Geochemical Characteristics Of Eocene Pinghu Formation Coal-bearing Source Rocks In The Xihu Depression,East China Sea Basin

Xihu Depression is located in the east central of Zhedong downwarpingregion, the East China Sea Shelf Basin, it is one of the biggest oil and gas exploration potential depressions in this basin, it also has an excellent local structures with the development of the tertiary hydrocarbon source rocksets such as Pinghu Formation, which is the main source rocks in Xihu Depression.Combined with the geological background of Xihu Depression, the main contents of this paper including geochemical characteristics of Pinghu Formation which includes distribution of source rocks, abundance、types and maturity of organic matters, the hydrocarbon genesis potential of coals and mudrocks in Pinghu Formation has also been evalued. On the basis of analysis in experiment, according to the geochemical characteristics of saturated hydrocarbon and aromatic hydrocarbon. Sedimentary enviroments, organic source and characteristics of biomarkers have been discussed.The lithology of source rocks in Pinghu Formation are dark mudstone and coal & carbonaceous mudstone. The average TOC value of dark mudstone is 1.03%;the average genetic potential (S1+S2) value is 1.86 mg/g; the average TOC values of carbonaceous mudstone and coal are 12.6%and 51.3% respectively; genetic potential average are 35.4 mg/g,143.3mg/g. H/C atomic ratio (0.84-1.08) of coal and carbonaceous mudstone samples are higher than mudstone (0.64-0.99).Obvious difference in the HI values exist between coals and mudstones, averaging 286 and 140 mg HC/g TOC, respectively. Coals have higher HI values than mudstones. Hydrocarbon source rocks of Pinghu Field are typical coal-bearing source rocks, mudstone samples contain type-Ⅲkerogen, while coals and carbonaceous mudstones contain typeⅡandⅢ. Ro values of this source rocks is ranged from 0.5～1.3%, the corresponding rock pyrolysis temperature Tmax peak value is mainly distributed in the 430～460℃, and the maturity is belong to condensate gas and generate oil evolutionary stages.The GC/MS analytical data of 60 saturate and aromatic fractions reveals that the saturated hydrocarbons of Pinghu coal-bearing source rocks in the Xihu depression of the East China Sea shelf basin are characterized by abundant diterpenoid compounds originated from the resins of coniferous trees and dominant C29 in steranes (>45%) Moreover, their aromatic fractions are abnormally rich in the terrestrial biomarkers such as retene and 1,7-dimethylphenanthrene, indicating a predominantly organic matter source derived from terrigenous higher plant. Compared with the coals and carbonaceous mudstones, the mudstones have relatively higher abundance of C27 sterane and bimodal distribution in n-alkanes, suggesting a certain proportion contribution of aquatic organism to the organic source. The source rocks as a whole are believed to deposited in an oxidizing swamp-lacustrine environment, as reflected by high Pr/Ph ratios ranging mostly from 3.5 to 8.5 and abundant dibenzofuranes. However, some mudstones show relatively lower Pr/Ph values (round 1.5), and thus appear to be formed in a weak oxic to reducing semi-deep lake setting. Coal and carbonaceous mudstone could be distinguished by higher relative concentration of C29 sterane (>80%) and lower gammacerane/C30 hopane values (<0.05) from mudstone.