GEOCHEMICAL CHARACTERISTICS AND ORIGIN OF CRUDE OIL FROM JZ-A STRUCTURE ON THE LIAOXI UPLIFT, BOHAI SEA

Geochemical analyses of crude oil samples from the 2nd Member of Shahejie Formation of the JZ-A structure on the Liaoxi Uplift were carried out, upon which physical properties, maturities and characteristics of biomarkers of the crude oil were studied. Comparison of geochemical characteristics of the crude oil is conducted for the oil and gas fields discovered in the Liaoxi and Lizhong depressions. The research results show that the crude oil in the western block is dominated by condensate oil with low wax and low sulfur, while the crude oil in the eastern block is characterized by light oil with high wax and low sulfur. The saturated hydrocarbon chromatograms of the crude oil from JZ-A structure show a complete distribution pattern, indicating a lack of biodegradation. The results also show that organic matters in the 2nd Member of Shahejie Formation came from a mixed source with minor terrigenous input of organic matter. Crude oils were formed in a reduction to weak reduction and brackish or saline water environment. Two source-related oil classes are identified by hierarchical cluster analysis according to the biomarkers of the crude oil from JZ-A structure. The Class Ⅰ oil has higher gammacerane index, low-medium 4-methyl sterane indices(4-MSI), low hopane/sterane, high C₂₇/C₂₉ sterane and high C₂₈/C₂₉ sterane, which is most similar with the E₂s¹ source rock of Liaoxi Depression. However, the Class Ⅱ oil displays low-medium gammacerane index, medium-high 4-MSI, high hopane/sterane, medium C₂₇/C₂₉ sterane and low C₂₈/C₂₉ sterane, mostly similar with the E₂s³ source rock of Liaozhong Depression.