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Origin Analysis on Anomalies in Enriched 25-Norhopanes in Crude Oil From the Karamay Formation of Santai Oilfield in Junggar Basin, Northwest China


25-norhopanes, a series of C26-C34 compounds that are structurally equivalent to the regular hopanes, has been frequently utilized to indicate severe biodegradation of crude oil. The crude oil from Karamay Formation of Santai Oilfield contained abundant 25-norhopanes, which indicated the crude oil had been severely biodegraded. As compared to other compounds, n-alkanes are prone to be preferentially removed in the earliest stages of biodegradation. Consequently, n-alkanes are depleted gradually with increasing biodegradation. However, it appeared anomalously that n-alkanes showed on the total ion chromatogram of the “biodegraded” oil were not significantly removed and the chromatographic baseline hump was not easily detectable. To figure out the anomaly in enriched 25-norhopanes and weakly consumed n-alkanes, oil-source correlation and hydrocarbon accumulation periods of the crude oil were analyzed based on the biomarkers with strong resistance to biodegradation, burial history simulation and the analysis of fluid inclusions. The results indicated that the crude oil of the Karamay Formation originated from shales of Karamay Formation and Lucaogou Formation. In other words, the crude oil from Karamay Formation was the mixture of two source rocks. The reservoirs of Karamay Formation in Santai Oilfield were firstly charged by hydrocarbon from shales of the Lucaogou Formation at 172 Ma. During the period of oil accumulation, the sedimentary strata were uplifted rapidly and appeared to be eroded. Consequently, the accumulated oil was biodegraded severely due to the lack of well preservation condition for oil. At 102 Ma, the reservoirs were charged by hydrocarbon from shales of the Karamay Formation again. The continuous subsidence of sedimentary strata provided a better preservation condition for oil. Therefore, the anomaly in enriched 25-norhopanes and weakly consumed n-alkanes was a response to the mixture of the biodegraded oil and the undegraded oil. Due to a great deal of undegraded oil charging in the later period, the signal that the crude oil from the Karamay Formation had been biodegraded was concealed by the low biodegradation degree reflected by weakly consumed n-alkanes.