Organic geochemical traits and paleo-depositional conditions of source rocks from the Carboniferous to Permian sediments of the northern Mahu Sag, Junggar Basin, China

摘要

The organic geochemical traits of Carboniferous to Permian hydrocarbon source rocks (SRs) from the Mahu Sag, Junggar basin, have been investigated in terms of thermal maturity (TM), hydrocarbon potential (HP), organic matter (OM) source input and paleoenvironment. A total of forty-seven (47) core samples from five (5) wells have been surveyed through standard organic geochemical approaches, including the total organic carbon (TOC) and the rock pyrolysis (RP) measurement. Fifteen (15) of the samples were bitumen extraction and fractionation, and their aliphatic hydrocarbons and aromatic hydrocarbons were carried out for the gas chromatography-mass spectrometry (GC-MS) analysis. The results indicate that the studied samples came from the C and the P(1)f are the moderate or good SRs, but samples from the P(2)w and P(1)j are not good enough, most of them are poor or moderate SRs. The TM was indicated by the temperature of maximum pyrolysis yield (Tmax), the Carbon Preference Index (CPI) and Odd to Even Preference (OEP) values, the C-30 M/H versus Tm/Ts, the C-29 beta beta/(beta beta+alpha alpha) versus C-29 20S/(20S + 20R), and the alkyl naphthalene ratios. Result shows that almost all the samples from the P(1)f are in the mature stage at present; some samples of P(1)j and C are in the premature or mature stage, others are completely immature; all P(2)w samples are in the immature stage at present. The OM source was indicated by the n-Alkane distribution, the relative quantity of C-27-C-28-C-29 steranes, the (C-19 + C-20)/C-23 tricyclic terpane (TT), and the Terrigenous/aquatic ratio (TAR). The deposition environment was inferred by the pristane/phytane (Pr/Ph), the Gammacerane index, and the relative quantity of fluorene compounds (Fls), dibenzothiophen compounds (DBTs) and dibenzofuran compounds (DBFs). Result shows that the mainly OM source of the P(2)w, P(1)f, P(1)j, and C were the saline lacustrine algal, but some samples of the P(1)j were mixed with the higher plants; most samples deposited in the strong reduction environment except the samples came from the P(2)w and some samples came from the P(1)j. Considering the OMs' content, type, origination, deposited paleo-environment and TM, the SRs of the P(1)f are considered the best sources rocks with the best conditions of the oil and gas generation.