Organic Matter Preservation of Mudstone from Chengzihe Formation in Jixi Basin

Key words: 
• Jixi Basin

Abstract: Jixi basin is the MesozoicCenozoic residue basin and the important coal base in the northeast China. At present, several drilling wells in the coal fields discovered the oil and gas shows in this area. Chengzihe Formation is the coalbearing formation in the lower Cretaceous of the Jixi basin, which deposited in the circumstance of coastalshallow lake. The previous studies found the mussels fossil of brackish water and a lot of dinoflagellates of marine facies, which reflected the marine interlayer existed in the Chengzihe Formation. It infers that the transgression occurred in this period. At home and abroad, many studies indicate that the main source rock layers in the majority of oil and gas bearing basin are related closely with the transgression. We choose some mud rocks from the Chengzihe Formation and perform analysis on the major element and trace element, combining with the previous organic geochemistry data, to reveal the character of water circumstance when the mud rock of Chengzihe Formation deposits, and discuss how the transgression influences on the water circumstance and organic matter preservation. The analysis result indicates that the values of Sr/Ba which is salinity index ranging from 0.11 to 0.38, 0.26 average; the (Ca+Mg)*20/(Si+Al), as the alkalinity index, is from 0.22 to 1.33, 1.68 average; as the oxidationreduction indexes, V/Cr is 1.17~3.23, and Ni/Co is 0.89~3.71. Compared with the typical transgression sequence from Quantou Formation to Nenjiang Formation in Songliao basin, the values are low obviously in the Chengzihe Formation, which reflects the sediment and preservation circumstances of low salinity, low alkalinity and weak oxidation. In recent years, the analysis results on the biomarkers from the Chengzihe Formation indicate that as the oxidationreduction index, the value of Pr/Ph is 2.4~2.8, 2.2 average; the G/C30H which indicates the salinity stratification of water column is 0.11 average; the pregnane+homopregnane / C27sterane is 0.88 average, C27diasterane/C27sterane is 0.34 average, reflecting the salinity of water is low, which is coincide with the analytical result of the major element and trace element. All kinds of indexes do not show the character of the typical transgression sequence, such as the high salinity water and the strong reducive sediment circumstance. It indicates that the sediment environment is not fit for the organic matter preservation. It is coincident with the Gaofuhong(2007)'study on the typical transgression sequence of Yuliangzigou section in the Chengzihe Formation.
In addition, the previous study on the potential of generating hydrocarbon reveals the TOC value of the black mud rock is between 0.11% and 3.87, which is lower than the value of the coal and the carbonaceous mud rock, the TOC of which ranges from 18.58% to 80.27%. Besides, the hydrogen index (HI) of the black mud stone is 0.02~144.00 mg/g, and the generating hydrocarbon potential (S1+S2) is between 0.04~4.16 mg/g, all of which is lower than the value of the coal and the carbonaceous mud rock that the IH is 53.8 mg/g~207 mg/g, and the S1+S2 is 16.93~179.99 mg/g. The organic matter type (ⅡB )of the coal and carbonaceous muddy rock is better than the organic matter type(Ⅲ) of the black mud rock. It indicates that the coal and carbonaceous mud rock have the advantageous potential of generating hydrocarbon.
Some evidence of sedimenary facies study indicate that the water area reached the maximum during the whole periods in this basin, and it is probably that the Jixi basin, Boli basin in the north and the Hulin basin in the northeast form the uniform basin, suffering the transgression from the east. However, a lot of coal in Chengzihe Formation of Jixi basin and the geochemical evidence from the previous and present studies indicate that the transgression do not improve sediment environment evidently. Contrasting with Songliao basin, the scale of transgression is smaller in the Jixi basin, which do not improve the salinity and alkalinity evidently, so the water environment of the Chengzihe Formation is not good for the organic matter preservation. Due to the advantageous generating hydrocarbon potential of the coal and carbonaceous mud rock, it infers the oil and gas show in this area is probably related to the coal and carbonate mud rock.