Abstract
A suite of oil-sand bitumen and crude oil samples, collected from the Laizhouwan depression, Bohai Bay basin, were geochemically investigated for molecular compositions. Three oil families (A, B, and C) were classified by hierarchical cluster analysis (HCA) and principal component analysis on nine typical biomarker ratios. Typically, family A oils are characterized by relatively low values of gammacerane/C31 homohopane 22R (G/H31, 0.13–0.76) and C35/C34 homohopane (H35/H34, 0.39–0.78), suggesting a source from freshwater depositional environments. In contrast, family C oils show a relatively high G/H31 (2.49–5.41) and H35/H34 (1.43–2.45) ratios, indicating a source from hypersaline water depositional environments. Family B oils display ratios of G/H31 and H35/H34 in-between the range of families A and C, suggesting mixed origin. In addition, family A oils can be further classified into four subfamilies (A1, A2, A3, and A4) and family B oils into two subfamilies (B1 and B2) by HCA. The A1 oils characterized by a high C24 tetracyclic terpane/C26 tricyclic terpane (TeT24/TT26) ratio (1.02–1.39) are mainly distributed in the northeast, B1 oils characterized by relatively low TeT24/TT26 ratio are in the west, and A2, A3, and A4 oils with an intermediate TeT24/TT26 ratio are in the center of the depression. Oils in well L16-1-2 in the southern depression, however, show vertical variations with family C oils in the deeper reservoirs, subfamily B2 oils in the shallower reservoirs, and subfamily A4 oils in the middle-depth reservoirs. Based on the biomarker compositions, at least three oil charges were indicated: family C oils are likely sourced from the Es4 rock in the southern sag, B2 oils may be a mixture of family C with family A oils, and A4 oils without biodegradation influence may be the latest charge derived from the Es3 source rock in the northern sag. The oil families and/or subfamilies with typical genetic affinities, as well as the regular occurrence in different blocks, may indicate two major petroleum systems or multiple subsidiary oil systems existing in the Laizhouwan depression.
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