Distribution and significance of C 40 + alkanes in the extracts of Cretaceous source rocks from the Songliao Basin

Abstract

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

A biomarker study of depositional paleoenvironments and source inputs for the massive formation of Upper Cretaceous lacustrine source rocks in the Songliao Basin, China

“…The Songliao Basin is located in north-eastern China and covers a vast area of 260,000 km 2 , with a total thickness of up to 10 km of Cretaceous lacustrine sediments (Feng et al, 2007) (Fig. 1).…”

Paleoenvironmental reconstruction of the Late Santonian Songliao Paleo-lake

“…The biomarker composition in the mudstones of Upper K 2 n 1 and in the black shales and oil shales in Lower K 2 n 2 supports the idea of diverse OM sources. High concentrations of C 23 n-alkanes, relatively high gammacerane and aryl isoprenoids in the black shales suggest mixed OM sources, mainly originating from algae, bacteria, and higher plants (Feng et al, 2007;Bechtel et al, 2012). While the n-alkanes of Upper K 2 n 1 are dominated by medium-to long-chain lengths, suggesting an increase in terrestrial OM input while aquatic OM input decreased.…”

“…Long-chain n-alkanes were originally thought to be derived predominantly from higher plants (Tissot and Welte, 1984;Tegelaar et al, 1989), but increasingly more studies revealed that algae and other aquatic organisms can also produce large amounts of these high molecular-weight waxy alkanes (e.g., Huang et al, 2003;Feng et al, 2007;Bechtel et al, 2012). Feng et al (2007) and Bechtel et al (2012) suggested that Botryococcus braunii may have contributed long-chain n-alkanes to the Songliao Paleo-lake deposits. In addition, a certain amount of dinosterane was detected in Lower K 2 n 2 (Figs.…”

“…The presence of aryl isoprenoids in Lower K 2 n 2 samples could therefore indicate OM input from green sulphur bacteria. Feng et al (2007) suggested a highly diversified OM input in K 2 n 1 + 2 , ranging from aquatic plants, blue-green algae, and bacteria to terrestrial plant matter. The biomarker composition in the mudstones of Upper K 2 n 1 and in the black shales and oil shales in Lower K 2 n 2 supports the idea of diverse OM sources.…”

Characteristics of remaining oil viscosity in water- and polymer-flooding reservoirs in Daqing Oilfield

“…At the same time, Carlson et al [15], Nguyen et al [16] and Boukadi et al [17] studied the possible origin of wax precipitation in crude oil by high-molecular weight alkanes. Feng et al [18] pointed out that the relative abundance of C 40+ hydrocarbons in crude oil appears to relate closely with oil viscosity. Wang et al [19] and Huang et al [20] studied the high molecular-weight n-alkanes of heavily biodegraded oil and precursor of high-wax oil, respectively.…”