The enhanced presence of tricyclic terpanes in the lacustrine rock extracts of the Upper Triassic Yanchang Formation, Qingcheng area, Ordos Basin, China

Abstract

The predominance of tricyclic terpanes over hopanes was observed for the first time in the lacustrine rock extracts of the Upper Yanchang Formation collected from Well CYX in the Qingcheng area, Ordos Basin, China. Specifically, hopanes are dominant terpanes in the Chang 7-2 member, while tricyclic terpanes ranging from C19 to C39 predominate over hopanes in the Chang 7–3 and Upper Chang 8-1 member. The distributions of terpanes, steranes, as well as n-alkanes vary significantly in these thermally mature rock extracts, however, are closely correlated in each sample. Generally, rock extracts comprising more abundant tricyclic terpanes and their lower molecular weight homologues are characterized by having lower Pr/n-C17 ratios and by containing more abundant low molecular weight n-alkanes (n-C21-) and short-chain C21-22 steranes. Furthermore, among the Chang 7–2 and Chang 7-3 samples, black shales contain more abundant low molecular weight tricyclic terpanes and n-alkanes (n-C21-), and short-chain C21-22 steranes when compared with siltstones and mudstones, which comprise more abundant hopanes, diasteranes and regular C27-29 steranes. The similar geochemical characteristics could also be found in other rock extracts that have variable abundances of tricyclic terpanes and hopanes, especially in the East Berlin Formation (Hartford Basin, Connecticut, U.S.A), which suggests that the causes for the variable distributions of terpanes, steranes and n-alkanes in these strata are similar.On the one hand, it is determined by the proportion of aquatic versus terrigenous source input and depositional environment. The predominance of tricyclic terpanes over hopanes and short-chain C21-22 steranes over regular C27-29 steranes in the Chang 7-3 member, especially in black shales, could be resulted from algal bloom and possibly associated microbial metabolic process in stable or stratified water column, which may be triggered by plentiful nutrients provided by runoff, volcanic or hydrothermal events. On the other hand, high hydrocarbon-generation intensity and petroleum expulsion efficiency for source rocks, especially for black shales, may be another significant driving force for the enrichment of tricyclic terpanes. In such cases, significant oils have been generated and expelled, an early or faster rate of generation/expulsion of hopanes relative to tricyclic terpanes may occurred, which is evidenced by the extended hopanes are depleted in all the studied rock extracts. Furthermore, to our knowledge, the predominance of tricyclic terpanes over hopanes has not been observed in source rocks of the Yanchang Formation in other regions of the Ordos Basin. The observation of the enhanced presence of tricyclic terpanes in the current study may suggest that the hydrocarbon-generating intensity and petroleum expulsion efficiency of source rocks in the Qingcheng area are higher than that of other regions in the basin. Thus, the Qingcheng area has tremendous potential for petroleum resources and is an important area for future exploration.