To determine geochemical indicators for depositional environment favored by terrestrial petroleum source rocks, we selected 40 source rock samples from the Late Cretaceous Qingshankou Formation (K 2 qn) and the first member of Nenjiang Formation (K 2 n 1 ) in the Songliao Basin to qualify saturate fraction and aromatic fraction using GC-HRT (gas chromatography high resolution time-of-flight mass spectrometry) and quantify important biomarkers using GC-MS. The results reveal that source rocks from the 1st member of Qingshankou Formation (K 2 qn 1 ) are characterized by not only high contents of terpanes, regular steranes and 4-methylsteranes but also high contents of dinosteranes, C 31 steranes and aryl isoprenoids. Presence of specific biomarkers like elementary sulfur and lanostanes indicates a depositional environment of lagoon characterized by water stratification and high salinity. In the 2nd-3rd members of Qingshankou Formation (K 2 qn 2+3 ), source rocks contain lower contents of biomarkers, indicating a depositional environment of shallow fresh-water lake delta. Source rocks in the K 2 n 1 contain high contents of terpanes, regular steranes and 4-methylsteranes but lower contents of dinosteranes, C 31 steranes and aryl isoprenoids, indicating a depositional environment of fresh-brackish open lake characterized by low salinity and poor water stratification, where organic matter is seriously altered by bacteria. Overall analysis shows that primary geochemical indicators for terrestrial petroleum source rocks are as follows: 1) C 30 hopanes > 1500 ppm; 2) gammacerane >190 ppm; 3) C 27 steranes >200 ppm; 4) 4-methylsteranes >100 ppm; 5) aryl isoprenoids > 3 ppm; 6) dehydroxyl-vitamin E >10 ppm.Songliao Basin, source rock, depositional environment, geochemistry, biomarker Citation:Feng Z H, Fang W, Li Z G, et al. Depositional environment of terrestrial petroleum source rocks and geochemical indicators in the Songliao Basin.

Source rock extracts and crude oils from the Songliao Basin were analyzed by high-temperature gas chromatography (HTGC), gas chromatography-mass spectrometry (HTGC-MS) and gas chromatography-isotope ratio-mass spectrometry (GC-IRMS), for high molecular-weight alkanes. The distributions of n-alkanes in the Nenjiang Formation extracts are in the C 14 -C 63 range; a bimodal distribution occurs in the 21 C − and C 21 -40 regions. The C 30 -C 37 n-alkanes are accompanied by C 29 -C 35 hopanes, whereas the high molecular-weight C 45 -C 47 n-alkanes co-occur with abundant isoalkanes, alkylcyclohexanes and alkylcyclopentanes. The high δ 13 C values of the n-alkanes and the microscopic maceral compositions indicate a highly diversified organic source input for the Nenjiang Formation source rocks, ranging from aquatic plants, blue alge-bacteria, to land plant material. In contrast, n-alkanes in the rock extracts of the Qingshankou Formation are characterized by a single modal distribution, with relatively low abundances of C 29 -C 35 hopanes, but high molecular-weight isoalkanes, alkylcyclohexanes and alkylcyclopentanes. The relatively low δ 13 C values of C 22 -C 44 n-alkanes and organic material compositions indicate that the source rocks in the Qingshankou Formation contain dominantly type I algal organic matter. The relative abundance of 40 C + compounds in source rocks changes little at low maturity stage, but decreases drastically at higher maturity levels, with a concurrent reduction in the odd/even carbon predominance. In crude oils, in contrast, the relative abundance of 40 C + compounds appears to relate closely with the oil source and oil viscosity.Songliao Basin, source rock, high-temperature gas chromatography (HTGC), high-temperature gas chromatography-mass spectrometry (HTGC-MS), gas chromatography-isotope ratio-mass spectrometry (GC-IRMS).The 40 C + high molecular-weight alkanes in crude oil and source rock samples can be readily examined using HTGC [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] . This molecular-weight range shows quite different carbon number distributions from those of the C 22 -C 35 alkanes in routine conventional GC analysis [17] , thus providing important information on oil wax deposition [2,11,13] , organic source input, source rock depositional environment [5,7,12,15] , and oil biodegradation [3,6,14] .Available data indicate that sometimes it is necessary to pre-concentrate the high molecular weight fractions from oil and source rock extract prior to HTGC analysis of the so-called ceresin wax (dominated by 40 C + ) or phaneritic wax fraction (middle and low molecularweight 40 C − compounds) [7] . As a result, the lack of compounds in other molecular-weight ranges often jeopardizes a meaningful comparison and subsequent

