Geochemical studies of the Silurian oil reservoir in the Well Shun-9 prospect area, Tarim Basin, NW China

Commercial oil flow has been obtained from the sandstone reservoir of the Lower Silurian Kelpintag Formation in the Well Shun-9 prospect area. In the present studies, 10 Silurian oil and oil sand samples from six wells in the area were analyzed for their molecular and carbon isotopic compositions, oil alteration (biodegradation), oil-source rock correlation and oil reservoir filling direction. All the Silurian oils and oil sands are characterized by low Pr/Ph and C-21/C-23 tricyclic terpane (< 1.0) ratios, "V"-pattern C-27-C-29 steranes distribution, low C-28-sterane and triaromatic dinosterane abundances and light delta C-13 values, which can be correlated well with the carbonate source rock of the O-3 l Lianglitage Formation. Different oil biodegradation levels have also been confirmed for the different oils/oil sands intervals. With the S-1 k (2) seal, oils and oil sands from the S-1 k (1) interval of the Kelpintag Formation have only suffered light biodegradation as confirmed by the presence of "UCM" and absence of 25-norhopanes, whereas the S-1 k (3-1) oil sands were heavily biodegraded (proved by the presence of 25-norhopanes) due to the lack of the S-1 k (2) seal, which suggests a significant role of the S-1 k (2) seal in the protection of the Silurian oil reservoir. Based on the Ts/(Ts+Tm) and 4-/1-MDBT ratios as reservoir filling tracers, a general oil filling direction from NW to SE has been also estimated for the Silurian oil reservoir in the Well Shun-9 prospect area.