Molecular markers of Neoproterozoic-Lower Paleozoic petroleum systems and their geological significance: A case study of the cratonic basins in western China

In this article, we report the investigation of the Ediacaran and Lower Paleozoic petroleum systems using their molecular markers. The cratonic basins located in western China were selected as a case study, whereas worldwide published samples where compiled for comparison. The study aimed in characterizing specifically and generally the molecular markers, and providing their geological significance as well. Gas chromatography-mass spectrometry experiments (GC-MS) were applied on Ediacaran reservoir solid bitumen extracts (Dengying Formation, member-Z2dn2) and Cambrian oil/rock extracts samples respectively from Sichuan Basin and Tarim Basin. The results show a predominance of n-alkanes over isoprenoids (e.g. Pr/nC17, Ph/nC18), a noticeable absence of 25-norhopane and monoaromatic steroids among all the samples. The values of pristane to phytane ratio (Pr/Ph) fall below 0.8 for all the solid bitumen samples, whereas they are equal to 0.9, on average, for the Lower-Middle Cambrian oils and rock extracts. Relatively high values of gammacerane to C30 17 alpha (H)-hopane ratio (G/C30H), low values of C30 17 alpha (H)-diahopane/C30H (average = 0.09), and a V-shaped distribution pattern for the following C29-C28-C27 regular steranes with C27 C29 > C28 and C28 < C27 < C29 respectively for Ediacaran and Cambrian samples, have been noticed as well. Moreover, the values of the following ratios: beta beta/(beta beta+alpha alpha) and 20S/(20S + 20R) C29 steranes, tricyclic terpanes/(tricyclic terpanes +17 alpha-hopanes), and Ts/(Ts + Tm) fall below their respective equilibrium endpoints, notwithstanding the calculated equivalent vitrinite reflectance values range within 1.69%-2.20%. The data shows that the source rocks of all the Ediacaran and Lower-Middle Cambrian samples of the western China were deposited in an anoxic and stratified environment, with the initial organic matter showing a prevalence of type II kerogen (marine algal). The organic matter have experienced none or at least a very slight biodegradation rate, in spite that the maturity rate corresponds to postmaturity level.