Geochemical Characteristics Of Crude Oils And Oil Source Correlation From GaSiKuLe Oilfield Of Qaidam Basin

The Tertiary system in the south area of western Qaidam Basin is saline lacustrine sediment, the area of about40,000square kilometers, where abundant oils and gas is showed. The region is currently focused on the accounting for the whole basin more than80%～90%of oil production and reserves. Therefore been listed as key areas for exploration in Qaidam Basin for a long period of time. GaSiKuLe oilfield is located in the east of the Gaskule Lake in the western Qaidam Basin, belong to Mangya depression Gasi rift,and to be at the Shizigou-Youshashan regicide overthrusts footwall. GaSiKuLe oilfield not only have good geological features, but also have rich reserves.Now GaSiKuLe oilfield is the largest oil field of the industrial exploitation of the value in the basin.This paper comprehensive analysis29source rocks samples of the Tertiary strata of the western Qaidam Basin and27crude oil samples of GaSiKuLe oilfield samples, combined with the geological background of western Qaidam Basin.Integrating geology setting in the south area of western Qaidam Basin, the Tertiary source rocks in the south area of western Qaidam Basin are assessed synthetically. Occurrence environment, biological sources and organic matter maturity of the Tertiary oil in this area are studied. And in the basis of these reseaches, oil-rock correlation in the GaSiKuLe oilfield of western Qaidam Basin is studied. The results and innovation points are as fellows:1.By the type of organic matter of source rocks of the Tertiary of the western Qaidam Basin found that:the samples of the Shizigou-Yuejin area n-alkanes have apparently even-odd predominance of C20～C28,and CPI is between0.70～1.32.The samples of the Arar-Hongliuquan area n-alkanes are bimodal characteristics, C27～C31have odd carbon number predominance, CPI is between0.98～2.31,and have apparently even-odd predominance of C20～C28.The Wunan area n-alkanes has a single peak characteristic, haven’t apparently odd-even predominance of n-alkane, CPI is between0.82～1.65. All Pr/Ph of n-alkanes of source rocks extracts are less than0.7, between0.09～0.57, and showed a strong advantage of phytane. It is indicated that most of C28αααR sterane relative content less than30%, C27αααR sterane and C29αααR sterane relative content of greater than30%, C27αααR steranes a slight advantage than C29αααR sterane,and some samples reached80%.It show that biogenic composition of organic matter is aquatic organisms, particularly the upper section of the last Ganchaigou group (E32). Overall, the next Ganchaigou group (E31andE32) and lower part of the last Ganchaigou group (N1) all little change in type of organic matter, basically are sapropel-humic type,and biogenic input are aquatic organisms dominant.2. Geochemical studies of the Tertiary source rocks of the western Qaidam Basin reveals the salt lake facies typical characteristics, obvious gammacerane advantage, homohopane cocky characteristics and aromatics three fluorene series showed significant advantages of sulfur fluorene. Through different parts of the source rocks of chloroform bitumen "A" biomarker and aromatic, inferred that the Tertiary sedimentary environment of the western Qaidam Basin is the deposition of higher salinity water environment and a strong reduction.The Shizigou-Yuejin area Pr/Ph<0.53,most ratio of C35homohopane/C34homohopane>1.0, average value is1.28; Gammacerane/C3ohopane distribution range is wide, from0.07to1.88, and average value is0.59. The Wunan area,0.14<Pr/Ph<0.57, average value is0.34, and most ratio of C35homohopane/C34homohopane>0.5,average value is0.85; ratio of Gammacerane/C3ohopane from0.2to1.25,average value is0.52.The Arar-Hongliuquan area Pr/Ph<0.3, ratio of C35homohopane/C34homohopane from0.59to2.39, average value is1.55; ratio of Gammacerane/C30hopane from0.75to1.12, average value is0.95.3. After the study that crude oil samples of the GaSiKuLe oilfield found most samples C21-/-C22+>1.0, indicates that crude oil sources have some degree of higher plants input of terrigenous. The crude oil samples of the GaSiKuLe oilfield OEP<1.2,from0.88to1.11;CPI from0.93to1.01,no regularity,no obvious even-odd predominance,indicates that the crude oil of theGaSiKuLe oilfield is immature. All crude oil samples with a strong phytane advantage, Pr/Ph ratio are lower, from0.25to0.33, indicates that the environment is the strong reduction marine and salt lake. C27aaaR、 C28αααR and C29aaaR steranes relative composition changed little, C27αααR steranes dominant, it show that biogenic composition of organic matter is aquatic organisms. The crude oil samples of the GaSiKuLe oilfield C35homohopane dominant, with high gammacerane characteristics,Gammacerane/C3ohopane ratio is generally large, basically in the more than0.7, reveals the crude oils of the GaSiKuLe oilfield formation in the salt lake depositional environments.Study that combination characteristics of the biomarker and aromatic hydrocarbon molecules, found that crude oil also showed the characteristics of the typical lagoons phase to oil but there are also subtle differences. The regional distribution are difference. The crude oils of the GaSiKuLe oilfield according to the depositional environment and maturity shows in the same category:C35homohopane/C34homohopane>1.2, Gammacerane/C3ohopane>0.8. The crude oil from the environment of high salinity and reduction. 4. With effective maturity indexes choosed,which are C3122S/(S+R)、C2920S/(S+R) and C29αββ/(ααα+αββ).Indicates that the crude oils of the GaSiKuLe oilfield should be immature-low mature oil. Maturity characterized as:C2920S/(S+R) from0.25to0.42, C29αββ/(ααα+αββ) from0.27to0.43.5. In the basis of study of geochemical characteristics of source rocks and crued oil, combining differentiation of oil, oil-rocks correlation is made. Pointed out the GaSiKuLe oilfield of the main oil source:he crude oils of the GaSiKuLe oilfield are mainly derived from the upper part of E31and E32source rocks in Shizigou-Hongliuquan area, and partly derived from E32source rocks in Wunan area.