Through tectonostratigraphic analysis of the nonmarine, intracontinental Songliao basin in northeast China, four episodes of deformation are recognized: mantle upwelling, rift, postrift thermal subsidence and structural inversion.The episodes are related to regional geodynamics and plate motions. Each episode is associated with a speci¢c stratigraphic signature.The ¢rst period of deformation occurred during the Middle and Late Jurassic when asthenospheric upwelling heated, thinned and stretched the lithosphere.These events may have been caused by the narrowing of the Okhotsk Sea through subduction.This deformation is characterized by doming, extension, widespread volcanism and intrusion, and erosion.Volcanics inter¢nger with alluvial fan and alluvial plain facies systems tracts.The second rifting episode began in the latest Jurassic and continued into the Early Cretaceous. It resulted in the formation of a large number of isolated, NNE-trending fault blocks of 'basin-and-range' style. Rifting may have been caused by the formation and subduction of the Izanagi and Paci¢c Plates. Coal-bearing £uvial, £oodplain, lacustrine and fan-delta strata and widespread volcanic rocks ¢lled the fault-block basins.Volcanic strata hundreds to several thousand meters thick in the Huoshiling and Yingcheng Formations record multiple intrusive events during the rifting stage in the basin.These events were concurrent with episodes of intrusion and volcanic eruption in northeast China.The third phase of regional postrift deformation and subsidence, which began with the Lower Cretaceous Denglouku Formation, was caused by lithospheric cooling and extension, modulated by multiple compressional events. Subsidence in the Songliao basin permitted accumulation of thick postrift deposits, in contrast with other Cretaceous basins in Mongolia and northeast Asia.Three compressional episodes, which episodically interrupted the long-term cooling subsidence, originated from development of the Okhotsk suture and subduction of the Paci¢c plate. In the Early Cretaceous, pronounced compression originated from closure of Okhotsk Sea, forming the mountain ranges of Daxinganling, which provided sediment to the northern part of the basin. In the Late Cretaceous, the intensity of compression from the Paci¢c margin increased through time, causing westward migration of depocentres and uplift in the east until the end of Cretaceous. Postrift strata, typically 3000^4000 m thick with a maximum thickness of 6000 m, extend beyond the rift blocks and onlap the basin margins to form a large uniform basin. Early thermal subsidence strata include alluvial fan, £uvial, £oodplain, shallow lacustrine and delta facies tracts, overlain by large deltaic and lacustrine facies. Late postrift environments featured by large lakes in the basin centre rimmed by delta, £uvial and £oodplain environments. Re£ection seismic pro¢les show that strong structural inversion, including folding and uplift, began at the end of the Nenjiang Formation and culminated at the end of the Cretaceous...

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The Songliao Basin is the largest oil-bearing basin in China. In the absence of sufficient evidence, the possibility of seawater incursion(s) into the Songliao Basin remains controversial. Recently, we discovered relatively abundant foraminifera fossils from units 1 and 2 of the Nenjiang Formation of borehole SK-1. Benthic foraminifera (Gavlinella sp., Anomalinoides sp., Pullenia sp., Haplophragmoides sp., Karrorulina hokkaidoana, Clavulinoides sp.), as well as planktonic foraminifera (Archaeoglobigerina blowi, Archaeoglobigerina cretacea and Hedbergella flandrini), were identified. These fossils were widely distributed in the marine Cretaceous. According to the global distribution of the above-mentioned planktonic foraminifera, the age of these fossil bearing strata appears to be Late Coniacian to Santonian. More importantly, these foraminifera provide direct evidence for marine water incursions into the Songliao Basin during deposition of the Lower Nenjiang Formation.

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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

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.