Geochemical Characteristics and Evaluation on Hydrocarbon Generation Potentials of Source Rocks in Jurassic Eastern Qaidam Basin

Key words: 
• Qaidam basin /
• Jurassic source rocks /
• petroleum geochemistry /
• biomarker featuwes /
• individual hydrocarbon isotope /
• evaluation of source rocks

Abstract: The geochemical characters of 13 Jurassic source rock samples in the eastern Qaidam basin are studied by means of Rock-Eval, GC-MS, GC-MS-MAT and so on Based on the comprehensive analyses of the organic carbon content, the content and conversion ratio of organic matter, parameters of pyrolysis, organic petrology parameters, biomarker characters of saturated hydrocarbon and stable carbon isotope composition of individual hydrocarbon, the organic matter's abundance, type and maturity are studied, and the potential of generation hydrocarbon are evaluated.1．Abundance of the organic matter：The variety range of the organic carbon content is wide, which is 0.20%～42.41%. Of which, the coal samples' organic carbon content is bigger than shale samples'. And only 31% samples' organic carbon content is less than 1.0%.The variation range of the soluble organic matter's content and total hydrocarbon is 0.0045%～1.0326% and 12.1×10-6～5 443.9×10-6 respectively, only 53.8% samples' soluble organic matter content exceeds 0.015%, 46.2% samples' total hydrocarbon exceeds 50×10-6, and have commonly low values. The conversion ratio of organic matter ("A"/TOC% and HC/TOC%) have also low values. Of which, the largest "A"/TOC% value is 4.20%, and only one sample's HC/TOC% is larger than 1.00%. 2．Type of the organic matter：According to the result of pyrolysis analysis, the variety range of IH、IO and S2/S3 is 25～1　700mg/gTOC、4～1　350mg/gTOC and 0.29～42.96 respectively. According to the organic petrology analysis of the maceral the major part is vitrinite, ranging from 17% to 80%, averaging being 65%. The second part is inertinite, ranging from 16% to 81%, averaging being 27%. The smallest part is exinite, ranging from 25% to 21%, averaging being 8%. 3．Maturity of the organic matter：Except three samples whose Tmax is above 512℃, all other samples'Tmax is 411～481℃,being the stage of lower-maturity to maturity. According to the vitrinite reflectance Ro, which is from 0.694 to 0.865, the organic matter is also lower-maturity to maturity stage, most of the source rocks being the highest stage of generating hydrocarbon. 4.Biomarker characteristics of saturated hydrocarbon：The characteristic of normal alkane are, from the TIC, the saturated hydrocarbon's dominant form is latter single peak, part of them is double peak or front single peak , having common features with other coal-bearing sedimentary basin. The isoprene's characteristics are that, the Pr/Ph of 54% samples is more than 1.0, which show that the Jurassic source rocks are not completely from the typical marsh environment, but much like the lake-beach and shallow lake depositional environment. CPI and OEP of the source rocks are more than 1.0, of which, CPI from 1.19 to 2.62, OEP from 1.05 to 2.40. ∑C⁺₂₁/∑C^_₂₁ of the samples are lower. All these, on the one hand, show that the source rocks come from continental higher plants, on the other hand, show that the maturity of the source rocks is not high. The existence of bicyclic sesquiterpanes and diterpenoids show the source rocks have the characteristics of coal-measure formation.The characteristics of tricyclic terpanes and hopanes of Jurassic samples are the amount of the tricyclic terpanes is less than hopanes, in which, commonly C29 and C30the is major part, C27(Tm or Ts) is second part, the C32+ homohopanes' amount is low, and gammacerane can't be checked. The characteristics of steranes are that C29-steranes is major part, C27-steranes is second part, C27-steranes is lowest, which forming non-symmetrical V shape. The maturity parameters of steranes and hopanes doesn't reach the equilibrium point of evolution. 5．Stable carbon isotope of individual hydrocarbon：This feature is that δC¹³ of the coal samples is high, ranging from -23.2‰ to -33‰, mostly in -24‰～-27‰. The δC¹³ of shale and mudstone is less than coal samples, however, the δC¹³ of shale and mudstone is bigger than other areas, mostly in -20‰～-27‰. The variation of individual hydrocarbon isotope is obvious, and the curve has a jaggy shape. With the increase of carbon numbers, δC¹³ has a decreasing tendency. The distribution curve of shale is below that of mudstone, and that of mudstone is below the curve of coal-measure rocks. The δC¹³ of different samples have bigger difference. Above features show that the depositional imentary environment of the source rocks with their different bio-input being continental higher plants are marsh lake-beach and shallow lake, and the maturity of the source rocks is low. Summing up above all the characteristics, the following results are obtained： (1) The Jurassic source rocks have relatively high content of organic carbon, and most of the samples belong to medium-good source rock. (2) Most of the organic matters are of type Ⅲ,few of them are of typeⅠor type Ⅱ.(3) The organic matter is at low-maturity to maturity stage. (4) The sedimentary environment of the source rocks with their bio-inputs being continental higher plants were marsh, lake-beach and shallow lake, and the source rocks possess the characters of coal-measure rocks. (5)There is a hydrocarbon generation potentials with the Jurassic source rocks in some areas in the basin.