26TT value suggests that the algae contribution is more than higher plants. The depositional environment and the characteristics of the lithology for Meem formation source rocks is benefit for the formation of rearranged biomarkers resulting in the higher abundance of the diadrimane、diahopane and diastrane. It is predicted that the distribution and composition of different biomarkers in four sets of the source rocks would be printed in their oils, it is basis for the correlation between crude oils and the source rocks.There are some differences in the distributions and compositions of the biomarkers in crude oils and oil sands from Sa’ar, Sabatayn, Shuqra Formaiton. In the crude oils and oil sands from Sa’ar and Sabatayn Formation, the homohopane series display C35>C34, and the abundance of gammacerane is higher. In the oilsands from Shuqra Formation, the relative abundance of homohopane sereis decrease with increasing carbon number, and the abundance of gammacerane is quite low, but the abundance of diahopane and diasterane is abnormally higher than in oils and oil sands from Sa’ar and Sabatayn Formation, suggesting that there are differences in depositional environment and organic matter input between them. The former is derived from the source rocks from a kind of salty-reduced environment, and the later from a kind of fresh and oxidizing environment. In addition, the crude oils from shallow Sa’ar formation was slightly biodegraded leading to part loss of normal alkanes resulting in higher Pr/C17and Ph/C18ratios.The vitrinite reflectance values Ro、biomarker maturity parameter such as CPI, C29sterane ratio, methyldibenzothiophone ratios indicate that the source rocks from Sa’ar formation still is immature, from the upper Nayfa formation in low mature stage、but from the lower Nayfa、Lam、Meem formation in mature stage. Other maturity parameters, such as Ts/Tm, methylphenanthrene index and methylphenanthrene ratio, affected by the depositional-organic facies, can not reflect the maturity of the organic matter in the source rock accurately. Based on the maturity parameters such as hopaneC3122S/(22S+22R)、sterane C2920S/(20S+20R)、C29ββ/(ββ+αα) suggest that all the oils and oil sands belong to mature oils.According to the distribution characteristics of terpanes and the related fingerprint parameters in oils and source rocks from different formations in Abyed-1well, it shows that oils and oil sands from Sa’ar and Sabatayn may be derived from the source rocks of Lam formation, and oilsand from Shuqra formation may be derived from Meem formation. Certainly, the crude oils in Abyed-1well may not be generated from the source rocks in the same well. Combined with the background of tectonic geology in study area, it could guess that the oils in Abyed-1well may come from the west sag.