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

Abstract

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 P1f are the moderate or good SRs, but samples from the P2w and P1j 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 C30 M/H versus Tm/Ts, the C29 ββ/(ββ+αα) versus C29 20S/(20S + 20R), and the alkyl naphthalene ratios. Result shows that almost all the samples from the P1f are in the mature stage at present; some samples of P1j and C are in the premature or mature stage, others are completely immature; all P2w samples are in the immature stage at present. The OM source was indicated by the n-Alkane distribution, the relative quantity of C27–C28–C29 steranes, the (C19+C20)/C23 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 P2w, P1f, P1j, and C were the saline lacustrine algal, but some samples of the P1j were mixed with the higher plants; most samples deposited in the strong reduction environment except the samples came from the P2w and some samples came from the P1j. Considering the OMs’ content, type, origination, deposited paleo-environment and TM, the SRs of the P1f are considered the best sources rocks with the best conditions of the oil and gas generation.