GENERATION AND APPLICATION OF HYDROCARBONS PRESERVED IN THE CLOSED PORES FROM THE JURASSIC FORMATION, SICHUAN BASIN

The low molecular weight hydrocarbons (including gaseous alkanes and light hydrocarbons) offer a unique opportunity to study the petroleum generation processes and its migration from source to reservoir rocks. However, significant loss of low molecular weight hydrocarbons has occurred during core and cuttings collection, storage and subsequent sample preparation.

In this paper, an experiment was presented for getting the hydrocarbons without loss in closed pores. A considerable number of low molecular weight hydrocarbons were corrected by using the pseudo-components of hydrocarbons in closed pores. The gas-oil ratio (GOR) of closed pores hydrocarbon is in good agreement with the gas-oil ratio (GOR) generated by microscale sealed vessel (MSSV) pyrolysis. The feasibility of correcting the loss composition of light hydrocarbon and gas hydrocarbon with closed pores hydrocarbon is proved.

The hydrocarbon in closed pores provides practical geochemical evidence for the mechanisms of light hydrocarbon generation (carbenium ion and free radical mechanisms) and its controlling factors. Based on the cross-plot of light hydrocarbon parameters and the distribution of pseudo-components, the main control mechanism of hydrocarbon generation in Fuxing area can be divided into three series: (1) Free radical type: a preferential yield of nC₁, the products are dominated by straight-chain hydrocarbons, nC₁/ nC_1-5 shows the highest ratio and iC₅/nC₅ shows the lowest ratio; (2) Carbenium ion type: the gaseous hydrocarbon has the preferential yield of C_3-5 while producing abundant branched hydrocarbons, nC₁/ nC_1-5 shows the lowest ratio and iC₅/nC₅ shows the highest ratio; (3) Mixed type: the gaseous hydrocarbon has the preferential yield of C_3-5 while producing abundant branched hydrocarbons, nC1 is the main light hydrocarbon, and a certain amount of C_3-5 and branched hydrocarbons are produced at the same time, nC₁/ nC_1-5 and iC₅/nC₅ are between the results of the first two reaction mechanisms.

The present results from hydrocarbon components in closed pores indicate the catalytic reaction of acidic clays play an important role in the hydrocarbon generation process of Jurassic source rocks in the Fuxing area, which may change our view on the generation and distribution of hydrocarbon in the Fuxing area of the Sichuan Basin.