Molecular Markers Of Neoproterozoic-Lower Paleozoic Petroleum Systems And Their Geological Significance

In this study,we reported 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 were compiled for comparison.The research aimed at characterizing the molecular markers and providing their geological significance.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,a noticeable absence of25-norhopane and monoaromatic steroids among all the samples.The value of pristane to phytane ratio（Pr/Ph）falls below 0.8 for all the solid bitumen samples,whereas it is equal to0.9,on average,for the Lower-Middle Cambrian oils and rock extracts.Low values of C₃₀17α（H）-diahopane/C₃₀H（average=0.09）,relatively high values of gammacerane to hopane ratio(G/C₃₀H),a V-shaped distribution pattern for the following regular steranes:C₂₉-C₂₈-C₂₇,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:ββ/（ββ+αα）and20S/（20S+20R）C₂₉ steranes,TT/（TT+17α-hopanes）,and Ts/（Ts+Tm）fall below their respective equilibrium endpoints.While the calculated equivalent vitrinite reflectance values range within 1.69-2.20%.We show 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,while the maturity rate corresponds to post-maturity level.In addition,we reported a comparative study of sulfur and oxygen-containing polycyclic aromatic hydrocarbons（PAHs）using a large dataset that includes reservoir solid bitumen and produced oil samples from four basins.Data from two hundred sixty-two published well samples from Tarim Basin,Beibuwan Basin and Bohai Basin were collected from literature,which were combined with data from sixty-two additional well samples from the Ediacaran-Lower Cambrian strata in the Sichuan Basin.Moreover,eighteen representative new samples were collected and analyzed by gas chromatography-mass spectrometry（GC-MS）.Based essentially on Linear correlation coefficients between sulfur,oxygen and nitrogen-containing PAH parameters,we found non-uniform results among the studied basins.Indeed,the coefficient correlation of given parameters changes widely from one basin to another.Furthermore,oxygen and nitrogen-containing PAH parameters on the one hand,and MPI-1 on the other hand,exhibited correlation coefficient varying essentially from no linear relationship to weak linear relationship,while sulfur-containing PAH parameters showed wide range of results.Despite the disparity of the numerical linear correlation,Methyl DiBenzo Furans（MDBFs）and DiBenzo Thiophenes（DBTs）have been applied successfully in the reconstruction of oil filling history through the Neoproterozoic-Lower Paleozoic reservoirs in the Sichuan Basin.