Citation: Feng Z H, Fang W, Wang X, et al. Microfossils and molecular records in oil shales of the Songliao Basin and implications for paleo-depositional environment.Several oil shale beds, over 10 m thick, occur at the base of the first member of the Upper Cretaceous Qingshankou Formation (K 2 qn 1 ) in the Songliao Basin. They act both as excellent source rocks for conventional oil and as potential oil deposit for shale oil production. Here we combine micropaleontology with organic geochemistry to investigate the paleo-depositional environment and organic source characteristics of the oil shales and black shales. Our results indicate that algal remains are dominant microfossils in K 2 qn 1 oil shales, and their relatively high abundance suggests a major algal thriving event during the oil shale deposition. The presence of fresh water and brackish water species, Sentusidinium, Vesperopsis and Nyktericysta, and marine or brackish water deltaic and lagoonal species such as Kiokansium and Dinogymniopsis demonstrate that this paleo-continental lake was influenced by marine transgressions at the time of K 2 qn 1 oil shale formation. The extremely low pristine/phytane ratios, relatively high abundance of gammacerane and 4-methyl steranes, and low δ 13 C values of C 14 -C 37 n-alkanes in the oil shale organic extracts indicate the deposition of oil shales in anoxic and highly stratified water columns and the significant contribution of lacustrine algae to sedimentary organic matter. High molecular-weight paraffinic hydrocarbons with unusually high abundance of nC 43 , nC 45, and nC 47 may be related to special algal species associated with marine transgression events. The giant water body of Songliao paleo-lake and the change in the organic and chemical environment (such as nutrition source and water column salinity) associated with seawater transgression into the lake are among the most important reasons for oil shales in the Songliao Basin being different from mudstone and oil shale in other rifted basins.

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The experimental analysis of 21 crude oil samples shows a good correlation between high molecular-weight hydrocarbon components (C 40 + ) and viscosity. Forty-four remaining oil samples extracted from oil sands of oilfield development coring wells were analyzed by high-temperature gas chromatography (HTGC), for the relative abundance of C 21 − , C 21 -C 40 and C 40 + hydrocarbons. The relationship between viscosity of crude oil and C 40 + (%) hydrocarbons abundance is used to expect the viscosity of remaining oil. The mobility characteristics of remaining oil, the properties of remaining oil, and the next displacement methods in reservoirs either water-flooded or polymer-flooded are studied with rock permeability, oil saturation of coring wells, etc. The experimental results show that the hydrocarbons composition, viscosity, and mobility of remaining oil from both polymer-flooding and water-flooding reservoirs are heterogeneous, especially the former. Relative abundance of C 21 − and C 21 -C 40 hydrocarbons in polymer-flooding reservoirs is lower than that of water-flooding, but with more abundance of C 40 + hydrocarbons. It is then suggested that polymer flooding must have driven more C 40 − hydrocarbons out of reservoir, which resulted in relatively enriched C 40 + , more viscous oils, and poorer mobility. Remaining oil in water-flooding reservoirs is dominated by moderate viscosity oil with some low viscosity oil, while polymer-flooding mainly contained moderate viscosity oil with some high viscosity oil. In each oilfield and reservoir, displacement methods of remaining oil, viscosity, and concentration by polymer-solution can be adjusted by current viscosity of remaining oil and mobility ratio in a favorable range. A new basis and methods are suggested for the further development and enhanced oil recovery of remaining oil. remaining oil, viscosity, mobility ratio, water-flooding, polymer-flooding, heterogeneity, polymer-solution viscosity Citation: Xu Z S, Zhang J H, Feng Z H, et al. Characteristics of remaining oil viscosity in water-and polymer-flooding reservoirs in